On Wed, Nov 16, 2016 at 9:00 PM, Tsunakawa, Takayuki
<tsunakawa.ta...@jp.fujitsu.com> wrote:
> Do we really want to enable libpq failover against pre-V10 servers?  I don't 
> think so, as libpq is a part of PostgreSQL and libpq failover is a new 
> feature in PostgreSQL 10.  At least, as one user, I don't want PostgreSQL to 
> sacrifice another round trip to establish a connection.  As a developer, I 
> don't want libpq code more complex than necessary (the proposed patch adds a 
> new state to the connection state machine.)  And I think it's natural for the 
> server to return the server attribute (primary/standby, writable, etc.) as a 
> response to the Startup message like server_version, 
> standard_conforming_strings and server_encoding.

Well, generally speaking, a new feature that works against older
server is better than one that doesn't.  Of course, if that entails
making other compromises then you have to decide whether it's worth
it, but SELECT pg_is_in_recovery() and SHOW transaction_read_only
exist in older versions so if we pick either of those methods then it
will just work.  If we decide to invent some completely new method of
distinguishing masters from standbys, then it might not, but that
would be a drawback of such a choice, not a benefit.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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