corda/docs/source/clientrpc.rst

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.

RPC permissions

If a node's owner needs to interact with their node via RPC (e.g. to read the contents of the node's storage), they must define one or more RPC users. Each user is authenticated with a username and password, and is assigned a set of permissions that RPC can use for fine-grain access control.

These users are added to the node's node.conf file.

The simplest way of adding an RPC user is to include it in the rpcUsers list:

rpcUsers=[
    {
        username=exampleUser
        password=examplePass
        permissions=[]
    }
    ...
]

Users need permissions to invoke any RPC call. By default, nothing is allowed. These permissions are specified as follows:

rpcUsers=[
    {
        username=exampleUser
        password=examplePass
        permissions=[
            "StartFlow.net.corda.flows.ExampleFlow1",
            "StartFlow.net.corda.flows.ExampleFlow2"
        ]
    }
    ...
]

Permissions Syntax

Fine grained permissions allow a user to invoke a specific RPC operation, or to start a specific flow. The syntax is:

• to start a specific flow: StartFlow.<fully qualified flow name> e.g., StartFlow.net.corda.flows.ExampleFlow1.

- to invoke a RPC operation: InvokeRpc.<rpc method name> e.g., InvokeRpc.nodeInfo. .. note:: Permission InvokeRpc.startFlow allows a user to initiate all flows.

RPC security management

Setting rpcUsers provides a simple way of granting RPC permissions to a fixed set of users, but has some obvious shortcomings. To support use cases aiming for higher security and flexibility, Corda offers additional security features such as:

• Fetching users credentials and permissions from an external data source (e.g.: a remote RDBMS), with optional in-memory caching. In particular, this allows credentials and permissions to be updated externally without requiring nodes to be restarted.
• Password stored in hash-encrypted form. This is regarded as must-have when security is a concern. Corda currently supports a flexible password hash format conforming to the Modular Crypt Format provided by the Apache Shiro framework

These features are controlled by a set of options nested in the security field of node.conf. The following example shows how to configure retrieval of users credentials and permissions from a remote database with passwords in hash-encrypted format and enable in-memory caching of users data:

security = {
    authService = {
        dataSource = {
            type = "DB",
            passwordEncryption = "SHIRO_1_CRYPT",
            connection = {
               jdbcUrl = "<jdbc connection string>"
               username = "<db username>"
               password = "<db user password>"
               driverClassName = "<JDBC driver>"
            }
        }
        options = {
             cache = {
                expireAfterSecs = 120
                maxEntries = 10000
             }
        }
    }
}

It is also possible to have a static list of users embedded in the security structure by specifying a dataSource of INMEMORY type:

security = {
    authService = {
        dataSource = {
            type = "INMEMORY",
            users = [
                {
                    username = "<username>",
                    password = "<password>",
                    permissions = ["<permission 1>", "<permission 2>", ...]
                },
                ...
            ]
        }
    }
}

Warning

A valid configuration cannot specify both the rpcUsers and security fields. Doing so will trigger an exception at node startup.

Authentication/authorisation data

The dataSource structure defines the data provider supplying credentials and permissions for users. There exist two supported types of such data source, identified by the dataSource.type field:

INMEMORY

A static list of user credentials and permissions specified by the users field.

DB

An external RDBMS accessed via the JDBC connection described by connection. Note that, unlike the INMEMORY case, in a user database permissions are assigned to roles rather than individual users. The current implementation expects the database to store data according to the following schema:

• Table users containing columns username and password. The username column must have unique values.
• Table user_roles containing columns username and role_name associating a user to a set of roles.
• Table roles_permissions containing columns role_name and permission associating a role to a set of permission strings.

Note

There is no prescription on the SQL type of each column (although our tests were conducted on username and role_name declared of SQL type VARCHAR and password of TEXT type). It is also possible to have extra columns in each table alongside the expected ones.

Password encryption

Storing passwords in plain text is discouraged in applications where security is critical. Passwords are assumed to be in plain format by default, unless a different format is specified by the passwordEncryption field, like:

passwordEncryption = SHIRO_1_CRYPT

SHIRO_1_CRYPT identifies the Apache Shiro fully reversible Modular Crypt Format, it is currently the only non-plain password hash-encryption format supported. Hash-encrypted passwords in this format can be produced by using the Apache Shiro Hasher command line tool.

Caching user accounts data

A cache layer on top of the external data source of users credentials and permissions can significantly improve performances in some cases, with the disadvantage of causing a (controllable) delay in picking up updates to the underlying data. Caching is disabled by default, it can be enabled by defining the options.cache field in security.authService, for example:

options = {
     cache = {
        expireAfterSecs = 120
        maxEntries = 10000
     }
}

This will enable a non-persistent cache contained in the node's memory with maximum number of entries set to maxEntries where entries are expired and refreshed after expireAfterSecs seconds.

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

CorDapps must whitelist any classes used over RPC with Corda's serialization framework, unless they are whitelisted by default in DefaultWhitelist. The whitelisting is done either via the plugin architecture or by using the @CordaSerializable annotation. See serialization. An example is shown in tutorial-clientrpc-api.