On Mon, Oct 15, 2012 at 2:33 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> Sequences, as defined by SQL Standard, provide a series of unique
> values. The current implementation on PostgreSQL isolates the
> generation mechanism to only a single node, as is common on many
> RDBMS.
>
> For sharded or replicated systems it forces people to various hackish
> mechanisms in user space for emulating a global or cluster-wide
> sequence.
>
> The solution to this problem is an in-core solution that allows
> coordination between nodes to guarantee unique values.
>
> There are a few options
> 1) Manual separation of the value space, so that N1 has 50% of
> possible values and N2 has 50%. That has problems when we reconfigure
> the cluster, and requires complex manual reallocation of values. So it
> starts good but ends badly.
> 2) Automatic separation of the value space. This could mimic the
> manual operation, so it does everything for you - but thats just
> making a bad idea automatic
> 3) Lazy allocation from the value space. When a node is close to
> running out of values, it requests a new allocation and coordinates
> with all nodes to confirm the new allocation is good.
While useful to some people, it seems like a way to avoid a crazy
amount of complexity whereas most people are fine just using
uncoordinated 128-bit integers. There are some who want temporal
locality and smaller datums, but couldn't the problem be minimized
somewhat by presuming non-coordinated identifier generation?
It seems like a proper subset of what you propose, so perhaps that's a
nice way to bisect the problem. I agree with you that an in-database
way to do this -- even if in principle it could be done by clients
most of the time -- would lend a lot more cohesion to the system.
FWIW, I like "3" otherwise -- much like the foibles of most
partitioning schemes and some of the stated design considerations of
segment exclusion, I think absolute rigidity at all times makes it
really hard to gradually move things into a desired alignment
incrementally while the system is online.
--
fdr
