Re: libpq changes for synchronous replication

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: libpq changes for synchronous replication
Date: 2010-11-27 00:49:10
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Excerpts from Fujii Masao's message of jue nov 25 10:47:12 -0300 2010:

> The attached patch s/CopyXLog/CopyBoth/g and adds the description
> about CopyBoth into the COPY section.

I gave this a look. It seems good, but I'm not sure about this bit:

+ case 'W': /* Start Copy Both */
+ /*
+ * We don't need to use getCopyStart here since CopyBothResponse
+ * specifies neither the copy format nor the number of columns in
+ * the Copy data. They should be always zero.
+ */
+ conn->result = PQmakeEmptyPGresult(conn, PGRES_COPY_OUT);
+ if (!conn->result)
+ return;
+ conn->asyncStatus = PGASYNC_COPY_BOTH;
+ conn->copy_already_done = 0;
+ break;

I guess this was OK when this was conceived as CopyXlog, but since it's
now a generic mechanism, this seems a bit unwise. Should this be
reconsidered so that it's possible to change the format or number of

(The paragraph added to the docs is also a bit too specific about this
being used exclusively in streaming replication, ISTM)

> While modifying the code, it occurred to me that we might have to add new
> ExecStatusType like PGRES_COPY_BOTH and use that for CopyBoth mode,
> for the sake of consistency. But since it's just alias of PGRES_COPY_BOTH
> for now, i.e., there is no specific behavior for that ExecStatusType, I don't
> think that it's worth adding that yet.

I'm not so sure about this. If we think that it's worth adding a new
possible state, we should do so now; we will not be able to change this
behavior later.

Álvaro Herrera <alvherre(at)commandprompt(dot)com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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