Postgres Source replication slot safe_wal_size only reset when a change occurs

  • Is this your first time deploying Airbyte?: Yes
  • OS Version / Instance: Oracle Linux 8 Ec2 Instance
  • Memory / Disk: 2Gb / 100GB
  • Deployment: Docker
  • Airbyte Version: 0.40.18
  • Source name/version: Postgres
  • Destination name/version: S3
  • Step: The issue is happening during sync
  • Description:

When Airbyte syncs and successfully pulls and records a change, the safe_wal_size metric in postgresql’s replication slot is reset, and any extra disk space used for recording the wal is free’d, however, When no change in the data source is made the sync completes but the safe_wal_size metric continues to decrease.
Should a sync that pulls no changes still reset the wal as it has been read?
This could cause a disk usage issue where the replication slot is not reset unless data has changed at the source.

Postgres version 13
Parameter group wal options;

  • max_slot_wal_keep_size 1024
  • max_wal_size 250
  • wal_keep_size 4096

Here are the logs for the sync with data which successfully resets the wal replication slot
airbyte-sync-data.txt (4.3 MB)

Here are the logs for the sync with no data which does not reset the wal replication slot
airbyte-sync-nodata.txt (4.2 MB)

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I’ve had this issue with postgres source connectors versions;
1.0.22 and 1.0.25

From poking around release notes, I’m going to try upgrading airbyte version from 0.40.18 to the latest 0.40.22, as the following fix in release 0.40.19 may resolve the issue;

Investigate usage of heartbeat property in debezium for CDC · Issue #15040 · airbytehq/airbyte · GitHub

Upgrading did not resolve the issue.

Hello Billy, it’s been a while without an update from us. Are you still having problems or did you find a solution?

Hey, This is still a problem.

We’ve now got our production Airbyte instance running and have created a few connections with production databases.

Our replication slots are configured via RDS parameter group with the following parameters.

  • max_slot_wal_keep_size 2048 (MB) Sets the maximum WAL size that can be reserved by replication slots.
  • max_wal_senders 20 Sets the maximum number of simultaneously running WAL sender processes.
  • max_wal_size 2048 (MB) Sets the WAL size that triggers a checkpoint.
  • min_wal_size 192 (MB) Sets the minimum size to shrink the WAL to.
  • wal_compression 1 Compresses full-page writes written in WAL file.
  • wal_keep_size 2048 (MB) Sets the size of WAL files held for standby servers.

We are only syncing 9 tables of data via the publication which are not updated overnight.
Airbyte is syncing once an hour.
Airbyte appears to be only resetting a replication slot if there are changes to the data, which means overnight the replication slot will eventually fill and then become lost, causing us to have to reset the replication slot and then perform the initial sync of all data again.

We’re at the point where creating a table which we update every 30 minutes and adding this into the publication should hopefully cause airbyte to clear the replication slot as a workaround.

Along with increasing the RDS parameter max_slot_wal_keep_size from 2048 to 3072 I’ve implemented a table into the publication called airbyteworkaround for each database which gets updated once an hour.

This ensures that there is at least 1 data change in every hourly sync and the replication slot is now being cleared.