dajac commented on code in PR #22487:
URL: https://github.com/apache/kafka/pull/22487#discussion_r3521584846


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/TargetAssignmentRecordsBuilder.java:
##########
@@ -0,0 +1,483 @@
+/*
+ * 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.kafka.coordinator.group;
+
+import org.apache.kafka.common.utils.internals.LogContext;
+import org.apache.kafka.coordinator.common.runtime.CoordinatorRecord;
+import org.apache.kafka.coordinator.group.modern.Assignment;
+import 
org.apache.kafka.coordinator.group.streams.StreamsCoordinatorRecordHelpers;
+import org.apache.kafka.coordinator.group.streams.TasksTuple;
+
+import org.slf4j.Logger;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+
+/**
+ * Builds the records to persist a new target assignment for a group.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * @param <A> The member's target assignment type.
+ */
+public abstract class TargetAssignmentRecordsBuilder<A> {
+
+    /**
+     * The logger.
+     */
+    private final Logger log;
+
+    /**
+     * The group id.
+     */
+    private final String groupId;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int assignmentEpoch;
+
+    /**
+     * The time at which the target assignment calculation finished.
+     */
+    private long assignmentTimestampMs;
+
+    /**
+     * The static members in the group at the time the new target assignment 
was computed.
+     */
+    private Map<String, String> previousStaticMembers = Map.of();
+
+    /**
+     * The current member ids in the group.
+     */
+    private Set<String> currentMemberIds;
+
+    /**
+     * The current static members in the group.
+     */
+    private Map<String, String> currentStaticMembers = Map.of();
+
+    /**
+     * The current target assignment.
+     */
+    private Map<String, A> currentTargetAssignment = Map.of();
+
+    /**
+     * The new target assignment.
+     */
+    private Map<String, A> newTargetAssignment = Map.of();
+
+    /**
+     * Constructs the object.
+     *
+     * @param logContext    The log context.
+     * @param groupId       The group id.
+     */
+    public TargetAssignmentRecordsBuilder(
+        LogContext logContext,
+        String groupId
+    ) {
+        this.log = logContext.logger(this.getClass());
+        this.groupId = Objects.requireNonNull(groupId);
+    }
+
+    /**
+     * Sets the target assignment epoch.
+     *
+     * @param assignmentEpoch The target assignment epoch.
+     * @return This object.
+     */
+    public TargetAssignmentRecordsBuilder<A> withAssignmentEpoch(int 
assignmentEpoch) {
+        this.assignmentEpoch = assignmentEpoch;
+        return this;
+    }
+
+    /**
+     * Sets the time at which the target assignment calculation finished.
+     *
+     * @param assignmentTimestampMs The time at which the target assignment 
calculation finished.
+     * @return This object.
+     */
+    public TargetAssignmentRecordsBuilder<A> withAssignmentTimestampMs(long 
assignmentTimestampMs) {
+        this.assignmentTimestampMs = assignmentTimestampMs;
+        return this;
+    }
+
+    /**
+     * Sets the static members in the group at the time the new target 
assignment was computed.
+     *
+     * @param previousStaticMembers The static members in the group at the 
time the new target assignment was computed.
+     * @return This object.
+     */
+    public TargetAssignmentRecordsBuilder<A> 
withPreviousStaticMembers(Map<String, String> previousStaticMembers) {
+        this.previousStaticMembers = previousStaticMembers;
+        return this;
+    }
+
+    /**
+     * Sets the current member ids in the group.
+     *
+     * @param currentMemberIds The current member ids in the group.
+     * @return This object.
+     */
+    public TargetAssignmentRecordsBuilder<A> withCurrentMemberIds(Set<String> 
currentMemberIds) {
+        this.currentMemberIds = currentMemberIds;
+        return this;
+    }
+
+    /**
+     * Sets the current static members in the group.
+     *
+     * @param currentStaticMembers The current static members in the group.
+     * @return This object.
+     */
+    public TargetAssignmentRecordsBuilder<A> 
withCurrentStaticMembers(Map<String, String> currentStaticMembers) {
+        this.currentStaticMembers = currentStaticMembers;
+        return this;
+    }
+
+    /**
+     * Sets the current target assignment.
+     *
+     * @param currentTargetAssignment The current target assignment.
+     * @return This object.
+     */
+    public TargetAssignmentRecordsBuilder<A> 
withCurrentTargetAssignment(Map<String, A> currentTargetAssignment) {
+        this.currentTargetAssignment = currentTargetAssignment;
+        return this;
+    }
+
+    /**
+     * Sets the new target assignment.
+     *
+     * @param newTargetAssignment The new target assignment.
+     * @return This object.
+     */
+    public TargetAssignmentRecordsBuilder<A> 
withNewTargetAssignment(Map<String, A> newTargetAssignment) {
+        this.newTargetAssignment = newTargetAssignment;
+        return this;
+    }
+
+    /**
+     * Builds the records for the new target assignment.
+     *
+     * @return The records for the new target assignment.
+     */
+    public List<CoordinatorRecord> build() {
+        List<CoordinatorRecord> records = new ArrayList<>();
+        build(records);
+        return records;
+    }
+
+    /**
+     * Builds the records for the new target assignment.
+     *
+     * @param records The list to accumulate records.
+     */
+    @SuppressWarnings({"CyclomaticComplexity", "NPathComplexity"})
+    public void build(List<CoordinatorRecord> records) {

Review Comment:
   > The current target assignment won't contain entries for newly joined 
members and I want us to write empty assignment records for those. The new 
target assignment may come from a client-side assignor and may not contain 
entries for all members.
   
   Why? New members will be taken care of by the next one anyway.



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