ZOOKEEPER-3427: Introduce SnapshotComparer that assists debugging with snapshots.

bin/zkSnapshotComparer.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+
+# 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.
+
+#
+# If this scripted is run out of /usr/bin or some other system bin directory
+# it should be linked to and not copied. Things like java jar files are found
+# relative to the canonical path of this script.
+#
+
+# use POSIX interface, symlink is followed automatically
+ZOOBIN="${BASH_SOURCE-$0}"
+ZOOBIN="$(dirname "${ZOOBIN}")"
+ZOOBINDIR="$(cd "${ZOOBIN}"; pwd)"
+
+if [ -e "$ZOOBIN/../libexec/zkEnv.sh" ]; then
+  . "$ZOOBINDIR"/../libexec/zkEnv.sh
+else
+  . "$ZOOBINDIR"/zkEnv.sh
+fi
+
+"$JAVA" -cp "$CLASSPATH" $JVMFLAGS \
+     org.apache.zookeeper.server.SnapshotComparer "$@"

zookeeper-docs/src/main/resources/markdown/zookeeperTools.md

@@ -23,6 +23,7 @@ limitations under the License.
     * [zkCleanup.sh](#zkCleanup)
     * [zkTxnLogToolkit.sh](#zkTxnLogToolkit)
     * [zkSnapShotToolkit.sh](#zkSnapShotToolkit)
+    * [zkSnapshotComparer.sh](#zkSnapshotComparer)
 
 * [Testing](#Testing)
     * [Jepsen Test](#jepsen-test)
@@ -205,6 +206,67 @@ USAGE: SnapshotFormatter [-d|-json] snapshot_file
 [[1,0,{"progname":"SnapshotFormatter.java","progver":"0.01","timestamp":1559788148637},[{"name":"\/","asize":0,"dsize":0,"dev":0,"ino":1001},[{"name":"zookeeper","asize":0,"dsize":0,"dev":0,"ino":1002},{"name":"config","asize":0,"dsize":0,"dev":0,"ino":1003},[{"name":"quota","asize":0,"dsize":0,"dev":0,"ino":1004},[{"name":"test","asize":0,"dsize":0,"dev":0,"ino":1005},{"name":"zookeeper_limits","asize":52,"dsize":52,"dev":0,"ino":1006},{"name":"zookeeper_stats","asize":15,"dsize":15,"dev":0,"ino":1007}]]],{"name":"test","asize":0,"dsize":0,"dev":0,"ino":1008}]]
 ```
 
+### zkSnapshotComparer.sh
+SnapshotComparer is a tool that loads and compares two snapshots, with configurable threshold and various filters. It's useful in use cases that involve snapshot analysis, such as offline data consistency checking, and data trending analysis (e.g. what's growing under which zNode path during when).
+
+This tool only outputs information about permanent nodes, ignoring both sessions and ephemeral nodes.
+
+It provides two tuning parameters to help filter out noise: 
+1. `--nodes` Threshold number of children added/removed; 
+2. `--bytes` Threshold number of bytes added/removed.
+
+#### Locate Snapshots
+Snapshots can be found in [Zookeeper Data Directory](zookeeperAdmin.html#The+Data+Directory) which configured in [conf/zoo.cfg](zookeeperStarted.html#sc_InstallingSingleMode) when set up Zookeeper server. 
+
+#### Supported Snapshot Formats
+This tool supports uncompressed snapshot format, and compressed snapshot file formats: `snappy` and `gz`. Snapshots with different formats can be compared using this tool directly without decompression.
+
+#### Running the Tool
+Running the tool with no command line argument or an unrecognized argument, it outputs the following help page:
+```
+usage: java -cp <classPath> org.apache.zookeeper.server.SnapshotComparer
+ -b,--bytes <BYTETHRESHOLD>       (Required) The node data delta size threshold, in bytes, for printing the node.
+ -d,--debug <DEBUG>               Use debug output.
+ -i,--interactive <INTERACTIVE>   Enter interactive mode.
+ -l,--left <LEFT>                 (Required) The left snapshot file.
+ -n,--nodes <NODETHRESHOLD>       (Required) The descendant node delta size threshold, in nodes, for printing the node.
+ -r,--right <RIGHT>               (Required) The right snapshot file.
+```
+Example Command:
+```
+$ bin/zkSnapshotComparer.sh -l /zookeeper-data/backup/snapshot.0.snappy -r /zookeeper-data/backup/snapshot.8.gz -b 100000 -n 100
+```
+Example Output:
+```
+...
+Deserialized snapshot in snapshot.0.snappy in 0.039392 seconds
+Processed data tree in 0.072455 seconds
+2020-01-12 20:16:00,479 [myid:] - INFO  [main:WatchManagerFactory@42] - Using org.apache.zookeeper.server.watch.WatchManager as watch manager
+2020-01-12 20:16:00,479 [myid:] - INFO  [main:WatchManagerFactory@42] - Using org.apache.zookeeper.server.watch.WatchManager as watch manager
+Deserialized snapshot in snapshot.8.gz in 0.001962 seconds
+Processed data tree in 0.000141 seconds
+Node count: 4
+Total size: 0
+Max depth: 3
+Count of nodes at depth 1: 1
+Count of nodes at depth 2: 1
+Count of nodes at depth 3: 2
+
+Node count: 8
+Total size: 0
+Max depth: 4
+Count of nodes at depth 1: 1
+Count of nodes at depth 2: 2
+Count of nodes at depth 3: 4
+Count of nodes at depth 4: 1
+
+Analysis for depth 0
+Analysis for depth 1
+Analysis for depth 2
+Analysis for depth 3
+All layers compared.
+```
+
 <a name="Testing"></a>
 
 ## Testing