How to fix FATAL: the database system is shutting down in PostgreSQL

First, verify whether the server is actually running and in what state:

sudo systemctl status postgresql

Or if using a custom installation:

pg_isready -h localhost -p 5432

The output will indicate:
- accepting connections - server is running and ready
- rejecting connections - server is running but not ready (startup/shutdown)
- no response - server is down

If the shutdown was intentional, the database will complete within seconds or minutes depending on:
- Shutdown mode (smart, fast, or immediate)
- Number of active connections
- Any long-running transactions

Monitor the PostgreSQL log for completion:

sudo tail -f /var/log/postgresql/postgresql-*.log

Look for the message LOG: database system is shut down which indicates completion. Then restart the service:

sudo systemctl start postgresql

If the shutdown was unexpected, examine the logs for root causes:

sudo tail -100 /var/log/postgresql/postgresql-*.log

Look for messages like:
- LOG: database system was not properly shut down - indicates a crash
- LOG: starting crash recovery - recovery is in progress
- ERROR: out of memory - resource exhaustion
- WARNING: terminating connection because of crash - another process crashed

Crash recovery may take several minutes on large databases. Do not interrupt this process.

Unexpected shutdowns can occur when the system runs out of disk space or memory:

# Check disk space
df -h /var/lib/postgresql

# Check available memory
free -h

# Check CPU load
uptime

If disk space is critically low (95%+), delete old logs and backups:

sudo find /var/log/postgresql -name "*.log" -mtime +7 -delete

If memory is exhausted, consider increasing available RAM or adjusting PostgreSQL memory settings in postgresql.conf:

shared_buffers = 256MB  # Conservative for limited memory

In replication setups (primary/standby), high network latency or slow standbys can trigger walsender timeouts, causing shutdown:

sudo -u postgres psql -c "SHOW wal_sender_timeout;"
sudo -u postgres psql -c "SHOW wal_receiver_timeout;"

Default is 60 seconds. If your standby is slow or the network is unreliable, increase these values in postgresql.conf:

wal_sender_timeout = 300s      # increased from 60s
wal_receiver_timeout = 300s

Then reload the configuration:

sudo systemctl reload postgresql

To avoid future slow shutdowns during maintenance:

1. Before shutting down, run a manual checkpoint to flush data to disk:

sudo -u postgres psql -c "CHECKPOINT;"

2. Then perform a fast shutdown (which will be quick since checkpoint is done):

sudo systemctl stop postgresql

Alternatively, use pg_ctl with explicit mode:

sudo -u postgres pg_ctl -D /var/lib/postgresql/[version]/main stop -m fast

Avoid immediate mode unless absolutely necessary, as it forces recovery on next startup.