PostgreSQL Process Architecture Diagram
+------------------+
| POSTMASTER |
| (Parent PID) |
+---------+--------+
|
----------------------------------------------------------------
| | | | | |
v v v v v v
+-------------+ +-----------+ +-----------+ +-----------+ +---------+ +---------+
| Background | | WAL | |Checkpointer| | Autovac | | Archiver| | Startup |
| Writer | | Writer | | | | Launcher | | | | Process |
+-------------+ +-----------+ +-----------+ +-----------+ +---------+ +---------+
|
----------------------
| | |
v v v
+----------+ +----------+ +----------+
|Vacuum W1 | |Vacuum W2 | |Vacuum W3 |
+----------+ +----------+ +----------+
Client Connections
|
----------------------------------------------------------------
| | | |
v v v v
+--------------+ +--------------+ +--------------+ +--------------+
| Backend PID | | Backend PID | | Backend PID | | Backend PID |
| User Session | | User Session | | User Session | | User Session |
+--------------+ +--------------+ +--------------+ +--------------+
Shared Memory:
--------------------------------------------------------
| Shared Buffers | WAL Buffers | Locks | Proc Array |
--------------------------------------------------------
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>
or
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
