ChangeMachine is a critical component in the Steelmesh stack. It is responsible for monitoring and responding to changes in a number of couchdb instances and taking appropriate actions in response to those changes.

The implementation of ChangeMachine is reasonably simple thanks to the flatiron neuron queueing library and through leveraging changemate notifiers.

Online Documentation

Documentation for changemachine is available at changemachine.readthedocs.org

System Internals

• Attempts to act intelligently when a machine does not have a process event handler. In this case, items are queued in the ready state and we make use of the newListener node event to wait for a process event to be connected. At this stage, similar action is not taken if the process listener is removed.

• Makes use of changemate notifier pause and resume methods to ensure efficient operation even in the case of a massive _changes feed from couch, etc. In the case that a notifier does not support these operations, items will be queued.

On ChangeMate vs Follow

While at the present stage an implementation of ChangeMachine would have been possible with the excellent follow library the long term plan is to support monitoring changes from the filesystem and other sources so work changemate has been integrated instead.

Alternative Systems

Some alternative systems that do similar things are:

• banzai - Banzai looks like an excellent system for processing documents through a number of various states. Banzai's implementation has definitely influenced parts of ChangeMachine.

• hook.io - If you are building, loosely-coupled, distributed systems then hook.io is one of the best choices in the #nodejs space. There are instances where ChangeMachine could perform a similar function to hook, but in most cases ChangeMachine is designed for simpler scenarios.

• kue - Job processor built around redis, has a pretty admin panel to boot.

Additionally, as previously stated, ChangeMachine does not attempt to compete with full-blown messaging and queuing systems and if you require a more distributed system then be sure to check out the likes of RabbitMQ and 0MQ.