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Client RPC
There are multiple ways to interact with a node from a client program, but if your client is written in a JVM compatible language the easiest way to do so is using the client library. The library connects to your running node using a message queue protocol and then provides a simple RPC interface to interact with it. You make calls on a Java object as normal, and the marshalling back and forth is handled for you.
The starting point for the client library is the CordaRPCClient class. This provides a start method that returns a CordaRPCConnection, holding an implementation of the CordaRPCOps that may be accessed with proxy in Kotlin and getProxy() in Java. Observables that are returned by RPCs can be subscribed to in order to receive an ongoing stream of updates from the node. More detail on how to use this is provided in the docs for the proxy method.
Warning
The returned CordaRPCConnection is somewhat expensive to create and consumes a small amount of server side resources. When you're done with it, call close on it. Alternatively you may use the use method on CordaRPCClient which cleans up automatically after the passed in lambda finishes. Don't create a new proxy for every call you make - reuse an existing one.
For a brief tutorial on how one can use the RPC API see tutorial-clientrpc-api.
Security
Users wanting to use the RPC library are first required to authenticate themselves with the node using a valid username and password. These are specified in the configuration file. Each user can be configured with a set of permissions which the RPC can use for fine-grain access control.
Observables
The RPC system handles observables in a special way. When a method returns an observable, whether directly or as a sub-object of the response object graph, an observable is created on the client to match the one on the server. Objects emitted by the server-side observable are pushed onto a queue which is then drained by the client. The returned observable may even emit object graphs with even more observables in them, and it all works as you would expect.
This feature comes with a cost: the server must queue up objects emitted by the server-side observable until you download them. Note that the server side observation buffer is bounded, once it fills up the client is considered slow and kicked. You are expected to subscribe to all the observables returned, otherwise client-side memory starts filling up as observations come in. If you don't want an observable then subscribe then unsubscribe immediately to clear the client-side buffers and to stop the server from streaming. If your app quits then server side resources will be freed automatically.
Warning
If you leak an observable on the client side and it gets garbage collected, you will get a warning printed to the logs and the observable will be unsubscribed for you. But don't rely on this, as garbage collection is non-deterministic.
Futures
A method can also return a ListenableFuture in its object graph and it will be treated in a similar manner to observables. Calling the cancel method on the future will unsubscribe it from any future value and release any resources.
Versioning
The client RPC protocol is versioned using the node's Platform Version (see versioning). When a proxy is created the server is queried for its version, and you can specify your minimum requirement. Methods added in later versions are tagged with the @RPCSinceVersion annotation. If you try to use a method that the server isn't advertising support of, an UnsupportedOperationException is thrown. If you want to know the version of the server, just use the protocolVersion property (i.e. getProtocolVersion in Java).
Thread safety
A proxy is thread safe, blocking, and allows multiple RPCs to be in flight at once. Any observables that are returned and you subscribe to will have objects emitted in order on a background thread pool. Each Observable stream is tied to a single thread, however note that two separate Observables may invoke their respective callbacks on different threads.
Error handling
If something goes wrong with the RPC infrastructure itself, an RPCException is thrown. If you call a method that requires a higher version of the protocol than the server supports, UnsupportedOperationException is thrown. Otherwise, if the server implementation throws an exception, that exception is serialised and rethrown on the client side as if it was thrown from inside the called RPC method. These exceptions can be caught as normal.
Wire protocol
The client RPC wire protocol is defined and documented in net/corda/client/rpc/RPCApi.kt.
Whitelisting classes with the Corda node
To avoid the RPC interface being wide open to all classes on the classpath, Cordapps have to whitelist any classes they require with the serialization framework of Corda, if they are not one of those whitelisted by default in DefaultWhitelist, via either the plugin architecture or simply with the annotation @CordaSerializable. See creating-a-cordapp or serialization. An example is shown in tutorial-clientrpc-api.
Warning
We will be replacing the use of Kryo in the serialization framework and so additional changes here are likely.