27min

How to Fix Replication Slot Performance or Errors?

What (Question)?

How can we fix any PostgreSQL performance problems (or errors) from Logical Replication slots?

Where?

Any PostgreSQL source connector where the logical replication change data capture method is used. This means also where a replication slot object in the PostgreSQL database has been created.

How (Answer)?

PostgreSQL like any other database can have constraints that control its behavior. One of which in this scenario is that the PostgreSQL database uses several settings to control the behavior of the logical replication process. This includes but is not limited to the configurations associated with the Write Ahead Log (WAL). In addition, older versions versus newer versions of PostgreSQL can handle a solution differently. Because there are many possible scenarios and several approaches for resolving these types of issues, the below recommendations are provided as general approaches to handling the topic. When logical replication slot issues or performance is concerned the following terms are often associated with the same problem:

  • Stuck
  • Slow
  • Sizing

Recommendations

A replication slot may have performance issues, setting issues, or configuration issues. These recommended troubleshooting steps are not in any particular order, and they should all be reviewed:

  1. Check that the slot exists
    • Run the basic query from your favorite PostgreSQL IDE. We like PgAdmin here at DLH.io so we'll reference that interface when we mention running queries, etc.
      • SELECT COUNT(1) FROM pg_replication_slots WHERE slot_name = '<replication_slot>';
      • Change the name of the placeholder, <replication_slot> to your replication slot name. The default we use if one is not entered is datalakehouseio_replication_slot the result is 0 the replication you thought existed does not exist.
  2. Check Configuration Settings
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  11. In DLH.io run a Historical Re-Sync on this table to complete the operation.

NB: these responses are in no way a substitute for a qualified PostgreSQL Database Administrator (DBA). We advise that if your team is not comfortable making changes, especially in a production environment, that they contact our support team for options to assist with configuring your database.

Who?

Your PostgreSQL database administrator (DBA) is best suited for the job.

Use your support credits with us by contacting your engagement manager or using the support portal.

When?

It may be necessary to use the answers and instructions above:

  • Anytime you begin noticing that some data is not synchronizing with your target/destination on a timely basis.
  • Anytime the synchronization of data from your PostgreSQL database tables noticeably takes longer than average
  • When alerts regarding timeout are visible or you've been notified to address a possible issue with your source connection database
  • When checking the replication slot on your PostgreSQL server, the slot seems stuck and dodoes not return any response from the WAL
  • Your replication slot size has reached a disproportionate level compared to the size of your database, for example it is in multiple GBs or TBs
  • The size of the replication slot borders the max_wal_size configuration limit



Updated 09 Jul 2022
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