Apache Flink CDC 批流融合技術原理分析

本文轉載自「好未來技術」公眾號，以 Flink SQL 案例來介紹 Flink CDC 2.0 的使用，並解讀 CDC 中的核心設計。主要內容為：
案例
核心設計
程式碼詳解

8 月份 Flink CDC 釋出 2.0.0 版本，相較於 1.0 版本，在全量讀取階段支援分散式讀取、支援 checkpoint，且在全量 + 增量讀取的過程在不鎖表的情況下保障資料一致性。詳細介紹參考 Flink CDC 2.0 正式釋出，詳解核心改進。

Flink CDC 2.0 資料讀取邏輯並不複雜，複雜的是 FLIP-27: Refactor Source Interface 的設計及對 Debezium Api 的不瞭解。本文重點對 Flink CDC 的處理邏輯進行介紹， FLIP-27 的設計及 Debezium 的 API 呼叫不做過多講解。

本文使用 CDC 2.0.0 版本，先以 Flink SQL 案例來介紹 Flink CDC 2.0 的使用，接著介紹 CDC 中的核心設計包含切片劃分、切分讀取、增量讀取，最後對資料處理過程中涉及 flink-mysql-cdc 介面的呼叫及實現進行程式碼講解。

一、案例

全量讀取 + 增量讀取 Mysql 表資料，以changelog-json 格式寫入 kafka，觀察 RowKind 型別及影響的資料條數。

public static void main(String[] args) {
        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
        EnvironmentSettings envSettings = EnvironmentSettings.newInstance()
                .useBlinkPlanner()
                .inStreamingMode()
                .build();
        env.setParallelism(3);
        // note: 增量同步需要開啟CK
        env.enableCheckpointing(10000);
        StreamTableEnvironment tableEnvironment = StreamTableEnvironment.create(env, envSettings);
            
        tableEnvironment.executeSql(" CREATE TABLE demoOrders (\n" +
                "         `order_id` INTEGER ,\n" +
                "          `order_date` DATE ,\n" +
                "          `order_time` TIMESTAMP(3),\n" +
                "          `quantity` INT ,\n" +
                "          `product_id` INT ,\n" +
                "          `purchaser` STRING,\n" +
                "           primary key(order_id)  NOT ENFORCED" +
                "         ) WITH (\n" +
                "          'connector' = 'mysql-cdc',\n" +
                "          'hostname' = 'localhost',\n" +
                "          'port' = '3306',\n" +
                "          'username' = 'cdc',\n" +
                "          'password' = '123456',\n" +
                "          'database-name' = 'test',\n" +
                "          'table-name' = 'demo_orders'," +
                            //  全量 + 增量同步   
                "          'scan.startup.mode' = 'initial'      " +
                " )");

              tableEnvironment.executeSql("CREATE TABLE sink (\n" +
                "         `order_id` INTEGER ,\n" +
                "          `order_date` DATE ,\n" +
                "          `order_time` TIMESTAMP(3),\n" +
                "          `quantity` INT ,\n" +
                "          `product_id` INT ,\n" +
                "          `purchaser` STRING,\n" +
                "          primary key (order_id)  NOT ENFORCED " +
                ") WITH (\n" +
                "    'connector' = 'kafka',\n" +
                "    'properties.bootstrap.servers' = 'localhost:9092',\n" +
                "    'topic' = 'mqTest02',\n" +
                "    'format' = 'changelog-json' "+
                ")");

             tableEnvironment.executeSql("insert into sink select * from demoOrders");}

全量資料輸出：

{"data":{"order_id":1010,"order_date":"2021-09-17","order_time":"2021-09-22 10:52:12.189","quantity":53,"product_id":502,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1009,"order_date":"2021-09-17","order_time":"2021-09-22 10:52:09.709","quantity":31,"product_id":500,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1008,"order_date":"2021-09-17","order_time":"2021-09-22 10:52:06.637","quantity":69,"product_id":503,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1007,"order_date":"2021-09-17","order_time":"2021-09-22 10:52:03.535","quantity":52,"product_id":502,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1002,"order_date":"2021-09-17","order_time":"2021-09-22 10:51:51.347","quantity":69,"product_id":503,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1001,"order_date":"2021-09-17","order_time":"2021-09-22 10:51:48.783","quantity":50,"product_id":502,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1000,"order_date":"2021-09-17","order_time":"2021-09-17 17:40:32.354","quantity":30,"product_id":500,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1006,"order_date":"2021-09-17","order_time":"2021-09-22 10:52:01.249","quantity":31,"product_id":500,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1005,"order_date":"2021-09-17","order_time":"2021-09-22 10:51:58.813","quantity":69,"product_id":503,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1004,"order_date":"2021-09-17","order_time":"2021-09-22 10:51:56.153","quantity":50,"product_id":502,"purchaser":"flink"},"op":"+I"}
{"data":{"order_id":1003,"order_date":"2021-09-17","order_time":"2021-09-22 10:51:53.727","quantity":30,"product_id":500,"purchaser":"flink"},"op":"+I"}

修改表資料，增量捕獲：

## 更新 1005 的值 
{"data":{"order_id":1005,"order_date":"2021-09-17","order_time":"2021-09-22 02:51:58.813","quantity":69,"product_id":503,"purchaser":"flink"},"op":"-U"}
{"data":{"order_id":1005,"order_date":"2021-09-17","order_time":"2021-09-22 02:55:43.627","quantity":80,"product_id":503,"purchaser":"flink"},"op":"+U"}

## 刪除 1000 
{"data":{"order_id":1000,"order_date":"2021-09-17","order_time":"2021-09-17 09:40:32.354","quantity":30,"product_id":500,"purchaser":"flink"},"op":"-D"}

二、核心設計

1. 切片劃分

全量階段資料讀取方式為分散式讀取，會先對當前表資料按主鍵劃分成多個Chunk，後續子任務讀取Chunk 區間內的資料。根據主鍵列是否為自增整數型別，對錶資料劃分為均勻分佈的Chunk及非均勻分佈的Chunk。

1.1 均勻分佈

主鍵列自增且型別為整數型別（int,bigint,decimal）。查詢出主鍵列的最小值，最大值，按 chunkSize 大小將資料均勻劃分，因為主鍵為整數型別，根據當前chunk 起始位置、chunkSize 大小，直接計算 chunk 的結束位置。

注意：最新版本均勻分佈的觸發條件不再依賴主鍵列是否自增，要求主鍵列衛整數型別且根據 max(id) - min(id)/rowcount 計算出資料分佈係數，只有分佈係數 <= 配置的分佈係數 (evenly-distribution.factor 預設為 1000.0d) 才會進行資料均勻劃分。

