----------------------------------
----------------------------------
Details on using the Mob* Monitor:
----------------------------------
----------------------------------


Overview:
---------

The Mob* Monitor provides a way to monitor the health state of a particular
service. Service health is defined by a set of satisfiable checks, called
health checks.

The Mob* Monitor executes health checks that are written for a particular
service and collects information on the health state. Users can query
the health state of a service via an RPC/RESTful interface.

When a service is unhealthy, the Mob* Monitor can be requested to execute
repair actions that are defined in the service's check file package.


Check Files and Check File Packages:
------------------------------------

Check file packages are located in the check file directory. Each 'package'
is a Python package.

The layout of the checkfile directory is as follows:

checkfile_directory:
    service1:
        __init__.py
        service_actions.py
        more_service_actions.py
        easy_check.py
        harder_check.py
        ...
    service2:
        __init__.py
        service2_actions.py
        service_check.py
        ....
    .
    .
    .
    serviceN:
        ...

Each service check file package should be flat, that is, no subdirectories will
be walked to collect health checks.

Check files define health checks and must end in '_check.py'. The Mob* Monitor
does not enforce how or where in the package you define repair actions.


Health Checks:
--------------

Health checks are the basic conditions that altogether define whether or not a
service is healthy from the perspective of the Mob* Monitor.

A health check is a python object that implements the following interface:

  - Check()

    Tests the health condition.

    -> Returns 0 if the health check was completely satisfied.
    -> Returns a positive integer if the check was successfuly, but could
       have been better.
    -> Returns a negative integer if the check was unsuccessful.

  - Diagnose(errocode)

    Maps an error code to a description and a set of actions that can be
    used to repair or improve the condition.

    -> Returns a tuple of (description, actions) where:
         description is a string describing the state.
         actions is a list of repair functions.


Health checks can (optionally) also define the following attributes:

  - CHECK_INTERVAL: Defines the interval (in seconds) between health check
                    executions. This defaults to 30 seconds if not defined.


A check file may contain as many health checks as the writer feels is
necessary. There is no restriction on what else may be included in the
check file. The writer is free to write many health check files.


Repair Actions:
---------------

Repair actions are used to repair or improve the health state of a service. The
appropriate repair actions to take are returned in a health check's Diagnose
method.

Repair actions are functions and can be defined anywhere in the service check
package.

It is suggested that repair actions are defined in files ending in 'actions.py'
which are imported by health check files.


Health Check and Action Example:
--------------------------------

Suppose we have a service named 'myservice'. The check file package should have
the following layout:

checkdir:
    myservice:
        __init__.py
        myservice_check.py
        repair_actions.py


The 'myservice_check.py' file should look like the following:

    from myservice import repair_actions

    def IsKeyFileInstalled():
      """Checks if the key file is installed.

      Returns:
        True if USB key is plugged in, False otherwise.
      """
      ....
      return result


    class MyHealthCheck(object):

      CHECK_INTERVAL = 10

      def Check(self):
        if IsKeyFileInstalled():
          return 0

        return -1

      def Diagnose(self, errcode):
        if -1 == errcode:
          return ('Key file is missing.' [repair_actions.InstallKeyFile])

        return ('Unknown failure.', [])


And the 'repair_actions.py' file should look like:


    def InstallKeyFile(**kwargs):
      """Installs the key file."""
      ...


TODO (msartori): Implement crbug.com/505066 for handling inputs to repair
                 actions.


Communicating with the Mob* Monitor:
------------------------------------

TODO (msartori): Fill in once RPCs are fully implemented.
                 See crbug.com/493318, crbug.com/493319, crbug.com/293320.