docs/hooks.html.in

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   4     <h1>Hooks for specific system management</h1>
   5
   6     <ul id="toc"></ul>
   7
   8     <h2><a name="intro">Custom event scripts</a></h2>
   9     <p>Beginning with libvirt 0.8.0, specific events on a host system will
  10        trigger custom scripts.</p>
  11     <p>These custom <b>hook</b> scripts are executed when any of the following
  12        actions occur:</p>
  13     <ul>
  14       <li>The libvirt daemon starts, stops, or reloads its
  15           configuration<br/><br/></li>
  16       <li>A QEMU guest is started or stopped<br/><br/></li>
  17       <li>An LXC guest is started or stopped<br/><br/></li>
  18     </ul>
  19
  20     <h2><a name="location">Script location</a></h2>
  21     <p>The libvirt hook scripts are located in the directory
  22        <code>$SYSCONFDIR/libvirt/hooks/</code>.</p>
  23     <ul>
  24       <li>In Linux distributions such as Fedora and RHEL, this is
  25           <code>/etc/libvirt/hooks/</code>.  Other Linux distributions may do
  26           this differently.</li>
  27       <li>If your installation of libvirt has instead been compiled from
  28           source, it is likely to be
  29           <code>/usr/local/etc/libvirt/hooks/</code>.</li>
  30     </ul>
  31     <p>To use hook scripts, you will need to create this <code>hooks</code>
  32        directory manually, place the desired hook scripts inside, then make
  33        them executable.</p>
  34     <br/>
  35
  36     <h2><a name="names">Script names</a></h2>
  37     <p>At present, there are three hook scripts that can be called:</p>
  38     <ul>
  39       <li><code>/etc/libvirt/hooks/daemon</code><br/><br/>
  40           Executed when the libvirt daemon is started, stopped, or reloads
  41           its configuration<br/><br/></li>
  42       <li><code>/etc/libvirt/hooks/qemu</code><br/><br/>
  43           Executed when a QEMU guest is started, stopped, or migrated<br/><br/></li>
  44       <li><code>/etc/libvirt/hooks/lxc</code><br /><br/>
  45           Executed when an LXC guest is started or stopped</li>
  46     </ul>
  47     <br/>
  48
  49     <h2><a name="structure">Script structure</a></h2>
  50     <p>The hook scripts are executed using standard Linux process creation
  51        functions.  Therefore, they must begin with the declaration of the
  52        command interpreter to use.</p>
  53     <p>For example:</p>
  54     <pre>#!/bin/bash</pre>
  55     <p>or:</p>
  56     <pre>#!/usr/bin/python</pre>
  57     <p>Other command interpreters are equally valid, as is any executable
  58        binary, so you are welcome to use your favourite languages.</p>
  59     <br/>
  60
  61     <h2><a name="arguments">Script arguments</a></h2>
  62     <p>The hook scripts are called with specific command line arguments,
  63        depending upon the script, and the operation being performed.</p>
  64     <p>The guest hook scripts, qemu and lxc, are also given the <b>full</b>
  65        XML description for the domain on their stdin. This includes items
  66        such the UUID of the domain and its storage information, and is
  67        intended to provide all the libvirt information the script needs.</p>
  68
  69     <p>The command line arguments take this approach:</p>
  70     <ol>
  71       <li>The first argument is the name of the <b>object</b> involved in the
  72           operation, or '-' if there is none.<br/><br/>
  73           For example, the name of a guest being started.<br/><br/></li>
  74       <li>The second argument is the name of the <b>operation</b> being
  75           performed.<br/><br/>
  76           For example, "start" if a guest is being started.<br/><br/></li>
  77       <li>The third argument is a <b>sub-operation</b> indication, or '-' if there
  78           is none.<br/><br/></li>
  79       <li>The last argument is an <b>extra argument</b> string, or '-' if there is
  80           none.</li>
  81     </ol>
  82
  83     <h4><a name="arguments_specifics">Specifics</a></h4>
  84     <p>This translates to the following specifics for each hook script:</p>
  85
  86     <h5><a name="daemon">/etc/libvirt/hooks/daemon</a></h5>
  87     <ul>
  88       <li>When the libvirt daemon is started, this script is called as:<br/>
  89           <pre>/etc/libvirt/hooks/daemon - start - start</pre></li>
  90       <li>When the libvirt daemon is shut down, this script is called as:<br/>
