---
title: 'Monitoring replication'
---

Monitoring replication lag is important and there are 3 ways to do this:

1. Dashboard - Under the [Reports](/docs/guides/platform/reports) of the dashboard, you can view the replication lag of your project
2. Database -
   - pg_stat_subscription (subscriber) - if PID is null, then the subscription is not active
   - pg_stat_subscription_stats - look here for error_count to see if there were issues applying or syncing (if yes, check the logs for why)
   - pg_replication_slots - use this to check if the slot is active and you can also calculate the lag from here
3. [Metrics](/docs/guides/telemetry/metrics) - Using the prometheus endpoint for your project
   - replication_slots_max_lag_bytes - this is the more important one
   - pg_stat_replication_replay_lag - lag to replay WAL files from the source DB on the target DB (throttled by disk or high activity)
   - pg_stat_replication_send_lag - lag in sending WAL files from the source DB (a high lag means that the publisher is not being asked to send new WAL files OR a network issues)

## Primary

### Replication status and lag

The `pg_stat_replication` table shows the status of any replicas connected to the primary database.

```sql
select pid, application_name, state, sent_lsn, write_lsn, flush_lsn, replay_lsn, sync_state
from pg_stat_replication;
```

### Replication slot status

A replication slot can be in one of three states:

- `active` - The slot is active and is receiving data
- `inactive` - The slot is not active and is not receiving data
- `lost` - The slot is lost and is not receiving data

The state can be checked using the `pg_replication_slots` table:

```sql
select slot_name, active, state from pg_replication_slots;
```

### WAL size

The WAL size can be checked using the `pg_ls_waldir()` function:

```sql
select * from pg_ls_waldir();
```

### Check LSN

```sql
select pg_current_wal_lsn();
```

## Subscriber

### Subscription status

The `pg_subscription` table shows the status of any subscriptions on a replica and the `pg_subscription_rel` table shows the status of each table within a subscription.

The `srsubstate` column in `pg_subscription_rel` can be one of the following:

- `i` - Initializing - The subscription is being initialized
- `d` - Data Synchronizing - The subscription is synchronizing data for the first time (i.e. doing the initial copy)
- `s` - Synchronized - The subscription is synchronized
- `r` - Replicating - The subscription is replicating data

```sql
SELECT
    sub.subname AS subscription_name,
    relid::regclass AS table_name,
    srel.srsubstate AS replication_state,
    CASE srel.srsubstate
        WHEN 'i' THEN 'Initializing'
        WHEN 'd' THEN 'Data Synchronizing'
        WHEN 's' THEN 'Synchronized'
        WHEN 'r' THEN 'Replicating'
        ELSE 'Unknown'
    END AS state_description,
    srel.srsyncedlsn AS last_synced_lsn
FROM
    pg_subscription sub
JOIN
    pg_subscription_rel srel ON sub.oid = srel.srsubid
ORDER BY
    table_name;
```

### Check LSN

```sql
select pg_last_wal_replay_lsn();
```