Re: Unexpected data when subscribing to logical replication slot

I would not find this behavior surprising in particular if you have a synchronous replica. According to the documentation of synchronous_commit:

   The local behavior of all non-off modes is to wait for local flush of WAL to disk.

This is when the logical decoder sees the item. But that does not mean the change is visible to other transactions in the MVCC sense. So, if wal2json and the rest of your stuff is fast enough, then the enrichment query may very well read old data.

A transaction being committed means all the WAL has been written (and usually synced) to disk including the bit in the pg_xact directory.

On Thu, May 9, 2024 at 9:32 AM Daniel McKenzie <daniel.mckenzie@curvedental.com> wrote:

Asynchronous commit introduces the risk of data loss. There is a short
time window between the report of transaction completion to the client
and the time that the transaction is truly committed.

The documentation speaks about synchronous_commit changing how transactions change behaviour for the client. So in this case, my psql terminal is the client, and I would expect a faster commit (from its perspective) and then a period of risk (as a process usually done as part of the commit is now being done in the background) but it's not clear how that affects a replication slot subscriber.

What we're struggling to understand is: why are we seeing any updates in the replication slot before they have been "truly committed"?

There appears to be a state of limbo between updating data and that data being available to query (and our subscriber is picking up changes in this period of time) but I can't pin down any documentation which describes it.

We've had this running in live now for years without a hiccup so we are surprised to learn that we have this massive race condition and it just so happens that the hardware is fast enough to process the transaction before the .NET application can react to replication slot changes.

Daniel McKenzie

Software Developer

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On Wed, May 8, 2024 at 5:28 PM Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 5/8/24 08:24, Daniel McKenzie wrote:
> It's running both (in docker containers) and also quite a few more
> docker containers running various .NET applications.

I think what you found is that the r7a.medium instance is not capable
enough to do all that it is asked without introducing lag under load.
Answering the questions posed by Tomas Vondra would help get to the
actual cause of the lag.

In meantime my suspicion is this part:

"For example, when I use a psql terminal to update a user's last name
from "Jones" to "Smith" then I would expect the enrichment query to find
"Smith" but it will sometimes still find "Jones". It finds the old data
perhaps 1 in 50 times."

If this is being run against the Postgres server my guess is that
synchronous_commit=on is causing the commit on the server to wait for
the WAL records to be flushed to disk and this is not happening in a
timely manner in the '... 1 in 50 times' you mention. In that case you
see the old values not the new committed values. This seems to be
confirmed when you set synchronous_commit=off and don't see old values.
For completeness per:

https://www.postgresql.org/docs/current/wal-async-commit.html

"However, for short transactions this delay is a major component of the
total transaction time. Selecting asynchronous commit mode means that
the server returns success as soon as the transaction is logically
completed, before the WAL records it generated have actually made their
way to disk. This can provide a significant boost in throughput for
small transactions.

Asynchronous commit introduces the risk of data loss. There is a short
time window between the report of transaction completion to the client
and the time that the transaction is truly committed (that is, it is
guaranteed not to be lost if the server crashes).  ...
"

>
> Daniel McKenzie
> Software Developer

--
Adrian Klaver
adrian.klaver@aklaver.com