# The Corda plugin framework

The intention is that Corda is a common platform, which will be extended by numerous application extensions (CorDapps). These extensions will package together all of the Corda contract code, state structures, protocols/flows to create and modify state as well as RPC extensions for node clients. Details of writing these CorDapps is given elsewhere CorDapp basics.

To enable these plugins to register dynamically with the Corda framework the node uses the Java `ServiceLoader` to locate and load the plugin components during the `AbstractNode.start` call. Therefore, to be recognised as a plugin the component must:

1. Include a default constructable class extending from `net.corda.core.node.CordaPluginRegistry` which overrides the relevant registration methods.

2. Include a resource file named `net.corda.core.node.CordaPluginRegistry` in the `META-INF.services` path. This must include a line containing the fully qualified name of the `CordaPluginRegistry` implementation class. Multiple plugin registries are allowed in this file if desired.

3. The plugin component must be on the classpath. In the normal use this means that it should be present within the plugins subfolder of the node’s workspace.

4. As a plugin the registered components are then allowed access to some of the node internal subsystems.

5. The overridden properties on the registry class information about the different extensions to be created, or registered at startup. In particular:

> a. The `webApis` property is a list of JAX-RS annotated REST access classes. These classes will be constructed by the bundled web server and must have a single argument constructor taking a `CordaRPCOps` reference. This will allow it to communicate with the node process via the RPC interface. These web APIs will not be available if the bundled web server is not started.

> b. The `staticServeDirs` property maps static web content to virtual paths and allows simple web demos to be distributed within the CorDapp jars. These static serving directories will not be available if the bundled web server is not started.

> c. The `requiredFlows` property is used to declare new protocols in the plugin jar. Specifically the property must return a map with a key naming each exposed top level flow class and a value which is a set naming every parameter class that will be passed to the flow’s constructor. Standard `java.lang.*` and `kotlin.*` types do not need to be included, but all other parameter types, or concrete interface implementations need declaring. Declaring a specific flow in this map white lists it for activation by the `FlowLogicRefFactory`. White listing is not strictly required for `subFlows` used internally, but is required for any top level flow, or a flow which is invoked through the scheduler.

> d. The `servicePlugins` property returns a list of classes which will be instantiated once during the `AbstractNode.start` call. These classes must provide a single argument constructor which will receive a `PluginServiceHub` reference. They must also extend the abstract class `SingletonSerializeAsToken` which ensures that if any reference to your service is captured in a flow checkpoint (i.e. serialized by Kryo as part of Quasar checkpoints, either on the stack or by reference within your flows) it is stored as a simple token representing your service. When checkpoints are restored, after a node restart for example, the latest instance of the service will be substituted back in place of the token stored in the checkpoint.

> > i. Firstly, they can call `PluginServiceHub.registerFlowInitiator` and register flows that will be initiated locally in response to remote flow requests.

> > ii. Second, the service can hold a long lived reference to the PluginServiceHub and to other private data, so the service can be used to provide Oracle functionality. This Oracle functionality would typically be exposed to other nodes by flows which are given a reference to the service plugin when initiated (as defined by the `registerFlowInitiator` call). The flow can then call into functions on the plugin service singleton. Note, care should be taken to not allow flows to hold references to fields which are not also `SingletonSerializeAsToken`, otherwise Quasar suspension in the `StateMachineManager` will fail with exceptions. An example oracle can be seen in `NodeInterestRates.kt` in the irs-demo sample.

> > iii. The final use case for service plugins is that they can spawn threads, or register to monitor vault updates. This allows them to provide long lived active functions inside the node, for instance to initiate workflows when certain conditions are met.

> e. The `customizeSerialization` function allows classes to be whitelisted for object serialisation, over and above those tagged with the `@CordaSerializable` annotation. In general the annotation should be preferred. For instance new state types will need to be explicitly registered. This will be called at various points on various threads and needs to be stable and thread safe. See Object Serialization.