Reading/Web/EventLogging best practices

Also see WMF wide EvetnLogging best practices: Extension:EventLogging/Guide

Schemas

 * Schemas in MobileFrontend should be prefixed with `MobileWeb`
 * The talk page should be edited with the `SchemaDoc` template. For example Schema_talk:MobileWebSearch

Sampling rate

 * When logging events that occur on every page view a sampling rate of 0.01%
 * If that schema is likely to track more than 1 event on a given page a more conservative measure of 0.005% is advised. When running the meta:SchemaMobileWebSectionUsage schema for example 1% was too high.
 * Be sure to check events for any newly deployed schema in grafana - above 40 events per second is typically a warning flag
 * Work on the assumption that at 2 million records a database table becomes unusable, so if you want 6 months work of data 5 events per second would get you there
 * When click tracking events on prominently used features, 1% is usually enough e.g. clicks to menu items or search usage (given that not all page views result in an event).

Use of session ids

 * Some events need to be linkable to other events, for example Schema:MobileWebSectionUsage records clicks to section headings in a given user session. This is usually done via a unique token that is generated once and used until the user closes the browser.
 * In some situations where workflows span multiple pages it may be necessary to persist such a token a little longer. When this is the case please ensure you minimise the duration the session is stored for.