Send new protocol keepalive messages to standby servers.

Allows streaming replication users to calculate transfer latency
and apply delay via internal functions. No external functions yet.

Author: simonat2ndQuadrant

doc/src/sgml/protocol.sgml

@@ -1463,6 +1463,54 @@ The commands accepted in walsender mode are:
        CopyData message):
      </para>
 
+     <para>
+      <variablelist>
+      <varlistentry>
+      <term>
+          Primary keepalive message (B)
+      </term>
+      <listitem>
+      <para>
+      <variablelist>
+      <varlistentry>
+      <term>
+          Byte1('k')
+      </term>
+      <listitem>
+      <para>
+          Identifies the message as a sender keepalive.
+      </para>
+      </listitem>
+      </varlistentry>
+      <varlistentry>
+      <term>
+          Byte8
+      </term>
+      <listitem>
+      <para>
+          The current end of WAL on the server, given in
+          XLogRecPtr format.
+      </para>
+      </listitem>
+      </varlistentry>
+      <varlistentry>
+      <term>
+          Byte8
+      </term>
+      <listitem>
+      <para>
+          The server's system clock at the time of transmission,
+          given in TimestampTz format.
+      </para>
+      </listitem>
+      </varlistentry>
+      </variablelist>
+      </para>
+      </listitem>
+      </varlistentry>
+      </variablelist>
+     </para>
+
      <para>
       <variablelist>
       <varlistentry>

src/backend/access/transam/xlog.c

@@ -452,6 +452,9 @@ typedef struct XLogCtlData
 	XLogRecPtr	recoveryLastRecPtr;
 	/* timestamp of last COMMIT/ABORT record replayed (or being replayed) */
 	TimestampTz recoveryLastXTime;
+	/* timestamp of when we started replaying the current chunk of WAL data,
+	 * only relevant for replication or archive recovery */
+	TimestampTz currentChunkStartTime;
 	/* end of the last record restored from the archive */
 	XLogRecPtr	restoreLastRecPtr;
 	/* Are we requested to pause recovery? */
@@ -606,6 +609,7 @@ static void exitArchiveRecovery(TimeLineID endTLI,
 static bool recoveryStopsHere(XLogRecord *record, bool *includeThis);
 static void recoveryPausesHere(void);
 static void SetLatestXTime(TimestampTz xtime);
+static void SetCurrentChunkStartTime(TimestampTz xtime);
 static void CheckRequiredParameterValues(void);
 static void XLogReportParameters(void);
 static void LocalSetXLogInsertAllowed(void);
@@ -5847,6 +5851,41 @@ GetLatestXTime(void)
 	return xtime;
 }
 
+/*
+ * Save timestamp of the next chunk of WAL records to apply.
+ *
+ * We keep this in XLogCtl, not a simple static variable, so that it can be
+ * seen by all backends.
+ */
+static void
+SetCurrentChunkStartTime(TimestampTz xtime)
+{
+	/* use volatile pointer to prevent code rearrangement */
+	volatile XLogCtlData *xlogctl = XLogCtl;
+
+	SpinLockAcquire(&xlogctl->info_lck);
+	xlogctl->currentChunkStartTime = xtime;
+	SpinLockRelease(&xlogctl->info_lck);
+}
+
+/*
+ * Fetch timestamp of latest processed commit/abort record.
+ * Startup process maintains an accurate local copy in XLogReceiptTime
+ */
+TimestampTz
+GetCurrentChunkReplayStartTime(void)
+{
+	/* use volatile pointer to prevent code rearrangement */
+	volatile XLogCtlData *xlogctl = XLogCtl;
+	TimestampTz xtime;
+
+	SpinLockAcquire(&xlogctl->info_lck);
+	xtime = xlogctl->currentChunkStartTime;
+	SpinLockRelease(&xlogctl->info_lck);
+
+	return xtime;
+}
+
 /*
  * Returns time of receipt of current chunk of XLOG data, as well as
  * whether it was received from streaming replication or from archives.
@@ -6390,6 +6429,7 @@ StartupXLOG(void)
 		xlogctl->replayEndRecPtr = ReadRecPtr;
 		xlogctl->recoveryLastRecPtr = ReadRecPtr;
 		xlogctl->recoveryLastXTime = 0;
+		xlogctl->currentChunkStartTime = 0;
 		xlogctl->recoveryPause = false;
 		SpinLockRelease(&xlogctl->info_lck);
 
