Ignoring Unchanged Layer 2 Subscriptions

In the "subs" tab, you have the option to ignore unchanged layer 2 subscriptions. This document explains why.

Background

re-frame makes a distinction between layer 2 and layer 3 subscriptions:

• layer 2 subscriptions extract data directly from app-db. They should be trivial in nature, which is to say there should be no computation in them beyond what is necessary to extract a value from app-db
• layer 3 subscriptions never take values from app-db. Instead, they have layer 2 nodes as inputs (or other layer 3), and they do the more serious CPU work of computing a materialised view of those values.

We never want to run a layer 3 subscriptions unless it is necessary, whereas layer 2 subscriptions are so trivial that we don't mind running them unnecessarily.

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. That would be 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-10x 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-10x 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.