Fully distributed services
- Each service assumes at the outset that it's the only one in existence.
- Each service periodicially probes for the existence of other services.
- If two services of the same type find one another, they'll link up and synchronize.
- If three services of the same type exist, but A knows about B and B knows about A and C, B and C will synch, and then A and B will synch the content of B and C.
- Service foo wants to search for services of the same type on other nodes that may or may not exist.
- Service foo wants to use another piece of software running on the same node to search for other instances.
- The datastore backing service foo isn't capable of synchronizing on its own with others of its type.
- The datastore backing service foo cannot grow without bound.
- Service foo isn't designed to have a web front end, but needs one.
- Service foo needs a plug-in that exposes the service over HTTP.
- Each instance of service foo needs a unique identifier.
 Nonspecific solutions
- Designate a service on the node that maintains a catalogue of other services running on the node and make them available on the local network.
- Designate a service on the node that listens for announcements of other services on neighboring nodes.
- Designate a service on the node that watches for new instances of the same service to appear.
- The monitoring daemon checks to see if the service announcement has already been picked up.
- The monitoring daemon contacts the service's administrative interface and executes the necessary commands to inform it of a new instance to contact.
- Service foo contacts the new instance and initiates synchronization.
I guess I'm talking about ejabberd and avahi here. Maybe with some glue code and the odd exposed administrative interface.