Flink-CDC(Change Data Capture)指定位点消费binlog

前言:

Flink从1.10开始支持从canal或者debezium消费binlog数据,从而实现数据的同步;但是无论是canal和debezium,都需要预先搭建相对应的服务和创建kafka topic,使用上造成的链路比较长。那么能否直接通过flink的source直接读取binlog呢?Flink-CDC就是这样一个连接器,它借助于debezium engine来捕获数据更改,目前支持的数据库有MySQL和PostgreSQL;详细介绍及使用可以访问github的wiki:https://github.com/ververica/flink-cdc-connectors

Exacty-Once

Flink-CDC默认启动时执行一次全量snapshot,把所有数据读取做为INSERT的change mode,在1.1版本还可以指定snapshot.mode为schema_only来禁用第一次启动时的snapshot。但是现在还不支持指定位点消费,不过Flink-CDC执行checkpoint时,会把位点存储到state以支持exacty-once语义。所以从另一个角度来讲,指定位点消费就类似于从savepoint恢复启动,想法上是可行的,但是实现上却少了一步。这就与debezium的实现有关了,拿mysql connector来讲,debezium会保存mysql表的schema信息以便能监听到ddl的变更,会把这些数据封装成HistoryDabase对象存储,所以恢复位点的同时,必须先恢复HistoryDatabase,Flink执行checkpoint存储offset的同时,也把database存储到state。相关代码如下:

// DebeziumSourceFunction.java
private void snapshotHistoryRecordsState() throws Exception {
        historyRecordsState.clear();

        if (engineInstanceName != null) {
            historyRecordsState.add(engineInstanceName);
            ConcurrentLinkedQueue historyRecords = FlinkDatabaseHistory.getRegisteredHistoryRecord(engineInstanceName);
            if (historyRecords != null) {
                DocumentWriter writer = DocumentWriter.defaultWriter();
                for (HistoryRecord record : historyRecords) {
                    historyRecordsState.add(writer.write(record.document()));
                }
            }
        }
    }
指定位点恢复

现在开始讲指定位点消费binlog的实现,目前只实现了mysql,postgresql的应该大同小异。通过代码的debug,我发现mysql的位点信息是一个json格式的数据,主要内容如下:

{
    "sourcePartition":{"server":"mysql-binlog-source"},
    "sourceOffset":{
        "file":"mysql-binlog-000001",
        "pos":2344,
        "ts_sec":1599147495831
    }
}

目前测试mysql只能通过file和pos来恢复位点读取。所以我的步骤如下:

  1. 增加两个options,分别指定binlog文件名称和位点:

    private static final ConfigOption SOURCE_OFFSET_FILE = ConfigOptions.key("source-offset-file")
             .stringType()
             .noDefaultValue()
             .withDescription("File Name of the MySQL binlog.");
    
     private static final ConfigOption SOURCE_OFFSET_POSITION = ConfigOptions.key("source-offset-pos")
             .intType()
             .noDefaultValue()
             .withDescription("Position of the MySQL binlog.");
    
  2. 组装位点json数据:

    // MysqlSource.java#build()
    if (sourceOffsetFile != null && sourceOffsetPosition != null) {
                 // 指定位点恢复时必须指定snapshot.mode为schema_only_recovery,让debezium恢复database,拿到表的schema信息
                 props.setProperty("snapshot.mode", "schema_only_recovery");
    
                 DebeziumState debeziumState = new DebeziumState();
                 Map sourcePartition = new HashMap<>();
                 sourcePartition.put("server", props.getProperty("database.server.name"));
                 debeziumState.setSourcePartition(sourcePartition);
    
                 Map sourceOffset = new HashMap<>();
                 sourceOffset.put("file", sourceOffsetFile);
                 sourceOffset.put("pos", sourceOffsetPosition);
                 debeziumState.setSourceOffset(sourceOffset);
    
                 try {
                     ObjectMapper objectMapper = new ObjectMapper();
                     String offsetJson = objectMapper.writeValueAsString(debeziumState);
                     // 覆盖OFFSET_STATE_VALUE,让debezium识别出从位点消费
                     props.setProperty(FlinkOffsetBackingStore.OFFSET_STATE_VALUE, offsetJson);
                     // 标记database不存在,原来的实现就去恒返回true,以至于debezium为schema_only_recovery时,却不能恢复database
                          props.setProperty("database.history.exists", "false");
                 } catch (IOException e) {
                     throw new RuntimeException("Can't serialize debezium offset state from Object: " + debeziumState, e);
                 }
             }
    
