timoninmaxim commented on code in PR #12301:
URL: https://github.com/apache/ignite/pull/12301#discussion_r2468022180


##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {

Review Comment:
   Add a comment for a reason for listening (in case coordinator changes)



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {
+                    verPair = (IgnitePair<IgniteProductVersion>)newVal;
+                });
+            }
+        });
+    }
+
+    /** {@inheritDoc} */
+    @Override public @Nullable IgniteNodeValidationResult 
validateNode(ClusterNode node) {
+        synchronized (lock) {

Review Comment:
   Add a comment for a reason to store this node in local var



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {
+                    verPair = (IgnitePair<IgniteProductVersion>)newVal;
+                });
+            }
+        });
+    }
+
+    /** {@inheritDoc} */
+    @Override public @Nullable IgniteNodeValidationResult 
validateNode(ClusterNode node) {
+        synchronized (lock) {
+            lastJoiningNode = node;
+
+            lastJoiningNodeTimestamp = U.currentTimeMillis();
+        }
+
+        ClusterNode locNode = ctx.discovery().localNode();
+
+        String locBuildVer = locNode.attribute(ATTR_BUILD_VER);
+        String rmtBuildVer = node.attribute(ATTR_BUILD_VER);
+
+        IgniteProductVersion rmtVer = 
IgniteProductVersion.fromString(rmtBuildVer);
+
+        IgnitePair<IgniteProductVersion> pair = verPair;
+
+        if (pair == null)
+            pair = F.pair(IgniteProductVersion.fromString(locBuildVer), null);
+
+        IgniteProductVersion curVer = pair.get1();
+        IgniteProductVersion targetVer = pair.get2();
+
+        if (validateVersionsForJoining(rmtVer, curVer) || 
validateVersionsForJoining(rmtVer, targetVer))
+            return null;
+
+        String errMsg = "Remote node rejected due to incompatible version for 
cluster join.\n"
+            + "Remote node info:\n"
+            + "  - Version     : " + rmtBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(node) + "\n"
+            + "  - Node ID     : " + node.id() + "\n"
+            + "Local node info:\n"
+            + "  - Version     : " + locBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(locNode) + "\n"
+            + "  - Node ID     : " + locNode.id() + "\n"
+            + "Allowed versions for joining: \n"
+            + " - " + curVer.major() + "." + curVer.minor() + "." + 
curVer.maintenance()
+            + (targetVer == null || targetVer.equals(curVer) ? "" :
+            "\n - " + targetVer.major() + "." + targetVer.minor() + "." + 
targetVer.maintenance());
+
+        LT.warn(log, errMsg);
+
+        if (log.isDebugEnabled())
+            log.debug(errMsg);
+
+        return new IgniteNodeValidationResult(node.id(), errMsg);
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     * This method can only be called on coordinator node with {@link 
TcpDiscoverySpi}.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If current and target versions are not 
compatible
+     * or if node is not coordinator or if discovery SPI is not {@link 
TcpDiscoverySpi}.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void enable(IgniteProductVersion target) throws 
IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be enabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (!(ctx.config().getDiscoverySpi() instanceof TcpDiscoverySpi))
+            throw new IgniteCheckedException("Rolling upgrade is supported 
only with TCP discovery SPI");
+
+        String curBuildVer = 
ctx.discovery().localNode().attribute(ATTR_BUILD_VER);
+        IgniteProductVersion curVer = 
IgniteProductVersion.fromString(curBuildVer);
+
+        if (!checkVersionsForEnabling(curVer, target))
+            return;
+
+        IgnitePair<IgniteProductVersion> newPair = F.pair(curVer, target);
+
+        if (!metastorage.compareAndSet(ROLLING_UPGRADE_VERSIONS_KEY, null, 
newPair)) {
+            IgnitePair<IgniteProductVersion> oldVerPair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+            if (newPair.equals(oldVerPair))
+                return;
+
+            if (oldVerPair == null)
+                throw new IgniteCheckedException("Could not enable rolling 
upgrade. Try again.");
+
+            throw new IgniteCheckedException("Rolling upgrade is already 
enabled with a different current and target version: " +
+                oldVerPair.get1() + " , " + oldVerPair.get2());
+        }
+
+        verPair = newPair;
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade enabled [current=" + curVer + ", 
target=" + target + ']');
