同步複製注意點

       同步複製情況下，先刷寫日誌，然後記錄clog並持久化，最後纔等待備機接收日誌後返回的ACK，如果備機有問題的話或複製鏈路異常，日誌傳輸失敗，此時用戶客戶端的commit會掛住，一旦主機異常宕機重啓後，這個異常的事務會從日誌中恢復出來，因爲事務在日誌中顯示已提交，而相對客戶而言是未提交的。這一點需要注意。

RecordTransactionCommit
  if ((wrote_xlog && markXidCommitted &&
     synchronous_commit > SYNCHRONOUS_COMMIT_OFF) ||
    forceSyncCommit || nrels > 0){//同步
    XLogFlush(XactLastRecEnd);
    /*
     * Now we may update the CLOG, if we wrote a COMMIT record above
     */
    if (markXidCommitted)
      TransactionIdCommitTree(xid, nchildren, children);
  }else{
    XLogSetAsyncXactLSN(XactLastRecEnd);
    if (markXidCommitted)
      TransactionIdAsyncCommitTree(xid, nchildren, children, XactLastRecEnd);
  }
  ...
  if (wrote_xlog && markXidCommitted)
    SyncRepWaitForLSN(XactLastRecEnd, true);
    |--  if (!SyncRepRequested())
    |    return;
    |  for(;;){
    |    等待確認
    |--  }
  ...
  
//同步複製
#define SyncRepRequested() \
  (max_wal_senders > 0 && synchronous_commit > SYNCHRONOUS_COMMIT_LOCAL_FLUSH)