Re: [Postgres-xc-general] Do you think the new feature is meaningful? - Read/Write Separation

[Mason S. <ms...@tr...>]

On Sun, May 25, 2014 at 2:04 PM, Koichi Suzuki <koi...@gm...>
wrote:

> I see.  Your have good usecase for read-only transactions.
>
> Because of the nature of log shipping and sharing/clustering, it is
> not simple to provide read-only transaction in XC.


There may be cases where it is being used as a data warehouse (thinking a
bit more about Postgres-XL here), where there is a nightly load process and
then reads are done during the day.  If the standbys are dedicated, being
able to load balance them could be useful.



>  Two essential
> reasons:
>
> 1. Delay in WAL playback in each slave may be different.   It makes
> providing consistent database view extremely difficult.
>


> 2. At present, slave calculates snapshot of the transaction from the
> WAL.   Current code does not allow missing XIDs.   There will be
> memory leak and crash by OOM if there's many missing XIDs in the WAL
> stream.  In XC, it is disabled and the database view may be
> inconsistent.   Please note that this does not affect recovery and
> promotion.
>
>
>
Agreed, there may be some level of effort for this.

-- 
Mason Sharp

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