Make sure that Postgres always get a consistent snapshot #929
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@Samira-El This PR is ready for review, I believe. |
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This change ensures that the LSN at which the replication slot is first consistent is availiable to the caller of the method.
There is a replication lag between a primary and replica database. We get information about what data the replication slot can provide from the primary but perform the initial data snapshot on the replica. Therefore, we must make sure that the replica has caught up to at least the LSN which we are going to begin streaming from the replication slot.
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Problem
It is currently possible for PipelineWise to select an LSN to stream from which will result in missing data once the initial snapshot has been conducted.
This is due to the current mechanism of just selecting either the most recent LSN on the primary or the most recent LSN which has been replayed on the replica: https://github.com/thread/pipelinewise/blob/cfd2c2091eb60a5b71ea38486594ccfb91a4fe1c/pipelinewise/fastsync/commons/tap_postgres.py#L297-L322
This is not a safe thing to do as it does not guarantee a consistent view of the database.
Fixes: #930
Proposed changes
Inspired by how Debezium (another CDC tool written in Java) deals with initial snapshots of Postgres databases we will take advantage of two behaviours:
confirmed_flush_lsnto this value). We will stream from that point forward but use the current state of the database as the snapshot.confirmed_flush_lsnto be the point at which we will start streaming.In both of these cases, we may have data which will also be streamed to us by the replication slot but this should not be an issue as we are relying on an 'at least once' delivery mechanism already.
Types of changes
What types of changes does your code introduce to PipelineWise?
Put an
xin the boxes that applyChecklist
setup.pyis an individual PR and not mixed with feature or bugfix PRs[AP-NNNN](if applicable. AP-NNNN = JIRA ID)AP-NNN(if applicable. AP-NNN = JIRA ID)