Re: Snapshot synchronization, again...

On 30.12.2010 16:49, Florian Pflug wrote:
> On Dec30, 2010, at 13:31 , Joachim Wieland wrote:
>> 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.
>
> We'd still have to stream these checksums to the standbys though,
> or would they be exempt from the checksum checks?
>
> I still wonder whether these checks are worth the complexity. I
> believe we'd only allow snapshot modifications for read-only queries
> anyway, so what point is there in preventing clients from setting
> broken snapshots?

Hmm, our definition of "read-only" is a bit fuzzy. While a transaction
doesn't modify the database itself, it could still send NOTIFYs or call
a PL function to do all sorts of things outside the database. Imagine
that you're paranoid about data integrity, and have a security definer
function that runs cross checks on the data. If it finds any
anomalities, it wakes up the operator or forces shutdown or similar.

Now a malicious user could set a snapshot that passes the basic validity
checks, ie. xmin >= GlobalXmin, but contains a combination of still
in-progress that never existed in reality. If he then calls the
paranoia-function, it would see an inconsistent state of committed
tuples and get upset.

Maybe that's a bit far-stretched, but it's not entirely clear that
running with an inconsistent snapshot is harmless.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com