//  計算主鍵列資料區間
select min(`order_id`), max(`order_id`) from demo_orders;

//  將資料劃分為 chunkSize 大小的切片
chunk-0： [min，start + chunkSize)
chunk-1： [start + chunkSize, start + 2chunkSize)
.......
chunk-last： [max，null)

1.2 非均勻分佈

主鍵列非自增或者型別為非整數型別。主鍵為非數值型別，每次劃分需要對未劃分的資料按主鍵進行升序排列，取出前 chunkSize 的最大值為當前 chunk 的結束位置。

注意：最新版本非均勻分佈觸發條件為主鍵列為非整數型別，或者計算出的分佈係數 (distributionFactor) > 配置的分佈係數 (evenly-distribution.factor)。

// 未拆分的資料排序後，取 chunkSize 條資料取最大值，作為切片的終止位置。
chunkend = SELECT MAX(`order_id`) FROM (
        SELECT `order_id`  FROM `demo_orders` 
        WHERE `order_id` >= [前一個切片的起始位置] 
        ORDER BY `order_id` ASC 
        LIMIT   [chunkSize]  
    ) AS T

2. 全量切片資料讀取

Flink 將表資料劃分為多個 Chunk，子任務在不加鎖的情況下，並行讀取 Chunk 資料。因為全程無鎖在資料分片讀取過程中，可能有其他事務對切片範圍內的資料進行修改，此時無法保證資料一致性。因此，在全量階段 Flink 使用快照記錄讀取 + Binlog 資料修正的方式來保證資料的一致性。

2.1 快照讀取

通過 JDBC 執行 SQL 查詢切片範圍的資料記錄。

## 快照記錄資料讀取SQL 
SELECT * FROM `test`.`demo_orders` 
WHERE order_id >= [chunkStart] 
AND NOT (order_id = [chunkEnd]) 
AND order_id <= [chunkEnd]

2.2 資料修正

在快照讀取操作前、後執行 SHOW MASTER STATUS 查詢 binlog 檔案的當前偏移量，在快照讀取完畢後，查詢區間內的 binlog 資料並對讀取的快照記錄進行修正。

快照讀取 + Binlog 資料讀取時的資料組織結構：

BinlogEvents 修正 SnapshotEvents 規則。

未讀取到 binlog 資料，即在執行 select 階段沒有其他事務進行操作，直接下發所有快照記錄。
讀取到 binlog 資料，且變更的資料記錄不屬於當前切片，下發快照記錄。
讀取到 binlog 資料，且資料記錄的變更屬於當前切片。delete 操作從快照記憶體中移除該資料，insert 操作向快照記憶體新增新的資料，update 操作向快照記憶體中新增變更記錄，最終會輸出更新前後的兩條記錄到下游。

修正後的資料組織結構：

![image.png](https://img.alicdn.com/imgextra/i1/O1CN01POsozI1WMXPFo2J0W_!!6000000002774-2-tps-1080-93.png)

以讀取切片 [1,11] 範圍的資料為例，描述切片資料的處理過程。c、d、u 代表 Debezium 捕獲到的新增、刪除、更新操作。

修正前資料及結構：

修正後資料及結構：

單個切片資料處理完畢後會向 SplitEnumerator 傳送已完成切片資料的起始位置（ChunkStart, ChunkStartEnd）、Binlog 的最大偏移量（High watermark），用來為增量讀取指定起始偏移量。

3. 增量切片資料讀取

全量階段切片資料讀取完成後，SplitEnumerator 會下發一個 BinlogSplit 進行增量資料讀取。BinlogSplit 讀取最重要的屬性就是起始偏移量，偏移量如果設定過小下游可能會有重複資料，偏移量如果設定過大下游可能是已超期的髒資料。而 Flink CDC 增量讀取的起始偏移量為所有已完成的全量切片最小的Binlog 偏移量，只有滿足條件的資料才被下發到下游。資料下發條件：

捕獲的 Binlog 資料的偏移量 > 資料所屬分片的 Binlog 的最大偏移量。

例如，SplitEnumerator 保留的已完成切片資訊為：

切片索引	Chunk 資料範圍	切片讀取的最大Binlog
0	[1,100]	1000
1	[101,200]	800
2	[201,300]	1500

增量讀取時，從偏移量 800 開始讀取 Binlog 資料，當捕獲到資料 <data:123, offset:1500> 時，先找到 123 所屬快照分片，並找到對應的最大 Binlog 偏移量 800。當前偏移量大於快照讀的最大偏移量，則下發資料，否則直接丟棄。

三、程式碼詳解

關於 FLIP-27: Refactor Source Interface 設計不做詳細介紹，本文側重對 flink-mysql-cdc 介面呼叫及實現進行講解。

1. MySqlSourceEnumerator 初始化

SourceCoordinator 作為 OperatorCoordinator 對 Source 的實現，執行在 Master 節點，在啟動時通過呼叫 MySqlParallelSource#createEnumerator 建立 MySqlSourceEnumerator 並呼叫 start 方法，做一些初始化工作。
SourceCoordinator 啟動.png

建立 MySqlSourceEnumerator，使用 MySqlHybridSplitAssigner 對全量+增量資料進行切片，使用 MySqlValidator 對 mysql 版本、配置進行校驗。
MySqlValidator 校驗：
1. mysql 版本必須大於等於 5.7。
2. binlog_format 配置必須為 ROW。
3. binlog_row_image 配置必須為 FULL。
MySqlSplitAssigner 初始化：
1. 建立 ChunkSplitter 用來劃分切片。
2. 篩選出要讀的表名稱。
啟動週期排程執行緒，要求 SourceReader 向 SourceEnumerator 傳送已完成但未傳送 ACK 事件的切片資訊。

private void syncWithReaders(int[] subtaskIds, Throwable t) {
    if (t != null) {
        throw new FlinkRuntimeException("Failed to list obtain registered readers due to:", t);
    }
    // when the SourceEnumerator restores or the communication failed between
    // SourceEnumerator and SourceReader, it may missed some notification event.
    // tell all SourceReader(s) to report there finished but unacked splits.
    if (splitAssigner.waitingForFinishedSplits()) {
        for (int subtaskId : subtaskIds) {
            // note: 傳送 FinishedSnapshotSplitsRequestEvent 
            context.sendEventToSourceReader(
                    subtaskId, new FinishedSnapshotSplitsRequestEvent());
        }
    }
}

