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doc/codewalk: Share Memory By Communicating
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Andrew Gerrand
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doc/codewalk/sharemem.xml
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doc/codewalk/sharemem.xml
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<codewalk title="Share Memory By Communicating">
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<step title="Introduction" src="doc/codewalk/urlpoll.go">
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Go's approach to concurrency differs from the traditional use of
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threads and shared memory. Philosophically, it can be summarized:
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<br/><br/>
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<i>Don't communicate by sharing memory; share memory by communicating.</i>
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<br/><br/>
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Channels allow you to pass references to data structures between goroutines.
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If you consider this as passing around ownership of the data (the ability to
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read and write it), they become a powerful and expressive synchronization
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mechanism.
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<br/><br/>
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In this codewalk we will look at a simple program that polls a list of
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URLs, checking their HTTP response codes and periodically printing their state.
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</step>
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<step title="State type" src="doc/codewalk/urlpoll.go:/State/,/}/">
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The State type represents the state of a URL.
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<br/><br/>
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The Pollers send State values to the StateMonitor,
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which maintains a map of the current state of each URL.
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</step>
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<step title="Resource type" src="doc/codewalk/urlpoll.go:/Resource/,/}/">
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A Resource represents the state of a URL to be polled: the URL itself
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and the number of errors encountered since the last successful poll.
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<br/><br/>
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When the program starts, it allocates one Resource for each URL.
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The main goroutine and the Poller goroutines send the Resources to
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each other on channels.
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</step>
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<step title="Poller function" src="doc/codewalk/urlpoll.go:/func Poller/,/\n}/">
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Each Poller receives Resource pointers from an input channel.
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In this program, the convention is that sending a Resource pointer on
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a channel passes ownership of the underlying data from the sender
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to the receiver. Because of this convention, we know that
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no two goroutines will access this Resource at the same time.
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This means we don't have to worry about locking to prevent concurrent
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access to these data structures.
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<br/><br/>
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The Poller processes the Resource by calling its Poll method.
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<br/><br/>
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It sends a State value to the status channel, to inform the StateMonitor
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of the result of the Poll.
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<br/><br/>
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Finally, it sends the Resource pointer to the out channel. This can be
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interpreted as the Poller saying "I'm done with this Resource" and
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returning ownership of it to the main goroutine.
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<br/><br/>
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Several goroutines run Pollers, processing Resources in parallel.
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</step>
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<step title="The Poll method" src="doc/codewalk/urlpoll.go:/Poll executes/,/\n}/">
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The Poll method (of the Resource type) performs an HTTP HEAD request
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for the Resource's URL and returns the HTTP response's status code.
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If an error occurs, Poll logs the message to standard error and returns the
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error string instead.
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</step>
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<step title="main function" src="doc/codewalk/urlpoll.go:/func main/,/\n}/">
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The main function starts the Poller and StateMonitor goroutines
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and then loops passing completed Resources back to the pending
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channel after appropriate delays.
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</step>
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<step title="Creating channels" src="doc/codewalk/urlpoll.go:/create our/,/complete/">
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First, main makes two channels of *Resource, pending and complete.
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<br/><br/>
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Inside main, a new goroutine sends one Resource per URL to pending
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and the main goroutine receives completed Resources from complete.
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<br/><br/>
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The pending and complete channels are passed to each of the Poller
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goroutines, within which they are known as in and out.
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</step>
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<step title="Initializing StateMonitor" src="doc/codewalk/urlpoll.go:/launch the StateMonitor/,/statusInterval/">
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StateMonitor will initialize and launch a goroutine that stores the state
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of each Resource. We will look at this function in detail later.
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<br/><br/>
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For now, the important thing to note is that it returns a channel of State,
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which is saved as status and passed to the Poller goroutines.
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</step>
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<step title="Launching Poller goroutines" src="doc/codewalk/urlpoll.go:/launch some Poller/,/}/">
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Now that it has the necessary channels, main launches a number of
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Poller goroutines, passing the channels as arguments.
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The channels provide the means of communication between the main, Poller, and
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StateMonitor goroutines.
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</step>
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<step title="Send Resources to pending" src="doc/codewalk/urlpoll.go:/send some Resources/,/}\(\)/">
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To add the initial work to the system, main starts a new goroutine
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that allocates and sends one Resource per URL to pending.
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<br/><br/>
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The new goroutine is necessary because unbuffered channel sends and
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receives are synchronous. That means these channel sends will block until
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the Pollers are ready to read from pending.
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<br/><br/>
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Were these sends performed in the main goroutine with fewer Pollers than
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channel sends, the program would reach a deadlock situation, because
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main would not yet be receiving from complete.
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<br/><br/>
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Exercise for the reader: modify this part of the program to read a list of
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URLs from a file. (You may want to move this goroutine into its own
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named function.)
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</step>
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<step title="Main Event Loop" src="doc/codewalk/urlpoll.go:/range complete/,/\n }/">
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When a Poller is done with a Resource, it sends it on the complete channel.
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This loop receives those Resource pointers from complete.
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For each received Resource, it starts a new goroutine calling
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the Resource's Sleep method. Using a new goroutine for each
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ensures that the sleeps can happen in parallel.
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<br/><br/>
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Note that any single Resource pointer may only be sent on either pending or
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complete at any one time. This ensures that a Resource is either being
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handled by a Poller goroutine or sleeping, but never both simultaneously.
