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All CloudStack APIs are implemented by annotated command class and PluggableService is a contract implemented by all the components such as the ManagementServer and all the plugins which provide an API. During load time, API discovery service asks all the pluggable services to return list of API cmd classes from whose fields and annotations it gathers information about each API, the information consists of name, description, parameter name, parameter description etc. The whole process takes less than a second and we can see messages like following in CloudStack's mgmt server logs:

Wiki Markup2013-03-06 18:48:45,160 INFO  \INFO  [cloudstack.discovery.ApiDiscoveryServiceImpl\] (main:null) Api Discovery Service: Annotation, docstrings, api relation graph processed in 491.386 ms

This was implemented for 4.1 and starting 4.1 release all versions will have the discovery service which is enabled by default. We chose to enable it by default because it made sense to provide a user list of APIs they can call.

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At present cloudmonkey, the CloudStack CLI uses this discovery feature to automatically generate a domain specific language (DSL) grammer, verbs and subjects. cloudmonkey provides an operation called "sync" to fetch the API information and which is then processed and cached, for more information: https://cwiki.apache.org/confluence/display/CLOUDSTACK/CloudStack+cloudmonkey+CLI#CloudStackcloudmonkeyCLI-Gettingstarted^{Image Removed}

In future, this service can be used by cloud-engine, the core orchestrating engine to publish APIs of several of its components on the fly and cloudmonkey can generate DSL for all of them on the fly.