2. MySqlSourceReader 初始化

SourceOperator 整合了 SourceReader，通過OperatorEventGateway 和 SourceCoordinator 進行互動。
SourceOperator 啟動 (1).png

SourceOperator 在初始化時，通過 MySqlParallelSource 建立 MySqlSourceReader。MySqlSourceReader 通過 SingleThreadFetcherManager 建立 Fetcher 拉取分片資料，資料以 MySqlRecords 格式寫入到 elementsQueue。

MySqlParallelSource#createReader

public SourceReader<T, MySqlSplit> createReader(SourceReaderContext readerContext) throws Exception {
    // note:  資料儲存佇列
FutureCompletingBlockingQueue<RecordsWithSplitIds<SourceRecord>> elementsQueue =
        new FutureCompletingBlockingQueue<>();
final Configuration readerConfiguration = getReaderConfig(readerContext);

    // note: Split Reader 工廠類
Supplier<MySqlSplitReader> splitReaderSupplier =
        () -> new MySqlSplitReader(readerConfiguration, readerContext.getIndexOfSubtask());

return new MySqlSourceReader<>(
        elementsQueue,
        splitReaderSupplier,
        new MySqlRecordEmitter<>(deserializationSchema),
        readerConfiguration,
        readerContext);
}

將建立的 MySqlSourceReader 以事件的形式傳遞給 SourceCoordinator 進行註冊。SourceCoordinator 接收到註冊事件後，將 reader 地址及索引進行儲存。

SourceCoordinator#handleReaderRegistrationEvent
// note: SourceCoordinator 處理Reader 註冊事件
private void handleReaderRegistrationEvent(ReaderRegistrationEvent event) {
    context.registerSourceReader(new ReaderInfo(event.subtaskId(), event.location()));
    enumerator.addReader(event.subtaskId());
}

MySqlSourceReader 啟動後會向 MySqlSourceEnumerator 傳送請求分片事件，從而收集分配的切片資料。
SourceOperator 初始化完畢後，呼叫 emitNext 由 SourceReaderBase 從 elementsQueue 獲取資料集合並下發給 MySqlRecordEmitter。介面呼叫示意圖：

3. MySqlSourceEnumerator 處理分片請求

MySqlSourceReader 啟動時會向 MySqlSourceEnumerator 傳送請求 RequestSplitEvent 事件，根據返回的切片範圍讀取區間資料。MySqlSourceEnumerator 全量讀取階段分片請求處理邏輯，最終返回一個 MySqlSnapshotSplit。
未命名檔案 (1).png

處理切片請求事件，為請求的 Reader 分配切片，通過傳送 AddSplitEvent 時間傳遞 MySqlSplit (全量階段MySqlSnapshotSplit、增量階段 MySqlBinlogSplit)。

MySqlSourceEnumerator#handleSplitRequest
public void handleSplitRequest(int subtaskId, @Nullable String requesterHostname) {
    if (!context.registeredReaders().containsKey(subtaskId)) {
        // reader failed between sending the request and now. skip this request.
        return;
    }
    // note:  將reader所屬的subtaskId儲存到TreeSet, 在處理binlog split時優先分配個task-0
    readersAwaitingSplit.add(subtaskId);

    assignSplits();
}

// note: 分配切片
private void assignSplits() {
    final Iterator<Integer> awaitingReader = readersAwaitingSplit.iterator();
    while (awaitingReader.hasNext()) {
        int nextAwaiting = awaitingReader.next();
        // if the reader that requested another split has failed in the meantime, remove
        // it from the list of waiting readers
        if (!context.registeredReaders().containsKey(nextAwaiting)) {
            awaitingReader.remove();
            continue;
        }

        //note: 由 MySqlSplitAssigner 分配切片
        Optional<MySqlSplit> split = splitAssigner.getNext();
        if (split.isPresent()) {
            final MySqlSplit mySqlSplit = split.get();
            //  note: 傳送AddSplitEvent, 為 Reader 返回切片資訊
            context.assignSplit(mySqlSplit, nextAwaiting);
            awaitingReader.remove();

            LOG.info("Assign split {} to subtask {}", mySqlSplit, nextAwaiting);
        } else {
            // there is no available splits by now, skip assigning
            break;
        }
    }
}

MySqlHybridSplitAssigner 處理全量切片、增量切片的邏輯。
1. 任務剛啟動時，remainingTables 不為空，noMoreSplits 返回值為false，建立 SnapshotSplit。
2. 全量階段分片讀取完成後，noMoreSplits 返回值為true, 建立 BinlogSplit。

MySqlHybridSplitAssigner#getNext
@Override
public Optional<MySqlSplit> getNext() {
    if (snapshotSplitAssigner.noMoreSplits()) {
        // binlog split assigning
        if (isBinlogSplitAssigned) {
            // no more splits for the assigner
            return Optional.empty();
        } else if (snapshotSplitAssigner.isFinished()) {
            // we need to wait snapshot-assigner to be finished before
            // assigning the binlog split. Otherwise, records emitted from binlog split
            // might be out-of-order in terms of same primary key with snapshot splits.
            isBinlogSplitAssigned = true;

            //note: snapshot split 切片完成後，建立BinlogSplit。
            return Optional.of(createBinlogSplit());
        } else {
            // binlog split is not ready by now
            return Optional.empty();
        }
    } else {
        // note: 由MySqlSnapshotSplitAssigner 建立 SnapshotSplit
        // snapshot assigner still have remaining splits, assign split from it
        return snapshotSplitAssigner.getNext();
    }
}

MySqlSnapshotSplitAssigner 處理全量切片邏輯，通過 ChunkSplitter 生成切片，並儲存到 Iterator 中。

@Override
public Optional<MySqlSplit> getNext() {
    if (!remainingSplits.isEmpty()) {
        // return remaining splits firstly
        Iterator<MySqlSnapshotSplit> iterator = remainingSplits.iterator();
        MySqlSnapshotSplit split = iterator.next();
        iterator.remove();
        
        //note: 已分配的切片儲存到 assignedSplits 集合
        assignedSplits.put(split.splitId(), split);

        return Optional.of(split);
    } else {
        // note: 初始化階段 remainingTables 儲存了要讀取的表名
        TableId nextTable = remainingTables.pollFirst();
        if (nextTable != null) {
            // split the given table into chunks (snapshot splits)
            //  note: 初始化階段建立了 ChunkSplitter，呼叫generateSplits 進行切片劃分
            Collection<MySqlSnapshotSplit> splits = chunkSplitter.generateSplits(nextTable);
            //  note: 保留所有切片資訊
            remainingSplits.addAll(splits);
            //  note: 已經完成分片的 Table
            alreadyProcessedTables.add(nextTable);
            //  note: 遞迴呼叫該該方法
            return getNext();
        } else {
            return Optional.empty();
        }
    }
}

