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Troubleshooting: Replication slot is active

This guide helps you troubleshoot and resolve the “replication slot… is active” error that can occur with PostgreSQL sources in Materialize.

What this error means

When you see an error like:

postgres: ERROR replication slot "materialize_1002f37c6eeb4c28b052fa3805d46baa" is active for PID 610

This means that PostgreSQL has detected an attempt to use a replication slot that is already in use by another active connection. Each replication slot can only have one active consumer at a time, and this error occurs when Materialize tries to connect to a slot that PostgreSQL considers already active. This error is generally transient and indicates a race condition during connection management between Materialize and PostgreSQL.

Common causes

  • PostgreSQL or Materialize maintenance: During maintenance windows, Materialize clusters or PostgreSQL instances may restart, causing active replication connections to be temporarily interrupted. When the cluster comes back online, it attempts to reconnect to the replication slot before PostgreSQL has fully released the previous connection.
  • Cluster replica changes: Adding replicas to your Materialize source cluster can cause the ingestion process to restart from a different replica, triggering this error as multiple replicas attempt to use the same replication slot.
  • Source restarts or failover: When a source cluster restarts or fails over to another replica, the new replica may attempt to connect to the replication slot before PostgreSQL has released the previous connection.
  • Network interruptions: Brief network disconnections can leave PostgreSQL thinking a connection is still active when Materialize has already moved to a new connection attempt.
  • PostgreSQL connection cleanup delays: PostgreSQL may not immediately release replication slots when connections are terminated, especially if the termination was not graceful.
  • Multiple environments or sources: Accidentally configuring multiple Materialize sources to use the same replication slot.

Diagnosing the issue

Check replication slot status in PostgreSQL

Connect to your PostgreSQL database and run:

-- Replace <slot_name> with the replication slot name shown in the error message
SELECT
  slot_name,
  active,
  active_pid,
  restart_lsn,
  confirmed_flush_lsn
FROM pg_replication_slots
WHERE slot_name = '<slot_name>';

Look for:

  • active = true: The slot is currently in use
  • active_pid: The PostgreSQL backend process ID that is using the slot
  • Multiple slots with similar names that might indicate configuration issues

Check for active connections

Check which connections are using the replication slot:

-- Replace <slot_name> with the replication slot name shown in the error message
SELECT
  pid,
  usename,
  application_name,
  client_addr,
  state,
  backend_start
FROM pg_stat_activity
WHERE pid IN (
  SELECT active_pid
  FROM pg_replication_slots
  WHERE slot_name = '<slot_name>'
);

Resolution

Immediate fix: Wait for automatic recovery

In most cases, Materialize will automatically reconnect and recover from this error. The error is typically transient and resolves itself within a few minutes as PostgreSQL releases the previous connection.

If automatic recovery fails: Terminate stale connections

If the error persists and you’ve verified that there are no legitimate active connections, you can terminate the stale connection in PostgreSQL:

WARNING! Only terminate connections if you’re certain they are stale. Terminating an active replication connection will interrupt data ingestion. 
-- First, identify the PID from the error message or from pg_replication_slots
-- Replace <slot_name> with the replication slot name shown in the error message
SELECT
  slot_name,
  active_pid
FROM pg_replication_slots
WHERE slot_name = '<slot_name>' AND active = true;

-- Terminate the connection (replace ### with the actual PID)
SELECT pg_terminate_backend(###);

After terminating the connection, Materialize should be able to reconnect to the replication slot.

Prevention

Best practices to avoid this error:

  • Avoid frequent replica changes: Minimize adding or removing replicas from source clusters during active replication. Plan cluster sizing changes during maintenance windows when possible.
  • Ensure stable network connectivity: Maintain reliable network connections between Materialize and PostgreSQL to prevent connection interruptions.
  • Monitor source health: Regularly check source status to detect and address issues early:
SELECT *
FROM mz_internal.mz_source_statuses;
  • Configure connection timeouts appropriately: Ensure PostgreSQL connection timeout settings allow for proper cleanup of disconnected sessions.
  • Use unique replication slots: Verify that each Materialize source uses a unique replication slot and avoid reusing slot names across different sources or environments.
  • Plan for graceful restarts: When performing maintenance that requires source restarts, allow sufficient time for PostgreSQL to release connections before restarting.
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