robocar-arduino/vendor/github.com/eclipse/paho.mqtt.golang/README.md

Eclipse Paho MQTT Go client

This repository contains the source code for the Eclipse Paho MQTT 3.1/3.11 Go client library.

This code builds a library which enable applications to connect to an MQTT broker to publish messages, and to subscribe to topics and receive published messages.

This library supports a fully asynchronous mode of operation.

A client supporting MQTT V5 is also available.

Installation and Build

The process depends upon whether you are using modules (recommended) or GOPATH.

Modules

If you are using modules then import "github.com/eclipse/paho.mqtt.golang" and start using it. The necessary packages will be download automatically when you run go build.

Note that the latest release will be downloaded and changes may have been made since the release. If you have encountered an issue, or wish to try the latest code for another reason, then run go get github.com/eclipse/paho.mqtt.golang@master to get the latest commit.

GOPATH

Installation is as easy as:

go get github.com/eclipse/paho.mqtt.golang

The client depends on Google's proxy package and the websockets package, also easily installed with the commands:

go get github.com/gorilla/websocket
go get golang.org/x/net/proxy

Usage and API

Detailed API documentation is available by using to godoc tool, or can be browsed online using the pkg.go.dev service.

Samples are available in the cmd directory for reference.

Note:

The library also supports using MQTT over websockets by using the ws:// (unsecure) or wss:// (secure) prefix in the URI. If the client is running behind a corporate http/https proxy then the following environment variables HTTP_PROXY, HTTPS_PROXY and NO_PROXY are taken into account when establishing the connection.

Troubleshooting

If you are new to MQTT and your application is not working as expected reviewing the MQTT specification, which this library implements, is a good first step. MQTT.org has some good resources that answer many common questions.

Error Handling

The asynchronous nature of this library makes it easy to forget to check for errors. Consider using a go routine to log these:

t := client.Publish("topic", qos, retained, msg)
go func() {
    _ = t.Wait() // Can also use '<-t.Done()' in releases > 1.2.0
    if t.Error() != nil {
        log.Error(t.Error()) // Use your preferred logging technique (or just fmt.Printf)
    }
}()

Logging

If you are encountering issues then enabling logging, both within this library and on your broker, is a good way to begin troubleshooting. This library can produce various levels of log by assigning the logging endpoints, ERROR, CRITICAL, WARN and DEBUG. For example:

func main() {
	mqtt.ERROR = log.New(os.Stdout, "[ERROR] ", 0)
	mqtt.CRITICAL = log.New(os.Stdout, "[CRIT] ", 0)
	mqtt.WARN = log.New(os.Stdout, "[WARN]  ", 0)
	mqtt.DEBUG = log.New(os.Stdout, "[DEBUG] ", 0)

	// Connect, Subscribe, Publish etc..
}

Common Problems

• Seemingly random disconnections may be caused by another client connecting to the broker with the same client identifier; this is as per the spec.
• Unless ordered delivery of messages is essential (and you have configured your broker to support this e.g. max_inflight_messages=1 in mosquitto) then set ClientOptions.SetOrderMatters(false). Doing so will avoid the below issue (deadlocks due to blocking message handlers).
• A MessageHandler (called when a new message is received) must not block (unless ClientOptions.SetOrderMatters(false) set). If you wish to perform a long-running task, or publish a message, then please use a go routine (blocking in the handler is a common cause of unexpected pingresp not received, disconnecting errors).
• When QOS1+ subscriptions have been created previously and you connect with CleanSession set to false it is possible that the broker will deliver retained messages before Subscribe can be called. To process these messages either configure a handler with AddRoute or set a DefaultPublishHandler. If there is no handler (or DefaultPublishHandler) then inbound messages will not be acknowledged. Adding a handler (even if it's opts.SetDefaultPublishHandler(func(mqtt.Client, mqtt.Message) {})) is highly recommended to avoid inadvertently hitting inflight message limits.
• Loss of network connectivity may not be detected immediately. If this is an issue then consider setting ClientOptions.KeepAlive (sends regular messages to check the link is active).
• Reusing a Client is not completely safe. After calling Disconnect please create a new Client (NewClient()) rather than attempting to reuse the existing one (note that features such as SetAutoReconnect mean this is rarely necessary).
• Brokers offer many configuration options; some settings may lead to unexpected results.
• Publish tokens will complete if the connection is lost and re-established using the default options.SetAutoReconnect(true) functionality (token.Error() will return nil). Attempts will be made to re-deliver the message but there is currently no easy way know when such messages are delivered.

If using Mosquitto then there are a range of fairly common issues:

• listener - By default Mosquitto v2+ listens on loopback interfaces only (meaning it will only accept connections made from the computer its running on).
• max_inflight_messages - Unless this is set to 1 mosquitto does not guarantee ordered delivery of messages.
• max_queued_messages / max_queued_bytes - These impose limits on the number/size of queued messages. The defaults may lead to messages being silently dropped.
• persistence - Defaults to false (messages will not survive a broker restart)
• max_keepalive - defaults to 65535 and, from version 2.0.12, SetKeepAlive(0) will result in a rejected connection by default.

Reporting bugs

Please report bugs by raising issues for this project in github https://github.com/eclipse/paho.mqtt.golang/issues

A limited number of contributors monitor the issues section so if you have a general question please see the resources in the more information section for help.

We welcome bug reports, but it is important they are actionable. A significant percentage of issues reported are not resolved due to a lack of information. If we cannot replicate the problem then it is unlikely we will be able to fix it. The information required will vary from issue to issue but almost all bug reports would be expected to include:

• Which version of the package you are using (tag or commit - this should be in your go.mod file)
• A full, clear, description of the problem (detail what you are expecting vs what actually happens).
• Configuration information (code showing how you connect, please include all references to ClientOption)
• Broker details (name and version).

If at all possible please also include:

• Details of your attempts to resolve the issue (what have you tried, what worked, what did not).
• A Minimal, Reproducible Example. Providing an example is the best way to demonstrate the issue you are facing; it is important this includes all relevant information (including broker configuration). Docker (see cmd/docker) makes it relatively simple to provide a working end-to-end example.
• Broker logs covering the period the issue occurred.
• Application Logs covering the period the issue occurred. Unless you have isolated the root cause of the issue please include a link to a full log (including data from well before the problem arose).

It is important to remember that this library does not stand alone; it communicates with a broker and any issues you are seeing may be due to:

• Bugs in your code.
• Bugs in this library.
• The broker configuration.
• Bugs in the broker.
• Issues with whatever you are communicating with.

When submitting an issue, please ensure that you provide sufficient details to enable us to eliminate causes outside of this library.

Contributing

We welcome pull requests but before your contribution can be accepted by the project, you need to create and electronically sign the Eclipse Contributor Agreement (ECA) and sign off on the Eclipse Foundation Certificate of Origin.

More information is available in the Eclipse Development Resources; please take special note of the requirement that the commit record contain a "Signed-off-by" entry.

More information

Stack Overflow has a range questions/answers covering a range of common issues (both relating to use of this library and MQTT in general). This is the best place to ask general questions (including those relating to the use of this library).

Discussion of the Paho clients takes place on the Eclipse paho-dev mailing list.

General questions about the MQTT protocol are discussed in the MQTT Google Group.

There is much more information available via the MQTT community site.