ChunkSplitter 將表劃分為均勻分佈 or 不均勻分佈切片的邏輯。讀取的表必須包含物理主鍵。

public Collection<MySqlSnapshotSplit> generateSplits(TableId tableId) {

    Table schema = mySqlSchema.getTableSchema(tableId).getTable();
    List<Column> primaryKeys = schema.primaryKeyColumns();
    // note: 必須有主鍵
    if (primaryKeys.isEmpty()) {
        throw new ValidationException(
                String.format(
                        "Incremental snapshot for tables requires primary key,"
                                + " but table %s doesn't have primary key.",
                        tableId));
    }
    // use first field in primary key as the split key
    Column splitColumn = primaryKeys.get(0);

    final List<ChunkRange> chunks;
    try {
         // note: 按主鍵列將資料劃分成多個切片
        chunks = splitTableIntoChunks(tableId, splitColumn);
    } catch (SQLException e) {
        throw new FlinkRuntimeException("Failed to split chunks for table " + tableId, e);
    }
    //note: 主鍵資料型別轉換、ChunkRange 包裝成MySqlSnapshotSplit。
    // convert chunks into splits
    List<MySqlSnapshotSplit> splits = new ArrayList<>();
    RowType splitType = splitType(splitColumn);
 
    for (int i = 0; i < chunks.size(); i++) {
        ChunkRange chunk = chunks.get(i);
        MySqlSnapshotSplit split =
                createSnapshotSplit(
                        tableId, i, splitType, chunk.getChunkStart(), chunk.getChunkEnd());
        splits.add(split);
    }
    return splits;
}

splitTableIntoChunks 根據物理主鍵劃分切片。

private List<ChunkRange> splitTableIntoChunks(TableId tableId, Column splitColumn)
        throws SQLException {
    final String splitColumnName = splitColumn.name();
    //  select min, max
    final Object[] minMaxOfSplitColumn = queryMinMax(jdbc, tableId, splitColumnName);
    final Object min = minMaxOfSplitColumn[0];
    final Object max = minMaxOfSplitColumn[1];
    if (min == null || max == null || min.equals(max)) {
        // empty table, or only one row, return full table scan as a chunk
        return Collections.singletonList(ChunkRange.all());
    }

    final List<ChunkRange> chunks;
    if (splitColumnEvenlyDistributed(splitColumn)) {
        // use evenly-sized chunks which is much efficient
        // note: 按主鍵均勻劃分
        chunks = splitEvenlySizedChunks(min, max);
    } else {
        // note: 按主鍵非均勻劃分
        // use unevenly-sized chunks which will request many queries and is not efficient.
        chunks = splitUnevenlySizedChunks(tableId, splitColumnName, min, max);
    }

    return chunks;
}

/** Checks whether split column is evenly distributed across its range. */
private static boolean splitColumnEvenlyDistributed(Column splitColumn) {
    // only column is auto-incremental are recognized as evenly distributed.
    // TODO: we may use MAX,MIN,COUNT to calculate the distribution in the future.
    if (splitColumn.isAutoIncremented()) {
        DataType flinkType = MySqlTypeUtils.fromDbzColumn(splitColumn);
        LogicalTypeRoot typeRoot = flinkType.getLogicalType().getTypeRoot();
        // currently, we only support split column with type BIGINT, INT, DECIMAL
        return typeRoot == LogicalTypeRoot.BIGINT
                || typeRoot == LogicalTypeRoot.INTEGER
                || typeRoot == LogicalTypeRoot.DECIMAL;
    } else {
        return false;
    }
}


/**
 *  根據拆分列的最小值和最大值將表拆分為大小均勻的塊，並以 {@link #chunkSize} 步長滾動塊。
 * Split table into evenly sized chunks based on the numeric min and max value of split column,
 * and tumble chunks in {@link #chunkSize} step size.
 */
private List<ChunkRange> splitEvenlySizedChunks(Object min, Object max) {
    if (ObjectUtils.compare(ObjectUtils.plus(min, chunkSize), max) > 0) {
        // there is no more than one chunk, return full table as a chunk
        return Collections.singletonList(ChunkRange.all());
    }

    final List<ChunkRange> splits = new ArrayList<>();
    Object chunkStart = null;
    Object chunkEnd = ObjectUtils.plus(min, chunkSize);
    //  chunkEnd <= max
    while (ObjectUtils.compare(chunkEnd, max) <= 0) {
        splits.add(ChunkRange.of(chunkStart, chunkEnd));
        chunkStart = chunkEnd;
        chunkEnd = ObjectUtils.plus(chunkEnd, chunkSize);
    }
    // add the ending split
    splits.add(ChunkRange.of(chunkStart, null));
    return splits;
}

/**   通過連續計算下一個塊最大值，將表拆分為大小不均勻的塊。
 * Split table into unevenly sized chunks by continuously calculating next chunk max value. */
private List<ChunkRange> splitUnevenlySizedChunks(
        TableId tableId, String splitColumnName, Object min, Object max) throws SQLException {
    final List<ChunkRange> splits = new ArrayList<>();
    Object chunkStart = null;

    Object chunkEnd = nextChunkEnd(min, tableId, splitColumnName, max);
    int count = 0;
    while (chunkEnd != null && ObjectUtils.compare(chunkEnd, max) <= 0) {
        // we start from [null, min + chunk_size) and avoid [null, min)
        splits.add(ChunkRange.of(chunkStart, chunkEnd));
        // may sleep a while to avoid DDOS on MySQL server
        maySleep(count++);
        chunkStart = chunkEnd;
        chunkEnd = nextChunkEnd(chunkEnd, tableId, splitColumnName, max);
    }
    // add the ending split
    splits.add(ChunkRange.of(chunkStart, null));
    return splits;
}

private Object nextChunkEnd(
        Object previousChunkEnd, TableId tableId, String splitColumnName, Object max)
        throws SQLException {
    // chunk end might be null when max values are removed
    Object chunkEnd =
            queryNextChunkMax(jdbc, tableId, splitColumnName, chunkSize, previousChunkEnd);
    if (Objects.equals(previousChunkEnd, chunkEnd)) {
        // we don't allow equal chunk start and end,
        // should query the next one larger than chunkEnd
        chunkEnd = queryMin(jdbc, tableId, splitColumnName, chunkEnd);
    }
    if (ObjectUtils.compare(chunkEnd, max) >= 0) {
        return null;
    } else {
        return chunkEnd;
    }
}

