Hi,
This patch introduces a new function pg_accept_connections_start_time().
Currently, pg_postmaster_start_time() is used to determine when the
database started. However, this is not accurate since the postmaster
process can sometimes be up whereas the database is not accepting
connections (for e.g. during child process crash [1],
long crash-recovery etc.)
This can lead to inaccurate database uptime calculations.
The new function, pg_accept_connections_start_time(), returns the
time when the database became ready to accept connections. This is
helpful, since in both of the above cases (quick crash-recovery on
child process crash, long crash-recovery on startup), this timestamp
would get reset - an example scenario given below [3].
This function can be used to tell:
1. Whether the database did a quick crash-recovery (without
a postmaster restart) in a production setup. In particular, this would
help a long-running client confirm whether a connection blip was a
server restart, or a session-abort / network / client-side issue [2].
2. Calculate database uptime (more accurately)
The patch passes `make check`, adds a brief function description
in func.sgml, works in single-user mode and applies cleanly on
master as of 9e17ac997 (14th Feb).
Look forward to feedback, but in particular:
- Good to have a second opinion on a better position to capture
timestamp during startup in single-user mode.
- Function name - I think it is too verbose, but it felt most unambiguous.
-
Thanks
Robins
1. pg_postmaster_start_time() doesn't tell when db became available:
https://www.postgresql.org/message-id/598d4a75-57d9-b41a-a927-7584be6278b2%40rblst.info
2. IIUC knowing that a crash-recovery happened may have helped here?
https://www.postgresql.org/message-id/954419.1623092217%40sss.pgh.pa.us
3. Sample usage of the function - Kill 'walwriter' to force postmaster
to do a quick crash-recovery - where pg_postmaster_start_time() does
not change, pg_accept_connections_start_time() does get updated
to the time when database (once again) became available for connections:
```
robins@camry:~/proj/postgres$ psql postgres -c "select
pg_accept_connections_start_time(), pg_postmaster_start_time();"
pg_accept_connections_start_time | pg_postmaster_start_time
----------------------------------+----------------------------------
2025-02-16 11:40:37.355906+10:30 | 2025-02-16 11:40:37.351776+10:30
(1 row)
robins@camry:~/proj/postgres$ ps -ef | grep postgres
robins 2935044 1 0 11:40 ? 00:00:00
/home/robins/proj/localpg/bin/postgres -D data
robins 2935045 2935044 0 11:40 ? 00:00:00 postgres: checkpointer
robins 2935046 2935044 0 11:40 ? 00:00:00 postgres: background
writer
robins 2935048 2935044 0 11:40 ? 00:00:00 postgres: walwriter
robins 2935049 2935044 0 11:40 ? 00:00:00 postgres: autovacuum
launcher
robins 2935050 2935044 0 11:40 ? 00:00:00 postgres: logical
replication launcher
robins 2937754 1769260 0 13:57 pts/1 00:00:00 grep --color=auto
postgres
robins@camry:~/proj/postgres$ kill -9 `ps -ef | grep postgres | grep
walwriter | awk '{print $2}'`
robins@camry:~/proj/postgres$ ps -ef | grep postgres
robins 2935044 1 0 11:40 ? 00:00:00
/home/robins/proj/localpg/bin/postgres -D data
robins 2937761 2935044 0 13:57 ? 00:00:00 postgres: checkpointer
robins 2937762 2935044 0 13:57 ? 00:00:00 postgres: background
writer
robins 2937763 2935044 0 13:57 ? 00:00:00 postgres: walwriter
robins 2937764 2935044 0 13:57 ? 00:00:00 postgres: autovacuum
launcher
robins 2937766 1769260 0 13:57 pts/1 00:00:00 grep --color=auto
postgres
robins@camry:~/proj/postgres$ psql postgres -c "select
pg_accept_connections_start_time(), pg_postmaster_start_time();"
pg_accept_connections_start_time | pg_postmaster_start_time
----------------------------------+----------------------------------
2025-02-16 13:57:52.914587+10:30 | 2025-02-16 11:40:37.351776+10:30
(1 row)
```
From 7521d666ce66347f894acf435d67ea69a6ecc677 Mon Sep 17 00:00:00 2001
From: Robins Tharakan <tharakan@gmail.com>
Date: Fri, 14 Feb 2025 20:51:10 +1030
Subject: [PATCH v1] Add support for pg_accept_connections_start_time()
It is not always reliable to depend on pg_postmaster_start_time()
database uptime calculations, owing to how postmaster catches
child process crashes, startup recovery etc. and continue without
a restart. This could lead to multiple seconds (or minutes/hours)
of difference when although postmaster was up, but the database
was not available for accepting connections.
This function returns the start time when the database was ready
to accept new database connections, allowing better calculation
of database availability.
