PostgreSQL Process Architecture Diagram

+------------------+

| POSTMASTER |

| (Parent PID) |

+---------+--------+

----------------------------------------------------------------

| | | | | |

v v v v v v

+-------------+ +-----------+ +-----------+ +-----------+ +---------+ +---------+

+-------------+ +-----------+ +-----------+ +-----------+ +---------+ +---------+

----------------------

| | |

v v v

+----------+ +----------+ +----------+

+----------+ +----------+ +----------+

Client Connections

----------------------------------------------------------------

| | | |

v v v v

+--------------+ +--------------+ +--------------+ +--------------+

+--------------+ +--------------+ +--------------+ +--------------+

Shared Memory:

--------------------------------------------------------

--------------------------------------------------------

How to See PostgreSQL Processes at OS Level

Find Postmaster (Parent Process)

ps -ef | grep postgres

Example:

postgres 1000 1 0 10:00 ? 00:00:00 postgres

postgres 1001 1000 0 10:00 ? 00:00:00 postgres: checkpointer

postgres 1002 1000 0 10:00 ? 00:00:00 postgres: background writer

postgres 1003 1000 0 10:00 ? 00:00:00 postgres: walwriter

postgres 1004 1000 0 10:00 ? 00:00:00 postgres: autovacuum launcher

Notice:

PID 1000 = Parent Process

PPID 1 = Started by OS

This is the Postmaster.

Better Process Tree

Linux

pstree -p | grep postgres

Example:

postgres(1000)

├─postgres(1001)

├─postgres(1002)

├─postgres(1003)

├─postgres(1004)

├─postgres(1200)

├─postgres(1201)

└─postgres(1202)

Much easier to visualize.

1. Postmaster

Responsibility

Starts all PostgreSQL processes
Accepts client connections
Handles crash recovery
Monitors child processes

Check:

ps -fp <postmaster_pid>

Example:

ps -fp 1000

If Backend Crashes

Backend dies

↓

Postmaster detects

↓

May restart required processes

Postmaster acts like a supervisor process.

2. Backend Process

Every client connection gets a backend process.

Connect:

psql postgres

Run:

SELECT pg_backend_pid();

Example:

pg_backend_pid

-----------------

14567

At OS level:

ps -fp 14567

Output:

postgres: appuser mydb 10.0.0.10 idle

Find Active Queries

SELECT pid,

usename,

state,

query

FROM pg_stat_activity;

Map PID to Linux:

top -Hp <pid>

ps -fp <pid>

3. WAL Writer

Responsibility

Writes WAL buffers to:

$PGDATA/pg_wal

Process appears as:

postgres: walwriter

Check:

ps -ef | grep walwriter

Example:

postgres 1003 1000 0 ? postgres: walwriter

Observe WAL Activity

Inside PostgreSQL:

SELECT *

FROM pg_stat_wal;

Important metrics:

wal_records

wal_fpi

wal_bytes

4. Checkpointer

Responsibility

Writes dirty pages from memory to data files.

Process:

postgres: checkpointer

View:

ps -ef | grep checkpointer

Monitor

SELECT *

FROM pg_stat_bgwriter;

Interesting columns:

checkpoints_timed

checkpoints_req

checkpoint_write_time

checkpoint_sync_time

5. Background Writer

Responsibility

Gradually writes dirty buffers before checkpoint.

Goal:

Reduce checkpoint spikes

Process:

postgres: background writer

Check:

ps -ef | grep "background writer"

Why Important

Without BGWriter:

Checkpoint writes millions of dirty pages

causing:

High I/O

Slow performance

6. Autovacuum Launcher

Responsibility

Monitors all databases.

Starts worker processes when necessary.

OS command:

ps -ef | grep autovacuum

Example:

postgres: autovacuum launcher

7. Autovacuum Workers

Process:

postgres: autovacuum worker

Check:

ps -ef | grep "autovacuum worker"

Example:

postgres: autovacuum worker mydb

Monitor Current Vacuum

SELECT *

FROM pg_stat_progress_vacuum;

Output:

pid

phase

heap_blks_total

heap_blks_scanned

8. Archiver Process

Only if:

archive_mode = on

Process:

postgres: archiver

Check:

ps -ef | grep archiver

Monitoring

SELECT *

FROM pg_stat_archiver;

Example:

archived_count

failed_count

9. Startup Process

Visible during:

Database startup

Crash recovery

Standby recovery

OS:

ps -ef | grep startup

Example:

postgres: startup recovering

During Recovery

Logs:

tail -f $PGDATA/log/postgresql.log

Example:

redo starts at ...

redo done at ...

10. WAL Sender

Primary Server Replication.

Process:

postgres: walsender

Check:

ps -ef | grep walsender

Monitoring

SELECT *

FROM pg_stat_replication;

Example:

application_name

client_addr

state

sync_state

11. WAL Receiver

Standby Server.

Process:

postgres: walreceiver

OS:

ps -ef | grep walreceiver

Monitoring

SELECT *

FROM pg_stat_wal_receiver;

Real Production Troubleshooting

Check Entire Process Architecture

ps -ef | grep postgres

Check Process Tree

pstree -p | grep postgres

Check CPU Consumer

top -p $(pgrep postgres | tr '\n' ',')

Match PostgreSQL PID with Query

SELECT pid,

usename,

state,

wait_event,

query

FROM pg_stat_activity;

Then:

ps -fp <pid>

Check Shared Memory

ipcs -m

Or:

cat /proc/<postmaster_pid>/status

Oracle vs PostgreSQL Process Mapping

PostgreSQL	Oracle
Postmaster	PMON (roughly supervisor role)
Backend Process	Dedicated Server Process
WAL Writer	LGWR
Checkpointer	CKPT
Background Writer	DBWR
Startup Recovery	SMON
WAL Sender	Data Guard Transport
WAL Receiver	Managed Recovery Process
Archiver	ARCn
Autovacuum	No direct equivalent

Interview Answer (2-Minute Version)

PostgreSQL uses a process-based architecture. The Postmaster is the parent process that starts and monitors all child processes. For each client connection, a dedicated backend process is created. Key background processes include WAL Writer (writes WAL records for durability), Checkpointer (flushes dirty pages to disk), Background Writer (reduces checkpoint I/O spikes), Autovacuum Launcher and Workers (clean dead tuples and freeze old rows), Archiver (archives WAL files), and WAL Sender/Receiver (streaming replication). At the OS level these can be viewed using commands such as:

ps -ef | grep postgres

pstree -p | grep postgres

top -H -p <pid>

and correlated with database views like:

pg_stat_activity

pg_stat_progress_vacuum

pg_stat_replication

pg_stat_bgwriter

Saturday, November 14, 2026