Event-Driven Flows

The concept of an event-driven flow is probably best explained by an example a lot of C and C++ programmers are familiar with: signal handlers.

signal(SIGINT, interrupt_handler);

SIGINT represents a signal called "interrupt". The program does not know if and when it will be risen. The program will not waste time waiting on the signal. Instead, it will define a callback (or a handler): a piece of code that needs to be run in the event of rising this signal. It will then register an association between the event (signal "interrupt") and the callback. The callback is not called right now. in fact, it may never be called. But we are guaranteed by the system that, should the corresponding event occur, the associated callback will be called. After the successful registration, the program can proceed to doing other things.

To achieve its performance goals, Capy employs a similar model. When a user-written server needs to obtain data from the network to process the request, the moment of this data arriving is an asynchronous event: the program does not know if and when it will occur. Rather than actively waiting, the server will register a request in the framework. The request is more-less something like this:

  • I provide the buffer where the data is expected to be filled.

  • I provide a callback to be called on the filled buffer.

  • I request that the framework awaits for the data and when it arrives, that it fills the buffer and calls the provided callback on the buffer.

Once the framework acknowledges the registration, the server never bothers with this anticipated read again: it moves on to doing other things. Any further data processing logic is encoded in the registered callback. This callback may interpret the data as a JSON request, call other servers, return response to the client, and likely do these things by registering other callbacks.

This means that the code which registered the request will never learn if the requested read succeeded or not. Any errors can only be recognized and responded to in the registered callback.

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