pulse

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Published: Apr 23, 2019 License: MPL-2.0 Imports: 8 Imported by: 4

Documentation

Overview

Package pulse provides operations for consuming mozilla pulse messages (see https://pulse.mozilla.org/).

For users that are interested in publishing messages, or having lower level control of the amqp interactions with pulse, take a look at http://godoc.org/github.com/streadway/amqp. This library is built on top of the amqp package.

For a user that is simply interesting in consuming pulse messages without wishing to acquire a detailed understanding of how pulse.mozilla.org has been designed, or how AMQP 0.9.1 works, this client provides basic utility methods to get you started off quickly.

Please note that parent package "github.com/taskcluster/pulse-go" provides a very simple command line interface into this library too, which can be called directly from a shell, for example, so that the user requires no go programming expertise, and can directly write e.g. shell scripts that process pulse messages.

To get started, we have created an example program which uses this library. The source code for this example is available at https://github.com/taskcluster/pulse-go/blob/master/pulsesniffer/pulsesniffer.go. Afterwards, we will describe how it works. Do not worry if none of it makes sense now. By the end of this overview it will all be explained.

// Package pulsesniffer provides a simple example program that listens to some
// real world pulse messages.
package main

import (
	"fmt"
	"github.com/taskcluster/pulse-go/pulse"
	"github.com/streadway/amqp"
)

func main() {
	// Passing all empty strings:
	// empty user => use PULSE_USERNAME env var
	// empty password => use PULSE_PASSWORD env var
	// empty url => connect to production
	conn := pulse.NewConnection("", "", "")
	conn.Consume(
		"taskprocessing", // queue name
		func(message interface{}, delivery amqp.Delivery) { // callback function to pass messages to
			fmt.Println("Received from exchange " + delivery.Exchange + ":")
			fmt.Println(string(delivery.Body))
			fmt.Println("")
			delivery.Ack(false) // acknowledge message *after* processing
		},
		1,     // prefetch 1 message at a time
		false, // don't auto-acknowledge messages
		pulse.Bind( // routing key and exchange to get messages from
			"*.*.*.*.*.*.gaia.#",
			"exchange/taskcluster-queue/v1/task-defined"),
		pulse.Bind( // another routing key and exchange to get messages from
			"*.*.*.*.*.aws-provisioner.#",
			"exchange/taskcluster-queue/v1/task-running"))
	conn.Consume( // a second workflow to manage concurrently
		"", // empty name implies anonymous queue
		func(message interface{}, delivery amqp.Delivery) { // simpler callback than before
			fmt.Println("Buildbot message received")
			fmt.Println("")
		},
		1,    // prefetch
		true, // auto acknowledge, so no need to call delivery.Ack
		pulse.Bind( // routing key and exchange to get messages from
			"#", // get *all* normalized buildbot messages
			"exchange/build/normalized"))
	// wait forever
	forever := make(chan bool)
	<-forever
}

The first thing we need to do is provide connection details for connecting to the pulse server, which we do like this:

conn := pulse.NewConnection("", "", "")

In this example, the provided strings (username, password, url) have all been left empty. This is because by default, if you provide no username or password, the NewConnection function will inspect environment variables PULSE_USERNAME and PULSE_PASSWORD, and an empty url will trigger the library to use the current production url. Another example call could be:

conn := pulse.NewConnection("guest", "guest", "amqp://localhost:5672/")

Typically we would set the username and password credentials via environment variables to avoid hardcoding them in the go code. For more details about managing the username, password and amqp url, see the documentation for the NewConnection function.

A call to NewConnection does not actually create a connection to the pulse server, it simply prepares the data that will be needed when we finally make the connection. Users and passwords can be created by going to the Pulse Guardian (https://pulse.mozilla.org) and registering an account.

You will see in the code above, that after creating a connection, there is only one more method we call - Consume - which we use for processing messages. This is the heart of the pulse library, and where all of the action happens.

In pulse, all messages are delivered to "topic exchanges" and the way to receive these messages is to request the ones you are interested in are copied onto a queue you can read from, and then to read them from the queue. This is called binding. To bind messages from an exchange to a queue, you specify the name of the exchange you want to receive messages from, and a matching criteria to define the ones you want. The matching process is handled by routing keys, which will now be explained.

Each message that arrives on an exchange has a "routing key" signature. The routing key comprises of several fields. For an example, see: https://docs.taskcluster.net/reference/platform/queue/exchanges#taskDefined. The fields are delimited by dots, and therefore the routing key of a message is represented as a '.' delimited string. In order to select the messages on an exchange that you wish to receive, you specify a matching routing key. For each field of the routing key, you can either match against a specific value, or match all entries with the '*' wildcard. Above, we specified the following routing key and exchange:

pulse.Bind( // routing key and exchange to get messages from
"*.*.*.*.*.*.gaia.#",
"exchange/taskcluster-queue/v1/task-defined"),

This would match all messages on the exchange "exchange/taskcluster-queue/v1/task-defined" which have a workerType of "gaia" (see the taskDefined link above). Notice also the '#' at the end of the string. This means "match all remaining fields" and can be used to match whatever comes after.