+    }
+
+    /**
+     * Disables rolling upgrade.
+     * This method can only be called on coordinator node.
+     *
+     * @throws IgniteCheckedException If cluster has two or more nodes with 
different versions or if node is not coordinator.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void disable() throws IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be disabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (verPair == null)
+            return;
+
+        IgnitePair<IgniteProductVersion> minMaxVerPair = 
ring.minMaxNodeVersions();
+
+        Set<IgniteProductVersion> vers = new HashSet<>();
+
+        vers.add(minMaxVerPair.get1());
+
+        vers.add(minMaxVerPair.get2());
+
+        if (vers.size() > 1)
+            throw new IgniteCheckedException("Can't disable rolling upgrade 
with different versions in cluster: " + vers);
+
+        synchronized (lock) {
+            if (lastJoiningNode != null) {
+                long timeout = 
((TcpDiscoverySpi)ctx.config().getDiscoverySpi()).getJoinTimeout();
+
+                if (ring.node(lastJoiningNode.id()) != null || 
U.currentTimeMillis() - lastJoiningNodeTimestamp > timeout)
+                    lastJoiningNode = null;
+            }
+
+            if (lastJoiningNode != null)
+                
vers.add(IgniteProductVersion.fromString(lastJoiningNode.attribute(ATTR_BUILD_VER)));
+
+            if (vers.size() > 1)
+                throw new IgniteCheckedException("Can't disable rolling 
upgrade with different versions in cluster: " + vers);
+
+            verPair = null;
+        }
+
+        metastorage.remove(ROLLING_UPGRADE_VERSIONS_KEY);
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade disabled");
+    }
+
+    /**
+     * Returns a pair containing the current and target versions of the 
cluster.
+     * <p>
+     * This method returns {@code null} if rolling upgrade has not been 
enabled yet
+     * or if version information has not been read from the distributed 
metastorage.
+     *
+     * @return A pair where:
+     *     <ul>
+     *         <li><b>First element</b> — current version of the cluster.</li>
+     *         <li><b>Second element</b> — target version to which the cluster 
is being upgraded.</li>
+     *     </ul>
+     *     or {@code null} if rolling upgrade is not active.
+     */
+    public IgnitePair<IgniteProductVersion> versions() {
+        return verPair;
+    }
+
+    /** Checks whether the cluster is in the rolling upgrade mode. */
+    public boolean enabled() {
+        return versions() != null;
+    }
+
+    /**
+     * @param ring TCP discovery nodes ring.
+     */
+    public void ring(TcpDiscoveryNodesRing ring) {
+        this.ring = ring;
+    }
+
+    /**
+     * Checks cur and target versions.
+     *
+     * @param cur Current cluster version.
+     * @param target Target cluster version.
+     * @return {@code false} if there is no need to update versions {@code 
true} otherwise.
+     * @throws IgniteCheckedException If versions are incorrect.
+     */
+    private boolean checkVersionsForEnabling(IgniteProductVersion cur, 
IgniteProductVersion target) throws IgniteCheckedException {
+        IgnitePair<IgniteProductVersion> oldVerPair = verPair;
+        if (oldVerPair != null) {
+            if (oldVerPair.get1().equals(cur) && 
oldVerPair.get2().equals(target))
+                return false;
+
+            throw new IgniteCheckedException("Rolling upgrade is already 
enabled with a different current and target version: " +
+                oldVerPair.get1() + " , " + oldVerPair.get2());
+        }
+
+        if (cur.major() != target.major())
+            throw new IgniteCheckedException("Major versions are different.");
+
+        if (cur.minor() != target.minor()) {
+            if (target.minor() == cur.minor() + 1 && target.maintenance() == 0)
+                return true;
+
+            throw new IgniteCheckedException("Minor version can only be 
incremented by 1.");
+        }
+
+        if (cur.maintenance() + 1 != target.maintenance())
+            throw new IgniteCheckedException("Patch version can only be 
incremented by 1.");
+
+        return true;
+    }
+
+    /** Checks if versions have same major, minor and maintenance versions. */
+    private boolean validateVersionsForJoining(IgniteProductVersion ver1, 
IgniteProductVersion ver2) {
+        if (ver1 == null || ver2 == null)
+            return false;
+
+        return ver1.major() == ver2.major() && ver1.minor() == ver2.minor() && 
ver1.maintenance() == ver2.maintenance();

Review Comment:
   equals (let's keep old logic here)



