JSON-RPC

JSON-RPC is a remote procedure call protocol encoded in JSON. It is a very simple protocol (and very similar to XML-RPC), defining only a handful of data types and commands. JSON-RPC allows for notifications (data sent to the server that does not require a response) and for multiple calls to be sent to the server which may be answered out of order.

History

Usage

JSON-RPC works by sending a request to a server implementing this protocol. The client in that case is typically software intending to call a single method of a remote system. Multiple input parameters can be passed to the remote method as an array or object, whereas the method itself can return multiple output data as well. (This depends on the implemented version.)

A remote method is invoked by sending a request to a remote service using HTTP or a TCP/IP socket (starting with version 2.0). When using HTTP, the content-type may be defined as application/json.^[1]

All transfer types are single objects, serialized using JSON.^[2] A request is a call to a specific method provided by a remote system. It must contain three certain properties:

• method - A String with the name of the method to be invoked.
• params - An Array of objects to be passed as parameters to the defined method.
• id - A value of any type, which is used to match the response with the request that it is replying to.

The receiver of the request must reply with a valid response to all received requests. A response must contain the properties mentioned below.

• result - The data returned by the invoked method. If an error occurred while invoking the method, this value must be null.
• error - A specified error code if there was an error invoking the method, otherwise null.
• id - The id of the request it is responding to.

Since there are situations where no response is needed or even desired, notifications were introduced. A notification is similar to a request except for the id, which is not needed because no response will be returned. In this case the id property should be omitted (Version 2.0) or be null (Version 1.0).

Examples

In these examples, --> denotes data sent to a service (request), while <-- denotes data coming from a service. (Although <-- often is called response in client–server computing, depending on the JSON-RPC version it does not necessarily imply answer to a request).

Version 1.0

A simple request and response:

--> {"method": "echo", "params": ["Hello JSON-RPC"], "id": 1}
<-- {"result": "Hello JSON-RPC", "error": null, "id": 1}

This example shows parts of a communication from an example chat application. The chat service sends notifications for each chat message the client peer should receive. The client peer sends requests to post messages to the chat and expects a positive reply to know the message has been posted.^[2]

...
--> {"method": "postMessage", "params": ["Hello all!"], "id": 99}
<-- {"result": 1, "error": null, "id": 99}
<-- {"method": "handleMessage", "params": ["user1", "we were just talking"], "id": null}
<-- {"method": "handleMessage", "params": ["user3", "sorry, gotta go now, ttyl"], "id": null}
--> {"method": "postMessage", "params": ["I have a question:"], "id": 101}
<-- {"method": "userLeft", "params": ["user3"], "id": null}
<-- {"result": 1, "error": null, "id": 101}
...

Because params field is an array of objects, the following format is also ok:

{
    "method": "methodnamehere",
    "params": [
        {
            "firstparam": "this contains information of the firstparam.",
            "secondparam": 1121211234,
            "thirdparam": "this contains information of the thirdparam."
        },
        {
            "fourthparam": "this is already a different object.",
            "secondparam": "there can be same name fields in different objects.",
            "thirdparam": "this contains information of the thirdparam."
        }
    ],
    "id": 1234
}

Version 1.1 (Working Draft)

The format of the contents of a request might be something like that shown below:

{
    "version": "1.1",
    "method": "confirmFruitPurchase",
    "id": "194521489",
    "params": [
        ["apple", "orange", "mangoes"],
        1.123
    ]
}

The format of a response might be something like this:

{
    "version": "1.1",
    "result": "done",
    "error": null,
    "id": "194521489"
}

Version 2.0

Procedure call with positional parameters:

--> {"jsonrpc": "2.0", "method": "subtract", "params": [42, 23], "id": 1}
<-- {"jsonrpc": "2.0", "result": 19, "id": 1}

--> {"jsonrpc": "2.0", "method": "subtract", "params": [23, 42], "id": 2}
<-- {"jsonrpc": "2.0", "result": -19, "id": 2}

Procedure call with named parameters:

--> {"jsonrpc": "2.0", "method": "subtract", "params": {"subtrahend": 23, "minuend": 42}, "id": 3}
<-- {"jsonrpc": "2.0", "result": 19, "id": 3}

--> {"jsonrpc": "2.0", "method": "subtract", "params": {"minuend": 42, "subtrahend": 23}, "id": 4}
<-- {"jsonrpc": "2.0", "result": 19, "id": 4}

Notification:

--> {"jsonrpc": "2.0", "method": "update", "params": [1,2,3,4,5]}

--> {"jsonrpc": "2.0", "method": "foobar"}

Procedure call of non-existent procedure:

--> {"jsonrpc": "2.0", "method": "foobar", "id": 10}
<-- {"jsonrpc": "2.0", "error": {"code": -32601, "message": "Procedure not found."}, "id": 10}

Procedure call with invalid JSON:

--> {"jsonrpc": "2.0", "method": "foobar", "params": "bar", "baz"]
<-- {"jsonrpc": "2.0", "error": {"code": -32700, "message": "Parse error"}, "id": null}

Procedure call with invalid JSON-RPC:

--> [1,2,3]
<-- [
  {"jsonrpc": "2.0", "error": {"code": -32600, "message": "Invalid Request"}, "id": null},
  {"jsonrpc": "2.0", "error": {"code": -32600, "message": "Invalid Request"}, "id": null},
  {"jsonrpc": "2.0", "error": {"code": -32600, "message": "Invalid Request"}, "id": null}
]

--> {"jsonrpc": "2.0", "method": 1, "params": "bar"}
<-- {"jsonrpc": "2.0", "error": {"code": -32600, "message": "Invalid Request"}, "id": null}

Implementations

The original official homepage^[3] has links to more implementations. CPAN lists Perl implementations.

See also

• Remote procedure call (RPC)
• SOAPjr - a hybrid of SOAP and JSON-RPC
• JSON-WSP - a JSON-RPC inspired protocol with a service description specification.

References

1. ↑ RFC 4627
2. 1 2 specification - JSON-RPC - Trac
3. ↑ JSON-RPC - Trac

External links

• Official website
• JSON-RPC Google Group discussing topics of and around the protocol
• JSON-RPC specifications, links etc.
• Official JSON-RPC 1.0 homepage (currently outdated)