117 lines
4.2 KiB
HTML
117 lines
4.2 KiB
HTML
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<html>
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<head>
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<title>Postfix Overview - Queue Management</title>
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</head>
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<body>
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<h1><a href="big-picture.html"><img src="small-picture.gif" width="115" height="45"></a>
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Postfix Overview - Queue Management</h1>
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<hr>
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<a href="index.html">Up one level</a> | <a
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href="motivation.html">Introduction</a> | <a href="goals.html">Goals
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and features</a> | <a href="architecture.html">Global architecture</a>
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| Queue Management | <a href="security.html">Security</a>
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<h2>Postfix mail queues</h2>
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Postfix has four different queues: <b>maildrop</b>, <b>incoming</b>,
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<b>active</b> and <b>deferred</b> (click the upper left-hand icon
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for the big picture). Locally-posted mail is deposited into the
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<b>maildrop</b>, and is copied to the <b>incoming</b> queue after
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some cleaning up. The <b>incoming</b> queue is for mail that is
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still arriving or that the queue manager hasn't looked at yet.
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The <b>active</b> queue is a limited-size queue for mail that the
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queue manager has opened for delivery. Mail that can't be delivered
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goes to the <b>deferred</b> queue, so that it does not get in the
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way of other deliveries.
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<p>
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The queue manager keeps information in memory about the <b>active</b>
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queue only. The active queue size is limited on purpose: <i>the
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queue manager should never run out of working memory because of a
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peak message workload</i>. Whenever there is space in the
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<b>active</b> queue, the queue manager lets in one message from the
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<b>incoming</b> queue and one from the <b>deferred</b> queue. This
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guarantees that new mail will get through even when there is a
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large backlog.
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<h2>No thundering herd</h2>
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Implementing a high-performance mail system is one thing. However,
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no-one would be pleased when Postfix connects to their site and
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overwhelms it with lots of simultaneous deliveries. This is an
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issue especially when a site has been down and mail is backed up
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elsewhere in the network.
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<p>
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Postfix tries to be a good network neighbor. When delivering mail
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to a site, Postfix will initially make no more than two simultaneous
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connections. As long as deliveries succeed, the concurrency slowly
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increases up to some configurable limit (or until the host or
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network is unable to handle the load); concurrency is decreased in
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the face of trouble. For those familiar with TCP/IP implementation
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details, Postfix implements its own analog of the TCP <i>slow
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start</i> algorithm
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<h2>Fairness</h2>
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Apart from the <i>thundering herd</i> controls, the Postfix delivery
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strategy is based on <i>round-robin</i> selection. The queue
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manager sorts message recipients in the active queue by destination,
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and makes round-robin walks <i>along</i> all destination queues.
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<p>
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On the average, Postfix will do simultaneous deliveries to the same
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domain only when there is not enough work to keep all outbound SMTP
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channels busy. So, when AOL goes off-line and comes back, it should
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not stop the system from delivering to other sites.
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<p>
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When mail arrives faster than Postfix can deliver it, Postfix will
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favor new mail over delayed mail. The idea is that new mail should
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be delivered with as little delay as possible; delayed mail can be
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delivered while the system would otherwise be idle.
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<h2>Exponential backoff</h2>
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Postfix implements per-message exponential backoff. When a message
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cannot be delivered upon the first attempt, the queue manager gives
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the queue file a time stamp that is offset into the future by some
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configurable amount of time. Queue files with future time stamps
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are normally ignored by the queue manager.
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<p>
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Whenever a repeat delivery attempt fails, the queue file time stamp
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is moved into the future by an amount of time equal to the age of
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the message. Thus, the time between delivery attempts doubles each
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time. This strategy effectively implements exponential backoff.
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<h2>Destination status cache</h2>
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The Postfix queue manager maintains a limited, short-term list of
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unreachable destinations. This list helps it to avoid unnecessary
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delivery attempts, especially with destinations that have a large
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mail backlog.
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<hr>
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<a href="index.html">Up one level</a> | <a
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href="motivation.html">Introduction</a> | <a href="goals.html">Goals
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and features</a> | <a href="architecture.html">Global architecture</a>
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| Queue Management | <a href="security.html">Security</a>
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</body>
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</html>
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