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {
+                    verPair = (IgnitePair<IgniteProductVersion>)newVal;
+                });
+            }
+        });
+    }
+
+    /** {@inheritDoc} */
+    @Override public @Nullable IgniteNodeValidationResult 
validateNode(ClusterNode node) {
+        synchronized (lock) {
+            lastJoiningNode = node;
+
+            lastJoiningNodeTimestamp = U.currentTimeMillis();
+        }
+
+        ClusterNode locNode = ctx.discovery().localNode();
+
+        String locBuildVer = locNode.attribute(ATTR_BUILD_VER);
+        String rmtBuildVer = node.attribute(ATTR_BUILD_VER);
+
+        IgniteProductVersion rmtVer = 
IgniteProductVersion.fromString(rmtBuildVer);
+
+        IgnitePair<IgniteProductVersion> pair = verPair;
+
+        if (pair == null)
+            pair = F.pair(IgniteProductVersion.fromString(locBuildVer), null);
+
+        IgniteProductVersion curVer = pair.get1();
+        IgniteProductVersion targetVer = pair.get2();
+
+        if (validateVersionsForJoining(rmtVer, curVer) || 
validateVersionsForJoining(rmtVer, targetVer))
+            return null;
+
+        String errMsg = "Remote node rejected due to incompatible version for 
cluster join.\n"
+            + "Remote node info:\n"
+            + "  - Version     : " + rmtBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(node) + "\n"
+            + "  - Node ID     : " + node.id() + "\n"
+            + "Local node info:\n"
+            + "  - Version     : " + locBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(locNode) + "\n"
+            + "  - Node ID     : " + locNode.id() + "\n"
+            + "Allowed versions for joining: \n"
+            + " - " + curVer.major() + "." + curVer.minor() + "." + 
curVer.maintenance()
+            + (targetVer == null || targetVer.equals(curVer) ? "" :
+            "\n - " + targetVer.major() + "." + targetVer.minor() + "." + 
targetVer.maintenance());
+
+        LT.warn(log, errMsg);
+
+        if (log.isDebugEnabled())
+            log.debug(errMsg);
+
+        return new IgniteNodeValidationResult(node.id(), errMsg);
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     * This method can only be called on coordinator node with {@link 
TcpDiscoverySpi}.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If current and target versions are not 
compatible
+     * or if node is not coordinator or if discovery SPI is not {@link 
TcpDiscoverySpi}.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void enable(IgniteProductVersion target) throws 
IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be enabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (!(ctx.config().getDiscoverySpi() instanceof TcpDiscoverySpi))
+            throw new IgniteCheckedException("Rolling upgrade is supported 
only with TCP discovery SPI");
+
+        String curBuildVer = 
ctx.discovery().localNode().attribute(ATTR_BUILD_VER);
+        IgniteProductVersion curVer = 
IgniteProductVersion.fromString(curBuildVer);
+
+        if (!checkVersionsForEnabling(curVer, target))
+            return;
+
+        IgnitePair<IgniteProductVersion> newPair = F.pair(curVer, target);
+
+        if (!metastorage.compareAndSet(ROLLING_UPGRADE_VERSIONS_KEY, null, 
newPair)) {
+            IgnitePair<IgniteProductVersion> oldVerPair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+            if (newPair.equals(oldVerPair))
+                return;
+
+            if (oldVerPair == null)
+                throw new IgniteCheckedException("Could not enable rolling 
upgrade. Try again.");
+
+            throw new IgniteCheckedException("Rolling upgrade is already 
enabled with a different current and target version: " +
+                oldVerPair.get1() + " , " + oldVerPair.get2());
+        }
+
+        verPair = newPair;
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade enabled [current=" + curVer + ", 
target=" + target + ']');
+    }
+
+    /**
+     * Disables rolling upgrade.
+     * This method can only be called on coordinator node.
+     *
+     * @throws IgniteCheckedException If cluster has two or more nodes with 
different versions or if node is not coordinator.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void disable() throws IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be disabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (verPair == null)
+            return;
+
+        IgnitePair<IgniteProductVersion> minMaxVerPair = 
ring.minMaxNodeVersions();
+
+        Set<IgniteProductVersion> vers = new HashSet<>();
+
+        vers.add(minMaxVerPair.get1());
+
+        vers.add(minMaxVerPair.get2());
+
+        if (vers.size() > 1)
+            throw new IgniteCheckedException("Can't disable rolling upgrade 
with different versions in cluster: " + vers);
+
+        synchronized (lock) {
+            if (lastJoiningNode != null) {
+                long timeout = 
((TcpDiscoverySpi)ctx.config().getDiscoverySpi()).getJoinTimeout();
+
+                if (ring.node(lastJoiningNode.id()) != null || 
U.currentTimeMillis() - lastJoiningNodeTimestamp > timeout)
+                    lastJoiningNode = null;
+            }
+
+            if (lastJoiningNode != null)
+                
vers.add(IgniteProductVersion.fromString(lastJoiningNode.attribute(ATTR_BUILD_VER)));
+
+            if (vers.size() > 1)
+                throw new IgniteCheckedException("Can't disable rolling 
upgrade with different versions in cluster: " + vers);
+
+            verPair = null;
+        }
+
+        metastorage.remove(ROLLING_UPGRADE_VERSIONS_KEY);
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade disabled");
+    }
+
+    /**
+     * Returns a pair containing the current and target versions of the 
cluster.
+     * <p>
+     * This method returns {@code null} if rolling upgrade has not been 
enabled yet
+     * or if version information has not been read from the distributed 
metastorage.
+     *
+     * @return A pair where:
+     *     <ul>
+     *         <li><b>First element</b> — current version of the cluster.</li>
+     *         <li><b>Second element</b> — target version to which the cluster 
is being upgraded.</li>
+     *     </ul>
+     *     or {@code null} if rolling upgrade is not active.
+     */
+    public IgnitePair<IgniteProductVersion> versions() {
+        return verPair;
+    }
+
+    /** Checks whether the cluster is in the rolling upgrade mode. */
+    public boolean enabled() {
+        return versions() != null;
+    }
+
+    /**
+     * @param ring TCP discovery nodes ring.
+     */
+    public void ring(TcpDiscoveryNodesRing ring) {
+        this.ring = ring;
+    }
+
+    /**
+     * Checks cur and target versions.
+     *
+     * @param cur Current cluster version.
+     * @param target Target cluster version.
+     * @return {@code false} if there is no need to update versions {@code 
true} otherwise.
+     * @throws IgniteCheckedException If versions are incorrect.
+     */
+    private boolean checkVersionsForEnabling(IgniteProductVersion cur, 
IgniteProductVersion target) throws IgniteCheckedException {
+        IgnitePair<IgniteProductVersion> oldVerPair = verPair;
+        if (oldVerPair != null) {
+            if (oldVerPair.get1().equals(cur) && 
oldVerPair.get2().equals(target))
+                return false;
+
+            throw new IgniteCheckedException("Rolling upgrade is already 
enabled with a different current and target version: " +
+                oldVerPair.get1() + " , " + oldVerPair.get2());
+        }
+
+        if (cur.major() != target.major())
+            throw new IgniteCheckedException("Major versions are different.");
+
+        if (cur.minor() != target.minor()) {
+            if (target.minor() == cur.minor() + 1 && target.maintenance() == 0)
+                return true;
+
+            throw new IgniteCheckedException("Minor version can only be 
incremented by 1.");
+        }
+
+        if (cur.maintenance() + 1 != target.maintenance())
+            throw new IgniteCheckedException("Patch version can only be 
incremented by 1.");
+
+        return true;
+    }
+
+    /** Checks if versions have same major, minor and maintenance versions. */
+    private boolean validateVersionsForJoining(IgniteProductVersion ver1, 
IgniteProductVersion ver2) {

