DBD::Pg: one ping to rule them all
Greg Sabino Mullane
January 7, 2015
How can you tell if your database connection is still valid? One way, when using Perl, is to use the ping() method. Besides backslash-escaped placeholders, a revamped ping() method is the major change in the recently released version 3.5.0 of DBD::Pg, the Perl/DBI interface to Postgres. Before 3.5.0, there was a chance of false positives when using this method. In particular, if you were inside of a transaction, DBD::Pg did not actually attempt to contact the Postgres backend. This was definitely an oversight, and DBD::Pg now does the right thing.
Detecting a dead backend is a little trickier than it sounds. While libpq stores some state information for us, the only way to be sure is to issue a command to the backend. Additionally, we check the value of PQstatus in case libpq has detected a problem. Realistically, it would be far better if the Postgres protocol supported some sort of ping itself, just a simple answer/response without doing anything, but there is nothing like that yet. Fortunately, the command that is issued, /* DBD::Pg ping test, v3.5.0 */, is very lightweight.
One small side effect is that the ping() method (and its stronger cousin, the pg_ping() method) will both cancel any COPY that happens to be in progress. Really, you should not be doing that anyway! :) Calling the next copy command, either pg_getline() or pg_putline(), will tell you if the connection is valid anyway. Since the copy system uses a completely different backend path, this side effect is unavoidable.
Even this small change may cause some problems for applications, which relied on the previous false positive behavior. Leaving as a basic no-op, however, was not a good idea, so check if your application is using ping() sanely. For most applications, simple exception handling will negate to use ping() in the first place.