4. MySqlSourceReader 處理切片分配請求

MySqlSourceReader 處理切片分配請求.png
MySqlSourceReader 接收到切片分配請求後，會為先建立一個 SplitFetcher 執行緒，向 taskQueue 新增、執行 AddSplitsTask 任務用來處理新增分片任務，接著執行 FetchTask 使用 Debezium API 進行讀取資料，讀取的資料儲存到 elementsQueue 中，SourceReaderBase 會從該佇列中獲取資料，並下發給 MySqlRecordEmitter。

處理切片分配事件時，建立 SplitFetcher 向 taskQueue 新增 AddSplitsTask。

SingleThreadFetcherManager#addSplits
public void addSplits(List<SplitT> splitsToAdd) {
    SplitFetcher<E, SplitT> fetcher = getRunningFetcher();
    if (fetcher == null) {
        fetcher = createSplitFetcher();
        // Add the splits to the fetchers.
        fetcher.addSplits(splitsToAdd);
        startFetcher(fetcher);
    } else {
        fetcher.addSplits(splitsToAdd);
    }
}

// 建立 SplitFetcher
protected synchronized SplitFetcher<E, SplitT> createSplitFetcher() {
    if (closed) {
        throw new IllegalStateException("The split fetcher manager has closed.");
    }
    // Create SplitReader.
    SplitReader<E, SplitT> splitReader = splitReaderFactory.get();

    int fetcherId = fetcherIdGenerator.getAndIncrement();
    SplitFetcher<E, SplitT> splitFetcher =
            new SplitFetcher<>(
                    fetcherId,
                    elementsQueue,
                    splitReader,
                    errorHandler,
                    () -> {
                        fetchers.remove(fetcherId);
                        elementsQueue.notifyAvailable();
                    });
    fetchers.put(fetcherId, splitFetcher);
    return splitFetcher;
}

public void addSplits(List<SplitT> splitsToAdd) {
    enqueueTask(new AddSplitsTask<>(splitReader, splitsToAdd, assignedSplits));
    wakeUp(true);
}

執行 SplitFetcher執行緒，首次執行 AddSplitsTask 執行緒新增分片，以後執行 FetchTask 執行緒拉取資料。

SplitFetcher#runOnce
void runOnce() {
    try {
        if (shouldRunFetchTask()) {
            runningTask = fetchTask;
        } else {
            runningTask = taskQueue.take();
        }
        
        if (!wakeUp.get() && runningTask.run()) {
            LOG.debug("Finished running task {}", runningTask);
            runningTask = null;
            checkAndSetIdle();
        }
    } catch (Exception e) {
        throw new RuntimeException(
                String.format(
                        "SplitFetcher thread %d received unexpected exception while polling the records",
                        id),
                e);
    }

    maybeEnqueueTask(runningTask);
    synchronized (wakeUp) {
        // Set the running task to null. It is necessary for the shutdown method to avoid
        // unnecessarily interrupt the running task.
        runningTask = null;
        // Set the wakeUp flag to false.
        wakeUp.set(false);
        LOG.debug("Cleaned wakeup flag.");
    }
}

AddSplitsTask 呼叫 MySqlSplitReader 的 handleSplitsChanges 方法，向切片佇列中新增已分配的切片資訊。在下一次 fetch() 呼叫時，從佇列中獲取切片並讀取切片資料。

AddSplitsTask#run
public boolean run() {
    for (SplitT s : splitsToAdd) {
        assignedSplits.put(s.splitId(), s);
    }
    splitReader.handleSplitsChanges(new SplitsAddition<>(splitsToAdd));
    return true;
}
MySqlSplitReader#handleSplitsChanges
public void handleSplitsChanges(SplitsChange<MySqlSplit> splitsChanges) {
    if (!(splitsChanges instanceof SplitsAddition)) {
        throw new UnsupportedOperationException(
                String.format(
                        "The SplitChange type of %s is not supported.",
                        splitsChanges.getClass()));
    }

    //note: 新增切片 到佇列。
    splits.addAll(splitsChanges.splits());
}

MySqlSplitReader 執行 fetch()，由 DebeziumReader 讀取資料到事件佇列，在對資料修正後以 MySqlRecords 格式返回。

MySqlSplitReader#fetch
@Override
public RecordsWithSplitIds<SourceRecord> fetch() throws IOException {
    // note: 建立Reader 並讀取資料
    checkSplitOrStartNext();

    Iterator<SourceRecord> dataIt = null;
    try {
        // note:  對讀取的資料進行修正
        dataIt = currentReader.pollSplitRecords();
    } catch (InterruptedException e) {
        LOG.warn("fetch data failed.", e);
        throw new IOException(e);
    }

    //  note: 返回的資料被封裝為 MySqlRecords 進行傳輸
    return dataIt == null
            ? finishedSnapshotSplit()   
            : MySqlRecords.forRecords(currentSplitId, dataIt);
}

private void checkSplitOrStartNext() throws IOException {
    // the binlog reader should keep alive
    if (currentReader instanceof BinlogSplitReader) {
        return;
    }

    if (canAssignNextSplit()) {
        // note:  從切片佇列讀取MySqlSplit
        final MySqlSplit nextSplit = splits.poll();
        if (nextSplit == null) {
            throw new IOException("Cannot fetch from another split - no split remaining");
        }

        currentSplitId = nextSplit.splitId();
        // note:  區分全量切片讀取還是增量切片讀取
        if (nextSplit.isSnapshotSplit()) {
            if (currentReader == null) {
                final MySqlConnection jdbcConnection = getConnection(config);
                final BinaryLogClient binaryLogClient = getBinaryClient(config);

                final StatefulTaskContext statefulTaskContext =
                        new StatefulTaskContext(config, binaryLogClient, jdbcConnection);
                // note: 建立SnapshotSplitReader，使用Debezium Api讀取分配資料及區間Binlog值
                currentReader = new SnapshotSplitReader(statefulTaskContext, subtaskId);
            }

        } else {
            // point from snapshot split to binlog split
            if (currentReader != null) {
                LOG.info("It's turn to read binlog split, close current snapshot reader");
                currentReader.close();
            }

            final MySqlConnection jdbcConnection = getConnection(config);
            final BinaryLogClient binaryLogClient = getBinaryClient(config);
            final StatefulTaskContext statefulTaskContext =
                    new StatefulTaskContext(config, binaryLogClient, jdbcConnection);
            LOG.info("Create binlog reader");
            // note: 建立BinlogSplitReader，使用Debezium API進行增量讀取
            currentReader = new BinlogSplitReader(statefulTaskContext, subtaskId);
        }
        // note: 執行Reader進行資料讀取
        currentReader.submitSplit(nextSplit);
    }
}