Review Comment:
   validate -> check, or vice versa



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {
+                    verPair = (IgnitePair<IgniteProductVersion>)newVal;
+                });
+            }
+        });
+    }
+
+    /** {@inheritDoc} */
+    @Override public @Nullable IgniteNodeValidationResult 
validateNode(ClusterNode node) {
+        synchronized (lock) {
+            lastJoiningNode = node;
+
+            lastJoiningNodeTimestamp = U.currentTimeMillis();
+        }
+
+        ClusterNode locNode = ctx.discovery().localNode();
+
+        String locBuildVer = locNode.attribute(ATTR_BUILD_VER);
+        String rmtBuildVer = node.attribute(ATTR_BUILD_VER);
+
+        IgniteProductVersion rmtVer = 
IgniteProductVersion.fromString(rmtBuildVer);
+
+        IgnitePair<IgniteProductVersion> pair = verPair;
+
+        if (pair == null)
+            pair = F.pair(IgniteProductVersion.fromString(locBuildVer), null);
+
+        IgniteProductVersion curVer = pair.get1();
+        IgniteProductVersion targetVer = pair.get2();
+
+        if (validateVersionsForJoining(rmtVer, curVer) || 
validateVersionsForJoining(rmtVer, targetVer))
+            return null;
+
+        String errMsg = "Remote node rejected due to incompatible version for 
cluster join.\n"
+            + "Remote node info:\n"
+            + "  - Version     : " + rmtBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(node) + "\n"
+            + "  - Node ID     : " + node.id() + "\n"
+            + "Local node info:\n"
+            + "  - Version     : " + locBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(locNode) + "\n"
+            + "  - Node ID     : " + locNode.id() + "\n"
+            + "Allowed versions for joining: \n"
+            + " - " + curVer.major() + "." + curVer.minor() + "." + 
curVer.maintenance()
+            + (targetVer == null || targetVer.equals(curVer) ? "" :
+            "\n - " + targetVer.major() + "." + targetVer.minor() + "." + 
targetVer.maintenance());

