Github user anew commented on a diff in the pull request:
https://github.com/apache/incubator-tephra/pull/20#discussion_r90759085
--- Diff:
tephra-core/src/main/java/org/apache/tephra/janitor/TransactionPruningPlugin.java
---
@@ -0,0 +1,90 @@
+/*
+ * 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.tephra.janitor;
+
+import org.apache.hadoop.conf.Configuration;
+
+import java.io.IOException;
+
+/**
+ * Data janitor interface to manage the invalid transaction list.
+ *
+ * <p/>
+ * An invalid transaction can only be removed from the invalid list after
the data written
+ * by the invalid transactions has been removed from all the data stores.
+ * The term data store is used here to represent a set of tables in a
database that have
+ * the same data clean up policy, like all Apache Phoenix tables in an
HBase instance.
+ *
+ * <p/>
+ * Typically every data store will have a background job which cleans up
the data written by invalid transactions.
+ * Prune upper bound for a data store is defined as the largest invalid
transaction whose data has been
+ * cleaned up from that data store.
+ * <pre>
+ * prune-upper-bound = min(max(invalid list), min(in-progress list) - 1)
+ * </pre>
+ * where invalid list and in-progress list are from the transaction
snapshot used to clean up the invalid data in the
+ * data store.
+ *
+ * <p/>
+ * There will be one such plugin per data store. The plugins will be
executed as part of the Transaction Service.
+ * Each plugin will be invoked periodically to fetch the prune upper bound
for its data store.
+ * Invalid transaction list can pruned up to the minimum of prune upper
bounds returned by all the plugins.
+ */
+public interface TransactionPruningPlugin {
+ /**
+ * Called once when the Transaction Service starts up.
+ *
+ * @param conf configuration for the plugin
+ */
+ void initialize(Configuration conf) throws IOException;
+
+ /**
+ * Called periodically to fetch prune upper bound for a data store. The
plugin examines the state of data cleanup
+ * in the data store and determines the smallest invalid transaction
whose writes no longer exist in the data
--- End diff --
or a greatest lower bound for transaction ids that may not be pruned?
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