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Metrics

Last updated: March 21st, 2022

Please read our metrics reference before this document as it has the context of our current system in it. The document below is about application metrics.

A brief history

The FxA metrics ecosystem is arcane and esoteric, so a brief history of how we got to where we are may be helpful to understand the landscape.

In the beginning there were simple activity events. These were basic events emitted from the auth and oauth servers, visualised via now defunct dashboards.

They provided useful insights from a high level about things like how many sign-ins and sign-ups there were. But they didn’t allow us to follow a user’s journey through a sign-in or sign-up flow, which include email verification steps that may take place across multiple devices.

So the flow events were created, stored in Redshift and visualised in Redash (and the existing activity metrics were moved to Redshift/Redash at the same time).

Flow events include two extra pieces of metadata, a flow id and a flow begin-time, which gave us more detailed insight into user behaviour. And, unlike the activity events, these were also emitted by client-side code in the content server.

The flow events proved handy for some use-cases that they weren’t meant for, so we started piggy-backing other things like performance metrics, post-sign-in activity and email events on them too. That stressed the infrastructure somewhat, but also gave us the ability to e.g. directly link perf metrics to user behaviour, which was an unexpected new superpower.

The stressed infrastructure meant queries ran pretty slowly though and they required SQL knowledge. And the organic growth of the event schema meant there were issues around inconsistent nomenclature confusing people. And they were ingested via Heka, which meant working with an unfamiliar language (Lua) in an environment that was hard to meaningfully test without just pushing your changes and seeing whether they worked. And we never got round to ingesting the events more frequently than daily, so people were frustrated about having to wait a day to see stuff show up in charts.

For all of those reasons, at the end of 2017 Amplitude was chosen as a paid-for replacement for the more producty metrics we were sending to Redshift. We called these Amplitude Events.

These had some really good properties:

  • The event schema was designed in advance, so it would be less confusing
  • We sent these events in real time. This was a huge improvement over the 24hr+ lag we were used to
  • Nobody had to wrestle with SQL any more

They also had some really bad properties:

  • The Amplitude service is expensive and we send a lot of events
  • Due to #1, we decided to only send a subset of events. This means there were now two places you have to look if you don't know exactly what you need.

In June 2020 we stopped using Amplitude due to cost. That leaves us with a lot of things referring to amplitude events still. That's fine for identifying that subset of events, but understand we don't actually use Amplitude anymore.

So, where does that leave us?

Well, it's a bit complicated and you should make sure you've read our metrics reference first.

In the firefox_accounts dataset in the mozdata project:

  • fxa_[server]_* tables will be most of the amplitude events
  • fxa_log_[server]_* tables will be most of the flow events

However, there are a bunch of other tables in there as well. In short, only experience can really help you find the data you are looking for if there isn't already a dashboard for it. Ask the FxA team or #data-help in slack.

How are metrics emitted?

Auth server

Activity events, flow events and amplitude events are all just mozlog-format log lines. There are functions for each of them in the module lib/log.js, respectively called activityEvent, flowEvent and amplitudeEvent. However, to reduce boilerplate in the codebase they’re actually invoked via calls to request.emitMetricsEvent. That function is defined in lib/metrics/events.js and is decorated onto the request object by some code in lib/server.js.

Activity events must have a uid property. Some call sites also set device_id. Properties containing user agent information and the OAuth client id are set automatically based on the request object.

Flow events have their properties set automatically too, either from the request object or, if there is an authentication token of some kind, from data stored against the uid and token id in memcached. This data is widely referred to as metricsContext throughout the codebase and has a lifetime of 2 hours, which is kind of an arbitrary limit that allows us to fetch data for most flows while ignoring dubious data from stale or non-authentic sources. The metricsContext is written to memcached via calls to request.stashMetricsContext (where it’s copied from the request data) and request.propagateMetricsContext (where it’s copied from some other stashed data). Those functions are both defined in lib/metrics/context.js and decorated onto the request object in lib/server.js.

Amplitude events are not explicitly emitted (note the exception in fxa-payments-server), because they can all be mapped to from previously-existing activity or flow events. Those mappings are defined by structures called EVENTS and FUZZY_EVENTS in lib/metrics/amplitude.js. Any event matching a string key from EVENTS or a regex key from FUZZY_EVENTS gets transformed into the corresponding event by common code in the fxa-shared package (it’s also used by the content server). Most of the properties on those events are set automatically, although there are still some cases where data must be set explicitly at the call site.

StatsD is used to capture latency performance metrics on SQS calls, SNS calls, Stripe calls, and any HTTP API requests handled by lib/backendService.js (e.g. requests to our customs server). It is also used to time the server's request handling on all routes.

Content server

Again, flow events and amplitude events in the content server are just mozlog-format log lines. But before they can be logged on the back-end, the front-end has to submit them to the endpoint POST /metrics first. This is handled by the module app/scripts/lib/metrics.js.

The front-end also needs to make sure it has a flowId and flowBeginTime, either from data- attributes on the body element that were set by the back-end, or from data propagated in the resume token. In both cases, the data is loaded by app/scripts/models/flow.js.

After they reach the back-end, events are processed by server/lib/flow-event.js. And, just like the auth server, there are EVENTS and FUZZY_EVENTS mappings to control transformations to amplitude events in server/lib/amplitude.js.

Event Broker

StatsD is used to collect SQS processing latency metrics and message type counts. Details are on the package's README.

Payments server

One thing to keep in mind is that users should not access the pages on Payments directly. The Content server will redirect them to the Payments server, along with metrics related query params. Those query params are:

  • flow_id
  • flow_begin_time
  • device_id

They are converted to camelCase once in Payments.

The main module on the front end for emitting events is src/lib/flow-events.ts. Its init function is called in src/App.tsx, during which the three query params above are passed into the module.

A call to logAmplitudeEvent results in a POST to /metrics, which is handled by server/lib/routes/post-metrics.js. The events are log lines in identitical formats to those emitted by Content server.

The Payments frontend reports the timing data surfaced by Navigation Timing Level 2 back to the server side, which then sends the calculated performance metrics to our statsD server. One major difference between these performance metrics and those in Redshift for Content Server is that these do not have a flow id to connect the timings into a performance overview for a particular request.

Relying Parties

See metrics for relying parties.

What other docs are there?

caution

These are pretty old and may not be accurate