Fescar - RM 全域性事務提交回滾流程

weixin_33843409發表於2019-02-20

開篇

 這篇文章的目的主要是講解RM在接收TC的請求後執行全域性分支事務提交（doBranchCommit）和全域性分支事務回滾（doBranchRollback）的流程。

 全域性的分支事務提交過程和回滾過程也算RM處理流程中核心的一環，瞭解以後並結合之前講解的本地事務提交流程就能夠較好的理解整個過程了。


全域性事務操作流程

整體流程

public class RMHandlerAT extends AbstractRMHandlerAT implements 
                   RMInboundHandler, TransactionMessageHandler {

    private DataSourceManager dataSourceManager = DataSourceManager.get();

    @Override
    protected void doBranchCommit(BranchCommitRequest request, BranchCommitResponse response)  
       throws TransactionException {
        String xid = request.getXid();
        long branchId = request.getBranchId();
        String resourceId = request.getResourceId();
        String applicationData = request.getApplicationData();
        LOGGER.info("AT Branch committing: " + xid + " " + branchId + " " + resourceId + " " + applicationData);
        BranchStatus status = dataSourceManager.branchCommit(xid, branchId, resourceId, applicationData);
        response.setBranchStatus(status);
        LOGGER.info("AT Branch commit result: " + status);

    }

    @Override
    protected void doBranchRollback(BranchRollbackRequest request, BranchRollbackResponse response)  
       throws TransactionException {
        String xid = request.getXid();
        long branchId = request.getBranchId();
        String resourceId = request.getResourceId();
        String applicationData = request.getApplicationData();
        LOGGER.info("AT Branch rolling back: " + xid + " " + branchId + " " + resourceId);
        BranchStatus status = dataSourceManager.branchRollback(xid, branchId, resourceId, applicationData);
        response.setBranchStatus(status);
        LOGGER.info("AT Branch rollback result: " + status);

    }
}

說明：

  • doBranchCommit()通過dataSourceManager.branchCommit()去執行分支事務提交。
  • doBranchRollback()通過dataSourceManager.branchRollback()去執行分支事務回滾。
  • dataSourceManager是DataSourceManager物件。


doBranchCommit流程

public class DataSourceManager implements ResourceManager {

    private ResourceManagerInbound asyncWorker;

    public void setAsyncWorker(ResourceManagerInbound asyncWorker) {
        this.asyncWorker = asyncWorker;
    }

    public BranchStatus branchCommit(String xid, long branchId, String resourceId, String applicationData)  
       throws TransactionException {
        return asyncWorker.branchCommit(xid, branchId, resourceId, applicationData);
    }
}


public class AsyncWorker implements ResourceManagerInbound {

    public BranchStatus branchCommit(String xid, long branchId, String resourceId, String applicationData) 
     throws TransactionException {
        if (ASYNC_COMMIT_BUFFER.size() < ASYNC_COMMIT_BUFFER_LIMIT) {
            ASYNC_COMMIT_BUFFER.add(new Phase2Context(xid, branchId, resourceId, applicationData));
        } else {
            LOGGER.warn("Async commit buffer is FULL. 
            Rejected branch [" + branchId + "/" + xid + "] will be handled by housekeeping later.");
        }
        return BranchStatus.PhaseTwo_Committed;
    }

    public synchronized void init() {
        LOGGER.info("Async Commit Buffer Limit: " + ASYNC_COMMIT_BUFFER_LIMIT);
        timerExecutor = new ScheduledThreadPoolExecutor(1,
            new NamedThreadFactory("AsyncWorker", 1, true));
        timerExecutor.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                try {

                    doBranchCommits();

                } catch (Throwable e) {
                    LOGGER.info("Failed at async committing ... " + e.getMessage());

                }
            }
        }, 10, 1000 * 1, TimeUnit.MILLISECONDS);
    }



    private void doBranchCommits() {
        if (ASYNC_COMMIT_BUFFER.size() == 0) {
            return;
        }
        Map<String, List<Phase2Context>> mappedContexts = new HashMap<>();
        Iterator<Phase2Context> iterator = ASYNC_COMMIT_BUFFER.iterator();
        while (iterator.hasNext()) {
            Phase2Context commitContext = iterator.next();
            List<Phase2Context> contextsGroupedByResourceId = mappedContexts.get(commitContext.resourceId);
            if (contextsGroupedByResourceId == null) {
                contextsGroupedByResourceId = new ArrayList<>();
                mappedContexts.put(commitContext.resourceId, contextsGroupedByResourceId);
            }
            contextsGroupedByResourceId.add(commitContext);

            iterator.remove();

        }

        for (String resourceId : mappedContexts.keySet()) {
            Connection conn = null;
            try {
                try {
                    DataSourceProxy dataSourceProxy = DataSourceManager.get().get(resourceId);
                    conn = dataSourceProxy.getPlainConnection();
                } catch (SQLException sqle) {
                    LOGGER.warn("Failed to get connection for async committing on " + resourceId, sqle);
                    continue;
                }

