notifyd

notifyd

Notifyd is a component that listens for configured notifications on the VCI bus and invokes the script that was configured. This will allow for feature defined scripts to be called when notifications are sent on the VCI bus. Ephemeral notifications can be configured by other scripts using RPCs into the notifier. Notifyd will execute an appropriate number of scripts in parallel using a queue of (events,handlers) pairs to ensure that execution order is preserved with regards to causal order of arrival. This does not mean that events are totally ordered but that if an event a caused event b to occur that a will always be processed before b. Order of called scripts for a given notification is arbitrary and not guaranteed to be constant.

The following diagram details the parts of the system involved with notification delivery when using notifyd.

              +---------------+
              |               |
              |   Emitter     |
              |               |
              +------+--------+
                     |
                     |
+--------------------v------------+
|                                 |
|             VCI Bus             |
|                                 |
+-------+-------------------------+
        |
        |
    +---v-------+
    |           |
    |  notifyd  +--+  +---------------+
    |           |  |  |               |
    +----+------+  +->+ .../*.notify  |
         |            |               |
         |            +---------------+
    +----v-----+
    |          |
    |  script  |
    |          |
    +----------+

Runtime notification subscriptions

Notifyd has 2 RPCs that allow manipulations of the runtime notification subscriptions. 'when' adds a new script to be called when a notification is emitted. 'done' removes the specified script from the list to be called for a notification.

Notifyd stores the ephemeral notification requests in a cache file '/run/vci/notifyd.cache'. This file will be restored if the daemon is restarted during the same boot of the system.

Static notification subscriptions

Notifyd supports static notification subscription using '.notify' files located in '/lib/vci/notify/static'. These files are loaded on every restart of the daemon. The file format is an 'ini' file that consists of 'When' sections that contain the script to call when the notification is heard. Below is an example.

[When module:name]
script=foo --arg1 --arg2

The script environement

Scripts are called using the UNIX environment and standard interfaces for interaction. The environment will be setup as follows.

Limitations

There are some scaling concerns to be aware of when using notifyd.

1. Scripts are called in parallel and will be limited by the number of processors available to the notifyd process. This is to ensure a 'fork bomb' can't happen when a notification is emitted.
2. Long executing scripts can hold up other notifications from being processed in a timely manner. Ensure your scripts are as fast as they can be when integrating with notifyd.
3. Even quick executing scripts may be too much for notifyd to process in a realistic timeframe if there are too many registered scripts. Notifyd should be used sparingly.

Conclusion

Notifyd is a piece of infrastucture that is meant to be used to enable small simple components that are ephemeral. It should not be used for components that can be implemented other ways. Notifyd will ensure that for simple small components no notifications are missed and their handlers will be called in a reasonable timeframe. This only remains true if it is used sparingly.