Expression language for Trigger filter

[slinkydeveloper]

Problem
Now the Trigger filtering abilities are poor, because they require direct match on specific CloudEvent attributes.

Persona:
User

Exit Criteria
Trigger APIs should have a more powerful filtering abilities

Time Estimate (optional):
2

Additional context (optional)
Together with @grantr we discussed the possibility to add an expression language to express the matching predicate. We need to find an expression language with implementations in different programming languages and that covers the basic string operations.

An example trigger could be:

apiVersion: eventing.knative.dev/v1beta1
kind: Trigger
metadata:
  name: my-service-trigger
spec:
  broker: default
  filter:
    predicate: my-extension-value == dev.knative.foo.bar || type == dev.knative.foo.bar
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: my-service

[matzew]

I'd highly appreciate this for our v1 API shape

[FWiesner]

❤️ this

[slinkydeveloper]

I'm starting to lean towards using Javascript. I know it may sound crazy, but:

• There are engines/engine bindings out there for every language, so it's not problematic from the Trigger API adoption POV.
• It's super easy to integrate with: the trigger impl just needs to parse the js, inject the attributes as variables into the scope, then execute it. Nice and smooth.
• It already has all the built-in functions we need, we don't need to add/extend it.
• For such simple expressions, It's readable and simple to use for everybody.
• It could event open the doors to start thinking adding some simple event transformation through triggers using a simple js expressions.

[slinkydeveloper]

Another alternative could be using Json Schema, but I feel this could make the predicates too much convoluted both to read and write:

predicate:
  allOf:
   - properties:
       attribute-a:
         const: aaa
   - properties:
       attribute-b:
         const: bbb

[nachocano]

@grantr put some thought on this sometime back. Might be worth syncing with him.
IIRC he used CEL https://github.com/google/cel-spec, but there were some problems regarding scalability.

[slinkydeveloper]

I checked out CEL, problem is that the only implementation out there is for Golang. I wish to avoid locking the "implementatibility" of Eventing APIs (in particular of the data-plane) to Golang only.

[FWiesner]

@slinkydeveloper The problem with the JavaScript filter engine is that you will have a hard time validating the content. Not only you need syntactical validation, but functional checks as well. Worst would be that a filter JS expression shows to be wrong in seldom situations that haven't been tested and where you're live in production already

[vaikas]

I'm totally on board with making the filter expressions more powerful. My $.02 is that this can be an additive (backwards compatible) change to the API and we do not have to get it in .16 where we bump the Trigger spec to v1.

wrt: javascript, I just can never remember how many = I have to specify to actually get the sane semantics :) jk

[slinkydeveloper]

Not only you need syntactical validation, but functional checks as well

We can perform a pretty simple validation on control plane side trying to execute the predicate, giving to it an dummy cloudevent and check if it fails badly with an error or returns a boolean value.

wrt: javascript, I just can never remember how many = I have to specify to actually get the sane semantics :) jk

Well, because all/most attributes (in the first iteration at least) are supposed to be compared strings, you just need ==

[FWiesner]

Not only you need syntactical validation, but functional checks as well

We can perform a pretty simple validation on control plane side trying to execute the predicate, giving to it an dummy cloudevent and check if it fails badly with an error or returns a boolean value.

well, the filter could behave differently on Wednesdays when the temperature is below freezing point ;) To prevent that you'd have to limit Javascript to be able to only use builtins that you provide