Metrics - Add actions/README.md

tools/metrics/actions/README.md

+# User Action Guidelines
+
+This document gives the best practices on how to use user actions in code and
+how to document them for the dashboard.  User actions come with only a name and
+a timestamp.  They are best used when you care about a sequence--which actions
+happen in what order.  If you don't care about the order, you should be using
+histograms (likely enumerated histograms).
+
+Often, you want both user actions and histogram logging in your code.  They
+enable different analyses.  They're complementary.
+
+[TOC]
+
+## Emitting to User Actions
+
+### Emit Once Per Action
+
+A user action should be tied to an actual action taken by a user.  Each
+meaningful unit of action should cause one emit.
+
+### Emit at a High-Level, not Deep in the Implementation
+
+Prefer to emit at the highest level reasonable, closest to the code that handles
+the UI interaction.  Emitting deep in implementation code can cause problems
+because that code may get reused (and thus called more times in more places) or
+may get called fewer times (due to caching for example).  In cases like this,
+the logged user action will not longer correspond with a meaningful action by
+the user.
+
+### Do Not Emit Redundantly
+
+Generally a meaningful user action should cause only one emit.  For example, if
+the browser already has a "Back" user action, it's poor practice to add a
+"BackViaKeyboardShortcut" user action.  This is mostly redundant.  (If you're
+trying to determine the breakdown of keyboard-shortcut backs versus all backs,
+use a histogram.)
+
+### Do Not Emit Excessively
+
+Again, choose an appropriately-sized meaningful unit.  For example, emit
+"DragScrolled" for a whole scroll action.  Don't emit this action every time the
+user pauses scrolling if the user remains in the process of scrolling (mouse
+button still down).
+
+As another example, you may want to emit "FocusOmnibox" (upon focus),
+"OmniboxEditInProgress" (upon first keystroke), and "OmniboxUse" (upon going
+somwhere) but forswear "OmniboxKeystroke".  That's probably more detailed than
+you need.
+
+### Generally, Do Not Emit Impressions
+
+It's okay to emit user actions such as "ShowTranslateInfobar" or
+"DisplayedImageLinkContextMenu".  However, more detailed impression information,
+especially those not caused by the user (as in the second example) and not as
+attention-grabbing (as in the first example), is often not useful for analyzing
+sequences of user actions.  For example, don't emit
+"ShowedSecureIconNextToOmnibox".
+
+### Testing
+
+Test your user actions using *chrome://user-actions*.  Make sure they're being
+emitted when you expect and not emitted at other times.
+
+If this is a general UI surface, please try to check every platform.  In
+particular, check Windows (Views-based platforms), Mac (non-Views), Android
+phone (yet other UI wrapper code), Android tablet (often triggers lookalike but
+different menus), and iOS (yet more different UI wrapper code).
+
+Also, check that your new user action is not mostly redundant with other user
+actions (see [advice above](#Do-Not-Emit-Redundantly)) and not emitted
+excessively (see [advice above](#Do-Not-Emit-Excessively)).
+
+### Revising User Actions
+
+If you're changing the semantics of a user action (when it's emitted), make it
+into a new user action with a new name.  Otherwise the dashboard will be mixing
+two different interpretations of the data and make no sense.
+
+## Documenting User Actions
+
+### Add User Actions and Documentation in the Same Changelist
+
+If possible, please add the actions.xml description in the same changelist in
+which you add the user-action-emitting code.  This has several benefits.  One,
+it sometimes happens that the actions.xml reviewer has questions or concerns
+about the user action description that reveal problems with interpretation of
+the data and call for a different recording strategy.  Two, it allows the user
+action reviewer to easily review the emission code to see if it comports with
+these best practices, and to look for other errors.
+ 
+### Understandable to Everyone
+
+User actions descriptions should be understandable to someone not familiar with
+your feature.  Please add a sentence or two of background if necessary.
+
+It is good practice to note caveats associated with your user actions in this
+section, such as which platforms are supported (if the set of supported
+platforms is surprising).  E.g., a desktop feature that happens not to be logged
+on Mac.
+
+### State When It Is Emitted
+
+User action descriptions should clearly state when the action is emitted.
+
+### Owners
+
+User actions need to be owned by a person or set of people.  These indicate who
+the current experts on it are.  Being the owner means you are responsible for
+answering questions about it, handling the maintenance if there are functional
+changes.  The owners should be added in the original user action description.
+If you are using a user action heavily and understand it intimately, feel free
+to add yourself as an owner. @chromium.org email addresses are preferred.
+
+### Beware `not_user_action="true"`
+
+actions.xml allows you to annotate an action as `not_user_action="true"`.  This
+feature should be used rarely.  If you think you want to annotate your action
+thusly, please re-review the best practices above.
+
+### Deleting User Action Entries
+
+Do not delete actions from actions.xml.  Instead, mark unused user actions as
+obsolete, annotating them with the associated date or milestone in the obsolete
+tag entry.  If your user action is being replaced by a new version, we suggest
+noting that in the previous user action's description.
+
+Deleting user action entries would be bad if someone accidentally reused your
+old user action name and which therefore corrupts new data with whatever old
+data is still coming in.  It's also useful to keep obsolete user action
+descriptions in actions.xml--that way, if someone is searching for a user action
+to answer a particular question, they can learn if there was a user action at
+some point that did so even if it isn't active now.