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Re: Snapshot synchronization, again...

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Joachim Wieland <joe(at)mcknight(dot)de>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Snapshot synchronization, again...
Date: 2011-02-22 12:24:22
Message-ID: AANLkTimS0wGSbAj7CjwBt2UM_+zsGg5ApjcQRCc+z3_O@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Feb 22, 2011 at 1:59 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> Really? The idea of the hash is to prevent you from importing arbitrary
> snapshots into the system, allowing only copying snapshots that are in use
> by another backend. The purpose of the hash is to make sure the snapshot the
> client passes in is identical to one used by another backend.

If that's the purpose of the hash, it is utterly useless.  The
computation performed by the backend can easily be replicated by an
adversary.

> If you don't use a cryptographically secure hash, it's easy to construct a
> snapshot with the same hash as an existing snapshot, with more or less
> arbitrary contents.

And if you did use a cryptographically secure hash, it would still be
easy, unless there is some plausible way of keeping the key a secret,
which I doubt.

The original reason why we went with this design (send the snapshot to
the client and then have the client send it back to the server) was
because some people thought it could be useful to take a snapshot from
the master and recreate it on a standby, or perhaps the other way
around.  If that's the goal, checking whether the snapshot being
imported is one that's already in use is missing the point.  We have
to rely on our ability to detect, when importing the snapshot, any
anomalies that could lead to system instability; and allow people to
import an arbitrary snapshot that meets those constraints, even one
that the user just created out of whole cloth.

Now, as I said before, I think that's a bad idea.  I think we should
NOT be designing the system to allow publication of arbitrary
snapshots; instead, the backend that wishes to export its snapshot
should so request, getting back a token (like "1") that other backends
can then pass to START TRANSACTION (SNAPSHOT '1'), or whatever syntax
we end up using.  In that design, there's no need to validate
snapshots because they never leave the server's memory space, which
IMHO is as it should be.  If someone can develop a convincing argument
that transmitting snapshots between the master and standby is a good
idea, we can still accommodate that: the master-standby connection is
now full-duplex.  In fact, we may want to do that anyway for other
reasons (see Kevin Grittner's musings on this point vs. true
serializability).

Another reason I don't like this approach is because it's possible
that the representation of snapshots might change in the future.  Tom
mused a while back about using an LSN as a snapshot (snapshots sees
all transactions committed prior to that LSN) and there are other
plausible changes, too.  If we expose the internal details of the
snapshot mechanism to users, then (1) it becomes a user-visible
feature with backward compatibility implications if we choose to break
it down the road and (2) all future snapshot representations must be
able to be fully sanity checked on import.  Right now it's fairly easy
to verify that the snapshot's xmin isn't too old and that the
remaining XIDs are in a reasonable range and that's probably all we
really need to be certain of, but it's not clear that some future
representation would be equally easy to validate.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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