Re: PostgreSQL HA config recommendations

Alex,
Note that you should be weary of suggestions to make your replication
synchronous. Synchronous replication is rarely used for this kind of use
case (Cisco Jabber) where the most complete durability of the standby is
not of the utmost concern (as it would be in a banking application). Not
only will it decrease performance, but since you expect to have only one
local standby it could actually decrease your availability because if your
standby went down no transactions would be able to commit on the master.
See the Synchronous Replication section of the docs for more details (
http://www.postgresql.org/docs/devel/static/warm-standby.html)

Also note that the suggestion provided by Fabio that you should not have
your application commit more than one transaction per user operation is
only applicable in synchronous replication (though since this is for a Cisco
Jabber, where you neither have control over nor much information regarding
the number of commits sent by the transaction per user operation, that
suggestion is not applicable anyway...). In the case of asynchronous
master-slave replication the typical issue with streaming replication
latency is that you have transactions going to the master and then the
application sends a read only transaction to the slave before the slave
receives the transaction. So long as you don't have the application
consider the user operation completed before all the transactions are
committed I don't think having multiple transactions would make your
replication latency issue any less.

For example, if you had a calendar application where a user enters event
details and creates an event for the calendar. The application may be set
up to execute 2 transactions, 1) Add the event and details to the calendar
events table and 2) once the event creation transaction returns add the
current user as an attendee for that event. In this case both transactions
would be going against the master, so how far the slave is behind wouldn't
be a factor. Of course it would be faster overall to send the inserts as a
single database procedure, but that all goes against the master database so
the streaming replication is not a factor in that consideration.

*William J. Dunn*
*willjdunn.com <http://willjdunn.com>*

*William J. Dunn*
*P* 978-844-4427 | *dunnwjr(at)gmail(dot)com <dunnwjr(at)gmail(dot)com>*
*dunnw(at)bu(dot)edu <dunnw(at)bu(dot)edu>*

On Thu, Apr 30, 2015 at 9:02 AM, Fabio Ugo Venchiarutti <fabio(at)vuole(dot)me>
wrote:

> > WAN delays can cause problems for any replication system; you just have
> > to be aware of that and not push things too hard (or try and violate the
> > laws of physics). For example, streaming replication set to be
> > synchronous crossing the planet is something you'd probably be rather
> > unhappy with. :)
>
>
> In my experience streaming replication fits most use cases due to inherent
> its simplicity and robustness, but you might need to adjust your software
> design to get the best out of it.
>
>
> More specifically, latency issues can be heavily mitigated by having
> application software commit no more than one transaction per user
> operation, provided 1 x "master<->sync_slave round trip time" is acceptable
> delay when they submit forms or the like.
>
> It can get much worse if the application server is on a different
> geographical node than the DB master. In such case it is realistically
> beneficial to batch multiple write operations in a single STATEMENT instead.
> If the replication synchronous slave is on yet another node, the best case
> (single statement) scenario would be 2 x round trip time. This
> configuration is more common than you might think as some setups feature
> remote app servers reading off synchronous slaves at their own physical
> location but committing against a master that is somewhere else.
>
>
> Cheers
>
>
>
>
>
>
>
> On 30/04/15 11:06, Jim Nasby wrote:
>
>> On 4/29/15 1:13 PM, Alex Gregory wrote:
>>
>>> I was thinking that I could use Slony but then I read that it does not
>>> like WAN replication. I have also read about streaming replication
>>> native to Postgres but was not sure how that would work over the WAN.
>>> Bucardo seems better for Data Warehousing or multimaster situations
>>> which this is not. That leaves pgpool ii which seems like it would
>>> add an extra layer of complexity.
>>>
>>
>> WAN delays can cause problems for any replication system; you just have
>> to be aware of that and not push things too hard (or try and violate the
>> laws of physics). For example, streaming replication set to be
>> synchronous crossing the planet is something you'd probably be rather
>> unhappy with. :)
>>
>> I haven't played with Slony in forever, but when I did it loved to lock
>> things. That would not play well with high latency.
>>
>> I have run londiste between sites within the same city, and that worked
>> well.
>>
>> Bucardo and pg_pool are both based on the idea of replaying SQL
>> statements instead of replicating actual data. They have their uses, but
>> I personally distrust that idea, especially for DR.
>>
>> When it comes down to to there are so many choices I am not sure if I
>>> need one or a combination of two. Any help you could provide could
>>> be greatly appreciated.
>>>
>>
>> If you want to replicate within a data center then streaming replication
>> is pretty nice, and as a bonus you might be able to do synchronous as
>> well. The downside to streaming rep is that it's binary, so if you ever
>> suffer data corruption you're practically guaranteed that corruption
>> will end up on the replica. Logical replication like londiste or Slony
>> are much more robust against that. You also can't use temporary tables
>> with streaming rep, and you have to replicate the details of ALL
>> activity, including maintenance like VACUUM. In some environments that
>> might be slower than logical replication.
>>
>
>
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