From 5659826e43349426d86ccc8816fbf02babe19065 Mon Sep 17 00:00:00 2001 From: Francisco Souza Date: Wed, 14 Mar 2012 13:03:11 +1100 Subject: [PATCH] doc: add Go Concurrency Patterns: Timing out, moving on article Originally published on The Go Programming Language Blog, September 23, 2010. http://blog.golang.org/2010/09/go-concurrency-patterns-timing-out-and.html Update #2547. R=golang-dev, adg CC=golang-dev https://golang.org/cl/5815044 --- doc/Makefile | 4 +- ...urrency_patterns_timing_out_moving_on.html | 79 +++++++++++++++++++ doc/docs.html | 2 +- doc/progs/run | 7 +- doc/progs/timeout1.go | 28 +++++++ doc/progs/timeout2.go | 27 +++++++ 6 files changed, 144 insertions(+), 3 deletions(-) create mode 100644 doc/articles/go_concurrency_patterns_timing_out_moving_on.html create mode 100644 doc/progs/timeout1.go create mode 100644 doc/progs/timeout2.go diff --git a/doc/Makefile b/doc/Makefile index b275dfe4dd..687f1b1eb5 100644 --- a/doc/Makefile +++ b/doc/Makefile @@ -7,6 +7,8 @@ RAWHTML=\ articles/error_handling.rawhtml\ articles/slices_usage_and_internals.rawhtml\ articles/laws_of_reflection.rawhtml\ + articles/c_go_cgo.rawhtml\ + articles/go_concurrency_patterns_timing_out_moving_on.rawhtml\ articles/image_draw.rawhtml\ effective_go.rawhtml\ go1.rawhtml\ @@ -17,4 +19,4 @@ all: $(RAWHTML) godoc -url /doc/$< >$@ clean: - rm -f $(RAWHTML) + rm -f $(RAWHTML) diff --git a/doc/articles/go_concurrency_patterns_timing_out_moving_on.html b/doc/articles/go_concurrency_patterns_timing_out_moving_on.html new file mode 100644 index 0000000000..63c8cd59e8 --- /dev/null +++ b/doc/articles/go_concurrency_patterns_timing_out_moving_on.html @@ -0,0 +1,79 @@ + + +

+Concurrent programming has its own idioms. A good example is timeouts. Although +Go's channels do not support them directly, they are easy to implement. Say we +want to receive from the channel ch, but want to wait at most one +second for the value to arrive. We would start by creating a signalling channel +and launching a goroutine that sleeps before sending on the channel: +

+ +{{code "/doc/progs/timeout1.go" `/timeout :=/` `/STOP/`}} + +

+We can then use a select statement to receive from either +ch or timeout. If nothing arrives on ch +after one second, the timeout case is selected and the attempt to read from +ch is abandoned. +

+ +{{code "/doc/progs/timeout1.go" `/select {/` `/STOP/`}} + +

+The timeout channel is buffered with space for 1 value, allowing +the timeout goroutine to send to the channel and then exit. The goroutine +doesn't know (or care) whether the value is received. This means the goroutine +won't hang around forever if the ch receive happens before the +timeout is reached. The timeout channel will eventually be +deallocated by the garbage collector. +

+ +

+(In this example we used time.Sleep to demonstrate the mechanics +of goroutines and channels. In real programs you should use +time.After, a function that returns +a channel and sends on that channel after the specified duration.) +

+ +

+Let's look at another variation of this pattern. In this example we have a +program that reads from multiple replicated databases simultaneously. The +program needs only one of the answers, and it should accept the answer that +arrives first. +

+ +

+The function Query takes a slice of database connections and a +query string. It queries each of the databases in parallel and +returns the first response it receives: +

+ +{{code "/doc/progs/timeout2.go" `/func Query/` `/STOP/`}} + +

+In this example, the closure does a non-blocking send, which it achieves by +using the send operation in select statement with a +default case. If the send cannot go through immediately the +default case will be selected. Making the send non-blocking guarantees that +none of the goroutines launched in the loop will hang around. However, if the +result arrives before the main function has made it to the receive, the send +could fail since no one is ready. +

+ +

+This problem is a textbook of example of what is known as a +race condition, but +the fix is trivial. We just make sure to buffer the channel ch (by +adding the buffer length as the second argument to make), +guaranteeing that the first send has a place to put the value. This ensures the +send will always succeed, and the first value to arrive will be retrieved +regardless of the order of execution. +

+ +

+These two examples demonstrate the simplicity with which Go can express complex +interactions between goroutines. +

diff --git a/doc/docs.html b/doc/docs.html index cc637b038a..39e4573eb6 100644 --- a/doc/docs.html +++ b/doc/docs.html @@ -105,7 +105,7 @@ Guided tours of Go programs.
  • JSON-RPC: a tale of interfaces
  • Go's Declaration Syntax
  • Defer, Panic, and Recover
    • -
  • Go Concurrency Patterns: Timing out, moving on
    • +
  • Go Concurrency Patterns: Timing out, moving on
  • Go Slices: usage and internals
  • A GIF decoder: an exercise in Go interfaces
  • Error Handling and Go
    • diff --git a/doc/progs/run b/doc/progs/run index 4d183530cb..3bd50beda7 100755 --- a/doc/progs/run +++ b/doc/progs/run @@ -41,7 +41,12 @@ if [ "$goos" == "freebsd" ]; then c_go_cgo="cgo3 cgo4" fi -all=$(echo $defer_panic_recover $effective_go $error_handling $law_of_reflection $c_go_cgo slices go1) +timeout=" + timeout1 + timeout2 +" + +all=$(echo $defer_panic_recover $effective_go $error_handling $law_of_reflection $c_go_cgo $timeout slices go1) for i in $all; do go build $i.go diff --git a/doc/progs/timeout1.go b/doc/progs/timeout1.go new file mode 100644 index 0000000000..a6c95624c8 --- /dev/null +++ b/doc/progs/timeout1.go @@ -0,0 +1,28 @@ +// Copyright 2012 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. +package timeout + +import ( + "time" +) + +func Timeout() { + ch := make(chan bool, 1) + timeout := make(chan bool, 1) + go func() { + time.Sleep(1e9) // one second + timeout <- true + }() + + // STOP OMIT + + select { + case <-ch: + // a read from ch has occurred + case <-timeout: + // the read from ch has timed out + } + + // STOP OMIT +} diff --git a/doc/progs/timeout2.go b/doc/progs/timeout2.go new file mode 100644 index 0000000000..7145bc93e1 --- /dev/null +++ b/doc/progs/timeout2.go @@ -0,0 +1,27 @@ +// Copyright 2012 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. +package query + +type Conn string + +func (c Conn) DoQuery(query string) Result { + return Result("result") +} + +type Result string + +func Query(conns []Conn, query string) Result { + ch := make(chan Result, 1) + for _, conn := range conns { + go func(c Conn) { + select { + case ch <- c.DoQuery(query): + default: + } + }(conn) + } + return <-ch +} + +// STOP OMIT