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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-5544) Implement Internal Timer Service in RocksDB
Date Fri, 17 Mar 2017 15:46:45 GMT

    [ https://issues.apache.org/jira/browse/FLINK-5544?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15930180#comment-15930180

ASF GitHub Bot commented on FLINK-5544:

Github user StefanRRichter commented on a diff in the pull request:

    --- Diff: flink-contrib/flink-timerserivce-rocksdb/src/main/java/org/apache/flink/contrib/streaming/api/operators/RocksDBInternalTimerService.java
    @@ -0,0 +1,797 @@
    + * 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.flink.contrib.streaming.api.operators;
    +import org.apache.flink.api.common.typeutils.base.IntSerializer;
    +import org.apache.flink.api.common.typeutils.base.LongSerializer;
    +import org.apache.flink.api.java.tuple.Tuple4;
    +import org.apache.flink.core.fs.FileSystem;
    +import org.apache.flink.core.fs.Path;
    +import org.apache.flink.core.memory.DataInputViewStreamWrapper;
    +import org.apache.flink.core.memory.DataOutputViewStreamWrapper;
    +import org.apache.flink.runtime.state.KeyGroupRange;
    +import org.apache.flink.runtime.state.KeyGroupRangeAssignment;
    +import org.apache.flink.streaming.api.operators.InternalTimer;
    +import org.apache.flink.streaming.api.operators.InternalTimerService;
    +import org.apache.flink.streaming.api.operators.KeyContext;
    +import org.apache.flink.streaming.runtime.tasks.ProcessingTimeService;
    +import org.apache.flink.util.Preconditions;
    +import org.rocksdb.ColumnFamilyDescriptor;
    +import org.rocksdb.ColumnFamilyHandle;
    +import org.rocksdb.ColumnFamilyOptions;
    +import org.rocksdb.CompactionStyle;
    +import org.rocksdb.DBOptions;
    +import org.rocksdb.RocksDB;
    +import org.rocksdb.RocksDBException;
    +import org.rocksdb.RocksIterator;
    +import org.rocksdb.StringAppendOperator;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +import java.io.ByteArrayInputStream;
    +import java.io.ByteArrayOutputStream;
    +import java.io.IOException;
    +import java.util.ArrayList;
    +import java.util.Collections;
    +import java.util.HashSet;
    +import java.util.List;
    +import java.util.Set;
    + * {@link InternalTimerService} that stores timers in RocksDB.
    + */
    +public class RocksDBInternalTimerService<K, N> extends InternalTimerService<K,
N> {
    +	private static Logger LOG = LoggerFactory.getLogger(RocksDBInternalTimerService.class);
    +	/** The data base where stores all timers */
    +	private final RocksDB db;
    +	/** The path where the rocksdb locates */
    +	private final Path dbPath;
    +	/**
    +	 * The in-memory heaps backed by rocksdb to retrieve the next timer to trigger. Each
    +	 * partition's leader is stored in the heap. When the timers in a partition is changed,
    +	 * will change the partition's leader and update the heap accordingly.
    +	 */
    +	private final int numPartitions;
    +	private final PersistentTimerHeap eventTimeHeap;
    +	private final PersistentTimerHeap processingTimeHeap;
    +	private static int MAX_PARTITIONS = (1 << 16);
    +	public RocksDBInternalTimerService(
    +			int totalKeyGroups,
    +			KeyGroupRange keyGroupRange,
    +			KeyContext keyContext,
    +			ProcessingTimeService processingTimeService,
    +			Path dbPath) {
    +		super(totalKeyGroups, keyGroupRange, keyContext, processingTimeService);
    +		this.dbPath = dbPath;
    +		try {
    --- End diff --
    I wonder if we could ensure all this already in the factory, so `IOException`s could be
more properly propagated or even handled? this problem would automatically disappear when
we reuse the RocksDB instance from the backend.

> Implement Internal Timer Service in RocksDB
> -------------------------------------------
>                 Key: FLINK-5544
>                 URL: https://issues.apache.org/jira/browse/FLINK-5544
>             Project: Flink
>          Issue Type: New Feature
>          Components: State Backends, Checkpointing
>            Reporter: Xiaogang Shi
>            Assignee: Xiaogang Shi
> Now the only implementation of internal timer service is HeapInternalTimerService which
stores all timers in memory. In the cases where the number of keys is very large, the timer
service will cost too much memory. A implementation which stores timers in RocksDB seems good
to deal with these cases.
> It might be a little challenging to implement a RocksDB timer service because the timers
are accessed in different ways. When timers are triggered, we need to access timers in the
order of timestamp. But when performing checkpoints, we must have a method to obtain all timers
of a given key group.
> A good implementation, as suggested by [~StephanEwen], follows the idea of merge sorting.
We can store timers in RocksDB with the format {{KEY_GROUP#TIMER#KEY}}. In this way, the timers
under a key group are put together and are sorted. 
> Then we can deploy an in-memory heap which keeps the first timer of each key group to
get the next timer to trigger. When a key group's first timer is updated, we can efficiently
update the heap.

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