Process lifecycle

process-lifecycle — postmaster, backends, aux processes, bgworkers

PostgreSQL uses a per-connection process model, not threads. Every client connection gets its own OS process, forked from the postmaster. New backends do NOT inherit query state — every connection starts fresh through InitPostgres. This model shapes almost every feature.

The five process classes

Class	Files	Lifetime	Example
Postmaster	postmaster/postmaster.c, pmchild.c, launch_backend.c, fork_process.c	Cluster-lifetime	The parent process — accepts connections, forks children, reaps exits, restarts on crash.
Regular backend	tcop/postgres.c, tcop/backend_startup.c	Connection-lifetime	The child that runs SQL for one client.
Auxiliary process	postmaster/auxprocess.c + one file each: checkpointer.c, bgwriter.c, walwriter.c, walsummarizer.c, startup.c, pgarch.c, syslogger.c, interrupt.c	Cluster-lifetime	Always-on infrastructure processes; not for user queries.
Bgworker	postmaster/bgworker.c + registered by extensions	Configurable (per session / for lifetime of cluster / restart on crash)	Autovacuum workers, parallel query workers, logical-rep apply workers, extension workers.
Autovacuum launcher/worker	postmaster/autovacuum.c	Cluster-lifetime launcher + short-lived workers	Special case: launcher is aux, workers are bgworkers.

Backend lifecycle (regular query-serving backend)

Every incoming connection follows this sequence:

1. Postmaster accepts — postmaster.c ServerLoop sees a new socket, calls BackendStartup in launch_backend.c.
2. Fork or exec-and-fork — On Unix, fork() copies the postmaster into a child. On Windows or EXEC_BACKEND builds, the child re-execs and re-attaches to shmem via SubPostmasterMain.
3. Auth handshake — backend_startup.c runs the startup message exchange, TLS/GSS, SCRAM/MD5/etc. authentication. Fails here → child exits before any query state exists.
4. InitPostgres — This is the big one (in utils/init/postinit.c). Loads GUC per-role/per-db settings, opens the database's relcache/syscache, checks CONNECT permission, runs the login _PG_init hooks, ends up in PostgresMain.
5. PostgresMain loop — the SQL query loop (in tcop/postgres.c). ReadCommand → exec → send RowDescription/DataRow/CommandComplete → ReadyForQuery → next iteration.
6. Shutdown — Client sends X (Terminate), or postmaster signals shutdown, or fatal error. proc_exit runs registered callbacks (see on_shmem_exit / on_proc_exit in storage/ipc/ipc.c), releases locks + LWLocks, detaches from shmem.

The interrupt / signal system

Because backends can be interrupted between statements (query cancel, admin shutdown, sighup, deadlock), there's a specific pattern:

• Signal handlers must be async-signal-safe. They set flags (InterruptPending, QueryCancelPending, ProcDiePending, ConfigReloadPending) and set the process latch (SetLatch(MyLatch)).
• CHECK_FOR_INTERRUPTS() — macros sprinkled through the code that check the flags at safe points and call ProcessInterrupts (in tcop/postgres.c) if any are set.
• ProcessInterrupts — the actual interrupt handler. Runs at safe points, may call ereport(FATAL/ERROR) to unwind or LATCH_WAIT_TIMEOUT handling.
• Safe interruption points — the code base has thousands. Long-running loops need to include CHECK_FOR_INTERRUPTS() — a missing one → uncancelable query.

Common signals + their flags:

• SIGINT (query cancel) → QueryCancelPending.
• SIGTERM (shutdown) → ProcDiePending.
• SIGHUP (reload conf) → ConfigReloadPending.
• SIGUSR1 (procsignal — multiplexed) → reason-specific flags via procsignal.c.

Auxiliary processes at a glance

Each aux process has its own file with a Main function that the postmaster spawns via SubPostmasterMain (Windows / EXEC_BACKEND) or forks directly:

File	Function	Purpose
startup.c	StartupProcessMain	Runs crash recovery / WAL replay at startup. Exits once redo completes.
checkpointer.c	CheckpointerMain	Runs periodic checkpoints; writes the shutdown stats file.
bgwriter.c	BackgroundWriterMain	Writes dirty buffers to smooth checkpoint I/O.
walwriter.c	WalWriterMain	Flushes WAL buffers asynchronously.
walsummarizer.c	WalSummarizerMain	(PG 17+) Summarizes WAL for incremental backup.
pgarch.c	PgArchiverMain	Archives completed WAL segments (archive_command / archive_library).
syslogger.c	SysLoggerMain	Rotates + captures postmaster/backend stderr when logging_collector=on.
interrupt.c	Shared aux-process signal helpers	Not a process itself — the signal handlers shared across aux processes.