  3. 重写database的exists方法

    // FlinkDatabaseHistory.java
    public void configure(Configuration config, HistoryRecordComparator comparator, DatabaseHistoryListener listener, boolean useCatalogBeforeSchema) {
         super.configure(config, comparator, listener, useCatalogBeforeSchema);
         this.instanceName = config.getString(DATABASE_HISTORY_INSTANCE_NAME);
         this.records = getRegisteredHistoryRecord(instanceName);
         if (records == null) {
             throw new IllegalStateException(
                 String.format("Couldn't find engine instance %s in the global records.", instanceName));
         }
         this.databaseexists = config.getBoolean("database.history.exists", true);
     }
    
     @Override
     public boolean exists() {
         return databaseexists;
     }
    

    以上为实现Flink-CDC指定位点消费mysql binlog的相关代码,代码已经Pull Request到github: https://github.com/ververica/flink-cdc-connectors/pull/39

测试
  1. 第一次启动:

    CREATE TABLE orders (
      order_id INT,
      order_date TIMESTAMP(0),
      customer_name STRING,
      price DECIMAL(10, 5),
      product_id INT,
      order_status BOOLEAN
    ) WITH (
      'connector' = 'mysql-cdc',
      'hostname' = 'localhost',
      'port' = '3306',
      'username' = 'root',
      'password' = '123456',
      'database-name' = 'flink',
      'table-name' = 'orders'
    );
    
    CREATE TABLE print_test (
      order_id INT,
      order_date TIMESTAMP(0),
      customer_name STRING,
      price DECIMAL(10, 5),
      product_id INT,
      order_status BOOLEAN
    ) WITH (
      'connector' = 'print'
    );
    
    INSERT INTO print_test
    SELECT * FROM orders;
    

    输出:

    +I(1,2020-09-03T00:29:30,shizc,12.00000,1111,true)
    +I(2,2020-09-03T23:52:23,shizc1233,22.00000,1123,true)
    +I(3,2020-09-03T23:56:17,2333,3213.00000,11,false)
    +I(4,2020-09-03T23:56:43,shiz343,222.00000,11,true)
    
  2. 指定位点:关于位点怎么获取,这个我目前是通过在mysql执行 show binlog events in 'mysql-bin.000003' 得到。

    CREATE TABLE orders (
      order_id INT,
      order_date TIMESTAMP(0),
      customer_name STRING,
      price DECIMAL(10, 5),
      product_id INT,
      order_status BOOLEAN
    ) WITH (
      'connector' = 'mysql-cdc',
      'hostname' = 'localhost',
      'port' = '3306',
      'username' = 'root',
      'password' = '123456',
      'database-name' = 'flink',
      'table-name' = 'orders',
      'source-offset-file' = 'mysql-bin.000003',
      'source-offset-pos' = '1682'
    );
    

    输出:

    +I(3,2020-09-03T23:56:17,2333,3213.00000,11,false)
    +I(4,2020-09-03T23:56:43,shiz343,222.00000,11,true)
    -U(3,2020-09-03T23:56:17,2333,3213.00000,11,false)
    +U(3,2020-09-03T23:56:17,2333,3213.00000,11,false)
    

    从输出可以看到,从指定位点后,增加了两条数据,并且修改了一条数据,和我测试的数据保持一致。

PS: 很多同学可能以为binlog的offset可以随便指定一个大概的范围的值,这其实是不行的,offset必须指定为一个有效的值,也就是能通过show binlog events in 'xxx' 查到的值,要不然会报一系列的异常,比如:

org.apache.kafka.connect.errors.ConnectException: binlog truncated in the middle of event; consider out of disk space on master; the first event 'mysql-bin.000003' at 1681, the last event read from '.\mysql-bin.000003' at 123, the last byte read from '.\mysql-bin.000003' at 1700. Error code: 1236; SQLSTATE: HY000.
    at io.debezium.connector.mysql.AbstractReader.wrap(AbstractReader.java:230)
    at io.debezium.connector.mysql.AbstractReader.failed(AbstractReader.java:196)
    at io.debezium.connector.mysql.BinlogReader$ReaderThreadLifecycleListener.onCommunicationFailure(BinlogReader.java:1125)
    at com.github.shyiko.mysql.binlog.BinaryLogClient.listenForEventPackets(BinaryLogClient.java:985)
    at com.github.shyiko.mysql.binlog.BinaryLogClient.connect(BinaryLogClient.java:581)
    at com.github.shyiko.mysql.binlog.BinaryLogClient$7.run(BinaryLogClient.java:860)
    at java.lang.Thread.run(Thread.java:748)
Caused by: com.github.shyiko.mysql.binlog.network.ServerException: binlog truncated in the middle of event; consider out of disk space on master; the first event 'mysql-bin.000003' at 1681, the last event read from '.\mysql-bin.000003' at 123, the last byte read from '.\mysql-bin.000003' at 1700.
    at com.github.shyiko.mysql.binlog.BinaryLogClient.listenForEventPackets(BinaryLogClient.java:949)
    ... 3 common frames omitted

目前也没有找到有效的解决方案,如果有好的解决方案,希望大家指出。


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