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80 lines
3.1 KiB
HTML
80 lines
3.1 KiB
HTML
<!--{
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"Title": "Go Concurrency Patterns: Timing out, moving on",
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"Template": true
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}-->
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<p>
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Concurrent programming has its own idioms. A good example is timeouts. Although
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Go's channels do not support them directly, they are easy to implement. Say we
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want to receive from the channel <code>ch</code>, but want to wait at most one
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second for the value to arrive. We would start by creating a signalling channel
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and launching a goroutine that sleeps before sending on the channel:
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</p>
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{{code "/doc/progs/timeout1.go" `/timeout :=/` `/STOP/`}}
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<p>
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We can then use a <code>select</code> statement to receive from either
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<code>ch</code> or <code>timeout</code>. If nothing arrives on <code>ch</code>
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after one second, the timeout case is selected and the attempt to read from
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<code>ch</code> is abandoned.
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</p>
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{{code "/doc/progs/timeout1.go" `/select {/` `/STOP/`}}
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<p>
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The <code>timeout</code> channel is buffered with space for 1 value, allowing
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the timeout goroutine to send to the channel and then exit. The goroutine
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doesn't know (or care) whether the value is received. This means the goroutine
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won't hang around forever if the <code>ch</code> receive happens before the
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timeout is reached. The <code>timeout</code> channel will eventually be
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deallocated by the garbage collector.
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</p>
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<p>
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(In this example we used <code>time.Sleep</code> to demonstrate the mechanics
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of goroutines and channels. In real programs you should use <code>
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<a href="/pkg/time/#After">time.After</a></code>, a function that returns
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a channel and sends on that channel after the specified duration.)
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</p>
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<p>
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Let's look at another variation of this pattern. In this example we have a
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program that reads from multiple replicated databases simultaneously. The
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program needs only one of the answers, and it should accept the answer that
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arrives first.
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</p>
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<p>
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The function <code>Query</code> takes a slice of database connections and a
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<code>query</code> string. It queries each of the databases in parallel and
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returns the first response it receives:
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</p>
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{{code "/doc/progs/timeout2.go" `/func Query/` `/STOP/`}}
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<p>
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In this example, the closure does a non-blocking send, which it achieves by
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using the send operation in <code>select</code> statement with a
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<code>default</code> case. If the send cannot go through immediately the
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default case will be selected. Making the send non-blocking guarantees that
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none of the goroutines launched in the loop will hang around. However, if the
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result arrives before the main function has made it to the receive, the send
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could fail since no one is ready.
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</p>
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<p>
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This problem is a textbook example of what is known as a
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<a href="https://en.wikipedia.org/wiki/Race_condition">race condition</a>, but
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the fix is trivial. We just make sure to buffer the channel <code>ch</code> (by
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adding the buffer length as the second argument to <a href="/pkg/builtin/#make">make</a>),
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guaranteeing that the first send has a place to put the value. This ensures the
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send will always succeed, and the first value to arrive will be retrieved
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regardless of the order of execution.
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</p>
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<p>
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These two examples demonstrate the simplicity with which Go can express complex
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interactions between goroutines.
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</p>
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