Re: The plan for FDW-based sharding

On Fri, Feb 26, 2016 at 2:19 PM, Konstantin Knizhnik
<k(dot)knizhnik(at)postgrespro(dot)ru> wrote:

> pg_tsdtm is based on another approach: it is using system time
> as CSN

Which brings up an interesting point, if we want logical
replication to be free of serialization anomalies for those using
serializable transactions, we need to support applying transactions
in an order which may not be the same as commit order -- CSN (as
such) would be the wrong thing. If serializable transaction 1 (T1)
modifies a row and concurrent serializable transaction 2 (T2) reads
the old version of the row, and modifies something based on that,
T2 must be applied to a logical replica first even if T1 commits
before it; otherwise the logical replica could see a state not
consistent with business rules and which could not have been seen
(due to SSI) on the source database. Any DTM API which does not
support some mechanism to rearrange the order of transactions from
commit order to some other order (based on, for example, read-write
dependencies) is not complete. If it does support that, it gives
us a way forward for presenting consistent data on logical
replicas.

To avoid confusion, it might be best to reserve CSN for actual
commit sequence numbers, or at least values which increase
monotonically with each commit. The term of art for what I
described above is "apparent order of execution", so maybe we want
to use AOE or AOoE for the order we choose to use in a particular
implementation. It doesn't seem to me to be outright inaccurate
for cases where the system time on the various systems is used.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company