Event payload filtering

[farshidtz]

The Notification API draft describes filtering of events based on their types.

The filtering based on payload was suggested by in #28 (comment). But there are different filtering requirements:

1. Observe an attribute and trigger events whenever it changes. E.g. $.properties will trigger event when properties change.
2. Filter the event payload and give the selected attributes to the client. E.g. $.properties will only give events with properties values.
3. Combination of 1 and 2. E.g. $.properties will give only the changed properties.

Notes:

• The update event produces only a diff. The order of applying selection and diff affects the final results.
• JSONPath (and maybe also XPath) cannot be used to filter events based on assertions, e.g. to get TD events that have a specific type. Unlike in the search API, here, the expression is executed on a single TD.
• This kind of event filtering is usually NOT possible in publish-subscribe exchanges (e.g. MQTT).

[farshidtz]

The current draft describes the first way. But this is confusing when considering what JSONPath and XPath are really meant for. Going to make a PR to remove that for now: #177.

[relu91]

Thank you for creating the issue 👍🏻 and I agree that selecting while also filtering events is tricky. I just want to clarify a point. The use case in point 1 I slightly narrowed with respect to what we had in mind. It would be nice to have the ability to subscribe to facts about the whole collection of TDs, not just a specific one. Like with $[?(@.name === "Lillo")]. Translated in human-readable text: Please let me know when a TD named Lillo is popping up or is leaving the TDD.

Do you think is still confusing?

[farshidtz]

I fully understand your point and have also thought about applying the expression to a hypothetical list, but this is really a hack to make JSONPath work.

I particularly didn't pursue that approach for the following reasons:

• JSONPath (based on common specs) is for filtering and selection, not assertion.
• The server has to place each changed TD in an empty list and then apply JSONPath.
• The API has to take one argument applied to individual events (event filtering by type) and another one (jsonpath expression) applied to a list of events data.

I personally think the best use of JSONPath in events is to filter and select parts of data from individual events (bullet number 2 above). Assertions on TD attributes can still be done on the client side.