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Snapshot synchronization, again...

From: Joachim Wieland <joe(at)mcknight(dot)de>
To: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Snapshot synchronization, again...
Date: 2010-12-30 12:31:47
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
The snapshot synchronization discussion from the parallel pg_dump
patch somehow ended without a clear way to go forward.

Let me sum up what has been brought up and propose a short- and
longterm solution.


Passing snapshot sync information can be done either:

a) by returning complete snapshot information from the backend to the
client so that the client can pass it along to a different backend
b) or by returning only a unique identifier to the client and storing
the actual snapshot data somewhere on the server side

Advantage of a: no memory is used in the backend and no memory needs
to get cleaned up, it is also theoretically possible that we could
forward that data to a hot standby server and do e.g. a dump partially
on the master server and partially on the hot standby server or among
several hot standby servers.
Disadvantage of a: The snapshot must be validated to make sure that
its information is still current, it might be difficult to cover all
cases of this validation. A client can not only access exactly a
published snapshot, but just about any snapshot that fits and passes
the validation checks (this is more a disadvantage than an advantage
because it allows to see a database state that never existed in

Advantage of b: No validation necessary, as soon as the transaction
that publishes the snapshot loses that snapshot, it will also revoke
the snapshot information (either by removing a temp file or deleting
it from shared memory)
Disadvantage of b: It doesn't allow a snapshot to be installed on a
different server. It requires a serializable open transaction to hold
the snapshot.

What I am proposing now is the following:

We return snapshot information as a chunk of data to the client. At
the same time however, we set a checksum in shared memory to protect
against modification of the snapshot. A publishing backend can revoke
its snapshot by deleting the checksum and a backend that is asked to
install a snapshot can verify that the snapshot is correct and current
by calculating the checksum and comparing it with the one in shared

This only costs us a few bytes for the checksum * max_connection in
shared memory and apart from resetting the checksum it does not have
cleanup and verification issues. Note that we are also free to change
the internal format of the chunk of data we return whenever we like,
so we are free to enhance this feature in the future, transparently to
the client.




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