mirror of
https://gerrit.wikimedia.org/r/mediawiki/extensions/Popups
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9a94300858
For logging to work: 1. $wgWMEStatsdBaseUri needs to point to a valid statsv endpoint, e.g. 'https://en.wikipedia.org/beacon/statsv'. 2. $wgPopupsStatsvSamplingRate needs to be set. Note that the codebase already contains the EventLogging functionality, which is configured separately. Separately configuring different logging mechanisms allows us to avoid sampling mistakes that may arise while choosing one or the other. For example, let's say we want to use EventLogging for 10% of users and statsv for 5%. We'd sample all users into two buckets: 50/50. And then we'd have to set the sampling rates as 20% and 10% respectively, only because of the bucketing above. To avoid this kind of complications, separate sampling rates are used for each logging mechanism. This, of course, may result in situations where a session is logged via both EventLogging and statsv. 3. The WikimediaEvents extension needs to be installed. The extension adds the `ext.wikimediaEvents` module to the output page. The logging functionality is delegated to this module. Notable changes: * The FETCH_START and FETCH_END actions are converted to a timed action. * The experiments stub used in tests has been extracted to the stubs file. Logged data is visualized at https://grafana.wikimedia.org/dashboard/db/reading-web-page-previews Bug: T157111 Change-Id: If3f1a06f1f623e8e625b6c30a48b7f5aa9de24db
28 lines
2.8 KiB
Markdown
28 lines
2.8 KiB
Markdown
# Instrumentation
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Page Previews is thoroughly instrumented. Currently, there's one [Event Logging](https://www.mediawiki.org/wiki/Extension:EventLogging) ("EL") schema that captures all of the data that we record about a user's interactions with the Page Previews extension, the [Schema:Popups](https://meta.wikimedia.org/wiki/Schema:Popups) schema. There is also a statsv instrumentation, which is visualized as a [Graphana dashboard](https://grafana.wikimedia.org/dashboard/db/reading-web-page-previews). The primary purpose of the statsv instrumentation is to monitor the performance of PagePreviews in production.
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Tilman Bayer captured the high level state and user action's that should trigger an event to be logged via EL [here](https://www.mediawiki.org/wiki/File:State_diagram_for_Schema-Popups_(Hovercards_instrumentation).svg) – indeed, this diagram was a catalyst for rewriting the Page Previews application as a large finite state machine.
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## Implementation
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### EventLogging
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Events need to be queued and dequeued in response to [actions](http://redux.js.org/docs/basics/Actions.html) dispatched to the store. This could be implemented in either a [Redux middleware](http://redux.js.org/docs/advanced/Middleware.html) or as a [reducer](http://redux.js.org/docs/basics/Reducers.html), an [action](http://redux.js.org/docs/basics/Actions.html), and a [change listener](./change_listener.md). Both approaches satisfy the general requirement that instrumentation should be transparent to the rest of the codebase but the latter is the approach we're taking for the rest of the application and instrumentation isn't a special case. Moreover, given the amount of time it took to get the original instrumentation under test, we can leverage the constraint the [reducers](http://redux.js.org/docs/basics/Reducers.html) must be pure to test the majority of the instrumentation logic in isolation.
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Since the event data varies with the value of the `action` property, events are represented by a blob of `action`-specific data and a blob of data that's shared between all events. Very nearly all of the latter can and should be initialized when the Page Previews application boots.
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#### Data Flow
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When enqueuing and logging an event, data flows between the reducer and the change listener as follows:
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1. The state is initialized to `null`..
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2. An event is enqueued by the reducer as a result of an action.
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3. The change listener sees that the state tree has changed and logs the queued event via `mw.eventLog.Schema#log`.
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4. The change listener dispatches the `EVENT_LOGGED` action.
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5. The reducer resets the state (read: `GOTO 1`).
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### Statsv
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Statsv instrumentation works similar to the EventLogging instrumentation, but it logs fewer pieces of data.
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