---
doc/src/sgml/func.sgml | 13 +++++++++++++
src/backend/postmaster/launch_backend.c | 3 +++
src/backend/postmaster/postmaster.c | 5 +++++
src/backend/tcop/postgres.c | 5 +++++
src/backend/utils/adt/timestamp.c | 9 +++++++++
src/include/catalog/pg_proc.dat | 6 ++++++
src/include/utils/timestamp.h | 3 +++
7 files changed, 44 insertions(+)
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 7efc81936ab..3bfbc384ea8 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -24935,6 +24935,19 @@ SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
</para></entry>
</row>
+ <row>
+ <entry role="func_table_entry"><para role="func_signature">
+ <indexterm>
+ <primary>pg_accept_connections_start_time</primary>
+ </indexterm>
+ <function>pg_accept_connections_start_time</function> ()
+ <returnvalue>timestamp with time zone</returnvalue>
+ </para>
+ <para>
+ Returns the time when the server was ready to accept connections.
+ </para></entry>
+ </row>
+
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c
index a97a1eda6da..985cda2a032 100644
--- a/src/backend/postmaster/launch_backend.c
+++ b/src/backend/postmaster/launch_backend.c
@@ -110,6 +110,7 @@ typedef struct
ProcSignalHeader *ProcSignal;
pid_t PostmasterPid;
TimestampTz PgStartTime;
+ TimestampTz PgAcceptConnStartTime;
TimestampTz PgReloadTime;
pg_time_t first_syslogger_file_time;
bool redirection_done;
@@ -738,6 +739,7 @@ save_backend_variables(BackendParameters *param,
param->PostmasterPid = PostmasterPid;
param->PgStartTime = PgStartTime;
+ param->PgAcceptConnStartTime = PgAcceptConnStartTime;
param->PgReloadTime = PgReloadTime;
param->first_syslogger_file_time = first_syslogger_file_time;
@@ -998,6 +1000,7 @@ restore_backend_variables(BackendParameters *param)
PostmasterPid = param->PostmasterPid;
PgStartTime = param->PgStartTime;
+ PgAcceptConnStartTime = param->PgAcceptConnStartTime;
PgReloadTime = param->PgReloadTime;
first_syslogger_file_time = param->first_syslogger_file_time;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index bb22b13adef..7bcecdbb830 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -2329,6 +2329,11 @@ process_pm_child_exit(void)
*/
StartWorkerNeeded = true;
+ /*
+ * Remember time when database was ready to accept connections
+ */
+ PgAcceptConnStartTime = GetCurrentTimestamp();
+
/* at this point we are really open for business */
ereport(LOG,
(errmsg("database system is ready to accept connections")));
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 1149d89d7a1..308930d5d76 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -4117,6 +4117,11 @@ PostgresSingleUserMain(int argc, char *argv[],
*/
PgStartTime = GetCurrentTimestamp();
+ /*
+ * Remember time when stand-alone came up to accept user commands.
+ */
+ PgAcceptConnStartTime = GetCurrentTimestamp();
+
/*
* Create a per-backend PGPROC struct in shared memory. We must do this
* before we can use LWLocks.
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index ba9bae05069..56cd4bfd3d9 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -52,6 +52,9 @@
/* Set at postmaster start */
TimestampTz PgStartTime;
+/* Set when database is ready to accept connections */
+TimestampTz PgAcceptConnStartTime;
+
/* Set at configuration reload */
TimestampTz PgReloadTime;
@@ -1628,6 +1631,12 @@ pg_postmaster_start_time(PG_FUNCTION_ARGS)
PG_RETURN_TIMESTAMPTZ(PgStartTime);
}
+Datum
+pg_accept_connections_start_time(PG_FUNCTION_ARGS)
+{
+ PG_RETURN_TIMESTAMPTZ(PgAcceptConnStartTime);
+}
+
Datum
pg_conf_load_time(PG_FUNCTION_ARGS)
{
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 9e803d610d7..2a1f27ea600 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -8650,6 +8650,12 @@
prorettype => 'timestamptz', proargtypes => '',
prosrc => 'pg_postmaster_start_time' },
+# accept connections start time function
+{ oid => '8600', descr => 'accept connections start time',
+ proname => 'pg_accept_connections_start_time', provolatile => 's',
+ prorettype => 'timestamptz', proargtypes => '',
+ prosrc => 'pg_accept_connections_start_time' },
+
# config reload time function
{ oid => '2034', descr => 'configuration load time',
proname => 'pg_conf_load_time', provolatile => 's', proparallel => 'r',
diff --git a/src/include/utils/timestamp.h b/src/include/utils/timestamp.h
index d26f023fb87..e1eae7ddfc8 100644
--- a/src/include/utils/timestamp.h
+++ b/src/include/utils/timestamp.h
@@ -89,6 +89,9 @@ IntervalPGetDatum(const Interval *X)
/* Set at postmaster start */
extern PGDLLIMPORT TimestampTz PgStartTime;
+/* Set when database is ready to accept connections */
+extern PGDLLIMPORT TimestampTz PgAcceptConnStartTime;
+
/* Set at configuration reload */
extern PGDLLIMPORT TimestampTz PgReloadTime;
--
2.43.0