@@ -9696,7 +9736,10 @@ XLogPageRead(XLogRecPtr *RecPtr, int emode, bool fetching_ckpt,
 						{
 							havedata = true;
 							if (!XLByteLT(*RecPtr, latestChunkStart))
+							{
 								XLogReceiptTime = GetCurrentTimestamp();
+								SetCurrentChunkStartTime(XLogReceiptTime);
+							}
 						}
 						else
 							havedata = false;

src/backend/replication/walreceiver.c

@@ -124,6 +124,7 @@ static void XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr);
 static void XLogWalRcvFlush(bool dying);
 static void XLogWalRcvSendReply(void);
 static void XLogWalRcvSendHSFeedback(void);
+static void ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime);
 
 /* Signal handlers */
 static void WalRcvSigHupHandler(SIGNAL_ARGS);
@@ -218,6 +219,10 @@ WalReceiverMain(void)
 	/* Fetch information required to start streaming */
 	strlcpy(conninfo, (char *) walrcv->conninfo, MAXCONNINFO);
 	startpoint = walrcv->receiveStart;
+
+	/* Initialise to a sanish value */
+	walrcv->lastMsgSendTime = walrcv->lastMsgReceiptTime = GetCurrentTimestamp();
+
 	SpinLockRelease(&walrcv->mutex);
 
 	/* Arrange to clean up at walreceiver exit */
@@ -433,12 +438,28 @@ XLogWalRcvProcessMsg(unsigned char type, char *buf, Size len)
 							 errmsg_internal("invalid WAL message received from primary")));
 				/* memcpy is required here for alignment reasons */
 				memcpy(&msghdr, buf, sizeof(WalDataMessageHeader));
+
+				ProcessWalSndrMessage(msghdr.walEnd, msghdr.sendTime);
+
 				buf += sizeof(WalDataMessageHeader);
 				len -= sizeof(WalDataMessageHeader);
-
 				XLogWalRcvWrite(buf, len, msghdr.dataStart);
 				break;
 			}
+		case 'k':				/* Keepalive */
+			{
+				PrimaryKeepaliveMessage keepalive;
+
+				if (len != sizeof(PrimaryKeepaliveMessage))
+					ereport(ERROR,
+							(errcode(ERRCODE_PROTOCOL_VIOLATION),
+							 errmsg_internal("invalid keepalive message received from primary")));
+				/* memcpy is required here for alignment reasons */
+				memcpy(&keepalive, buf, sizeof(PrimaryKeepaliveMessage));
+
+				ProcessWalSndrMessage(keepalive.walEnd, keepalive.sendTime);
+				break;
+			}
 		default:
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
@@ -711,3 +732,27 @@ XLogWalRcvSendHSFeedback(void)
 	memcpy(&buf[1], &feedback_message, sizeof(StandbyHSFeedbackMessage));
 	walrcv_send(buf, sizeof(StandbyHSFeedbackMessage) + 1);
 }
+
+/*
+ * Keep track of important messages from primary.
+ */
+static void
+ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime)
+{
+	/* use volatile pointer to prevent code rearrangement */
+	volatile WalRcvData *walrcv = WalRcv;
+
+	TimestampTz lastMsgReceiptTime = GetCurrentTimestamp();
+
+	/* Update shared-memory status */
+	SpinLockAcquire(&walrcv->mutex);
+	walrcv->lastMsgSendTime = sendTime;
+	walrcv->lastMsgReceiptTime = lastMsgReceiptTime;
+	SpinLockRelease(&walrcv->mutex);
+
+	elog(DEBUG2, "sendtime %s receipttime %s replication apply delay %d transfer latency %d",
+					timestamptz_to_str(sendTime),
+					timestamptz_to_str(lastMsgReceiptTime),
+					GetReplicationApplyDelay(),
+					GetReplicationTransferLatency());
+}

src/backend/replication/walreceiverfuncs.c

@@ -28,6 +28,7 @@
 #include "replication/walreceiver.h"
 #include "storage/pmsignal.h"
 #include "storage/shmem.h"
+#include "utils/timestamp.h"
 
 WalRcvData *WalRcv = NULL;
 
@@ -238,3 +239,65 @@ GetWalRcvWriteRecPtr(XLogRecPtr *latestChunkStart)
 
