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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 proxy method that returns an implementation of the CordaRPCOps interface. A timeout parameter can be specified, and 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 object is somewhat expensive to create and consumes a small amount of server side resources. When you're done with it, cast it to Closeable or AutoCloseable and close it. Don't create one for every call you make - create a proxy and reuse it.
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 kept in rpc-users.properties in the node base directory. This file also specifies permissions for each user, which the RPC implementation can use to control access. The file format is described in corda-configuration-files.
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. Therefore RPCs that use this feature are marked with the @RPCReturnsObservables annotation, and you are expected to subscribe to all the observables returned. If you don't want an observable then subscribe then unsubscribe immediately to clear the buffers and indicate that you aren't interested. If your app quits then server side resources will be freed automatically.
When all the observables returned by an RPC are unsubscribed on the client side, that unsubscription propagates through to the server where the corresponding server-side observables are also unsubscribed.
Warning
If you leak an observable or proxy on the client side and it gets garbage collected, you will get a warning printed to the logs and the proxy will be closed for you. But don't rely on this, as garbage collection is non-deterministic.
Versioning
The client RPC protocol is versioned with a simple incrementing integer. When a proxy is created the server is queried for its protocol 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 will only allow a single RPC to be in flight at once. Any observables that are returned and you subscribe to will have objects emitted on a background thread. Observables returned as part of one RPC and observables returned from another may have their callbacks invoked in parallel, but observables returned as part of the same specific RPC invocation are processed serially and will not be invoked in parallel.
If you want to make multiple calls to the server in parallel you can do that by creating multiple proxies, but be aware that the server itself may not process your work in parallel even if you make your requests that way.
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 not currently documented. To use it you must use the client library provided. This is likely to change in a future release.