[Tarantool-patches] [RFC v7 2/2] relay: provide information about downstream lag

[Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

We already have `box.replication.upstream.lag` entry for monitoring
sake. Same time in synchronous replication timeouts are key properties
for quorum gathering procedure. Thus we would like to know how long
it took of a transaction to traverse `initiator WAL -> network ->
remote applier -> ACK` path.

Typical ouput is

 | tarantool> box.info.replication[2].downstream
 | ---
 | - status: follow
 |   idle: 0.61753897101153
 |   vclock: {1: 147}
 |   lag: 0
 | ...
 | tarantool> box.space.sync:insert{69}
 | ---
 | - [69]
 | ...
 |
 | tarantool> box.info.replication[2].downstream
 | ---
 | - status: follow
 |   idle: 0.75324084801832
 |   vclock: {1: 151}
 |   lag: 0.0011014938354492
 | ...

Closes #5447

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

@TarantoolBot document
Title: Add `box.info.replication[n].downstream.lag` entry

`replication[n].downstream.lag` is the time difference between
last transactions been written to the WAL journal of the transaction
initiator and the transaction written to WAL on the `n` replica.

In other words this is a lag in seconds between the main node writes
data to own WAL and replica `n` get this data replicated to the own
WAL journal.
---
 .../unreleased/gh-5447-downstream-lag.md      |  6 ++
 src/box/lua/info.c                            |  3 +
 src/box/relay.cc                              | 51 ++++++++++
 src/box/relay.h                               |  6 ++
 .../replication/gh-5447-downstream-lag.result | 93 +++++++++++++++++++
 .../gh-5447-downstream-lag.test.lua           | 41 ++++++++
 6 files changed, 200 insertions(+)
 create mode 100644 changelogs/unreleased/gh-5447-downstream-lag.md
 create mode 100644 test/replication/gh-5447-downstream-lag.result
 create mode 100644 test/replication/gh-5447-downstream-lag.test.lua

diff --git a/changelogs/unreleased/gh-5447-downstream-lag.md b/changelogs/unreleased/gh-5447-downstream-lag.md
new file mode 100644
index 000000000..726175c6c
--- /dev/null
+++ b/changelogs/unreleased/gh-5447-downstream-lag.md
@@ -0,0 +1,6 @@
+#feature/replication
+
+ * Introduced `box.info.replication[n].downstream.lag` field to monitor
+   state of replication. This member represents time spent between
+   transaction been written to initiator's WAL file and reached WAL
+   file of a replica (gh-5447).
diff --git a/src/box/lua/info.c b/src/box/lua/info.c
index 0eb48b823..f201b25e3 100644
--- a/src/box/lua/info.c
+++ b/src/box/lua/info.c
@@ -143,6 +143,9 @@ lbox_pushrelay(lua_State *L, struct relay *relay)
 		lua_pushnumber(L, ev_monotonic_now(loop()) -
 			       relay_last_row_time(relay));
 		lua_settable(L, -3);
+		lua_pushstring(L, "lag");
+		lua_pushnumber(L, relay_txn_lag(relay));
+		lua_settable(L, -3);
 		break;
 	case RELAY_STOPPED:
 	{
diff --git a/src/box/relay.cc b/src/box/relay.cc
index b1571b361..18c1ac06b 100644
--- a/src/box/relay.cc
+++ b/src/box/relay.cc
@@ -158,6 +158,19 @@ struct relay {
 	struct stailq pending_gc;
 	/** Time when last row was sent to peer. */
 	double last_row_time;
+	/**
+	 * A time difference between the moment when we
+	 * wrote a transaction to the local WAL and when
+	 * this transaction has been replicated to remote
+	 * node (ie written to node's WAL).
+	 */
+	double txn_lag;
+	/**
+	 * Last timestamp observed from remote node to
+	 * persist @a txn_lag value.
+	 */
+	double txn_acked_tm;
+
 	/** Relay sync state. */
 	enum relay_state state;
 
@@ -217,6 +230,12 @@ relay_last_row_time(const struct relay *relay)
 	return relay->last_row_time;
 }
 
+double
+relay_txn_lag(const struct relay *relay)
+{
+	return relay->txn_lag;
+}
+
 static void
 relay_send(struct relay *relay, struct xrow_header *packet);
 static void
@@ -284,6 +303,16 @@ relay_start(struct relay *relay, int fd, uint64_t sync,
 	relay->state = RELAY_FOLLOW;
 	relay->row_count = 0;
 	relay->last_row_time = ev_monotonic_now(loop());
+	relay->txn_lag = 0;
+	/*
+	 * We assume that previously written rows in WAL
+	 * are older than current node real time which allows
+	 * to simplify @a txn_lag calculation. In worst
+	 * scenario when runtime has been adjusted backwards
+	 * between restart we simply get some big value in
+	 * @a txn_lag until next transaction get replicated.
+	 */
+	relay->txn_acked_tm = ev_now(loop());
 }
 
 void
@@ -336,6 +365,8 @@ relay_stop(struct relay *relay)
 	 * upon cord_create().
 	 */
 	relay->cord.id = 0;
+	relay->txn_lag = 0;
+	relay->txn_acked_tm = ev_now(loop());
 }
 
