[ome-users] pgpool-II with OMERO?

[Harri Jäälinoja]

Hi,

On 01/05/12 19:53, Roger Leigh wrote:
>>
>> http://www.postgresql.org/docs/9.1/static/hot-standby.html
>
> I think this is probably the best solution if the primary concern is
> high availability. Being built into the core, it's also the most
> performant, and also the most simple to configure. However, if you want
> to be running (read only) queries on the hot standby, there is of course
> the chance that the data will be outdated compared with the master due
> to there being a delay shipping over the WAL. And you can only do
> writes on the master.

this sounds good. Of course there are trade-offs, so we have to discuss 
first a bit. Also with the IT department, it seems that in the standard 
RedHat repositories we have, PostgreSQL 8.4 is the latest available.

>
> A solution such as pgpool-II would give you the ability to do both read
> and write on either node, due to the two-phase commit. However, there
> is a performance cost, and additional complexity in the replication.
> Unless you definitely need to do load balancing of writes in addition to
> having the high availability of multiple replicas, this is probably both
> more complex and slower.

 From the point of view of OMERO, is there any reason we would need 
this? Or put differently, which OMERO features will fail in case the 
user is connected to the hot-standby database that cannot perform 
certain actions (as listed in the Hot Standby documentation)?

If it is only data import that will fail, I think this is acceptable. 
Because that is something that is easy to repeat after the master db is 
back.

The hot standby documentation also says:
"Users will be able to tell whether their session is read-only by 
issuing SHOW transaction_read_only."
Can OMERO check if this is the case, and give an approroate error 
message to the user?

Thanks, regards,
Harri

-- 
__________________________________________________
Harri Jäälinoja
Light Microscopy Unit
Institute of Biotechnology, University of Helsinki
http://www.biocenter.helsinki.fi/bi/lmu/
+358 9 191 59370 fax +358 9 191 59366