Re: logical decoding and replication of sequences

On 8/29/22 12:21, Thomas Munro wrote:
> On Mon, Aug 8, 2022 at 8:56 PM Kyotaro Horiguchi
> <horikyota.ntt@gmail.com> wrote:
>> At Mon, 08 Aug 2022 17:33:22 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in
>>> If WaitForWALToBecomeAvailable returned by promotion, ReadPageInteral
>>> misses the chance to inavlidate reader-state.  That state is not an
>>> error while in StandbyMode.
>>
>> Mmm... Maybe I wanted to say:  (Still I'm not sure the rewrite works..)
>>
>> If WaitForWALToBecomeAvailable returned by promotion, ReadPageInteral
>> would miss the chance to invalidate reader-state.  When XLogPageRead
>> is called in blocking mode while in StandbyMode (that is, the
>> traditional condition) , the function continues retrying until it
>> succeeds, or returns XLRAD_FAIL if promote is triggered.  In other
>> words, it was not supposed to return non-failure while the header
>> validation is failing while in standby mode.  But while in nonblocking
>> mode, the function can return non-failure with lastSourceFailed =
>> true, which seems wrong.
> 
> New ideas:
> 
> 0001:  Instead of figuring out when to invalidate the cache, let's
> just invalidate it before every read attempt.  It is only marked valid
> after success (ie state->readLen > 0).  No need to worry about error
> cases.
> 

Maybe I misunderstand how all this works, but won't this have a really
bad performance impact. If not, why do we need the cache at all?

regards

-- 
Tomas Vondra
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