 void
@@ -629,6 +660,26 @@ relay_reader_f(va_list ap)
 			/* vclock is followed while decoding, zeroing it. */
 			vclock_create(&relay->recv_vclock);
 			xrow_decode_vclock_xc(&xrow, &relay->recv_vclock);
+			/*
+			 * Replica send us last replicated transaction
+			 * timestamp which is needed for relay lag
+			 * monitoring. Note that this transaction has
+			 * been written to WAL with our current realtime
+			 * clock value, thus when it get reported back we
+			 * can compute time spent regardless of the clock
+			 * value on remote replica.
+			 *
+			 * An interesting moment is replica restart - it will
+			 * send us value 0 after that but we can preserve
+			 * old reported value here since we *assume* that
+			 * timestamp is not going backwards on properly
+			 * set up nodes, otherwise the lag get raised.
+			 * After all this is a not tamper-proof value.
+			 */
+			if (relay->txn_acked_tm < xrow.tm) {
+				relay->txn_acked_tm = xrow.tm;
+				relay->txn_lag = ev_now(loop()) - xrow.tm;
+			}
 			fiber_cond_signal(&relay->reader_cond);
 		}
 	} catch (Exception *e) {
diff --git a/src/box/relay.h b/src/box/relay.h
index b32e2ea2a..615ffb75d 100644
--- a/src/box/relay.h
+++ b/src/box/relay.h
@@ -93,6 +93,12 @@ relay_vclock(const struct relay *relay);
 double
 relay_last_row_time(const struct relay *relay);
 
+/**
+ * Returns relay's transaction's lag.
+ */
+double
+relay_txn_lag(const struct relay *relay);
+
 /**
  * Send a Raft update request to the relay channel. It is not
  * guaranteed that it will be delivered. The connection may break.
diff --git a/test/replication/gh-5447-downstream-lag.result b/test/replication/gh-5447-downstream-lag.result
new file mode 100644
index 000000000..8586d0ed3
--- /dev/null
+++ b/test/replication/gh-5447-downstream-lag.result
@@ -0,0 +1,93 @@
+-- test-run result file version 2
+--
+-- gh-5447: Test for box.info.replication[n].downstream.lag.
+-- We need to be sure that if replica start been back of
+-- master node reports own lagging and cluster admin would
+-- be able to detect such situation.
+--
+
+fiber = require('fiber')
+ | ---
+ | ...
+test_run = require('test_run').new()
+ | ---
+ | ...
+engine = test_run:get_cfg('engine')
+ | ---
+ | ...
+
+box.schema.user.grant('guest', 'replication')
+ | ---
+ | ...
+
+test_run:cmd('create server replica with rpl_master=default, \
+             script="replication/replica.lua"')
+ | ---
+ | - true
+ | ...
+test_run:cmd('start server replica')
+ | ---
+ | - true
+ | ...
+
+s = box.schema.space.create('test', {engine = engine})
+ | ---
+ | ...
+_ = s:create_index('pk')
+ | ---
+ | ...
+
+--
+-- The replica should wait some time (wal delay is 1 second
+-- by default) so we would be able to detect the lag, since
+-- on local instances the lag is minimal and usually transactions
+-- are handled instantly.
+test_run:switch('replica')
+ | ---
+ | - true
+ | ...
+box.error.injection.set("ERRINJ_WAL_DELAY", true)
+ | ---
+ | - ok
+ | ...
+
+test_run:switch('default')
+ | ---
+ | - true
+ | ...
+box.space.test:insert({1})
+ | ---
+ | - [1]
+ | ...
+test_run:wait_cond(function() return box.info.replication[2].downstream.lag ~= 0 end, 10)
+ | ---
+ | - true
+ | ...
+
+test_run:switch('replica')
+ | ---
+ | - true
+ | ...
+box.error.injection.set("ERRINJ_WAL_DELAY", false)
+ | ---
+ | - ok
+ | ...
+--
+-- Cleanup everything.
+test_run:switch('default')
+ | ---
+ | - true
+ | ...
+
+test_run:cmd('stop server replica')
+ | ---
+ | - true
+ | ...
+test_run:cmd('cleanup server replica')
+ | ---
+ | - true
+ | ...
+test_run:cmd('delete server replica')
+ | ---
+ | - true
+ | ...
diff --git a/test/replication/gh-5447-downstream-lag.test.lua b/test/replication/gh-5447-downstream-lag.test.lua
new file mode 100644
index 000000000..650e8e215
--- /dev/null
+++ b/test/replication/gh-5447-downstream-lag.test.lua
@@ -0,0 +1,41 @@
+--
+-- gh-5447: Test for box.info.replication[n].downstream.lag.
+-- We need to be sure that if replica start been back of
+-- master node reports own lagging and cluster admin would
+-- be able to detect such situation.
+--
+
+fiber = require('fiber')
+test_run = require('test_run').new()
+engine = test_run:get_cfg('engine')
+
+box.schema.user.grant('guest', 'replication')
+
+test_run:cmd('create server replica with rpl_master=default, \
+             script="replication/replica.lua"')
+test_run:cmd('start server replica')
+
+s = box.schema.space.create('test', {engine = engine})
+_ = s:create_index('pk')
+
+--
+-- The replica should wait some time (wal delay is 1 second
+-- by default) so we would be able to detect the lag, since
+-- on local instances the lag is minimal and usually transactions
+-- are handled instantly.
+test_run:switch('replica')
+box.error.injection.set("ERRINJ_WAL_DELAY", true)
+
+test_run:switch('default')
+box.space.test:insert({1})
+test_run:wait_cond(function() return box.info.replication[2].downstream.lag ~= 0 end, 10)
+
+test_run:switch('replica')
+box.error.injection.set("ERRINJ_WAL_DELAY", false)
+--
+-- Cleanup everything.
+test_run:switch('default')
+
+test_run:cmd('stop server replica')
+test_run:cmd('cleanup server replica')
+test_run:cmd('delete server replica')
-- 
2.31.1