A possible implementation for the idea expressed in the previous post -
https://github.com/stuartsierra/component/pull/25
On Wednesday, March 18, 2015 at 2:41:46 PM UTC+1, platon...@gmail.com wrote:
>
> I've also been investigating the nested system approach/problem.
>
> The primary use case that I have is composing subsystems which are mostly
> independent modules using a higher order system to run in one process. The
> modules themselves can be easily extracted into separate applications thus
> becoming their own top level systems which makes it desirable to keep their
> system maps intact.
>
> Components inside modules might depend on the *whole* modules, not their
> constituent parts. This allows to have modules call each other through the
> API's in-process as well as being easily replaced by remote endpoints when
> separated into multiple processes.
>
> This mostly works except for the components depending on other
> modules/systems, e.g.:
>
> (require '[com.stuartsierra.component :as cmp])
>
> (defrecord X [x started]
> cmp/Lifecycle
> (start [this] (if started (println "Already started " x) (println
> "Starting " x " " this)) (assoc this :started true))
> (stop [this] (println "Stopping " x " " this) this))
>
> (def sys1 (cmp/system-map :x (cmp/using (X. "depends on dep" nil) [:dep])))
> (def sys2 (cmp/system-map :y (cmp/using (X. "depends on sys1" nil)
> [:sys1])))
> (def hsys (cmp/system-map :sys1 (cmp/using sys1 [:dep]), :sys2 (cmp/using
> sys2 [:sys1]) :dep (X. "dependency" nil)))
>
> (cmp/start hsys)
>
> Starting dependency #user.X{:x dependency, :started nil}
> Already started dependency
> Starting depends on dep #user.X{:x depends on dep, :started nil, :dep
> #user.X{:x dependency, :started true}}
>
> clojure.lang.ExceptionInfo: Error in component :sys2 in system
> com.stuartsierra.component.SystemMap calling
> #'com.stuartsierra.component/start
> clojure.lang.ExceptionInfo: Missing dependency :dep of
> clojure.lang.Keyword expected in system at :dep
>
> This happens because of the following:
> 1. Dependency :*dep* of *sys1* is started in *hsys*
> 2. *sys1* is started (:*dep* is started again, so the start/stop should
> be idempotent)
> 3. Dependency :*sys1* of *sys2* is started in *hsys*
> 4. :*sys1* cannot be started as it depends on :*dep* which isn't present
> in *sys2*
>
> This scenario could be supported by the Component library in several ways:
>
> 1. introduce an IdempotentLifecycle protocol which will be respected by
> the Component library. Implement the protocol for the SystemMap.
> IdempotentLifecycles will not be started or stopped for the second time,
> also their dependencies will not be updated if they are already started.
> 2. do not fail if a component already has a dependency under the specified
> key. This is a hack compared to the first solution, but I might go with it
> in the short term.
>
> Stuart, what do you think about that? Would you consider a PR implementing
> the first proposal?
>
> On Wednesday, March 18, 2015 at 10:18:36 AM UTC+1, Stuart Sierra wrote:
>>
>>
>> On Tue, Mar 17, 2015 at 5:47 PM, James Gatannah <james.g...@gmail.com>
>> wrote:
>>
>>> FWIW, we've been using something that smells an awful lot like nested
>>> systems for months now. I never realized we weren't supposed to.
>>>
>>
>>
>> It's not that nested systems *never* work, but from what I've seen they
>> cause more complications than they're worth. The 'component' model doesn't
>> forbid it, but it does not support dependencies between components in
>> different "subsystems."
>>
>> I've found it easier to keep system maps "flat" and use namespaced
>> keywords to distinguish "subsystem" groups, even in large systems with 30+
>> components.
>>
>> –S
>>
>>
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