Re: Patch: Implement failover on libpq connect level.

From: pgsql-hackers-owner@postgresql.org
> [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Mithun Cy
> If you are suggesting me to change in protocol messages, I think that would
> not be backward compatible to older version servers. I also think such level
> of protocol changes will not be allowed. with connection status
> CONNECTION_SETENV used for protocol version 2.0 setup, we sent some query
> like "select pg_catalog.pg_client_encoding()" for same. So I think using
> "SELECT pg_is_in_recovery()" should be fine.

No, there's no concern about compatibility.  Please look at this:

https://www.postgresql.org/docs/devel/static/protocol-flow.html#PROTOCOL-ASYNC

[Excerpt]
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ParameterStatus messages will be generated whenever the active value changes for any of the parameters the backend believes the frontend should know about. Most commonly this occurs in response to a SET SQL command executed by the frontend, and this case is effectively synchronous — but it is also possible for parameter status changes to occur because the administrator changed a configuration file and then sent the SIGHUP signal to the server. Also, if a SET command is rolled back, an appropriate ParameterStatus message will be generated to report the current effective value.

At present there is a hard-wired set of parameters for which ParameterStatus will be generated: they are server_version, server_encoding, client_encoding, application_name, is_superuser, session_authorization, DateStyle, IntervalStyle, TimeZone, integer_datetimes, and standard_conforming_strings. (server_encoding, TimeZone, and integer_datetimes were not reported by releases before 8.0; standard_conforming_strings was not reported by releases before 8.1; IntervalStyle was not reported by releases before 8.4; application_name was not reported by releases before 9.0.) Note that server_version, server_encoding and integer_datetimes are pseudo-parameters that cannot change after startup. This set might change in the future, or even become configurable. Accordingly, a frontend should simply ignore ParameterStatus for parameters that it does not understand or care about.
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Regards
Takayuki Tsunakawa