Review Comment:
   use comma as a separator for second version



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {
+                    verPair = (IgnitePair<IgniteProductVersion>)newVal;
+                });
+            }
+        });
+    }
+
+    /** {@inheritDoc} */
+    @Override public @Nullable IgniteNodeValidationResult 
validateNode(ClusterNode node) {
+        synchronized (lock) {
+            lastJoiningNode = node;
+
+            lastJoiningNodeTimestamp = U.currentTimeMillis();
+        }
+
+        ClusterNode locNode = ctx.discovery().localNode();
+
+        String locBuildVer = locNode.attribute(ATTR_BUILD_VER);
+        String rmtBuildVer = node.attribute(ATTR_BUILD_VER);
+
+        IgniteProductVersion rmtVer = 
IgniteProductVersion.fromString(rmtBuildVer);
+
+        IgnitePair<IgniteProductVersion> pair = verPair;
+
+        if (pair == null)
+            pair = F.pair(IgniteProductVersion.fromString(locBuildVer), null);
+
+        IgniteProductVersion curVer = pair.get1();
+        IgniteProductVersion targetVer = pair.get2();
+
+        if (validateVersionsForJoining(rmtVer, curVer) || 
validateVersionsForJoining(rmtVer, targetVer))
+            return null;
+
+        String errMsg = "Remote node rejected due to incompatible version for 
cluster join.\n"
+            + "Remote node info:\n"
+            + "  - Version     : " + rmtBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(node) + "\n"
+            + "  - Node ID     : " + node.id() + "\n"
+            + "Local node info:\n"
+            + "  - Version     : " + locBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(locNode) + "\n"
+            + "  - Node ID     : " + locNode.id() + "\n"
+            + "Allowed versions for joining: \n"
+            + " - " + curVer.major() + "." + curVer.minor() + "." + 
curVer.maintenance()
+            + (targetVer == null || targetVer.equals(curVer) ? "" :
+            "\n - " + targetVer.major() + "." + targetVer.minor() + "." + 
targetVer.maintenance());
+
+        LT.warn(log, errMsg);
+
+        if (log.isDebugEnabled())
+            log.debug(errMsg);
+
+        return new IgniteNodeValidationResult(node.id(), errMsg);
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     * This method can only be called on coordinator node with {@link 
TcpDiscoverySpi}.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If current and target versions are not 
compatible
+     * or if node is not coordinator or if discovery SPI is not {@link 
TcpDiscoverySpi}.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void enable(IgniteProductVersion target) throws 
IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be enabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (!(ctx.config().getDiscoverySpi() instanceof TcpDiscoverySpi))
+            throw new IgniteCheckedException("Rolling upgrade is supported 
only with TCP discovery SPI");
+
+        String curBuildVer = 
ctx.discovery().localNode().attribute(ATTR_BUILD_VER);
+        IgniteProductVersion curVer = 
IgniteProductVersion.fromString(curBuildVer);
+
+        if (!checkVersionsForEnabling(curVer, target))
+            return;
+
+        IgnitePair<IgniteProductVersion> newPair = F.pair(curVer, target);
+
+        if (!metastorage.compareAndSet(ROLLING_UPGRADE_VERSIONS_KEY, null, 
newPair)) {
+            IgnitePair<IgniteProductVersion> oldVerPair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+            if (newPair.equals(oldVerPair))
+                return;
+
+            if (oldVerPair == null)
+                throw new IgniteCheckedException("Could not enable rolling 
upgrade. Try again.");
+
+            throw new IgniteCheckedException("Rolling upgrade is already 
enabled with a different current and target version: " +
+                oldVerPair.get1() + " , " + oldVerPair.get2());
+        }
+
+        verPair = newPair;
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade enabled [current=" + curVer + ", 
target=" + target + ']');
+    }
+
+    /**
+     * Disables rolling upgrade.
+     * This method can only be called on coordinator node.
+     *
+     * @throws IgniteCheckedException If cluster has two or more nodes with 
different versions or if node is not coordinator.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void disable() throws IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be disabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (verPair == null)
+            return;
+
+        IgnitePair<IgniteProductVersion> minMaxVerPair = 
ring.minMaxNodeVersions();
+
+        Set<IgniteProductVersion> vers = new HashSet<>();
+
+        vers.add(minMaxVerPair.get1());
+
+        vers.add(minMaxVerPair.get2());
+
+        if (vers.size() > 1)
+            throw new IgniteCheckedException("Can't disable rolling upgrade 
with different versions in cluster: " + vers);
+
+        synchronized (lock) {
+            if (lastJoiningNode != null) {
+                long timeout = 
((TcpDiscoverySpi)ctx.config().getDiscoverySpi()).getJoinTimeout();
+
+                if (ring.node(lastJoiningNode.id()) != null || 
U.currentTimeMillis() - lastJoiningNodeTimestamp > timeout)
+                    lastJoiningNode = null;
+            }
+
+            if (lastJoiningNode != null)
+                
vers.add(IgniteProductVersion.fromString(lastJoiningNode.attribute(ATTR_BUILD_VER)));
+
+            if (vers.size() > 1)
+                throw new IgniteCheckedException("Can't disable rolling 
upgrade with different versions in cluster: " + vers);
+
+            verPair = null;
+        }
+
+        metastorage.remove(ROLLING_UPGRADE_VERSIONS_KEY);
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade disabled");

