Re: Implement waiting for wal lsn replay: reloaded

Yura Sokolov <y.sokolov@postgrespro.ru>

From: Yura Sokolov <y.sokolov@postgrespro.ru>

To: pgsql-hackers@lists.postgresql.org

Date: 2025-03-12T14:44:28Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Clean up 019_replslot_limit.pl comments
- 0b866bb90368 19 (unreleased) landed
Stabilize 019_replslot_limit.pl: wait on slot restart_lsn
- cdb1d1cf1de6 19 (unreleased) landed
Use WAIT FOR LSN in PostgreSQL::Test::Cluster::wait_for_catchup()
- 7e8aeb9e483d 19 (unreleased) landed
- f30848cb05d4 19 (unreleased) landed
Add tab completion for the WAIT FOR LSN MODE option
- 76948337f724 19 (unreleased) landed
Add the MODE option to the WAIT FOR LSN command
- 49a181b5d634 19 (unreleased) landed
Extend xlogwait infrastructure with write and flush wait types
- 7a39f43d885b 19 (unreleased) landed
Unify error messages
- 502e256f2262 19 (unreleased) cited
Optimize shared memory usage for WaitLSNProcInfo
- 75e82b2f5a6f 19 (unreleased) landed
Fix WaitLSNWakeup() fast-path check for InvalidXLogRecPtr
- ede6acef4967 19 (unreleased) landed
Fix incorrect function name in comments
- 23792d738158 19 (unreleased) landed
Add infrastructure for efficient LSN waiting
- 3b4e53a075ea 19 (unreleased) landed
Add pairingheap_initialize() for shared memory usage
- 8af3ae0d4b36 19 (unreleased) landed
Implement WAIT FOR command
- 447aae13b030 19 (unreleased) landed

Attachments

v6-0001-Implement-WAIT-FOR-command.patch (text/x-patch) patch v6-0001

10.03.2025 14:30, Alexander Korotkov пишет:
> On Fri, Feb 28, 2025 at 3:55 PM Yura Sokolov <y.sokolov@postgrespro.ru> wrote:
>> 28.02.2025 16:03, Yura Sokolov пишет:
>>> 17.02.2025 00:27, Alexander Korotkov wrote:
>>>> On Thu, Feb 6, 2025 at 10:31 AM Yura Sokolov <y.sokolov@postgrespro.ru> wrote:
>>>>> I briefly looked into patch and have couple of minor remarks:
>>>>>
>>>>> 1. I don't like `palloc` in the `WaitLSNWakeup`. I believe it wont issue
>>>>> problems, but still don't like it. I'd prefer to see local fixed array, say
>>>>> of 16 elements, and loop around remaining function body acting in batch of
>>>>> 16 wakeups. Doubtfully there will be more than 16 waiting clients often,
>>>>> and even then it wont be much heavier than fetching all at once.
>>>>
>>>> OK, I've refactored this to use static array of 16 size.  palloc() is
>>>> used only if we don't fit static array.
>>>
>>> I've rebased patch and:
>>> - fixed compiler warning in wait.c ("maybe uninitialized 'result'").
>>> - made a loop without call to palloc in WaitLSNWakeup. It is with "goto" to
>>> keep indentation, perhaps `do {} while` would be better?
>>
>> And fixed:
>>    'WAIT' is marked as BARE_LABEL in kwlist.h, but it is missing from
>> gram.y's bare_label_keyword rule
> 
> Thank you, Yura.  I've further revised the patch.  Mostly added the
> documentation including SQL command reference and few paragraphs in
> the high availability chapter explaining the read-your-writes
> consistency concept.

Good day, Alexander.

Looking "for the last time" to the patch I found there remains
`pg_wal_replay_wait` function in documentation and one comment.
So I fixed it in documentation, and removed sentence from comment.

Otherwise v6 is just rebased v5.

-------
regards
Yura Sokolov aka funny-falcon