Caveat: my approach may have been all wrong. It's the first time I tried
stuartsierra components. Frankly I'd have been happier with some CLOS
objects and before and after methods, I'm still getting the hang of this
clojure stuff.
On Fri, Sep 4, 2015 at 7:57 AM, Dave Tenny <dave.te...@gmail.com> wrote:
> I'm using components to encapsulate Langohr/RabbitMQ interactions in cases
> where Langohr auto-recovery wasn't happy with what was going on.
>
> I make sure the Lifecycle methods are idempotent, built the component
> stack in the correct order.
>
> To make sure that I can deal with the exceptions that require restarting
> the component stack, I have some macros that wrap up exception handling,
> retries, and component stack restarts.
>
> So
>
> (with-rmq-publisher! [channel component-system]
> ... do my stuff ...)
>
> The body of the macro ("... do my stuff ...") is turned into a function
> and and will be re-executed if the system is restarted, so you have to
> beware
> of side effects or other things you may not want executed multiple times.
> Not a problem for me, all I'm doing is entering the macro long enough
> to get a channel and publish to it, or similar types of things.
>
> The component-system is a wrap of the system-map call with some other
> data, in an atom, that will be mutated
> if we do things like dynamically add subscribers to the system or call
> (restart! component-system).
> There are some thread safety/locking concerns, since a connection failure
> is likely to be seen by callers
> on multiple threads using the components, and I try to avoid race
> conditions on restarts (only one thread will do the restart until it
> succeeds,
> the unblock the others).
>
> Hope this helps with strategy, even if the code is omitted.
>
> The trick to me was not in restarting the component stack, but in managing
> the shared state across threads safely and making sure (with-stack-activity
> [system] <code>)
> code was prepared to re-execute on failures with new connections or other
> stack components.
>
> In my case I also needed to add components to the stack dynamically. Not
> often, but dynamcially, not at (system-map) call time. That required some
> lock consideration,
> and I'd have to call Lifecycle/start on the stack every time I added a
> component. They methods have to be idempotent.
>
>
>
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