postfix-master/queuing.html

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  12 <h1><a href="big-picture.html"><img src="small-picture.gif" width="115" height="45"></a>
  13 Postfix Overview - Queue Management</h1>
  14
  15 <hr>
  16
  17 <p> <strong> Note: this web page is no longer maintained. It exists
  18 only to avoid breaking links in web pages that describe earlier
  19 versions of the Postfix mail system. </strong> </p>
  20
  21 <a href="docs.html">Up one level</a> | <a
  22 href="motivation.html">Introduction</a> | <a href="goals.html">Goals
  23 and features</a> | <a href="architecture.html">Global architecture</a>
  24 | Queue Management | <a href="security.html">Security</a>
  25
  26 <h2>Postfix mail queues</h2>
  27
  28 Postfix has four main queues: <b>maildrop</b>, <b>incoming</b>,
  29 <b>active</b> and <b>deferred</b> (click the upper left-hand icon
  30 for the big picture).  Locally-posted mail is deposited into the
  31 <b>maildrop</b>, and is copied to the <b>incoming</b> queue after
  32 some cleaning up. The <b>incoming</b> queue is for mail that is
  33 still arriving or that the queue manager hasn't looked at yet.
  34 The <b>active</b> queue is a limited-size queue for mail that the
  35 queue manager has opened for delivery.  Mail that can't be delivered
  36 goes to the <b>deferred</b> queue, so that it does not get in the
  37 way of other deliveries.
  38
  39 <p>
  40
  41 The queue manager keeps information in memory about the <b>active</b>
  42 queue only.  The active queue size is limited on purpose:  <i>the
  43 queue manager should never run out of working memory because of a
  44 peak message workload</i>.  Whenever there is space in the
  45 <b>active</b> queue, the queue manager lets in one message from the
  46 <b>incoming</b> queue and one from the <b>deferred</b> queue. This
  47 guarantees that new mail will get through even when there is a
  48 large backlog.
  49
  50 <p>
  51
  52 In addition to the queues mentioned above Postfix also maintains
  53 two parking spaces. The <b>hold</b> queue is for mail that is frozen
  54 in the queue; no delivery attempts are made until someone releases
  55 these messages with the <a href="postsuper.1.html">postsuper</a>
  56 command. The <b>corrupt</b> directory is for damaged queue files.
  57 Rather than discarding these, Postfix leaves them here for human
  58 inspection.
  59
  60 <h2>No thundering herd</h2>
  61
  62 Implementing a high-performance mail system is one thing. However,
  63 no-one would be pleased when Postfix connects to their site and
  64 overwhelms it with lots of simultaneous deliveries.  This is an
  65 issue especially when a site has been down and mail is backed up
  66 elsewhere in the network.
  67
  68 <p>
  69
  70 Postfix tries to be a good network neighbor.  When delivering mail
  71 to a site, Postfix will initially make no more than two simultaneous
  72 connections. As long as deliveries succeed, the concurrency slowly
  73 increases up to some configurable limit (or until the host or
  74 network is unable to handle the load); concurrency is decreased in
  75 the face of trouble.  For those familiar with TCP/IP implementation
  76 details, Postfix implements its own analog of the TCP <i>slow
  77 start</i> algorithm
  78
  79 <h2>Fairness</h2>
  80
  81 Apart from the <i>thundering herd</i> controls, the Postfix delivery
  82 strategy is based on <i>round-robin</i> selection.  The queue
  83 manager sorts message recipients in the active queue by destination,
  84 and makes round-robin walks <i>along</i> all destination queues.
  85
  86 <p>
  87
  88 On the average, Postfix will do simultaneous deliveries to the same
  89 domain only when there is not enough work to keep all outbound SMTP
  90 channels busy. So, when AOL goes off-line and comes back, it should
  91 not stop the system from delivering to other sites.
  92
  93 <p>
  94
  95 When mail arrives faster than Postfix can deliver it, Postfix will
  96 favor new mail over delayed mail. The idea is that new mail should
  97 be delivered with as little delay as possible; delayed mail can be
  98 delivered while the system would otherwise be idle.
  99
 100 <h2>Exponential backoff</h2>
 101
 102 Postfix implements per-message exponential backoff.  When a message
 103 cannot be delivered upon the first attempt, the queue manager gives
 104 the queue file a time stamp that is offset into the future by some
 105 configurable amount of time.  Queue files with future time stamps
 106 are normally ignored by the queue manager.
 107
 108 <p>
 109
 110 Whenever a repeat delivery attempt fails, the queue file time stamp
 111 is moved into the future by an amount of time equal to the age of
 112 the message.  Thus, the time between delivery attempts doubles each
 113 time. This strategy effectively implements exponential backoff.
 114
 115 <h2>Destination status cache</h2>
 116
 117 The Postfix queue manager maintains a limited, short-term list of
 118 unreachable destinations. This list helps it to avoid unnecessary
 119 delivery attempts, especially with destinations that have a large
 120 mail backlog.
 121
 122 <hr>
 123
 124 <a href="docs.html">Up one level</a> | <a
 125 href="motivation.html">Introduction</a> | <a href="goals.html">Goals
 126 and features</a> | <a href="architecture.html">Global architecture</a>
 127 | Queue Management | <a href="security.html">Security</a>
 128
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