源码版本为1.3.0

分支事务执行前，需要把分支注册到seata server

代码 AbstractCore # branchRegister

@Override
    public Long branchRegister(BranchType branchType, String resourceId, String clientId, String xid,
                               String applicationData, String lockKeys) throws TransactionException {
        GlobalSession globalSession = assertGlobalSessionNotNull(xid, false);
        return SessionHolder.lockAndExecute(globalSession, () -> {
            globalSessionStatusCheck(globalSession);
            globalSession.addSessionLifecycleListener(SessionHolder.getRootSessionManager());
            BranchSession branchSession = SessionHelper.newBranchByGlobal(globalSession, branchType, resourceId,
                    applicationData, lockKeys, clientId);
            branchSessionLock(globalSession, branchSession);
            try {
                globalSession.addBranch(branchSession);

　　最重要的一步就是获取该分支事务的全局锁

　　如果使用mysql来做分布式锁就是这样的 代码在 DataBaseLocker # acquireLock

　　LockStoreDataBaseDAO # acquireLock(List<LockDO> lockDOs)

@Override
    public boolean acquireLock(List<LockDO> lockDOs) {
        Connection conn = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        Set<String> dbExistedRowKeys = new HashSet<>();
        boolean originalAutoCommit = true;
        if (lockDOs.size() > 1) {
            lockDOs = lockDOs.stream().filter(LambdaUtils.distinctByKey(LockDO::getRowKey)).collect(Collectors.toList());
        }
        try {
            conn = lockStoreDataSource.getConnection();
            if (originalAutoCommit = conn.getAutoCommit()) {
                conn.setAutoCommit(false);
            }
            //check lock
            StringJoiner sj = new StringJoiner(",");
            for (int i = 0; i < lockDOs.size(); i++) {
                sj.add("?");
            }
            boolean canLock = true;
            //query
            String checkLockSQL = LockStoreSqlFactory.getLogStoreSql(dbType).getCheckLockableSql(lockTable, sj.toString());
            ps = conn.prepareStatement(checkLockSQL);
            for (int i = 0; i < lockDOs.size(); i++) {
                ps.setString(i + 1, lockDOs.get(i).getRowKey());
            }
            rs = ps.executeQuery();
            String currentXID = lockDOs.get(0).getXid();
            while (rs.next()) {
                String dbXID = rs.getString(ServerTableColumnsName.LOCK_TABLE_XID);
                if (!StringUtils.equals(dbXID, currentXID)) {//如果存在行记录而且还不是一个全局事务ID，获取锁失败
                    if (LOGGER.isInfoEnabled()) {
                        String dbPk = rs.getString(ServerTableColumnsName.LOCK_TABLE_PK);
                        String dbTableName = rs.getString(ServerTableColumnsName.LOCK_TABLE_TABLE_NAME);
                        Long dbBranchId = rs.getLong(ServerTableColumnsName.LOCK_TABLE_BRANCH_ID);
                        LOGGER.info("Global lock on [{}:{}] is holding by xid {} branchId {}", dbTableName, dbPk, dbXID,
                            dbBranchId);
                    }
                    canLock &= false;
                    break;
                }
                dbExistedRowKeys.add(rs.getString(ServerTableColumnsName.LOCK_TABLE_ROW_KEY));
            }

            if (!canLock) {
                conn.rollback();
                return false;
            }
            List<LockDO> unrepeatedLockDOs = null;
            if (CollectionUtils.isNotEmpty(dbExistedRowKeys)) {
                unrepeatedLockDOs = lockDOs.stream().filter(lockDO -> !dbExistedRowKeys.contains(lockDO.getRowKey()))
                    .collect(Collectors.toList());
            } else {
                unrepeatedLockDOs = lockDOs;
            }
            if (CollectionUtils.isEmpty(unrepeatedLockDOs)) {
                conn.rollback();
                return true;
            }
            //lock
            if (unrepeatedLockDOs.size() == 1) {
                LockDO lockDO = unrepeatedLockDOs.get(0);
                if (!doAcquireLock(conn, lockDO)) {
                    if (LOGGER.isInfoEnabled()) {
                        LOGGER.info("Global lock acquire failed, xid {} branchId {} pk {}", lockDO.getXid(), lockDO.getBranchId(), lockDO.getPk());
                    }
                    conn.rollback();
                    return false;
                }
            } else {
                if (!doAcquireLocks(conn, unrepeatedLockDOs)) {
                    if (LOGGER.isInfoEnabled()) {
                        LOGGER.info("Global lock batch acquire failed, xid {} branchId {} pks {}", unrepeatedLockDOs.get(0).getXid(),
                            unrepeatedLockDOs.get(0).getBranchId(), unrepeatedLockDOs.stream().map(lockDO -> lockDO.getPk()).collect(Collectors.toList()));
                    }
                    conn.rollback();
                    return false;
                }
            }
            conn.commit();
            return true;

　　来看看语句 

/**
     * The constant CHECK_LOCK_SQL.
     */
    private static final String CHECK_LOCK_SQL = "select " + ALL_COLUMNS + " from " + LOCK_TABLE_PLACE_HOLD
        + " where " + ServerTableColumnsName.LOCK_TABLE_ROW_KEY + " in (" + IN_PARAMS_PLACE_HOLD + ")";

　　LOCK_TABLE的表模型如下

CREATE TABLE IF NOT EXISTS `lock_table`
(
    `row_key`        VARCHAR(128) NOT NULL,
    `xid`            VARCHAR(96),
    `transaction_id` BIGINT,
    `branch_id`      BIGINT       NOT NULL,
    `resource_id`    VARCHAR(256),
    `table_name`     VARCHAR(32),
    `pk`             VARCHAR(36),
    `gmt_create`     DATETIME,
    `gmt_modified`   DATETIME,
    PRIMARY KEY (`row_key`),
    KEY `idx_branch_id` (`branch_id`)
) ENGINE = InnoDB
  DEFAULT CHARSET = utf8;

　　row_key就是要锁定的行的主键值

　　查询的逻辑就是当前表里是不是有这行主键值等于入参的行数据，如果有全局事务id还不一样那说明另外的事务锁定该行，获取全局锁失败。