Review Comment:
   Lets's mention version of cluster



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {
+                    verPair = (IgnitePair<IgniteProductVersion>)newVal;
+                });
+            }
+        });
+    }
+
+    /** {@inheritDoc} */
+    @Override public @Nullable IgniteNodeValidationResult 
validateNode(ClusterNode node) {
+        synchronized (lock) {
+            lastJoiningNode = node;
+
+            lastJoiningNodeTimestamp = U.currentTimeMillis();
+        }
+
+        ClusterNode locNode = ctx.discovery().localNode();
+
+        String locBuildVer = locNode.attribute(ATTR_BUILD_VER);
+        String rmtBuildVer = node.attribute(ATTR_BUILD_VER);
+
+        IgniteProductVersion rmtVer = 
IgniteProductVersion.fromString(rmtBuildVer);
+
+        IgnitePair<IgniteProductVersion> pair = verPair;
+
+        if (pair == null)
+            pair = F.pair(IgniteProductVersion.fromString(locBuildVer), null);
+
+        IgniteProductVersion curVer = pair.get1();
+        IgniteProductVersion targetVer = pair.get2();
+
+        if (validateVersionsForJoining(rmtVer, curVer) || 
validateVersionsForJoining(rmtVer, targetVer))
+            return null;
+
+        String errMsg = "Remote node rejected due to incompatible version for 
cluster join.\n"
+            + "Remote node info:\n"
+            + "  - Version     : " + rmtBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(node) + "\n"
+            + "  - Node ID     : " + node.id() + "\n"
+            + "Local node info:\n"
+            + "  - Version     : " + locBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(locNode) + "\n"
+            + "  - Node ID     : " + locNode.id() + "\n"
+            + "Allowed versions for joining: \n"
+            + " - " + curVer.major() + "." + curVer.minor() + "." + 
curVer.maintenance()
+            + (targetVer == null || targetVer.equals(curVer) ? "" :
+            "\n - " + targetVer.major() + "." + targetVer.minor() + "." + 
targetVer.maintenance());
+
+        LT.warn(log, errMsg);
+
+        if (log.isDebugEnabled())
+            log.debug(errMsg);
+
+        return new IgniteNodeValidationResult(node.id(), errMsg);
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     * This method can only be called on coordinator node with {@link 
TcpDiscoverySpi}.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If current and target versions are not 
compatible
+     * or if node is not coordinator or if discovery SPI is not {@link 
TcpDiscoverySpi}.
+     * @throws NullPointerException If metastorage is not available.

Review Comment:
   Throwing NPE within Ignite node is bad practice.



