Seata AT 模式是如何处理悬挂和空回滚

空回滚描述：

摘自Seata官网（在未收到 Try 请求的情况下收到 Cancel 请求，这种场景被称为空回滚；空回滚在生产环境经常出现，用户在实现TCC服务时，应允许允许空回滚的执行，即收到空回滚时返回成功。）

AT 模式是利用 undo log 来处理空回滚的，具体细节见代码

    // io.seata.rm.datasource.DataSourceManager#branchRollback
    @Override
    public BranchStatus branchRollback(BranchType branchType, String xid, long branchId, String resourceId,
                                       String applicationData) throws TransactionException {
        DataSourceProxy dataSourceProxy = get(resourceId);
        if (dataSourceProxy == null) {
            throw new ShouldNeverHappenException();
        }
        try {
            // 获取 undo log 并执行 undo 操作
            UndoLogManagerFactory.getUndoLogManager(dataSourceProxy.getDbType()).undo(dataSourceProxy, xid, branchId);
        } catch (TransactionException te) {
            StackTraceLogger.info(LOGGER, te,
                "branchRollback failed. branchType:[{}], xid:[{}], branchId:[{}], resourceId:[{}], applicationData:[{}]. reason:[{}]",
                new Object[]{branchType, xid, branchId, resourceId, applicationData, te.getMessage()});
            if (te.getCode() == TransactionExceptionCode.BranchRollbackFailed_Unretriable) {
                return BranchStatus.PhaseTwo_RollbackFailed_Unretryable;
            } else {
                return BranchStatus.PhaseTwo_RollbackFailed_Retryable;
            }
        }
        return BranchStatus.PhaseTwo_Rollbacked;

    }

    // io.seata.rm.datasource.undo.AbstractUndoLogManager#undo
    @Override
    public void undo(DataSourceProxy dataSourceProxy, String xid, long branchId) throws TransactionException {
        Connection conn = null;
        ResultSet rs = null;
        PreparedStatement selectPST = null;
        boolean originalAutoCommit = true;

        for (; ; ) {
            try {
                conn = dataSourceProxy.getPlainConnection();

                // The entire undo process should run in a local transaction.
                if (originalAutoCommit = conn.getAutoCommit()) {
                    conn.setAutoCommit(false);
                }

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

                boolean exists = false;
                while (rs.next()) {
                    exists = true;

                    // It is possible that the server repeatedly sends a rollback request to roll back
                    // the same branch transaction to multiple processes,
                    // ensuring that only the undo_log in the normal state is processed.
                    // 判断是否能够回滚
                    int state = rs.getInt(ClientTableColumnsName.UNDO_LOG_LOG_STATUS);
                    if (!canUndo(state)) {
                        if (LOGGER.isInfoEnabled()) {
                            LOGGER.info("xid {} branch {}, ignore {} undo_log", xid, branchId, state);
                        }
                        return;
                    }

                    String contextString = rs.getString(ClientTableColumnsName.UNDO_LOG_CONTEXT);
                    Map<String, String> context = parseContext(contextString);
                    byte[] rollbackInfo = getRollbackInfo(rs);

                    String serializer = context == null ? null : context.get(UndoLogConstants.SERIALIZER_KEY);
                    UndoLogParser parser = serializer == null ? UndoLogParserFactory.getInstance()
                        : UndoLogParserFactory.getInstance(serializer);
                    BranchUndoLog branchUndoLog = parser.decode(rollbackInfo);

                    try {
                        // put serializer name to local
                        setCurrentSerializer(parser.getName());
                        List<SQLUndoLog> sqlUndoLogs = branchUndoLog.getSqlUndoLogs();
                        if (sqlUndoLogs.size() > 1) {
                            Collections.reverse(sqlUndoLogs);
                        }
                        for (SQLUndoLog sqlUndoLog : sqlUndoLogs) {
                            TableMeta tableMeta = TableMetaCacheFactory.getTableMetaCache(dataSourceProxy.getDbType()).getTableMeta(
                                conn, sqlUndoLog.getTableName(), dataSourceProxy.getResourceId());
                            sqlUndoLog.setTableMeta(tableMeta);
                            AbstractUndoExecutor undoExecutor = UndoExecutorFactory.getUndoExecutor(
                                dataSourceProxy.getDbType(), sqlUndoLog);
                            undoExecutor.executeOn(conn);
                        }
                    } finally {
                        // remove serializer name
                        removeCurrentSerializer();
                    }
                }

                // If undo_log exists, it means that the branch transaction has completed the first phase,
                // we can directly roll back and clean the undo_log
                // Otherwise, it indicates that there is an exception in the branch transaction,
                // causing undo_log not to be written to the database.
                // For example, the business processing timeout, the global transaction is the initiator rolls back.
                // To ensure data consistency, we can insert an undo_log with GlobalFinished state
                // to prevent the local transaction of the first phase of other programs from being correctly submitted.
                // See https://github.com/seata/seata/issues/489

                if (exists) {
                    // 删除回滚日志
                    deleteUndoLog(xid, branchId, conn);
                    conn.commit();
                    if (LOGGER.isInfoEnabled()) {
                        LOGGER.info("xid {} branch {}, undo_log deleted with {}", xid, branchId,
                            State.GlobalFinished.name());
                    }
                } else {
                    // 回滚日志不存在，插入一条全局事务已完成的回滚记录
                    insertUndoLogWithGlobalFinished(xid, branchId, UndoLogParserFactory.getInstance(), conn);
                    conn.commit();
                    if (LOGGER.isInfoEnabled()) {
                        LOGGER.info("xid {} branch {}, undo_log added with {}", xid, branchId,
                            State.GlobalFinished.name());
                    }
                }

                return;
            } catch (SQLIntegrityConstraintViolationException e) {
                // Possible undo_log has been inserted into the database by other processes, retrying rollback undo_log
                if (LOGGER.isInfoEnabled()) {
                    LOGGER.info("xid {} branch {}, undo_log inserted, retry rollback", xid, branchId);
                }
            } catch (Throwable e) {
                if (conn != null) {
                    try {
                        conn.rollback();
                    } catch (SQLException rollbackEx) {
                        LOGGER.warn("Failed to close JDBC resource while undo ... ", rollbackEx);
                    }
                }
                throw new BranchTransactionException(BranchRollbackFailed_Retriable, String
                    .format("Branch session rollback failed and try again later xid = %s branchId = %s %s", xid,
                        branchId, e.getMessage()), e);

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

详细的流程见下图

AT 空回滚.png

悬挂描述

摘自Seata官网（事务协调器在调用 TCC 服务的一阶段 Try 操作时，可能会出现因网络拥堵而导致的超时，此时事务管理器会触发二阶段回滚，调用 TCC 服务的 Cancel 操作，Cancel 调用未超时；在此之后，拥堵在网络上的一阶段 Try 数据包被 TCC 服务收到，出现了二阶段 Cancel 请求比一阶段 Try 请求先执行的情况，此 TCC 服务在执行晚到的 Try 之后，将永远不会再收到二阶段的 Confirm 或者 Cancel ，造成 TCC 服务悬挂。

用户在实现 TCC 服务时，要允许空回滚，但是要拒绝执行空回滚之后 Try 请求，要避免出现悬挂。）