RE: Libpq support to connect to standby server as priority

From: Haribabu Kommi [mailto:kommi.haribabu@gmail.com]
> IMO, if we try to use only pg_is_in_recovery() only to decide to connect,
> we may not
> support all the target_session_attrs that are possible. how about using
> both to decide?

I favor adding a new parameter like target_server_type whose purpose is to select the server role.  That aligns better
withthe PgJDBC's naming, which conveys consistency to PostgreSQL users.  Again, the original desire should have been to
connectto a standby to eliminate the burdon on the primary, where the primary may be read-only by default and only a
limitedgroup of users are allowed to write to the database.
 

I don't know what kind of realistic use cases there are that request read-only session in the logical replication
configuration. I think we can just leave target_session_attrs as what it is now in PostgreSQL 11, for compatibility and
possiblyfuture new use cases.
 


> Master/read-write -- recovery = false and default_transaction_read_only
> = false
> Standby/read-only -- recovery = true
> prefer-standby/prefer-read -- recovery = true or
> default_transaction_read_only = true
> any -- Nothing to be verified
> 
> 
> I feel above verifications can cover for both physical and logical
> replication.

As for prefer-standby/prefer-read, if host parameter specifies host1,host2 in this order, and host1 is the primary with
default_transaction_read_only=true,does the app get a connection to host1?  I want to connect to host2 (standby) only
ifhost1 is down.
 


Regards
Takayuki Tsunakawa