To see the list of available exchanges on pulse, visit https://wiki.mozilla.org/Auto-tools/Projects/Pulse/Exchanges.

After deciding which exchanges you are interested in, you need a queue to have them copied onto. This is also handled by the Consume method, with the first argument being the name of the queue to use. You will notice above there are two types of queues we create: named queues, and unnamed queues:

conn.Consume(
	"taskprocessing", // queue name

conn.Consume( // a second workflow to manage concurrently
	"", // empty name implies anonymous queue

To understand the difference, first we need to explain the different scenarios in which you might want to use them.

Scenario 1) You have one client reading from the queue, and when you disconnect, you don't want your queue to receive any more messages

Scenario 2) you have multiple clients that want to feed from the same queue (e.g. when multiple workers can perform the same task, and whichever one pops the message off the queue first should process it)

Scenario 3) you only have a single client reading from the queue, but if you go offline (crash, network interrupts etc) then you want pulse to keep updating your queue so your missed messages are there when you get back.

In scenario 1 above, your client only uses the queue for the scope of the connection, and as soon as it disconnects, does not require the queue any further. In this case, an unnamed queue can be created, by passing "" as the queue name. When the connection closes, the AMQP server will automatically delete the queue.

In scenarios 2 it is useful to have a friendly name for the queue that can be shared by all the clients using it. The queue also should not be deleted when one client disconnects, it needs to live indefinitely. By providing a name for the queue, this signifies to the pulse library, that the queue should persist after a disconnect, and pulse should continue to populate the queue, even if no pulse clients are connected to consume the messages. Please note eventually the Pulse Guardian will delete your queue if you leave it collecting messages without consuming them.

Scenario 3 is essentially the same as scenario 2 but with one consumer only. Again, a named queue is required.

So, we're nearly done now. We now have a means to consume messages, by calling the Consume method, and specifying a queue name, some bindings of exchanges and routing keys, but how to actually process messages arriving on the queue?

You will notice the Consume method takes a callback function. This can be an inline function, or point to any available function in your go code. You simply need to have a function that accepts an amqp.Delivery input, and pass it into the Consume method. Above, we did it like this:

func(message interface{}, delivery amqp.Delivery) { // callback function to pass messages to
	fmt.Println("Received from exchange " + delivery.Exchange + ":")
	fmt.Println(string(delivery.Body))
	fmt.Println("")
	delivery.Ack(false) // acknowledge message *after* processing
},

The two parameters of the callback function we have created are the message object, and the delivery object. The message object is the pulse message, but unmarshaled into an interface{}. Since the pulse messages are all json messages, the pulse library unmarshals it and give you back a go object with its contents. Please note if you require that the json is unmarshaled into something more specific than interface{}, such as a custom class, this is possible, and will be explained in the next paragraph. The other parameter, the delivery object, is an underlying amqp library type, which gives you access to some meta data for the message. Please see http://godoc.org/github.com/streadway/amqp#Delivery for more information. Among other things, it provides you with delivery.Body, which is the raw json of the message. You can therefore choose if you want to process the raw json or the unmarshaled json in your callback method.

You recall above that to describe the binding from an exchange to a queue with a given routing key, we specified pulse.Bind(routingKey, exchange) as a parameter of the Consume method. pulse.Bind(routingKey, exchange) returns an object of type Binding, where Binding is an interface. If you wish to unmarshal your json into something other than an interface{}, take a look at the Binding interface documentation (http://godoc.org/github.com/taskcluster/pulse-go/pulse#Binding). Instead of calling pulse.Bind(routingKey, exchange) you can provide your own Binding interface implementation which can enable custom handling of exchange names, routing keys, and unmarshaling of objects. The taskcluster go client relies heavily on this, for example. See http://godoc.org/github.com/taskcluster/taskcluster-client-go/queueevents#example-package--TaskclusterSniffer for inspiration.

In this example above, we simply output the information we receive, and then acknowledge receipt of the message. But why do we need to do this? To explain, take a look at the remaining parameters to Consume that we pass in. There are two more we have not discussed yet: they are the prefetch size (how many messages to fetch at once), and a bool to say whether to auto-acknowledge messages or not.

1,     // prefetch 1 message at a time
false, // don't auto-acknowledge messages

When you acknowledge a message, it gets popped off the queue. If you don't auto-acknowledge, and also don't manually acknowledge, your queue is going to grow until it gets deleted by Pulse Guardian, so better to acknowledge those messages! Auto-acknowledge happens when you receive the message; if you crash after receiving it but before processing it, you may have a problem. If it is important not to lose messages in such a scenario, you can acknowledge manually *after* processing the message. See above:

delivery.Ack(false) // acknowledge message *after* processing

This is "more work" for you to do, but guarantees that you don't lose messages. To handle situation of crashing after processing, but before acknowledging, having an idempotent message processing function (the callback) should help avoid the problem of processing a message twice.

Please note the Consume method will take care of connecting to the pulse server (if no connection has yet been established), creating an AMQP channel, creating or connecting to an existing queue, binding it to all the exchanges and routing keys that you specify, and spawning a dedicated go routine to process the messages from this queue and feed them back to the callback method you provide.

