Question

I’m currently facing an interesting problem in my OSGi application.
I’m implementing a configuration service that should retrieve the application’s configuration from multiple sources (file, registry or network). The configuration service should read from the sources in a special order until he got a value for the configuration property.

First I thought of putting each source in a separate bundle, create a super interface for them and let them provide a declarative service. This would also help to modularize the registry which is required since it is not available on every OS. When the configuration service is asked for a property’s value it queries for all source bundles according to the whiteboard pattern and reads the configuration until he got a non-null value. But it does it in a random order.

Does anyone have an idea how to implement a special order in traversing the bundles providing the configuration service?

Best regards

Answer 1

OSGi already has a concept of service ranking. When registering a service, you can provide a value for the property “service.ranking” (org.osgi.framework.Constants.SERVICE_RANKING).

I don’t think that this property has any effect on the order in which BundleContext#getServiceReferences() returns available service references (at least the spec doesn’t say anything about it) , but you could still use the property value to order the internal collection managed by your “super” config service.

However, to me it is generally a strange idea to let the service implementation itself determine its relative importance. I would rather split the problem in two parts.

1. A config service acting as a facacde for
2. a collection of config source providers

The second interface would include some characterization concept (e.g. an enum {file, registry, net}). I would then have the implementation of the first interface (the facade) perform the ordering based on the characteristic of each provider (as the first answer by Chris already suggests).