Re: Causal reads take II

From: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
To: Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>
Cc: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Causal reads take II
Date: 2017-01-19 22:11:58
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On Fri, Jan 20, 2017 at 3:01 AM, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee> wrote:
> Yes there is a race even with all transactions having the same
> synchronization level. But nobody will mind if we some day fix that
> race. :)

We really should fix that!

> With different synchronization levels it is much trickier to
> fix as either async commits must wait behind sync commits before
> becoming visible, return without becoming visible or visibility order
> must differ from commit record LSN order. The first makes the async
> commit feature useless, second seems a no-go for semantic reasons,
> third requires extra information sent to standby's so they know the
> actual commit order.

Thought experiment:

1. Log commit and make visible atomically (so DO and REDO agree on
visibility order).
2. Introduce flag 'not yet visible' to commit record for sync rep commits.
3. Introduce a new log record 'make all invisible commits up to LSN X
visible', which is inserted when enough sync rep standbys reply. Log
this + make visible on primary atomically (again, so DO and REDO agree
on visibility order).
4. Teach GetSnapshotData to deal with this using <insert magic here>.

Now standby and primary agree on visibility order of async and sync
transactions, and no standby will allow you to see transactions that
the primary doesn't yet consider to be durable (ie flushed on a quorum
of standbys etc). But... sync rep has to flush xlog twice on primary,
and standby has to wait to make things visible, and remote_apply would
either need to be changed or supplemented with a new level
remote_apply_and_visible, and it's not obvious how to actually do
atomic visibility + logging (I heard ProcArrayLock is kinda hot...).
Hmm. Doesn't sound too popular...

>>> In CSN based snapshot
>>> discussions we came to the conclusion that to make standby visibility
>>> order match master while still allowing for async transactions to
>>> become visible before they are durable we need to make the commit
>>> sequence a vector clock and transmit extra visibility ordering
>>> information to standby's. Having one more level of delay between wal
>>> logging of commit and making it visible would make the problem even
>>> worse.
>> I'd like to read that... could you please point me at the right bit of
>> that discussion?
> Some of that discussion was face to face at, some of it is
> here:
> Let me know if you have any questions.

Thanks! That may take me some time...

Thomas Munro

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