MQTT-JSON-RPC

JSON-RPC protocol over MQTT communication.

Installation

$ npm install mqtt mqtt-json-rpc

About

This is an addon API for the MQTT.js API of Node.js, for Remote Procedure Call (RPC) communication based on the JSON-RPC protocol. This allows a bi-directional request/response-style communication over the technically uni-directional message protocol MQTT.

Usage

Server:

const MQTT = require("mqtt")
const RPC  = require("mqtt-json-rpc")

const mqtt = MQTT.connect("wss://127.0.0.1:8889", { ... })
const rpc  = new RPC(mqtt)

mqtt.on("connect", () => {
    rpc.register("example/hello", (a1, a2) => {
        console.log("example/hello: request: ", a1, a2)
        return `${a1}:${a2}`
    })
})

Client:

const MQTT = require("mqtt")
const RPC  = require("mqtt-json-rpc")

const mqtt = MQTT.connect("wss://127.0.0.1:8889", { ... })
const rpc  = new RPC(mqtt)

mqtt.on("connect", () => {
    rpc.call("example/hello", "world", 42).then((response) => {
        console.log("example/hello response: ", response)
        mqtt.end()
    })
})

Application Programming Interface

The MQTT-JSON-RPC API provides the following methods (check out the corresponding TypeScript definition) file):

• constructor(mqtt: MQTT, options?: { encoding?: string, timeout?: number }): MQTT-JSON-RPC: The mqtt is the MQTT.js instance. The optional encoding option can be either json (default), msgpack or cbor. The optional timeout option is the timeout in seconds.

• MQTT-JSON-RPC#registered(method: string): boolean: Check for the previous registration of a method. The method has to be a valid MQTT topic name. The method returns true if method is already registered, else it returns false.

• MQTT-JSON-RPC#register(method: string, callback: (...args: any[]) => any): Promise: Register a method. The method has to be a valid MQTT topic name. The callback is called with the params passed to the remote MQTT-JSON-RPC#notify() or MQTT-JSON-RPC#call(). For a remote MQTT-JSON-RPC#notify(), the return value of callback will be ignored. For a remote MQTT-JSON-RPC#call(), the return value of callback will resolve the promise returned by the remote MQTT-JSON-RPC#call(). Internally, on the MQTT broker the topic ${method}/request is subscribed.

• MQTT-JSON-RPC#unregister(method: string): Promise: Unregister a previously registered method. Internally, on the MQTT broker the topic ${method}/request is unsubscribed.

• MQTT-JSON-RPC#notify(method: string, ...params: any[]): void: Notify a method. The remote MQTT-JSON-RPC#register() callback is called with params and its return value is silently ignored.

• MQTT-JSON-RPC#call(method: string, ...params: any[]): Promise: Call a method. The remote MQTT-JSON-RPC#register() callback is called with params and its return value resolves the returned Promise. If the remote callback throws an exception, this rejects the returned Promise. Internally, on the MQTT broker the topic ${method}/response/<cid> is temporarily subscribed for receiving the response (<cid> is a UUID v1 to uniquely identify the MQTT-JSON-RPC caller instance).

Internals

Internally, remote methods are assigned to MQTT topics. When calling a remote method named example/hello with parameters "world" and 42 via...

rpc.call("example/hello", "world", 42).then((result) => {
    ...
})

...the following JSON-RPC 2.0 request message is sent to the permanent MQTT topic example/hello/request:

{
    "jsonrpc": "2.0",
    "id":      "d1acc980-0e4e-11e8-98f0-ab5030b47df4:d1db7aa0-0e4e-11e8-b1d9-5f0ab230c0d9",
    "method":  "example/hello",
    "params":  [ "world", 42 ]
}

Beforehand, this example/hello method should have been registered with...

rpc.register("example/hello", (a1, a2) => {
    return `${a1}:${a2}`
})

...and then its result, in the above rpc.call example "world:42", is then sent back as the following JSON-RPC 2.0 success response message to the temporary (client-specific) MQTT topic example/hello/response/d1acc980-0e4e-11e8-98f0-ab5030b47df4:

{
    "jsonrpc": "2.0",
    "id":      "d1acc980-0e4e-11e8-98f0-ab5030b47df4:d1db7aa0-0e4e-11e8-b1d9-5f0ab230c0d9",
    "result":  "world:42"
}

The JSON-RPC 2.0 id field always consists of <cid>:<rid>, where <cid> is the UUID v1 of the MQTT-JSON-RPC instance and <rid> is the UUID v1 of the particular method request. The <cid> is used for sending back the JSON-RPC 2.0 response message to the requestor only.

Example

For a real test-drive of MQTT-JSON-RPC, install the Mosquitto MQTT broker with at least a "MQTT over Secure-WebSockets" lister in the mosquitto.conf file like...

[...]

password_file        mosquitto-pwd.txt
acl_file             mosquitto-acl.txt

[...]

#   additional listener (wss: MQTT over WebSockets+SSL/TLS)
listener             8889 127.0.0.1
max_connections      -1
protocol             websockets
cafile               mosquitto-ca.crt.pem
certfile             mosquitto-sv.crt.pem
keyfile              mosquitto-sv.key.pem
require_certificate  false

[...]

...and an access control list in mosquitto-acl.txt like...

user    example
topic   readwrite example/#

...and an example user (with password example) in mosquitto-pwd.txt like:

example:$6$awYNe6oCAi+xlvo5$mWIUqyy4I0O3nJ99lP1mkRVqsDGymF8en5NChQQxf7KrVJLUp1SzrrVDe94wWWJa3JGIbOXD9wfFGZdi948e6A==

Then test-drive MQTT-JSON-RPC with a complete sample to see MQTT-JSON-RPC in action and tracing its communication:

const MQTT = require("mqtt")
const RPC  = require("mqtt-json-rpc")

const mqtt = MQTT.connect("wss://127.0.0.1:8889", {
    rejectUnauthorized: false,
    username: "example",
    password: "example"
})

const rpc = new RPC(mqtt)

mqtt.on("error",     (err)            => { console.log("ERROR", err) })
mqtt.on("offline",   ()               => { console.log("OFFLINE") })
mqtt.on("close",     ()               => { console.log("CLOSE") })
mqtt.on("reconnect", ()               => { console.log("RECONNECT") })
mqtt.on("message",   (topic, message) => { console.log("RECEIVED", topic, message.toString()) })

mqtt.on("connect", () => {
    console.log("CONNECT")
    rpc.register("example/hello", (a1, a2) => {
        console.log("example/hello: request: ", a1, a2)
        return `${a1}:${a2}`
    })
    rpc.call("example/hello", "world", 42).then((result) => {
        console.log("example/hello sucess: ", result)
        mqtt.end()
    }).catch((err) => {
        console.log("example/hello error: ", err)
    })
})

The output will be:

$ node sample.js
CONNECT
RECEIVED example/hello/request {"jsonrpc":"2.0","id":"1099cb50-bd2b-11eb-8198-43568ad728c4:10bf7bc0-bd2b-11eb-bac6-439c565b651a","method":"example/hello","params":["world",42]}
example/hello: request:  world 42
RECEIVED example/hello/response/1099cb50-bd2b-11eb-8198-43568ad728c4 {"jsonrpc":"2.0","id":"1099cb50-bd2b-11eb-8198-43568ad728c4:10bf7bc0-bd2b-11eb-bac6-439c565b651a","result":"world:42"}
example/hello sucess:  world:42
CLOSE

License

Copyright (c) 2018-2023 Dr. Ralf S. Engelschall (http://engelschall.com/)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.