 	return recptr;
 }
+
+/*
+ * Returns the replication apply delay in ms
+ */
+int
+GetReplicationApplyDelay(void)
+{
+	/* use volatile pointer to prevent code rearrangement */
+	volatile WalRcvData *walrcv = WalRcv;
+
+	XLogRecPtr	receivePtr;
+	XLogRecPtr	replayPtr;
+
+	long	secs;
+	int		usecs;
+
+	SpinLockAcquire(&walrcv->mutex);
+	receivePtr = walrcv->receivedUpto;
+	SpinLockRelease(&walrcv->mutex);
+
+	replayPtr = GetXLogReplayRecPtr(NULL);
+
+	if (XLByteLE(receivePtr, replayPtr))
+		return 0;
+
+	TimestampDifference(GetCurrentChunkReplayStartTime(),
+						GetCurrentTimestamp(),
+						&secs, &usecs);
+
+	return (((int) secs * 1000) + (usecs / 1000));
+}
+
+/*
+ * Returns the network latency in ms, note that this includes any
+ * difference in clock settings between the servers, as well as timezone.
+ */
+int
+GetReplicationTransferLatency(void)
+{
+	/* use volatile pointer to prevent code rearrangement */
+	volatile WalRcvData *walrcv = WalRcv;
+
+	TimestampTz lastMsgSendTime;
+	TimestampTz lastMsgReceiptTime;
+
+	long	secs = 0;
+	int		usecs = 0;
+	int		ms;
+
+	SpinLockAcquire(&walrcv->mutex);
+	lastMsgSendTime = walrcv->lastMsgSendTime;
+	lastMsgReceiptTime = walrcv->lastMsgReceiptTime;
+	SpinLockRelease(&walrcv->mutex);
+
+	TimestampDifference(lastMsgSendTime,
+						lastMsgReceiptTime,
+						&secs, &usecs);
+
+	ms = ((int) secs * 1000) + (usecs / 1000);
+
+	return ms;
+}

src/backend/replication/walsender.c

@@ -131,6 +131,7 @@ static void ProcessStandbyMessage(void);
 static void ProcessStandbyReplyMessage(void);
 static void ProcessStandbyHSFeedbackMessage(void);
 static void ProcessRepliesIfAny(void);
+static void WalSndKeepalive(char *msgbuf);
 
 
 /* Main entry point for walsender process */
@@ -823,30 +824,24 @@ WalSndLoop(void)
 		 */
 		if (caughtup || pq_is_send_pending())
 		{
-			TimestampTz finish_time = 0;
-			long		sleeptime = -1;
+			TimestampTz timeout = 0;
+			long		sleeptime = 10000; /* 10 s */
 			int			wakeEvents;
 
 			wakeEvents = WL_LATCH_SET | WL_POSTMASTER_DEATH |
-				WL_SOCKET_READABLE;
+				WL_SOCKET_READABLE | WL_TIMEOUT;
+
 			if (pq_is_send_pending())
 				wakeEvents |= WL_SOCKET_WRITEABLE;
+			else
+				WalSndKeepalive(output_message);
 
 			/* Determine time until replication timeout */
 			if (replication_timeout > 0)
 			{
-				long		secs;
-				int			usecs;
-
-				finish_time = TimestampTzPlusMilliseconds(last_reply_timestamp,
+				timeout = TimestampTzPlusMilliseconds(last_reply_timestamp,
 														  replication_timeout);
-				TimestampDifference(GetCurrentTimestamp(),
-									finish_time, &secs, &usecs);
-				sleeptime = secs * 1000 + usecs / 1000;
-				/* Avoid Assert in WaitLatchOrSocket if timeout is past */
-				if (sleeptime < 0)
-					sleeptime = 0;
-				wakeEvents |= WL_TIMEOUT;
+				sleeptime = 1 + (replication_timeout / 10);
 			}
 
 			/* Sleep until something happens or replication timeout */
@@ -859,7 +854,7 @@ WalSndLoop(void)
 			 * timeout ... he's supposed to reply *before* that.
 			 */
 			if (replication_timeout > 0 &&
-				GetCurrentTimestamp() >= finish_time)
+				GetCurrentTimestamp() >= timeout)
 			{
 				/*
 				 * Since typically expiration of replication timeout means
@@ -1627,6 +1622,23 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
 	return (Datum) 0;
 }
 
+static void
+WalSndKeepalive(char *msgbuf)
+{
+	PrimaryKeepaliveMessage keepalive_message;
+
+	/* Construct a new message */
+	keepalive_message.walEnd = sentPtr;
+	keepalive_message.sendTime = GetCurrentTimestamp();
+
+	elog(DEBUG2, "sending replication keepalive");
+
+	/* Prepend with the message type and send it. */
+	msgbuf[0] = 'k';
+	memcpy(msgbuf + 1, &keepalive_message, sizeof(PrimaryKeepaliveMessage));
+	pq_putmessage_noblock('d', msgbuf, sizeof(PrimaryKeepaliveMessage) + 1);
+}
+
 /*
  * This isn't currently used for anything. Monitoring tools might be
  * interested in the future, and we'll need something like this in the

src/include/access/xlog.h

@@ -293,6 +293,7 @@ extern XLogRecPtr GetXLogWriteRecPtr(void);
 extern bool RecoveryIsPaused(void);
 extern void SetRecoveryPause(bool recoveryPause);
 extern TimestampTz GetLatestXTime(void);
+extern TimestampTz GetCurrentChunkReplayStartTime(void);
 
 extern void UpdateControlFile(void);
 extern uint64 GetSystemIdentifier(void);