Github user poornachandra commented on a diff in the pull request:
https://github.com/apache/incubator-tephra/pull/20#discussion_r90765092
--- Diff:
tephra-hbase-compat-1.1-base/src/main/java/org/apache/tephra/hbase/coprocessor/janitor/HBaseTransactionPruningPlugin.java
---
@@ -0,0 +1,289 @@
+/*
+ * 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.hbase.coprocessor.janitor;
+
+import com.google.common.base.Function;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Maps;
+import com.google.common.collect.Sets;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hbase.HRegionInfo;
+import org.apache.hadoop.hbase.HTableDescriptor;
+import org.apache.hadoop.hbase.TableName;
+import org.apache.hadoop.hbase.client.Admin;
+import org.apache.hadoop.hbase.client.Connection;
+import org.apache.hadoop.hbase.client.ConnectionFactory;
+import org.apache.hadoop.hbase.client.Table;
+import org.apache.hadoop.hbase.util.Bytes;
+import org.apache.tephra.TxConstants;
+import org.apache.tephra.hbase.coprocessor.TransactionProcessor;
+import org.apache.tephra.janitor.TransactionPruningPlugin;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.SortedSet;
+import java.util.TreeSet;
+
+/**
+ * Default implementation of the {@link TransactionPruningPlugin} for
HBase.
+ *
+ * This plugin determines the prune upper bound for transactional HBase
tables that use
+ * coprocessor {@link TransactionProcessor}.
+ *
+ * <h3>State storage:</h3>
+ *
+ * This plugin expects the TransactionProcessor to save the prune upper
bound for invalid transactions
+ * after every major compaction of a region. Let's call this <i>(region,
prune upper bound)</i>.
+ * In addition, the plugin also persists the following information on a
run at time <i>t</i>
+ * <ul>
+ * <li>
+ * <i>(t, set of regions)</i>: Set of transactional regions at time
<i>t</i>.
+ * Transactional regions are regions of the tables that have the
coprocessor TransactionProcessor
+ * attached to them.
+ * </li>
+ * <li>
+ * <i>(t, prune upper bound)</i>: This is the smallest not in-progress
transaction that
+ * will not have writes in any HBase regions that are created after
time <i>t</i>.
+ * This value is determined by the Transaction Service based on the
transaction state at time <i>t</i>
+ * and passed on to the plugin.
+ * </li>
+ * </ul>
+ *
+ * <h3>Computing prune upper bound:</h3>
+ *
+ * In a typical HBase instance, there can be a constant change in the
number of regions due to region creations,
+ * splits and merges. At any given time there can always be a region on
which a major compaction has not been run.
+ * Since the prune upper bound will get recorded for a region only after a
major compaction,
+ * using only the latest set of regions we may not be able to find the
+ * prune upper bounds for all the current regions. Hence we persist the
set of regions that exist at that time
+ * of each run of the plugin, and use historical region set for time
<i>t</i>, <i>t - 1</i>, etc.
+ * to determine the prune upper bound.
+ *
+ * From the regions saved at time <i>t</i>, <i>t - 1</i>, etc.,
+ * the plugin tries to find the latest <i>(t, set of regions)</i> where
all regions have been major compacted,
+ * i.e, all regions have prune upper bound recorded in <i>(region, prune
upper bound)</i>.
+ * <br/>
+ * If such a set is found for time <i>t1</i>, the prune upper bound
returned by the plugin is the minimum of
+ * <ul>
+ * <li>Prune upper bounds of regions in set <i>(t1, set of
regions)</i></li>
+ * <li>Prune upper bound from <i>(t1, prune upper bound)</i></li>
+ * </ul>
+ *
+ * <p/>
+ * Above, when we find <i>(t1, set of regions)</i>, there may a region
that was created after time <i>t1</i>,
+ * but has a data write from an invalid transaction that is smaller than
the prune upper bounds of all
+ * regions in <i>(t1, set of regions)</i>. This is possible because
<i>(region, prune upper bound)</i> persisted by
+ * TransactionProcessor is always the latest prune upper bound for a
region.
+ * <br/>
+ * However a region created after time <i>t1</i> cannot have writes from
an invalid transaction that is smaller than
+ * <i>min(max(invalid list), min(in-progress list) - 1)</i> at the time
the region was created.
+ * Since we limit the plugin prune upper bound using <i>(t1, prune upper
bound)</i>, there should be no invalid
--- End diff --
We need to define and enforce the maximum duration a transaction can be
used for writes. I have created TEPHRA-199 for this. I'll add it in the next PR.
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