Learn how to fix the PostgreSQL Error: “All Replication Slots Are in Use”. Our PostgreSQL Support team is here to help you with your questions and concerns.
How to Fix PostgreSQL Error: “All Replication Slots Are in Use”
According to our Experts, the PostgreSQL error “all replication slots are in use” indicates that the maximum number of allowed replication slots has been reached, preventing new replication processes from being created.
We can maintain database performance, replication integrity, and overall system stability by understanding and resolving this issue.
Impacts of the Error
- Prevents new replication processes from being established.
- Blocks data synchronization between primary and standby servers.
- Interrupts critical data replication workflows.
- Replication slots can cause uncontrolled Write-Ahead Log (WAL) file retention.
- Risk of disk space exhaustion, potentially bringing down the primary database server.
- WAL files can grow rapidly, consuming gigabytes of storage on highly active servers.
- Prevents new logical or physical replication connections, limiting scalability and high availability.
- Risk of losing recent transaction data and potential data inconsistency between primary and replica databases.
- It may hinder VACUUM operations, risk transaction ID wraparound, and cause operational overhead.
Causes and Fixes
1. Insufficient Replication Slot Configuration
By default, PostgreSQL allows only 10 replication slots, which may be insufficient for complex replication architectures.
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In this case, we have to increase the number of replication slots.
To modify `postgresql.conf`:
# Increase max replication slots (adjust based on requirements)
max_replication_slots = 35
max_sync_workers_per_subscription = 2
max_logical_replication_workers = 16
wal_level = replica # Ensure replication support
Here are the verification commands:
SHOW max_replication_slots;
SELECT * FROM pg_settings WHERE name = 'max_replication_slots';
To restart PostgreSQL:
sudo systemctl stop postgresql
sudo systemctl start postgresql
2. Abandoned Replication Slots
Failed or incomplete replication setups may leave behind orphaned slots.
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Here, we have to identify and drop unused slots.
To list all replication slots:
SELECT * FROM pg_replication_slots;
To identify inactive slots:
SELECT slot_name, plugin, slot_type FROM pg_replication_slots WHERE active = false;
We can drop specific unused slots with this command:
SELECT pg_drop_replication_slot('unused_slot_name');
To automate the cleanup of inactive slots:
DO $$
BEGIN
FOR slot_rec IN
SELECT slot_name FROM pg_replication_slots WHERE active = false
LOOP
EXECUTE format('SELECT pg_drop_replication_slot(%L)', slot_rec.slot_name);
END LOOP;
END $$;
3. Persistent Replication Worker Issues
If logical replication workers fail, they can prevent slots from being freed.
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In this case, restart replication workers.
To check logs for errors:
tail -n 100 /var/log/postgresql/postgresql-<version>-main.log
To disable and re-enable subscription:
ALTER SUBSCRIPTION subscription_name DISABLE;
ALTER SUBSCRIPTION subscription_name ENABLE;
Here are the network verification steps:
- Test connectivity between nodes.
- Validate firewall rules.
- Measure network latency.
- Confirm DNS resolution.
4. Resource Constraints Management
Insufficient system resources can lead to replication failure.
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Here, we have to optimize PostgreSQL performance.
To modify `postgresql.conf` based on available resources:
shared_buffers = 25% of total RAM
effective_cache_size = 50% of total RAM
work_mem = 16MB
maintenance_work_mem = 64MB
Here are a few monitoring strategies:
- Use `pg_stat_activity` to track worker processes.
- Monitor replication lag.
- Set up alerting for high resource utilization.</;i>
5. Slot Creation Strategy
Improper slot creation across databases can lead to unnecessary slot consumption.
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In this case, ensure correct slot creation.
To set proper WAL level:
ALTER SYSTEM SET wal_level = logical;
To create consistently named slots:
SELECT pg_create_logical_replication_slot('app_name_slot', 'pgoutput');
We can validate slot creation with a script as seen below:
DO $$
BEGIN
IF current_setting('wal_level') <> 'logical' THEN
RAISE EXCEPTION 'WAL level must be set to logical';
END IF;
END $$;
6. Version Compatibility Resolution
Replication issues may arise due to mismatched PostgreSQL versions or incompatible settings.
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Check Available Replication Extensions:
SELECT * FROM pg_available_extensions WHERE name LIKE '%replication%';
To ensure configuration synchronization:
- Match PostgreSQL major versions across nodes.
- Use Infrastructure-as-Code (IaC) tools for consistency.
- Implement centralized configuration management.
Prevention Strategies
- Periodically check the `pg_replication_slots` view.
- Monitor system resources.
- Review PostgreSQL logs for anomalies.
- Set appropriate `max_replication_slots` based on future scalability.
- Configure `max_logical_replication_workers` correctly.
- Use reserved slots for future expansion.
- Implement scripts to drop inactive slots periodically.
- Create maintenance routines to optimize replication infrastructure.
- Implement retry mechanisms for failed replication connections.
- Design fallback procedures for replication failures.
- Maintain detailed documentation of the replication setup.
- Track slot usage and monitor performance metrics.
Key Recommendations from our Experts
- Always allocate an extra buffer in `max_replication_slots` to accommodate future growth.
- Regularly audit and clean unused replication slots.
- Monitor system performance and replication lag.
- Ensure consistent replication configuration across environments.
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Conclusion
In brief, our Support Experts demonstrated how to resolve the PostgreSQL Error: “All Replication Slots Are in Use”.
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