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,310 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.ignite.internal.processors.rollingupgrade;
+
+import java.util.HashSet;
+import java.util.Set;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.cluster.ClusterNode;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.nodevalidation.DiscoveryNodeValidationProcessor;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.internal.util.typedef.internal.U;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.apache.ignite.spi.IgniteNodeValidationResult;
+import org.apache.ignite.spi.discovery.tcp.TcpDiscoverySpi;
+import org.apache.ignite.spi.discovery.tcp.internal.TcpDiscoveryNodesRing;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter implements 
DiscoveryNodeValidationProcessor {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** TCP discovery nodes ring. */
+    private TcpDiscoveryNodesRing ring;
+
+    /** Last joining node. */
+    private ClusterNode lastJoiningNode;
+
+    /** Last joining node timestamp. */
+    private long lastJoiningNodeTimestamp;
+
+    /** Lock for synchronization between tcp-disco-msg-worker thread and 
management operations. */
+    private final Object lock = new Object();
+
+    /** Pair with current and target versions. */
+    private volatile IgnitePair<IgniteProductVersion> verPair = null;
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    verPair = metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) -> {
+                    verPair = (IgnitePair<IgniteProductVersion>)newVal;
+                });
+            }
+        });
+    }
+
+    /** {@inheritDoc} */
+    @Override public @Nullable IgniteNodeValidationResult 
validateNode(ClusterNode node) {
+        synchronized (lock) {
+            lastJoiningNode = node;
+
+            lastJoiningNodeTimestamp = U.currentTimeMillis();
+        }
+
+        ClusterNode locNode = ctx.discovery().localNode();
+
+        String locBuildVer = locNode.attribute(ATTR_BUILD_VER);
+        String rmtBuildVer = node.attribute(ATTR_BUILD_VER);
+
+        IgniteProductVersion rmtVer = 
IgniteProductVersion.fromString(rmtBuildVer);
+
+        IgnitePair<IgniteProductVersion> pair = verPair;
+
+        if (pair == null)
+            pair = F.pair(IgniteProductVersion.fromString(locBuildVer), null);
+
+        IgniteProductVersion curVer = pair.get1();
+        IgniteProductVersion targetVer = pair.get2();
+
+        if (validateVersionsForJoining(rmtVer, curVer) || 
validateVersionsForJoining(rmtVer, targetVer))
+            return null;
+
+        String errMsg = "Remote node rejected due to incompatible version for 
cluster join.\n"
+            + "Remote node info:\n"
+            + "  - Version     : " + rmtBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(node) + "\n"
+            + "  - Node ID     : " + node.id() + "\n"
+            + "Local node info:\n"
+            + "  - Version     : " + locBuildVer + "\n"
+            + "  - Addresses   : " + U.addressesAsString(locNode) + "\n"
+            + "  - Node ID     : " + locNode.id() + "\n"
+            + "Allowed versions for joining: \n"
+            + " - " + curVer.major() + "." + curVer.minor() + "." + 
curVer.maintenance()
+            + (targetVer == null || targetVer.equals(curVer) ? "" :
+            "\n - " + targetVer.major() + "." + targetVer.minor() + "." + 
targetVer.maintenance());
+
+        LT.warn(log, errMsg);
+
+        if (log.isDebugEnabled())
+            log.debug(errMsg);
+
+        return new IgniteNodeValidationResult(node.id(), errMsg);
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     * This method can only be called on coordinator node with {@link 
TcpDiscoverySpi}.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If current and target versions are not 
compatible
+     * or if node is not coordinator or if discovery SPI is not {@link 
TcpDiscoverySpi}.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void enable(IgniteProductVersion target) throws 
IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be enabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (!(ctx.config().getDiscoverySpi() instanceof TcpDiscoverySpi))
+            throw new IgniteCheckedException("Rolling upgrade is supported 
only with TCP discovery SPI");
+
+        String curBuildVer = 
ctx.discovery().localNode().attribute(ATTR_BUILD_VER);
+        IgniteProductVersion curVer = 
IgniteProductVersion.fromString(curBuildVer);
+
+        if (!checkVersionsForEnabling(curVer, target))
+            return;
+
+        IgnitePair<IgniteProductVersion> newPair = F.pair(curVer, target);
+
+        if (!metastorage.compareAndSet(ROLLING_UPGRADE_VERSIONS_KEY, null, 
newPair)) {
+            IgnitePair<IgniteProductVersion> oldVerPair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+            if (newPair.equals(oldVerPair))
+                return;
+
+            if (oldVerPair == null)
+                throw new IgniteCheckedException("Could not enable rolling 
upgrade. Try again.");