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In this way, we share our Resource data by communicating.
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</step>
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<step title="The Sleep method" src="doc/codewalk/urlpoll.go:/Sleep/,/\n}/">
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Sleep calls time.Sleep to pause before sending the Resource to done.
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The pause will either be of a fixed length (pollInterval) plus an
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additional delay proportional to the number of sequential errors (r.errCount).
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<br/><br/>
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This is an example of a typical Go idiom: a function intended to run inside
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a goroutine takes a channel, upon which it sends its return value
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(or other indication of completed state).
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</step>
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<step title="StateMonitor" src="doc/codewalk/urlpoll.go:/StateMonitor/,/\n}/">
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The StateMonitor receives State values on a channel and periodically
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outputs the state of all Resources being polled by the program.
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</step>
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<step title="The updates channel" src="doc/codewalk/urlpoll.go:/updates :=/">
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The variable updates is a channel of State, on which the Poller goroutines
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send State values.
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<br/><br/>
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This channel is returned by the function.
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</step>
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<step title="The urlStatus map" src="doc/codewalk/urlpoll.go:/urlStatus/">
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The variable urlStatus is a map of URLs to their most recent status.
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</step>
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<step title="The Ticker object" src="doc/codewalk/urlpoll.go:/ticker/">
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A time.Ticker is an object that repeatedly sends a value on a channel at a
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specified interval.
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<br/><br/>
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In this case, ticker triggers the printing of the current state to
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standard output every updateInterval nanoseconds.
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</step>
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<step title="The StateMonitor goroutine" src="doc/codewalk/urlpoll.go:/go func/,/}\(\)/">
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StateMonitor will loop forever, selecting on two channels:
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ticker.C and update. The select statement blocks until one of its
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communications is ready to proceed.
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<br/><br/>
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When StateMonitor receives a tick from ticker.C, it calls logState to
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print the current state. When it receives a State update from updates,
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it records the new status in the urlStatus map.
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<br/><br/>
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Notice that this goroutine owns the urlStatus data structure,
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ensuring that it can only be accessed sequentially.
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This prevents memory corruption issues that might arise from parallel reads
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and/or writes to a shared map.
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</step>
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<step title="Conclusion" src="doc/codewalk/urlpoll.go">
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In this codewalk we have explored a simple example of using Go's concurrency
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primitives to share memory through commmunication.
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<br/><br/>
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This should provide a starting point from which to explore the ways in which
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goroutines and channels can be used to write expressive and concise concurrent
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programs.
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</step>
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</codewalk>
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117
doc/codewalk/urlpoll.go
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doc/codewalk/urlpoll.go
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// Copyright 2010 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package main
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import (
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"http"
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"log"
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"time"
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)
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const (
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numPollers = 2 // number of Poller goroutines to launch
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second = 1e9 // one second is 1e9 nanoseconds
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pollInterval = 60 * second // how often to poll each URL
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statusInterval = 10 * second // how often to log status to stdout
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errTimeout = 10 * second // back-off timeout on error
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)
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var urls = []string{
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"http://www.google.com/",
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"http://golang.org/",
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"http://blog.golang.org/",
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}
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// State represents the last-known state of a URL.
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type State struct {
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url string
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status string
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}
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// StateMonitor maintains a map that stores the state of the URLs being
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// polled, and prints the current state every updateInterval nanoseconds.
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// It returns a chan State to which resource state should be sent.
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func StateMonitor(updateInterval int64) chan<- State {
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updates := make(chan State)
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urlStatus := make(map[string]string)
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ticker := time.NewTicker(updateInterval)
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go func() {
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for {
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select {
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case <-ticker.C:
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logState(urlStatus)
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case s := <-updates:
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urlStatus[s.url] = s.status
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}
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}
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}()
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return updates
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}
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// logState prints a state map.
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func logState(s map[string]string) {
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log.Stdout("Current state:")
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for k, v := range s {
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log.Stdoutf(" %s %s", k, v)
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}
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}
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// Resource represents an HTTP URL to be polled by this program.
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type Resource struct {
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url string
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errCount int64
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}
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// Poll executes an HTTP HEAD request for url
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// and returns the HTTP status string or an error string.
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func (r *Resource) Poll() string {
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resp, err := http.Head(r.url)
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if err != nil {
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log.Stderr("Error", r.url, err)
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r.errCount++
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return err.String()
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}
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r.errCount = 0
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return resp.Status
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}
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// Sleep sleeps for an appropriate interval (dependant on error state)
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// before sending the Resource to done.
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func (r *Resource) Sleep(done chan *Resource) {
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time.Sleep(pollInterval + errTimeout*r.errCount)
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done <- r
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}
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func Poller(in <-chan *Resource, out chan<- *Resource, status chan<- State) {
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for r := range in {
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s := r.Poll()
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status <- State{r.url, s}
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out <- r
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}
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}
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func main() {
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// create our input and output channels
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pending, complete := make(chan *Resource), make(chan *Resource)
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// launch the StateMonitor
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status := StateMonitor(statusInterval)
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// launch some Poller goroutines
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for i := 0; i < numPollers; i++ {
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go Poller(pending, complete, status)
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}
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// send some Resources to the pending queue
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go func() {
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for _, url := range urls {
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pending <- &Resource{url: url}
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}
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}()
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for r := range complete {
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go r.Sleep(pending)
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}
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}
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