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32.30.1 Overview

Quoting from the spec, JSONRPC "is transport agnostic in that the concepts can be used within the same process, over sockets, over http, or in many various message passing environments."

To model this agnosticism, the jsonrpc library uses objects of a jsonrpc-connection class, which represent a connection to a remote JSON endpoint (for details on Emacs’s object system, see EIEIO in EIEIO). In modern object-oriented parlance, this class is “abstract”, i.e. the actual class of a useful connection object is always a subclass of jsonrpc-connection. Nevertheless, we can define two distinct APIs around the jsonrpc-connection class:

  1. A user interface for building JSONRPC applications

    In this scenario, the JSONRPC application selects a concrete subclass of jsonrpc-connection, and proceeds to create objects of that subclass using make-instance. To initiate a contact to the remote endpoint, the JSONRPC application passes this object to the functions jsonrpc-notify, jsonrpc-request, and/or jsonrpc-async-request. For handling remotely initiated contacts, which generally come in asynchronously, the instantiation should include :request-dispatcher and :notification-dispatcher initargs, which are both functions of 3 arguments: the connection object; a symbol naming the JSONRPC method invoked remotely; and a JSONRPC params object.

    The function passed as :request-dispatcher is responsible for handling the remote endpoint’s requests, which expect a reply from the local endpoint (in this case, the program you’re building). Inside that function, you may either return locally (a normal return) or non-locally (an error return). A local return value must be a Lisp object that can be serialized as JSON (see Parsing JSON). This determines a success response, and the object is forwarded to the server as the JSONRPC result object. A non-local return, achieved by calling the function jsonrpc-error, causes an error response to be sent to the server. The details of the accompanying JSONRPC error are filled out with whatever was passed to jsonrpc-error. A non-local return triggered by an unexpected error of any other type also causes an error response to be sent (unless you have set debug-on-error, in which case this calls the Lisp debugger, see Error Debugging).

  2. A inheritance interface for building JSONRPC transport implementations

    In this scenario, jsonrpc-connection is subclassed to implement a different underlying transport strategy (for details on how to subclass, see (eieio)Inheritance.). Users of the application-building interface can then instantiate objects of this concrete class (using the make-instance function) and connect to JSONRPC endpoints using that strategy.

    This API has mandatory and optional parts.

    To allow its users to initiate JSONRPC contacts (notifications or requests) or reply to endpoint requests, the subclass must have an implementation of the jsonrpc-connection-send method.

    Likewise, for handling the three types of remote contacts (requests, notifications, and responses to local requests), the transport implementation must arrange for the function jsonrpc-connection-receive to be called after noticing a new JSONRPC message on the wire (whatever that "wire" may be).

    Finally, and optionally, the jsonrpc-connection subclass should implement the jsonrpc-shutdown and jsonrpc-running-p methods if these concepts apply to the transport. If they do, then any system resources (e.g. processes, timers, etc.) used to listen for messages on the wire should be released in jsonrpc-shutdown, i.e. they should only be needed while jsonrpc-running-p is non-nil.

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