5. DebeziumReader 資料處理

DebeziumReader 包含全量切片讀取、增量切片讀取兩個階段，資料讀取後儲存到 ChangeEventQueue，執行pollSplitRecords 時對資料進行修正。

SnapshotSplitReader 全量切片讀取。全量階段的資料讀取通過執行 Select 語句查詢出切片範圍內的表資料，在寫入佇列前後執行 SHOW MASTER STATUS 時，寫入當前偏移量。

public void submitSplit(MySqlSplit mySqlSplit) {
    ......
    executor.submit(
            () -> {
                try {
                    currentTaskRunning = true;
                    // note: 資料讀取，在資料前後插入Binlog當前偏移量
                    // 1. execute snapshot read task。 
                    final SnapshotSplitChangeEventSourceContextImpl sourceContext =
                            new SnapshotSplitChangeEventSourceContextImpl();
                    SnapshotResult snapshotResult =
                            splitSnapshotReadTask.execute(sourceContext);

                    //  note: 為增量讀取做準備，包含了起始偏移量
                    final MySqlBinlogSplit appendBinlogSplit = createBinlogSplit(sourceContext);
                    final MySqlOffsetContext mySqlOffsetContext =
                            statefulTaskContext.getOffsetContext();
                    mySqlOffsetContext.setBinlogStartPoint(
                            appendBinlogSplit.getStartingOffset().getFilename(),
                            appendBinlogSplit.getStartingOffset().getPosition());

                    //  note: 從起始偏移量開始讀取           
                    // 2. execute binlog read task
                    if (snapshotResult.isCompletedOrSkipped()) {
                        // we should only capture events for the current table,
                        Configuration dezConf =
                                statefulTaskContext
                                        .getDezConf()
                                        .edit()
                                        .with(
                                                "table.whitelist",
                                                currentSnapshotSplit.getTableId())
                                        .build();

                        // task to read binlog for current split
                        MySqlBinlogSplitReadTask splitBinlogReadTask =
                                new MySqlBinlogSplitReadTask(
                                        new MySqlConnectorConfig(dezConf),
                                        mySqlOffsetContext,
                                        statefulTaskContext.getConnection(),
                                        statefulTaskContext.getDispatcher(),
                                        statefulTaskContext.getErrorHandler(),
                                        StatefulTaskContext.getClock(),
                                        statefulTaskContext.getTaskContext(),
                                        (MySqlStreamingChangeEventSourceMetrics)
                                                statefulTaskContext
                                                        .getStreamingChangeEventSourceMetrics(),
                                        statefulTaskContext
                                                .getTopicSelector()
                                                .getPrimaryTopic(),
                                        appendBinlogSplit);

                        splitBinlogReadTask.execute(
                                new SnapshotBinlogSplitChangeEventSourceContextImpl());
                    } else {
                        readException =
                                new IllegalStateException(
                                        String.format(
                                                "Read snapshot for mysql split %s fail",
                                                currentSnapshotSplit));
                    }
                } catch (Exception e) {
                    currentTaskRunning = false;
                    LOG.error(
                            String.format(
                                    "Execute snapshot read task for mysql split %s fail",
                                    currentSnapshotSplit),
                            e);
                    readException = e;
                }
            });
}

SnapshotSplitReader 增量切片讀取。增量階段切片讀取重點是判斷 BinlogSplitReadTask 什麼時候停止，在讀取到分片階段的結束時的偏移量即終止。

MySqlBinlogSplitReadTask#handleEvent
protected void handleEvent(Event event) {
    // note: 事件下發 佇列
    super.handleEvent(event);
    // note: 全量讀取階段需要終止Binlog讀取
    // check do we need to stop for read binlog for snapshot split.
    if (isBoundedRead()) {
        final BinlogOffset currentBinlogOffset =
                new BinlogOffset(
                        offsetContext.getOffset().get(BINLOG_FILENAME_OFFSET_KEY).toString(),
                        Long.parseLong(
                                offsetContext
                                        .getOffset()
                                        .get(BINLOG_POSITION_OFFSET_KEY)
                                        .toString()));
        // note: currentBinlogOffset > HW 停止讀取
        // reach the high watermark, the binlog reader should finished
        if (currentBinlogOffset.isAtOrBefore(binlogSplit.getEndingOffset())) {
            // send binlog end event
            try {
                signalEventDispatcher.dispatchWatermarkEvent(
                        binlogSplit,
                        currentBinlogOffset,
                        SignalEventDispatcher.WatermarkKind.BINLOG_END);
            } catch (InterruptedException e) {
                logger.error("Send signal event error.", e);
                errorHandler.setProducerThrowable(
                        new DebeziumException("Error processing binlog signal event", e));
            }
            //  終止binlog讀取
            // tell reader the binlog task finished
            ((SnapshotBinlogSplitChangeEventSourceContextImpl) context).finished();
        }
    }
}

SnapshotSplitReader 執行 pollSplitRecords 時對佇列中的原始資料進行修正。具體處理邏輯檢視 RecordUtils#normalizedSplitRecords。

public Iterator<SourceRecord> pollSplitRecords() throws InterruptedException {
    if (hasNextElement.get()) {
        // data input: [low watermark event][snapshot events][high watermark event][binlogevents][binlog-end event]
        // data output: [low watermark event][normalized events][high watermark event]
        boolean reachBinlogEnd = false;
        final List<SourceRecord> sourceRecords = new ArrayList<>();
        while (!reachBinlogEnd) {
            // note: 處理佇列中寫入的 DataChangeEvent 事件
            List<DataChangeEvent> batch = queue.poll();
            for (DataChangeEvent event : batch) {
                sourceRecords.add(event.getRecord());
                if (RecordUtils.isEndWatermarkEvent(event.getRecord())) {
                    reachBinlogEnd = true;
                    break;
                }
            }
        }
        // snapshot split return its data once
        hasNextElement.set(false);
        //  ************   修正資料  ***********
        return normalizedSplitRecords(currentSnapshotSplit, sourceRecords, nameAdjuster)
                .iterator();
    }
    // the data has been polled, no more data
    reachEnd.compareAndSet(false, true);
    return null;
}

