Tuesday, October 27, 2009

generic callback in a concurrent java/c++ sys

(Note OO is a development-time concept. Once compiled, an OO program looks no different from a C program with tons of structs and function pointers.)

Most financial systems today are concurrent OO systems. The lowly callback pattern is extremely common but actually requires a thorough understanding of both thread and object interaction. Let's examine some. Note just about every player in the callback scenarios is a pointer, including function pointers.

In the simplest 1-thread case, an observer pointee registers interest with a subject object, by giving its own address. After subject state changes in Thr 1, the same thread loops through the list of observers and invokes the callback method on each observer. Note the observer object must not be deallocated.

For onMsg() in tibrv or onMessage() in JMS, there's a remote "upstream" thread in the message broker's PID/runtime, and there's a separate downstream thread in the listener's PID. I believe this thread starts out blocking for messages. Upon receiving a msg, the runtime somehow wakes up this blocking thread and this thread invokes onMsg() on the listener pointee. Meanwhile what's the remote upstream thread doing? Simplest design is synchronous -- upstream thread waits for the listener thread to finish. This means the upstream thread can only dispatch the message to remote listeners sequentially. I feel it's much better to get the upstream thread to return right away after sending the one-way-message[3] (see Doug Lea's book) so this thread can contact the 2nd listener in the list. Rewind to registration time -- Any thread could register the listener with "Hollywood". This thread could terminate immediately but the listener pointee must live.

[3] in classic Reo, we actually send one-way messages to the market-facing gateway. We get the status message on a separate thread via onMessage().

In C/C++, you can also register into "Hollywood" a ptr to a free func. No object, usually no state (unless the free func uses stateful static objects). Free functions are free of destruction. Hollywood could save the ptr in any collection. When there's an event, Hollywood calls us back, in any thread.

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