Excerpts from Joachim Wieland's message of jue dic 30 09:31:47 -0300 2010:
> 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.
Why does it require a serializable transaction? You could simply
register the snapshot in any transaction. (Of course, the net effect
would be pretty similar to a serializable transaction).
> 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
> memory.
>
> 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.
So one registered snapshot per transaction? Sounds a reasonable
limitation (I doubt there's a use case for more than that, anyway).
--
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
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