BinlogSplitReader 資料讀取。讀取邏輯比較簡單，重點是起始偏移量的設定，起始偏移量為所有切片的 HW。
BinlogSplitReader 執行 pollSplitRecords 時對佇列中的原始資料進行修正，保障資料一致性。增量階段的Binlog讀取是無界的，資料會全部下發到事件佇列，BinlogSplitReader 通過 shouldEmit() 判斷資料是否下發。

BinlogSplitReader#pollSplitRecords
public Iterator<SourceRecord> pollSplitRecords() throws InterruptedException {
    checkReadException();
    final List<SourceRecord> sourceRecords = new ArrayList<>();
    if (currentTaskRunning) {
        List<DataChangeEvent> batch = queue.poll();
        for (DataChangeEvent event : batch) {
            if (shouldEmit(event.getRecord())) {
                sourceRecords.add(event.getRecord());
            }
        }
    }
    return sourceRecords.iterator();
}

事件下發條件：

新收到的 event post 大於 maxwm；
當前 data 值所屬某個 snapshot spilt & 偏移量大於 HWM,下發資料。

/**
 *
 * Returns the record should emit or not.
 *
 * <p>The watermark signal algorithm is the binlog split reader only sends the binlog event that
 * belongs to its finished snapshot splits. For each snapshot split, the binlog event is valid
 * since the offset is after its high watermark.
 *
 * <pre> E.g: the data input is :
 *    snapshot-split-0 info : [0,    1024) highWatermark0
 *    snapshot-split-1 info : [1024, 2048) highWatermark1
 *  the data output is:
 *  only the binlog event belong to [0,    1024) and offset is after highWatermark0 should send,
 *  only the binlog event belong to [1024, 2048) and offset is after highWatermark1 should send.
 * </pre>
 */
private boolean shouldEmit(SourceRecord sourceRecord) {
    if (isDataChangeRecord(sourceRecord)) {
        TableId tableId = getTableId(sourceRecord);
        BinlogOffset position = getBinlogPosition(sourceRecord);
        // aligned, all snapshot splits of the table has reached max highWatermark
       
        // note:  新收到的event post 大於 maxwm ,直接下發
        if (position.isAtOrBefore(maxSplitHighWatermarkMap.get(tableId))) {
            return true;
        }
        Object[] key =
                getSplitKey(
                        currentBinlogSplit.getSplitKeyType(),
                        sourceRecord,
                        statefulTaskContext.getSchemaNameAdjuster());

        for (FinishedSnapshotSplitInfo splitInfo : finishedSplitsInfo.get(tableId)) {
            /**
             *  note: 當前 data值所屬某個snapshot spilt &  偏移量大於 HWM,下發資料
             */
            if (RecordUtils.splitKeyRangeContains(
                            key, splitInfo.getSplitStart(), splitInfo.getSplitEnd())
                    && position.isAtOrBefore(splitInfo.getHighWatermark())) {
                return true;
            }
        }
        // not in the monitored splits scope, do not emit
        return false;
    }

    // always send the schema change event and signal event
    // we need record them to state of Flink
    return true;
}

6. MySqlRecordEmitter 資料下發

SourceReaderBase 從佇列中獲取切片讀取的 DataChangeEvent 資料集合，將資料型別由 Debezium 的 DataChangeEvent 轉換為 Flink 的 RowData 型別。

SourceReaderBase 處理切片資料流程。

org.apache.flink.connector.base.source.reader.SourceReaderBase#pollNext
public InputStatus pollNext(ReaderOutput<T> output) throws Exception {
    // make sure we have a fetch we are working on, or move to the next
    RecordsWithSplitIds<E> recordsWithSplitId = this.currentFetch;
    if (recordsWithSplitId == null) {
        recordsWithSplitId = getNextFetch(output);
        if (recordsWithSplitId == null) {
            return trace(finishedOrAvailableLater());
        }
    }

    // we need to loop here, because we may have to go across splits
    while (true) {
        // Process one record.
        // note:  通過MySqlRecords從迭代器中讀取單條資料
        final E record = recordsWithSplitId.nextRecordFromSplit();
        if (record != null) {
            // emit the record.
            recordEmitter.emitRecord(record, currentSplitOutput, currentSplitContext.state);
            LOG.trace("Emitted record: {}", record);

            // We always emit MORE_AVAILABLE here, even though we do not strictly know whether
            // more is available. If nothing more is available, the next invocation will find
            // this out and return the correct status.
            // That means we emit the occasional 'false positive' for availability, but this
            // saves us doing checks for every record. Ultimately, this is cheaper.
            return trace(InputStatus.MORE_AVAILABLE);
        } else if (!moveToNextSplit(recordsWithSplitId, output)) {
            // The fetch is done and we just discovered that and have not emitted anything, yet.
            // We need to move to the next fetch. As a shortcut, we call pollNext() here again,
            // rather than emitting nothing and waiting for the caller to call us again.
            return pollNext(output);
        }
        // else fall through the loop
    }
}

private RecordsWithSplitIds<E> getNextFetch(final ReaderOutput<T> output) {
    splitFetcherManager.checkErrors();

    LOG.trace("Getting next source data batch from queue");
    // note: 從elementsQueue 獲取資料
    final RecordsWithSplitIds<E> recordsWithSplitId = elementsQueue.poll();
    if (recordsWithSplitId == null || !moveToNextSplit(recordsWithSplitId, output)) {
        return null;
    }

    currentFetch = recordsWithSplitId;
    return recordsWithSplitId;
}

MySqlRecords 返回單條資料集合。

com.ververica.cdc.connectors.mysql.source.split.MySqlRecords#nextRecordFromSplit

public SourceRecord nextRecordFromSplit() {
    final Iterator<SourceRecord> recordsForSplit = this.recordsForCurrentSplit;
    if (recordsForSplit != null) {
        if (recordsForSplit.hasNext()) {
            return recordsForSplit.next();
        } else {
            return null;
        }
    } else {
        throw new IllegalStateException();
    }
}

