Horton

Re: [PATCH 10/16] Introduce the concept that wal has a 'origin' node

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>

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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>, Andres Freund <andres@2ndquadrant.com>, pgsql-hackers@postgresql.org, Robert Haas <robertmhaas@gmail.com>, Daniel Farina <daniel@heroku.com>, Tom Lane <tgl@sss.pgh.pa.us>
Date: 2012-06-20T08:23:31Z
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 →

  1. Don't waste the last segment of each 4GB logical log file.

  2. Stamp HEAD as 9.3devel.

  3. Wake WALSender to reduce data loss at failover for async commit.

  4. Make the visibility map crash-safe.

On 20.06.2012 11:17, Simon Riggs wrote:
> On 20 June 2012 15:45, Heikki Linnakangas
> <heikki.linnakangas@enterprisedb.com>  wrote:
>> On 20.06.2012 10:32, Simon Riggs wrote:
>>>
>>> On 20 June 2012 14:40, Heikki Linnakangas
>>>>
>>>> And I'm worried
>>>> it might not even be enough in more complicated scenarios.
>>>
>>> It is not the only required conflict mechanism, and has never been
>>> claimed to be so. It is simply one piece of information needed, at
>>> various times.
>>
>> So, if the origin id is not sufficient for some conflict resolution
>> mechanisms, what extra information do you need for those, and where do you
>> put it?
>
> As explained elsewhere, wal_level = logical (or similar) would be used
> to provide any additional logical information required.
>
> Update and Delete WAL records already need to be different in that
> mode, so additional info would be placed there, if there were any.
>
> In the case of reflexive updates you raised, a typical response in
> other DBMS would be to represent the query
>    UPDATE SET counter = counter + 1
> by sending just the "+1" part, not the current value of counter, as
> would be the case with the non-reflexive update
>    UPDATE SET counter = 1
>
> Handling such things in Postgres would require some subtlety, which
> would not be resolved in first release but is pretty certain not to
> require any changes to the WAL record header as a way of resolving it.
> Having already thought about it, I'd estimate that is a very long
> discussion and not relevant to the OT, but if you wish to have it
> here, I won't stop you.

Yeah, I'd like to hear briefly how you would handle that without any 
further changes to the WAL record header.

>>> Additional information required by logical information will be handled
>>> by a new wal_level.
>>>
>>> The discussion here is about adding origin_node_id *only*, which needs
>>> to be added on each WAL record.
>>
>> If that's all we can discuss here is, and all other options are off the
>> table, then I'll have to just outright object to this patch. Let's implement
>> what we can without the origin id, and revisit this later.
>
> As explained, we can do nothing without the origin id. It is not
> optional or avoidable in the way you've described.

It's only needed for multi-master replication, where the same table can 
be updated from multiple nodes. Just leave that out for now. There's 
plenty of functionality and issues left even without that.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com