How to fix FATAL: the database system is starting up in PostgreSQL

First, verify that the PostgreSQL server is actually running and observe the startup progress:

# Check if PostgreSQL service is running
sudo systemctl status postgresql

# Or check if the process is active
ps aux | grep postgres

# Use pg_isready to check connection status
pg_isready -h localhost

If pg_isready shows "accepting connections", the server is ready. If it shows "rejecting connections", the startup is still in progress.

In most cases, this error resolves itself. The database needs time to complete crash recovery by replaying transaction logs. Wait at least 2-5 minutes before taking further action:

# Monitor PostgreSQL logs to see recovery progress
tail -f /var/log/postgresql/postgresql-15-main.log
# Look for messages like "redo starts at" and "redo done at"

For larger databases with many transactions, recovery can take much longer. Monitor the logs to confirm progress rather than assuming the database is stuck.

If the server appears to be stuck, check the data directory for stale lock files or recovery indicators:

# Check for postmaster PID file and lock
ls -la /var/lib/postgresql/15/main/postmaster.pid

# Check if recovery process is active
ls -la /var/lib/postgresql/15/main/recovery.conf
ls -la /var/lib/postgresql/15/main/standby.signal

Note: Replace "15" with your actual PostgreSQL version and "/var/lib/postgresql" with your actual data directory location.

If the database has been starting up for an unreasonable amount of time, restart the service:

# Gracefully restart PostgreSQL
sudo systemctl restart postgresql

# Wait and check the status
sleep 10
sudo systemctl status postgresql

# Test the connection
psql -U postgres -d postgres -c "SELECT version();"

If a graceful restart doesn't work, try a more forceful approach:

sudo systemctl stop postgresql
sleep 5
sudo systemctl start postgresql

If this is a standby (replica) server, ensure hot_standby mode is enabled to allow connections during recovery:

# Check the postgresql.conf file
sudo grep hot_standby /etc/postgresql/15/main/postgresql.conf

# It should show: hot_standby = on

If hot_standby is off, enable it:

# Edit the configuration
sudo nano /etc/postgresql/15/main/postgresql.conf

# Find and uncomment/change to:
hot_standby = on

# Reload configuration (no restart needed)
sudo systemctl reload postgresql

If the startup process is unusually slow, check for resource constraints:

# Check available memory
free -h

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

# Check CPU usage and load
top -n 1 | head -15

If memory is severely limited (especially on VMs), you may need to:
- Allocate more memory to the VM
- Reduce other services to free up memory
- Consider recovering from backups on a server with more resources

Ensure there is adequate disk space for the recovery process to complete.