Bgworker registration

Extensions and core code both use RegisterBackgroundWorker (in postmaster/bgworker.c):

• Static — called from _PG_init at postmaster start. Fixed slot count (max_worker_processes GUC).
• Dynamic — RegisterDynamicBackgroundWorker at runtime; used by parallel query (from a backend) and by extensions.

Bgworkers have flags controlling: shared-memory access, database connection, restart-on-crash policy, restart interval. See knowledge/idioms/background-worker-startup.md for the flag matrix and lifecycle diagram.

Common patch shapes

Add a startup-lifecycle hook

The scenario add-startup-hook (see knowledge/scenarios/add-startup-hook.md) covers this end-to-end. Short version:

• Hook typically lives in PostmasterMain (postmaster-wide) or InitPostgres (per-backend).
• Decide: cluster-once vs backend-per-connection.
• Existing patterns: shared_preload_libraries (postmaster startup), local_preload_libraries + session_preload_libraries (per-backend), ClientAuthentication_hook (auth-time), emit_log_hook (per log record).

Add a new auxiliary process

Rare. Requires:

• New file under src/backend/postmaster/<name>.c with a <Name>Main function.
• Registration in postmaster/postmaster.c (grep for StartChildProcess / StartAuxiliaryProcess).
• Signal handlers (usually delegated to interrupt.c helpers).
• Shmem region if it exchanges data with backends (via ShmemInit).
• Consider whether it should restart on crash (postmaster's HandleChildCrash).

Note: for most "run something periodically" use cases, a bgworker is preferable — less core code to touch, extensions can add without a core patch.

Add a signal / interrupt reason

• New PROCSIGNAL_* constant in src/include/storage/procsignal.h.
• Signal-dispatcher procsignal_sigusr1_handler case in storage/ipc/procsignal.c.
• Flag + ProcessInterrupts case in tcop/postgres.c.
• Setter helper if the reason is per-target-backend (e.g. procsignal_ProcSendSignal(pid, PROCSIG_...))`.
• Docs for what "kills" or "cancels" this new interrupt cause.

Pitfalls

• Fork copies memory but not open file descriptors semantically — a palloc before fork is fine (COW), but a dsm_attach before fork is not (the child would double-detach). This is why aux processes and bgworkers do shmem init in their own Main, not inherited state.
• Signal handlers cannot log via ereport — that's elog(LOG, ...) calls internally allocating in ErrorContext, which is not async-signal-safe. Set a flag, return, let the main loop pick it up via CHECK_FOR_INTERRUPTS.
• InitPostgres reads pg_authid + pg_database + role/db GUC settings BEFORE the user has issued any SQL — extensions in _PG_init need to be careful about assuming database context is complete.
• shared_preload_libraries vs session_preload_libraries vs local_preload_libraries — different postmaster/backend load points; different capabilities (shmem allocation is only possible from shared_preload_libraries).
• Bgworker signal setup — a bgworker inherits SIG_IGN/SIG_DFL from postmaster. It MUST reset signal handlers in its Main via pqsignal calls before doing any interruptible work.
• proc_exit vs _exit vs abort — proc_exit(0) runs registered callbacks (releases locks, detaches shmem, closes files); _exit skips them; abort (PANIC) triggers a cluster-wide restart. Never call _exit in a place that's holding a lock.
• EXEC_BACKEND builds are Windows-native but also used for testing on Unix — a patch that "works" on your Linux dev box may still fail EXEC_BACKEND CI because the fork-then-exec path is different. Test with -DEXEC_BACKEND locally when touching startup code.

Related corpus

• Subsystems: tcop (the query loop side), main (backend entry), libpq-backend (auth handshake), access-transam (WAL / xact IDs, needed at startup).
• Idioms: background-worker-startup, apply-worker-loop, process-utility-hook-chain, abort-transaction-cleanup, crash-recovery-startup.
• Data structures: pgproc-fields (the per-backend PGPROC slot in shmem).
• Scenarios: add-startup-hook, add-new-bgworker.
• File docs: 20 files under knowledge/files/src/backend/postmaster/ + src/backend/tcop/.

Corpus-chain shortcut

python3 scripts/corpus-chain.py --scenario add-startup-hook
python3 scripts/corpus-chain.py --file src/backend/tcop/postgres.c
python3 scripts/corpus-chain.py --file src/backend/postmaster/postmaster.c

Third command in particular surfaces the full 20-file neighborhood of the postmaster tree.

Boundary

Use this skill for backend/aux/bgworker/postmaster lifecycle questions.

Don't use for:

• libpq / client-side driver — that's src/interfaces/libpq/; different codebase, different lifecycle.
• Individual SQL commands' handling — that's tcop (dispatch) + commands/ (per-statement).
• GUC config loading — use gucs-config skill; touches lifecycle but focused on the config side.
• Extension _PG_init details — use bgworker-and-extensions; this skill covers the invocation timing, not the extension-authoring surface.