This document briefly explains why re-frame-trace gives you an option to ignore unchanged layer 2 subscriptions.

Background

The re-frame docs make a distinction between layer 2 and layer 3 subscriptions:

• layer 2 subscriptions extract data directly from app-db and should be trivial in nature. There should be no computation in them beyond what is necessary to extract a value from app-db
• layer 3 subscriptions take values from layer 2 nodes as inputs, and compute a materialised view of those values. Just to repeat: they never directly extract values from app-db. They create new values where necessary, and because of it they to do more serious CPU work. So we never want to run a layer 3 subscriptions unless it is necessary.

This structure delivers efficiency. You see, all (currently instantiated) layer 2 subscriptions will run every time app-db changes in any way. All of them. Every time. And app-db changes on almost every event, so we want them to be computationally trivial.

If the value of a layer 2 subscription tests = to its previous value, then the further propagation of values through the signal graph will be pruned. The more computationally intensive layer 3 subscriptions, and ultimately the views, will only recompute if and when there has been a change in their data inputs.

We don't want your app recomputing views only to find that nothing has changed. Inefficient.

Back To Tracing

Because layer 2 subs run on every single modification of app-db, and because very often nothing has changed, their trace can be a bit noisy. Yes, it happened, but it just isn't that interesting.

So re-frame-trace gives you the option of filtering out trace for the layer 2 subscriptions where the value "this time" is the same as the value "last time".

On the other hand, if a layer 2 subscription runs and its value is different to last time, that's potentially fascinating and you'll want to be told all about it. :-)

Why do I sometimes see "Layer ?" when viewing a subscription?

To determine whether a subscription is a layer 2 or layer 3, re-frame-trace looks at the input signals to a subscription. If one of the input signals is app-db then the subscription is a layer 2 sub, otherwise it is a layer 3. If a subscription hasn't run yet, then we can't know if it is a layer 2 or 3.

In almost all cases, a subscription will be created (by (subscribe [:my-sub])) and run (by dereferencing the subscription) within the same epoch, providing the layer level. If you see "Layer ?" this means that a subscription was created but not used. This may indicate a bug in your application, although there are cases where this is ok.

In most cases, after a few more epochs, that subscription will have run, and we know it's layer level, and can use it for any subscriptions shown on any future (and past) epochs.