heikki.linnakangas@enterprisedb.com (Heikki Linnakangas) writes:
> On 24/05/10 19:51, Kevin Grittner wrote:
>> The only thing I'm confused about is what benefit anyone expects to
>> get from looking at data between commits in some way other than our
>> current snapshot mechanism. Can someone explain a use case where
>> what Jan is proposing is better than snapshot isolation? It doesn't
>> provide any additional integrity guarantees that I can see.
>
> Right, it doesn't. What it provides is a way to reconstruct a snapshot
> at any point in time, after the fact. For example, after transactions
> A, C, D and B have committed in that order, it allows you to
> reconstruct a snapshot just like you would've gotten immediately after
> the commit of A, C, D and B respectively. That's useful replication
> tools like Slony that needs to commit the changes of those
> transactions in the slave in the same order as they were committed in
> the master.
>
> I don't know enough of Slony et al. to understand why that'd be better
> than the current heartbeat mechanism they use, taking a snapshot every
> few seconds, batching commits.
I see two advantages:
a) Identifying things on a transaction-by-transaction basis means that the snapshots ("syncs") don't need to be
captured,which is presently an area of fragility. If the slon daemon falls over on Friday evening, and nobody
noticesuntil Monday, the "snapshot" reverts to being all updates between Friday and whenever SYNCs start to be
collectedagain.
Exposing commit orders eliminates that fragility. SYNCs don't need to be captured anymore, so they can't be
missed(which is today's problem).
b) The sequence currently used to control log application ordering is a bottleneck, as it is a single sequence shared
acrossall connections.
It could be eliminated in favor of (perhaps) an in-memory variable defined on a per-connection basis.
It's not a bottleneck that we hear a lot of complaints about, but the sequence certainly is a bottleneck.
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