MySqlRecordEmitter 通過 RowDataDebeziumDeserializeSchema 將資料轉換為Rowdata。

com.ververica.cdc.connectors.mysql.source.reader.MySqlRecordEmitter#emitRecord
public void emitRecord(SourceRecord element, SourceOutput<T> output, MySqlSplitState splitState)
    throws Exception {
if (isWatermarkEvent(element)) {
    BinlogOffset watermark = getWatermark(element);
    if (isHighWatermarkEvent(element) && splitState.isSnapshotSplitState()) {
        splitState.asSnapshotSplitState().setHighWatermark(watermark);
    }
} else if (isSchemaChangeEvent(element) && splitState.isBinlogSplitState()) {
    HistoryRecord historyRecord = getHistoryRecord(element);
    Array tableChanges =
            historyRecord.document().getArray(HistoryRecord.Fields.TABLE_CHANGES);
    TableChanges changes = TABLE_CHANGE_SERIALIZER.deserialize(tableChanges, true);
    for (TableChanges.TableChange tableChange : changes) {
        splitState.asBinlogSplitState().recordSchema(tableChange.getId(), tableChange);
    }
} else if (isDataChangeRecord(element)) {
    //  note: 資料的處理
    if (splitState.isBinlogSplitState()) {
        BinlogOffset position = getBinlogPosition(element);
        splitState.asBinlogSplitState().setStartingOffset(position);
    }
    debeziumDeserializationSchema.deserialize(
            element,
            new Collector<T>() {
                @Override
                public void collect(final T t) {
                    output.collect(t);
                }

                @Override
                public void close() {
                    // do nothing
                }
            });
} else {
    // unknown element
    LOG.info("Meet unknown element {}, just skip.", element);
}
}

RowDataDebeziumDeserializeSchema 序列化過程。

com.ververica.cdc.debezium.table.RowDataDebeziumDeserializeSchema#deserialize
public void deserialize(SourceRecord record, Collector<RowData> out) throws Exception {
    Envelope.Operation op = Envelope.operationFor(record);
    Struct value = (Struct) record.value();
    Schema valueSchema = record.valueSchema();
    if (op == Envelope.Operation.CREATE || op == Envelope.Operation.READ) {
        GenericRowData insert = extractAfterRow(value, valueSchema);
        validator.validate(insert, RowKind.INSERT);
        insert.setRowKind(RowKind.INSERT);
        out.collect(insert);
    } else if (op == Envelope.Operation.DELETE) {
        GenericRowData delete = extractBeforeRow(value, valueSchema);
        validator.validate(delete, RowKind.DELETE);
        delete.setRowKind(RowKind.DELETE);
        out.collect(delete);
    } else {
        GenericRowData before = extractBeforeRow(value, valueSchema);
        validator.validate(before, RowKind.UPDATE_BEFORE);
        before.setRowKind(RowKind.UPDATE_BEFORE);
        out.collect(before);

        GenericRowData after = extractAfterRow(value, valueSchema);
        validator.validate(after, RowKind.UPDATE_AFTER);
        after.setRowKind(RowKind.UPDATE_AFTER);
        out.collect(after);
    }
}

7. MySqlSourceReader 彙報切片讀取完成事件

MySqlSourceReader 處理完一個全量切片後，會向 MySqlSourceEnumerator 傳送已完成的切片資訊，包含切片 ID、HighWatermar ，然後繼續傳送切片請求。

com.ververica.cdc.connectors.mysql.source.reader.MySqlSourceReader#onSplitFinished
protected void onSplitFinished(Map<String, MySqlSplitState> finishedSplitIds) {
for (MySqlSplitState mySqlSplitState : finishedSplitIds.values()) {
    MySqlSplit mySqlSplit = mySqlSplitState.toMySqlSplit();

    finishedUnackedSplits.put(mySqlSplit.splitId(), mySqlSplit.asSnapshotSplit());
}
/**
 *   note: 傳送切片完成事件
 */
reportFinishedSnapshotSplitsIfNeed();

//  上一個spilt處理完成後繼續傳送切片請求
context.sendSplitRequest();
}

private void reportFinishedSnapshotSplitsIfNeed() {
    if (!finishedUnackedSplits.isEmpty()) {
        final Map<String, BinlogOffset> finishedOffsets = new HashMap<>();
        for (MySqlSnapshotSplit split : finishedUnackedSplits.values()) {
            // note: 傳送切片ID，及最大偏移量
            finishedOffsets.put(split.splitId(), split.getHighWatermark());
        }
        FinishedSnapshotSplitsReportEvent reportEvent =
                new FinishedSnapshotSplitsReportEvent(finishedOffsets);

        context.sendSourceEventToCoordinator(reportEvent);
        LOG.debug(
                "The subtask {} reports offsets of finished snapshot splits {}.",
                subtaskId,
                finishedOffsets);
    }
}

8. MySqlSourceEnumerator 分配增量切片

全量階段所有分片讀取完畢後，MySqlHybridSplitAssigner 會建立 BinlogSplit 進行後續增量讀取，在建立 BinlogSplit 會從全部已完成的全量切片中篩選最小 BinlogOffset。注意：2.0.0 分支 createBinlogSplit 最小偏移量總是從 0 開始，最新 master 分支已經修復這個 BUG。

private MySqlBinlogSplit createBinlogSplit() {
    final List<MySqlSnapshotSplit> assignedSnapshotSplit =
            snapshotSplitAssigner.getAssignedSplits().values().stream()
                    .sorted(Comparator.comparing(MySqlSplit::splitId))
                    .collect(Collectors.toList());

    Map<String, BinlogOffset> splitFinishedOffsets =
            snapshotSplitAssigner.getSplitFinishedOffsets();
    final List<FinishedSnapshotSplitInfo> finishedSnapshotSplitInfos = new ArrayList<>();
    final Map<TableId, TableChanges.TableChange> tableSchemas = new HashMap<>();

    BinlogOffset minBinlogOffset = null;
    // note: 從所有assignedSnapshotSplit中篩選最小偏移量
    for (MySqlSnapshotSplit split : assignedSnapshotSplit) {
        // find the min binlog offset
        BinlogOffset binlogOffset = splitFinishedOffsets.get(split.splitId());
        if (minBinlogOffset == null || binlogOffset.compareTo(minBinlogOffset) < 0) {
            minBinlogOffset = binlogOffset;
        }
        finishedSnapshotSplitInfos.add(
                new FinishedSnapshotSplitInfo(
                        split.getTableId(),
                        split.splitId(),
                        split.getSplitStart(),
                        split.getSplitEnd(),
                        binlogOffset));
        tableSchemas.putAll(split.getTableSchemas());
    }

    final MySqlSnapshotSplit lastSnapshotSplit =
            assignedSnapshotSplit.get(assignedSnapshotSplit.size() - 1).asSnapshotSplit();
       
    return new MySqlBinlogSplit(
            BINLOG_SPLIT_ID,
            lastSnapshotSplit.getSplitKeyType(),
            minBinlogOffset == null ? BinlogOffset.INITIAL_OFFSET : minBinlogOffset,
            BinlogOffset.NO_STOPPING_OFFSET,
            finishedSnapshotSplitInfos,
            tableSchemas);
}

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