The client is implemented in such a way that a new AMQP channel is created for each queue that you consume, and that a separate go routine handles calling the callback function you specify. This means you can take advantage of go's built in concurrency support, and call the Consume method as many times as you wish.

The aim of this library is to shield users from this lower-level resource management, and provide a simple interface in order to quickly and easily develop components that can interact with pulse.

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type Binding

type Binding interface {

	// This should return a routing key string for matching pulse messages
	RoutingKey() string

	// This should return the fully qualified name of the pulse exchange to
	// bind messages from
	ExchangeName() string

	// This should return a pointer to a new object for unmarshaling matching
	// pulse messages into
	NewPayloadObject() interface{}
}

Binding interface allows you to create custom types to describe exchange / routing key combinations. For example Binding types are generated in Task Cluster go client to avoid a library user referencing a non existent exchange, or an invalid routing key.

func Bind

func Bind(routingKey, exchangeName string) Binding

Convenience function for returning a Binding for the given routing key and exchange strings, which can be passed to the Consume method of *Connection. Typically this is used if you wish to refer to exchanges and routing keys with explicit strings, rather than generated types (e.g. Task Cluster go client generates custom types to avoid invalid exchange names or invalid routing keys). See the Consume method for more information.

type Connection

type Connection struct {
	User     string
	Password string
	URL      string
	AMQPConn *amqp.Connection
	// contains filtered or unexported fields
}

Connection manages the underlying AMQP connection, and provides an interface for performing further actions, such as creating a queue.

func NewConnection

func NewConnection(pulseUser string, pulsePassword string, amqpUrl string) Connection

NewConnection prepares a Connection object with a username, password and an AMQP URL, but does not actually make an outbound connection to the service. An actual network connection will be made the first time the Consume method is called.

The logic for deriving the AMQP url is as follows:

If the provided amqpUrl is a non-empty string, it will be used to set the AMQP URL. Otherwise, production will be used ("amqps://pulse.mozilla.org:5671")

The pulse user is determined as follows:

If the provided pulseUser is a non-empty string, it will be used for AMQP connection user. Otherwise, if the amqlUrl contains a user, it will be used. Otherwise, if environment variable PULSE_USERNAME is non empty, it will be used. Otherwise, the value "guest" will be used.

The pulse password is determined as follows:

If the provided pulsePassword is a non-empty string, it will be used for AMQP connection password. Otherwise, if the amqlUrl contains a password, it will be used. Otherwise, if environment variable PULSE_PASSWORD is non empty, it will be used. Otherwise, the value "guest" will be used.

Finally, the AMQP url is adjusted, by stripping out any user/password contained inside it, and then embedding the derived username and password above.

Typically, a call to this method would look like:

conn := pulse.NewConnection("", "", "")

whereby the client program would export PULSE_USERNAME and PULSE_PASSWORD environment variables before calling the go program, and the empty url would signify that the client should connect to the production instance.

func (*Connection) Consume

func (c *Connection) Consume(
	queueName string,
	callback func(interface{}, amqp.Delivery),
	prefetch int,
	autoAck bool,
	bindings ...Binding,
) (
	PulseQueue,
	error,
)

Consume is at the heart of the pulse library. After creating a connection with pulse.NewConnection(...) above, you can call the Consume method to register a queue, set a callback function to be called with each message received on the queue and bind the queue to a list of exchange / routing key pairs. See the package overview for a walkthrough example. A go routine will be spawned to take care of calling the callback function, and a new AMQP channel will be created behind-the-scenes to handle the processing.

queueName is the name of the queue to connect to or create; leave empty for an anonymous queue that will get auto deleted after disconnecting, or provide a name for a long-lived queue. callback specifies the function to be called with each message that arrives. prefetch specifies how many messages should be read from the queue at a time. autoAck is a bool to specify if auto acknowledgements should be sent or not; if not auto-acknowledging, remember to ack / nack in your callback method. bindings is a variadic input of the exchange names / routing keys that you wish pulse to copy to your queue.

type PulseError

type PulseError struct {
	Message         string
	LowerLevelError error
}

PulseError is used for describing problems that occurred when interacting with Pulse, caused by a lower-level error

func Error

func Error(err error, msg string) PulseError

Utility function for generating a PulseError

func (PulseError) Error

func (err PulseError) Error() string

Utility method used for checking an error condition, and failing with a given error message if the error is not nil. msg should contain a description of what activity could not be performed as required.

type PulseQueue

type PulseQueue struct {
}

PulseQueue manages an underlying AMQP queue, and provides methods for closing, deleting, pausing and resuming queues.

func (*PulseQueue) Close

func (pq *PulseQueue) Close()

TODO: not yet implemented

func (*PulseQueue) Delete

func (pq *PulseQueue) Delete()

TODO: not yet implemented

func (*PulseQueue) Pause

func (pq *PulseQueue) Pause()

TODO: not yet implemented

func (*PulseQueue) Resume

func (pq *PulseQueue) Resume()

TODO: not yet implemented

Source Files

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