Re: Streaming replication failover/failback

On Nov 18, 2016, at 5:48 AM, Jehan-Guillaume de Rorthais <ioguix(at)free(dot)fr> wrote:
>
> On Thu, 17 Nov 2016 08:26:59 -0900
> Israel Brewster <israel(at)ravnalaska(dot)net <mailto:israel(at)ravnalaska(dot)net>> wrote:
>
>>> On Nov 16, 2016, at 4:24 PM, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
>>> wrote:
>>>
>>> On 11/16/2016 04:51 PM, Israel Brewster wrote:
>>>> I've been playing around with streaming replication, and discovered that
>>>> the following series of steps *appears* to work without complaint:
>>>>
>>>> - Start with master on server A, slave on server B, replicating via
>>>> streaming replication with replication slots.
>>>> - Shut down master on A
>>>> - Promote slave on B to master
>>>> - Create recovery.conf on A pointing to B
>>>> - Start (as slave) on A, streaming from B
>>>>
>>>> After those steps, A comes up as a streaming replica of B, and works as
>>>> expected. In my testing I can go back and forth between the two servers
>>>> all day using the above steps.
>>>>
>>>> My understanding from my initial research, however, is that this
>>>> shouldn't be possible - I should need to perform a new basebackup from B
>>>> to A after promoting B to master before I can restart A as a slave. Is
>>>> the observed behavior then just a "lucky fluke" that I shouldn't rely
>>>
>>> You don't say how active the database is, but I going to say it is not
>>> active enough for the WAL files on B to go out for scope for A in the time
>>> it takes you to do the switch over.
>>
>> Yeah, not very - this was just in testing, so essentially no activity. So
>> between your response and the one from Jehan-Guillaume de Rorthais, what I'm
>> hearing is that my information about the basebackup being needed was
>> obsoleted with the patch he linked to, and as long as I do a clean shutdown
>> of the master, and don't do too much activity on the *new* master before
>> bringing the old master up as a slave (such that WAL files are lost)
>
> Just set up wal archiving to avoid this (and have PITR backup as a side effect).

Good point. Streaming replication may not *need* WAL archiving to work, but having it can provide other benefits than just replication. I'll have to look more into the PITR backup though - that's something that sounds great to have, but I have no clue, beyond the concept, how it works. :-)

-----------------------------------------------
Israel Brewster
Systems Analyst II
Ravn Alaska
5245 Airport Industrial Rd
Fairbanks, AK 99709
(907) 450-7293
-----------------------------------------------

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