                List<Phase2Context> contextsGroupedByResourceId = mappedContexts.get(resourceId);
                for (Phase2Context commitContext : contextsGroupedByResourceId) {
                    try {
                        UndoLogManager.deleteUndoLog(commitContext.xid, commitContext.branchId, conn);
                    } catch (Exception ex) {
                        LOGGER.warn("Failed to delete undo log [" + 
                         commitContext.branchId + "/" + commitContext.xid + "]", ex);
                    }
                }

            } finally {
                if (conn != null) {
                    try {
                        conn.close();
                    } catch (SQLException closeEx) {
                        LOGGER.warn("Failed to close JDBC resource while deleting undo_log ", closeEx);
                    }
                }
            }

        }

    }
}

說明：

  • doBranchCommit()操作的核心實現通過AsyncWorker完成，AsyncWorker類其實是一個生成消費模型。
  • doBranchCommit()把需要提交的任務新增到AsyncWorker的ASYNC_COMMIT_BUFFER佇列當中。
  • AsyncWorker內部timerExecutor負責啟動執行commit動作執行緒執行doBranchCommits()動作。
  • doBranchCommits動作內部負責刪除多餘的UndoLog， UndoLogManager.deleteUndoLog。
  • doBranchCommit()的本質任務就是刪除備份的回滾日誌而已。


doBranchRollback流程

public class DataSourceManager implements ResourceManager {

    public BranchStatus branchRollback(String xid, long branchId, String resourceId, String applicationData) 
       throws TransactionException {
        DataSourceProxy dataSourceProxy = get(resourceId);
        if (dataSourceProxy == null) {
            throw new ShouldNeverHappenException();
        }
        try {
            // 執行回滾操作
            UndoLogManager.undo(dataSourceProxy, xid, branchId);
        } catch (TransactionException te) {
            if (te.getCode() == TransactionExceptionCode.BranchRollbackFailed_Unretriable) {
                return BranchStatus.PhaseTwo_RollbackFailed_Unretryable;
            } else {
                return BranchStatus.PhaseTwo_RollbackFailed_Retryable;
            }
        }
        return BranchStatus.PhaseTwo_Rollbacked;

    }
}


public final class UndoLogManager {
    private static String SELECT_UNDO_LOG_SQL = 
    "SELECT * FROM " + UNDO_LOG_TABLE_NAME + " WHERE log_status = 0 AND branch_id = ? AND xid = ? FOR UPDATE";

    public static void undo(DataSourceProxy dataSourceProxy, String xid, long branchId) 
       throws TransactionException {

        assertDbSupport(dataSourceProxy.getTargetDataSource().getDbType());

        Connection conn = null;
        ResultSet rs = null;
        PreparedStatement selectPST = null;
        try {
            conn = dataSourceProxy.getPlainConnection();

            // The entire undo process should run in a local transaction.
            conn.setAutoCommit(false);

            // Find UNDO LOG
            selectPST = conn.prepareStatement(SELECT_UNDO_LOG_SQL);
            selectPST.setLong(1, branchId);
            selectPST.setString(2, xid);
            rs = selectPST.executeQuery();

            // 遍歷所有回滾日誌
            while (rs.next()) {
                Blob b = rs.getBlob("rollback_info");
                String rollbackInfo = StringUtils.blob2string(b);
                BranchUndoLog branchUndoLog = UndoLogParserFactory.getInstance().decode(rollbackInfo);

                for (SQLUndoLog sqlUndoLog : branchUndoLog.getSqlUndoLogs()) {
                    TableMeta tableMeta = TableMetaCache.getTableMeta(dataSourceProxy, sqlUndoLog.getTableName());
                    sqlUndoLog.setTableMeta(tableMeta);
                    AbstractUndoExecutor undoExecutor = UndoExecutorFactory.getUndoExecutor(
                                                      dataSourceProxy.getDbType(), sqlUndoLog);
                    undoExecutor.executeOn(conn);
                }

            }
            deleteUndoLog(xid, branchId, conn);

            conn.commit();

        } catch (Throwable e) {
            if (conn != null) {
                try {
                    conn.rollback();
                } catch (SQLException rollbackEx) {
                    LOGGER.warn("Failed to close JDBC resource while undo ... ", rollbackEx);
                }
            }
            throw new TransactionException(BranchRollbackFailed_Retriable, String.format("%s/%s", branchId, xid), e);

        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (selectPST != null) {
                    selectPST.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException closeEx) {
                LOGGER.warn("Failed to close JDBC resource while undo ... ", closeEx);
            }
        }
    }
}

說明：

  • doBranchRollback操作的核心實現通過UndoLogManager完成，UndoLogManager.undo()負責執行回滾。
  • undo()操作的核心是通過SELECT_UNDO_LOG_SQL日誌去獲取回滾日誌內容。
  • 根據undoLog物件通過UndoExecutorFactory.getUndoExecutor獲取回滾的執行者Executor物件。
  • undoExecutor.executeOn(conn)執行回滾操作，不同的回滾操作物件不同的undoExecutor。
  • deleteUndoLog(xid, branchId, conn)執行日誌刪除操作。


期待

 下篇文章會針對undoExecutor作具體的介紹。


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