Re: Connection terminated by the server causes deadlock in jdbc client side connection

Leonardo,

At this point I'm wondering if we get an I/O error if it is feasible to attempt to close it. I am leaning towards no.

What is above this in the stack trace. Do you see any information about the IOException error that caused it

Dave Cramer

dave.cramer(at)credativ(dot)ca
http://www.credativ.ca

On 7 October 2015 at 14:43, Leonardo Frittelli <leonardo.frittelli@gmail.com> wrote:

Dave,

Thank you very much for your prompt response.

I am using version postgresql-9.3-1101, but I have checked the latest available 9.4 sources and it appears to have the same code (the code example I pasted was from v2, but I see similar code in v3).

Could it be that in this particular scenario we should use ProtocolConnectionImpl.abort() instead of close()?

Thanks,

Leonardo

On 7 October 2015 at 15:23, Dave Cramer <pg@fastcrypt.com> wrote:

It could be a problem. What version are you using ?

Dave Cramer

dave.cramer(at)credativ(dot)ca
http://www.credativ.ca

On 7 October 2015 at 12:33, Leonardo Frittelli <leonardo.frittelli@gmail.com> wrote:

Hi,

We are experiencing very frequent deadlocks in pgsql jdbc connections. The scenario is a replicated database with hot_standby = on

At times of high volumes of queries in both the primary and the replicated server, we sometimes get the following log indicating that the server has terminated the connection in the replicated database.

FATAL:  terminating connection due to conflict with recovery

DETAIL:  User query might have needed to see row versions that must be removed.

HINT:  In a moment you should be able to reconnect to the database and repeat your command.

This is expected and we see no issue with that. What I did not expect, however, is that in the JDBC client side, the connection is deadlocked.

java.lang.Thread.State: RUNNABLE

at java.net.SocketOutputStream.socketWrite0(Native Method)

at java.net.SocketOutputStream.socketWrite(Unknown Source)

at java.net.SocketOutputStream.write(Unknown Source)

at java.io.BufferedOutputStream.flushBuffer(Unknown Source)

at java.io.BufferedOutputStream.flush(Unknown Source)

- locked <0x0000000700742e30> (a java.io.BufferedOutputStream)

at org.postgresql.core.PGStream.flush(PGStream.java:518)

at org.postgresql.core.v3.ProtocolConnectionImpl.close(ProtocolConnectionImpl.java:136)

at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:280)

- locked <0x0000000700742fd0> (a org.postgresql.core.v3.QueryExecutorImpl)

at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:547)

at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:417)

at org.postgresql.jdbc2.AbstractJdbc2Statement.executeQuery(AbstractJdbc2Statement.java:302)

...

Looking at the Postgres JDBC code, I notice that ProtocolConnectionImpl.close() (invoked by the exception handler in QueryExecutorImpl.execute) is trying to 'gracefully close' by sending an 'X' to the server before actually closing the socket.

...

if (logger.logDebug()) logger.debug(" FE=> Terminate"); pgStream.SendChar('X'); pgStream.flush(); pgStream.close(); ...

Does this make sense in a scenario of a connection which has already been terminated by the server side? 

At times of high load, all connections in the pool get eventually locked with exactly the same stack trace. 

I'd appreciate any advice on how to handle this. Could this be a bug in the JDBC driver?

Thanks,

Leonardo