Re: Walsender process patch v1 for Synch Rep

Hi, Simon

Thank you for the review.

On Fri, Nov 7, 2008 at 5:49 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>
> On Wed, 2008-11-05 at 23:17 +0900, Fujii Masao wrote:
>
>> Authentication
>> -----------------------
>> As pointed out at another thread, for authentication, I defined the database
>> only for replication (named "walsender" tentatively). walsender database is
>> not pseudo but created by initdb like postgres database, because the user
>> can re-create it easily even if it is lost accidentally.
>>
>> If the startup packet specifies walsender database, a backend declares
>> postmaster working as walsender. Then, the backend is removed from
>> BackendList and managed as background process by postmaster.
>>
>> Replication message
>> ---------------------------------
>> I defined new message type 'R', which means the start of replication. If the
>> message is received, walsender will perform the main routine. Of course,
>> a backend who is not walsender cannot perform the routine.
>
> I don't understand why you've done it this way. Can you explain? This
> stuff about a walsender database sounds like it has a purpose, but I'm
> not sure what it is.
>
> The route I would have taken would be to add walsender and walreceiver
> as new auxiliary processes. They would start via AuxiliaryProcessMain()
> in bootstrap/bootstrap.c. ISTM this would be slightly less code as well
> and not too much change from what you have now. After a quick look, most
> of the rest of the patch looks correct.
>
> I would hope that walsender and walreceiver would start like that.
> walsender would start at same time as walwriter. walreceiver can start
> earlier, for later discussion.

Yeah, I also add walsender as new auxiliary process at first. But,
during coding,
I made out that this is more complicated for code and user.

First problem : Which process accepts the connection from standby?
IMO, *postmaster* should accept it like normal database access. Since we
can use the existing connection establishment logic, the change of the code
is smaller and it's easier to use. So, I added walsender as a special backend
which is forked when standby connects to postmaster. Is there any advantage
which walsender or other processes accept the connection from standby?

Second problem : What should walsender do after the termination of the
connection from standby? should die?, or remain alive and wait for next
connection? IMO, we should handle it like normal database access, i.e.
exit walsender. This and adding walsender as an auxiliary process seldom
meet, I think.

Does that answer you? Am I missing something?

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center