Is asynchronous jdbc call possible?
It's impossible to make an asynchronous call to the database via JDBC, but you can make asynchronous calls to JDBC with Actors (e.g., actor makes calls to the DB via JDBC, and sends messages to the third parties, when the calls are over), or, if you like CPS, with pipelined futures (promises) (a good implementation is Scalaz Promises)
I don't consider that using a pool of threads is a solution because it doesn't scale, in the case of heavy concurrent requests this will spawn a very large number of threads.
Scala actors by default are event-based (not thread-based) - continuation scheduling allows creating millions of actors on a standard JVM setup.
If you're targeting Java, Akka Framework is an Actor model implementation that has a good API both for Java and Scala.
Aside from that, the synchronous nature of JDBC makes perfect sense to me. The cost of a database session is far higher than the cost of the Java thread being blocked (either in the fore- or background) and waiting for a response. If your queries run for so long that the capabilities of an executor service (or wrapping Actor/fork-join/promise concurrency frameworks) are not enough for you (and you're consuming too many threads) you should first of all think about your database load. Normally the response from a database comes back very fast, and an executor service backed with a fixed thread pool is a good enough solution. If you have too many long-running queries, you should consider upfront (pre-)processing - like nightly recalculation of the data or something like that.
I don't understand how any of the proposed approaches that wrap JDBC calls in Actors, executors or anything else can help here - can someone clarify.
Surely the basic problem is that the JDBC operations block on socket IO. When it does this it blocks the Thread its running on - end of story. Whatever wrapping framework you choose to use its going to end up with one thread being kept busy/blocked per concurrent request.
If the underlying database drivers (MySql?) offers a means to intercept the socket creation (see SocketFactory) then I imagine it would be possible to build an async event driven database layer on top of the JDBC api but we'd have to encapsulate the whole JDBC behind an event driven facade, and that facade wouldn't look like JDBC (after it would be event driven). The database processing would happen async on a different thread to the caller, and you'd have to work out how to build a transaction manager that doesn't rely on thread affinity.
Something like the approach I mention would allow even a single background thread to process a load of concurrent JDBC exec's. In practice you'd probably run a pool of threads to make use of multiple cores.
(Of course I'm not commenting on the logic of the original question just the responses that imply that concurrency in a scenario with blocking socket IO is possible without the user of a selector pattern - simpler just to work out your typical JDBC concurrency and put in a connection pool of the right size).
Looks like MySql probably does something along the lines I'm suggesting --- http://code.google.com/p/async-mysql-connector/wiki/UsageExample