Add mini FAQ on subscriptions
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@ -92,15 +92,14 @@ How did we ever live without it? It is a quite beautiful solution to one half of
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data conveyance needs, **but** we're cautious about taking it too far - as far as, say, cycle.js.
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It doesn't take over everything in re-frame - it just does part of the job.
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__Finally__, a long time ago in a galaxy far far away I programmed briefly in Eiffel where I learned
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about [command-query separation](https://en.wikipedia.org/wiki/Command%E2%80%93query_separation).
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Each generation of
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programmers seems destined to rediscover this principle, with CQRS as the latest iteration [see resources here](http://www.baeldung.com/cqrs-event-sourced-architecture-resources).
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And yet, even today, we still see read/write `cursors` and two-way data binding being promoted as a good thing.
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Please, just say no. As your programs get bigger, the use of these two-way constructs
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__Finally__, a long time ago in a galaxy far far away, I was lucky enough to program in Eiffel
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where I was exposed to the idea of [command-query separation](https://en.wikipedia.org/wiki/Command%E2%80%93query_separation).
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The modern rendering of this idea is CQRS ([see resources here](http://www.baeldung.com/cqrs-event-sourced-architecture-resources)).
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But, even today, we still see read/write `cursors` and two-way data binding being promoted as a good thing.
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Please, just say no. We already know where that goes. As your programs get bigger, the use of these two-way constructs
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will encourage control logic into all the
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wrong places and you'll end up with a tire-fire of an Architecture. IMO.
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wrong places and you'll end up with a tire-fire of an Architecture. <br>
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Sincerely, The Self-appointed President of the Cursor Skeptic's Society.
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## It does Event Sourcing
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@ -157,6 +157,57 @@ though they involve the same event id, `:some-event`, the query vectors do not t
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This feature shakes out nicely because re-frame has a data oriented design.
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### A Final FAQ
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The following issues comes up a bit.
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You will likely end up with a bunch of level 1 `reg-sub` which look the same (they directly extract a path within `app-db`):
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```clj
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(reg-sub
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:a
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(fn [db _]
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(:a db)))
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```
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```clj
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(reg-sub
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:b
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(fn [db _]
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(-> db :top :b)))
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```
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Lot's of them the same. Same pattern over and over.
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Now, you are a person who thinks abstractly, and that repetition will feel uncomfortable. It will
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call to you like a Siren: "refaaaaactoooor meeeee".
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So here's my tip: you will have to actively resist the urge to "refactor" out this common pattern.
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The repetition is fine. It is serving a purpose. It is deliberate. Take a deep breath.
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The very WORST thing you can do is to flex your magnificent abstraction muscles and create something like this:
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```clj
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(reg-sub
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:extract-any-path
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(fn [db path]
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(get-in db path))
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```
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Genius!, you think to yourself. Now I only need one direct `reg-sub` and I supply a path to it.
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A read-only cursor of sorts. Look at the code I can delete.
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Neat and minimal it might be, but genius it isn't. IMO. You are now asking the code USING the subscription
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to provide the path.
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The view which subscribes using `(subscribe [:extract-any-path [:a]])` now "knows" about the
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structure of `app-db`.
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What happens when you restructure `app-db` slightly and put that `:a` path under
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another high level branch of `app-db`? You will have to run around all the views,
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looking for the paths supplied, knowing which to alter and which to leave alone. Fragile.
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No! We want our views to declarative ask for data, but they should have
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no idea where it comes from.
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***
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Previous: [Infographic](SubscriptionInfographic.md)
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