+
+            throw new IgniteCheckedException("Rolling upgrade is already 
enabled with a different current and target version: " +
+                oldVerPair.get1() + " , " + oldVerPair.get2());
+        }
+
+        verPair = newPair;
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade enabled [current=" + curVer + ", 
target=" + target + ']');
+    }
+
+    /**
+     * Disables rolling upgrade.
+     * This method can only be called on coordinator node.
+     *
+     * @throws IgniteCheckedException If cluster has two or more nodes with 
different versions or if node is not coordinator.
+     * @throws NullPointerException If metastorage is not available.
+     */
+    public void disable() throws IgniteCheckedException {
+        if (!U.isLocalNodeCoordinator(ctx.discovery()))
+            throw new IgniteCheckedException("Rolling upgrade can be disabled 
only on coordinator node");
+
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        if (verPair == null)
+            return;
+
+        IgnitePair<IgniteProductVersion> minMaxVerPair = 
ring.minMaxNodeVersions();
+
+        Set<IgniteProductVersion> vers = new HashSet<>();
+
+        vers.add(minMaxVerPair.get1());
+
+        vers.add(minMaxVerPair.get2());
+
+        if (vers.size() > 1)
+            throw new IgniteCheckedException("Can't disable rolling upgrade 
with different versions in cluster: " + vers);
+
+        synchronized (lock) {
+            if (lastJoiningNode != null) {
+                long timeout = 
((TcpDiscoverySpi)ctx.config().getDiscoverySpi()).getJoinTimeout();
+
+                if (ring.node(lastJoiningNode.id()) != null || 
U.currentTimeMillis() - lastJoiningNodeTimestamp > timeout)
+                    lastJoiningNode = null;
+            }
+
+            if (lastJoiningNode != null)
+                
vers.add(IgniteProductVersion.fromString(lastJoiningNode.attribute(ATTR_BUILD_VER)));
+
+            if (vers.size() > 1)
+                throw new IgniteCheckedException("Can't disable rolling 
upgrade with different versions in cluster: " + vers);
+
+            verPair = null;
+        }
+
+        metastorage.remove(ROLLING_UPGRADE_VERSIONS_KEY);
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade disabled");
+    }
+
+    /**
+     * Returns a pair containing the current and target versions of the 
cluster.
+     * <p>
+     * This method returns {@code null} if rolling upgrade has not been 
enabled yet
+     * or if version information has not been read from the distributed 
metastorage.
+     *
+     * @return A pair where:
+     *     <ul>
+     *         <li><b>First element</b> — current version of the cluster.</li>
+     *         <li><b>Second element</b> — target version to which the cluster 
is being upgraded.</li>
+     *     </ul>
+     *     or {@code null} if rolling upgrade is not active.
+     */
+    public IgnitePair<IgniteProductVersion> versions() {
+        return verPair;
+    }
+
+    /** Checks whether the cluster is in the rolling upgrade mode. */
+    public boolean enabled() {
+        return versions() != null;
+    }
+
+    /**
+     * @param ring TCP discovery nodes ring.
+     */
+    public void ring(TcpDiscoveryNodesRing ring) {
+        this.ring = ring;
+    }
+
+    /**
+     * Checks cur and target versions.
+     *
+     * @param cur Current cluster version.
+     * @param target Target cluster version.
+     * @return {@code false} if there is no need to update versions {@code 
true} otherwise.
+     * @throws IgniteCheckedException If versions are incorrect.
+     */
+    private boolean checkVersionsForEnabling(IgniteProductVersion cur, 
IgniteProductVersion target) throws IgniteCheckedException {
+        IgnitePair<IgniteProductVersion> oldVerPair = verPair;
+        if (oldVerPair != null) {
+            if (oldVerPair.get1().equals(cur) && 
oldVerPair.get2().equals(target))
+                return false;
+
+            throw new IgniteCheckedException("Rolling upgrade is already 
enabled with a different current and target version: " +
+                oldVerPair.get1() + " , " + oldVerPair.get2());
+        }
+
+        if (cur.major() != target.major())
+            throw new IgniteCheckedException("Major versions are different.");
+
+        if (cur.minor() != target.minor()) {
+            if (target.minor() == cur.minor() + 1 && target.maintenance() == 0)
+                return true;
+
+            throw new IgniteCheckedException("Minor version can only be 
incremented by 1.");
+        }
+
+        if (cur.maintenance() + 1 != target.maintenance())
+            throw new IgniteCheckedException("Patch version can only be 
incremented by 1.");
+
+        return true;
+    }
+
+    /** Checks if versions have same major, minor and maintenance versions. */
+    private boolean validateVersionsForJoining(IgniteProductVersion ver1, 
IgniteProductVersion ver2) {
+        if (ver1 == null || ver2 == null)

Review Comment:
   When ver1 is null?



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