Обсуждение: JDBC Postgres Failover with quorum based synchronous replication

I guess the distinction between master and secondary logic can be looked up here(isMaster)

https://github.com/pgjdbc/pgjdbc/blob/f78a639d1ed3c64e80e1fa107691b4af5945cb84/pgjdbc/src/main/java/org/postgresql/core/v3/ConnectionFactoryImpl.java
and from within slaves iirc we did check the code and it is random shuffle and does not support any custom logic to
pickany slaves. 
I might be wrong but I do remember looking it up in the source :)

Regards,
Vijay

________________________________________
From: Debraj Manna <subharaj.manna@gmail.com>
Sent: Sunday, June 17, 2018 11:31:32 AM
To: pgsql-admin
Subject: [External] JDBC Postgres Failover with quorum based synchronous replication

Hi

Let's say I am using quorum based synchronous
replication<https://wiki.postgresql.org/wiki/New_in_postgres_10#Quorum_Commit_for_Synchronous_Replication>either ANY or
FIRSTin synchronous_standby_names 

I am trying to read from slave as discussed here<https://jdbc.postgresql.org/documentation/head/connect.html>.

jdbc:postgresql://node1,node2,node3/accounting?targetServerType=preferSlave&loadBalanceHosts=true

Can someone let me know

  1.  How does a slave is selected? Is a slave randomly selected or a slave is preferred which has zero lag or the
minimumlag? 
  2.  Is there any setting in jdbc url which will only select the slave with zero replication and if no such slave
existsthen read will be sent to master ? 

Thanks,

No I guess there is no such option.
Maybe you raise a PR if you can :)

what you can do at infra level is put your databases behind a Load balancer (like haproxy or nginx) and have a health
checkbased on lag etc and only then Route the connections to that standby node else take it off the load balancer. 



Regards,
Vijay

________________________________________
From: Debraj Manna <subharaj.manna@gmail.com>
Sent: Sunday, June 17, 2018 2:42:46 PM
To: Vijaykumar Jain
Cc: pgsql-admin
Subject: Re: [External] JDBC Postgres Failover with quorum based synchronous replication

Yes Vijaykumar you are right it is just a random shuffle while picking a slave.


https://github.com/pgjdbc/pgjdbc/blob/f78a639d1ed3c64e80e1fa107691b4af5945cb84/pgjdbc/src/main/java/org/postgresql/hostchooser/MultiHostChooser.java#L51

Is there a way I can specify in the connection parameters to pick a slave with zero lag else fall back to master?

On Sun, Jun 17, 2018 at 2:11 PM, Vijaykumar Jain <vjain@opentable.com<mailto:vjain@opentable.com>> wrote:
I guess the distinction between master and secondary logic can be looked up here(isMaster)

https://github.com/pgjdbc/pgjdbc/blob/f78a639d1ed3c64e80e1fa107691b4af5945cb84/pgjdbc/src/main/java/org/postgresql/core/v3/ConnectionFactoryImpl.java
and from within slaves iirc we did check the code and it is random shuffle and does not support any custom logic to
pickany slaves. 
I might be wrong but I do remember looking it up in the source :)

Regards,
Vijay

________________________________________
From: Debraj Manna <subharaj.manna@gmail.com<mailto:subharaj.manna@gmail.com>>
Sent: Sunday, June 17, 2018 11:31:32 AM
To: pgsql-admin
Subject: [External] JDBC Postgres Failover with quorum based synchronous replication

Hi

Let's say I am using quorum based synchronous
replication<https://wiki.postgresql.org/wiki/New_in_postgres_10#Quorum_Commit_for_Synchronous_Replication>either ANY or
FIRSTin synchronous_standby_names 

I am trying to read from slave as discussed here<https://jdbc.postgresql.org/documentation/head/connect.html>.

jdbc:postgresql://node1,node2,node3/accounting?targetServerType=preferSlave&loadBalanceHosts=true

Can someone let me know

  1.  How does a slave is selected? Is a slave randomly selected or a slave is preferred which has zero lag or the
minimumlag? 
  2.  Is there any setting in jdbc url which will only select the slave with zero replication and if no such slave
existsthen read will be sent to master ? 

Thanks,