A subscription is a way of telling an EffectManager, "Hey, let me know if anything
interesting happens over there!" So if you want to listen for messages on a web
socket, you would tell an EffectManager to create a subscription. If you want to get clock
ticks, you would tell an EffectManager to subscribe to that. The cool thing here is that
this means an EffectManager manages all the details of subscriptions instead of you.
So if a web socket goes down, you do not need to manually reconnect with an
exponential backoff strategy, the EffectManager does this all for you behind the scenes!
Every Sub specifies (1) which effects you need access to and (2) the type of
messages that will come back into your application.
If you are using a fractal approach where a Sub can come from a child's subscription function, you need to map the Sub in order for it to produce an action that can be routed back to the child.
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Note: TEA has managed effects, meaning that things like HTTP requests or writing to disk are all treated as data in TEA. When this data is given to the TEA runtime system, it can do some “query optimization” before actually performing the effect. Perhaps unexpectedly, this managed effects idea is the heart of why TEA is so nice for testing, reuse, reproducibility, etc.
TEA has two kinds of managed effects: commands and subscriptions.