  91           <pre>/etc/libvirt/hooks/daemon - shutdown - shutdown</pre></li>
  92       <li>When the libvirt daemon receives the SIGHUP signal, it reloads its
  93           configuration and triggers the hook script as:<br/>
  94           <pre>/etc/libvirt/hooks/daemon - reload begin SIGHUP</pre></li>
  95     </ul>
  96     <p>Please note that when the libvirt daemon is restarted, the <i>daemon</i>
  97        hook script is called once with the "shutdown" operation, and then once
  98        with the "start" operation.  There is no specific operation to indicate
  99        a "restart" is occurring.</p>
 100
 101     <h5><a name="qemu">/etc/libvirt/hooks/qemu</a></h5>
 102     <ul>
 103       <li>Before a QEMU guest is started, the qemu hook script is
 104         called in two locations; if either location fails, the guest
 105         is not started.  The first location, <span class="since">since
 106         0.9.0</span>, is before libvirt performs any resource
 107         labeling, and the hook can allocate resources not managed by
 108         libvirt such as DRBD or missing bridges.  This is called as:<br/>
 109         <pre>/etc/libvirt/hooks/qemu guest_name prepare begin -</pre>
 110         The second location, available <span class="since">Since
 111         0.8.0</span>, occurs after libvirt has finished labeling
 112         all resources, but has not yet started the guest, called as:<br/>
 113         <pre>/etc/libvirt/hooks/qemu guest_name start begin -</pre></li>
 114       <li>When a QEMU guest is stopped, the qemu hook script is called
 115         in two locations, to match the startup.
 116         First, <span class="since">since 0.8.0</span>, the hook is
 117         called before libvirt restores any labels:<br/>
 118         <pre>/etc/libvirt/hooks/qemu guest_name stopped end -</pre>
 119         Then, after libvirt has released all resources, the hook is
 120         called again, <span class="since">since 0.9.0</span>, to allow
 121         any additional resource cleanup:<br/>
 122         <pre>/etc/libvirt/hooks/qemu guest_name release end -</pre></li>
 123     </ul>
 124
 125     <h5><a name="lxc">/etc/libvirt/hooks/lxc</a></h5>
 126     <ul>
 127       <li>When an LXC guest is started, the lxc hook script is called as:<br/>
 128           <pre>/etc/libvirt/hooks/lxc guest_name start begin -</pre></li>
 129       <li>When a LXC guest is stopped, the lxc hook script is called
 130           as:<br/>
 131           <pre>/etc/libvirt/hooks/lxc guest_name stopped end -</pre></li>
 132     </ul>
 133     <br/>
 134
 135     <h2><a name="execution">Script execution</a></h2>
 136     <ul>
 137       <li>The "start" operation for the guest hook scripts, qemu and lxc,
 138           executes <b>prior</b> to the guest being created.  This allows the
 139           guest start operation to be aborted if the script returns indicating
 140           failure.<br/><br/></li>
 141       <li>The "shutdown" operation for the guest hook scripts, qemu and lxc,
 142           executes <b>after</b> the guest has stopped.  If the hook script
 143           indicates failure in its return, the shut down of the guest cannot
 144           be aborted because it has already been performed.<br/><br/></li>
 145       <li>Hook scripts execute in a synchronous fashion.  Libvirt waits
 146           for them to return before continuing the given operation.<br/><br/>
 147           This is most noticeable with the guest start operation, as a lengthy
 148           operation in the hook script can mean an extended wait for the guest
 149           to be available to end users.<br/><br/></li>
 150       <li>For a hook script to be utilised, it must have its execute bit set
 151           (ie. chmod o+rx <i>qemu</i>), and must be present when the libvirt
 152           daemon is started.<br/><br/></li>
 153       <li>If a hook script is added to a host after the libvirt daemon is
 154           already running, it won't be used until the libvirt daemon
 155           next starts.</li>
 156     </ul>
 157     <br/>
 158
 159     <h2><a name="qemu_migration">QEMU guest migration</a></h2>
 160     <p>Migration of a QEMU guest involves running hook scripts on both the
 161        source and destination hosts:</p>
 162     <ol>
 163       <li>At the beginning of the migration, the <i>qemu</i> hook script on
 164           the <b>destination</b> host is executed with the "start"
 165           operation.<br/><br/></li>
 166       <li>If this hook script returns indicating success (error code 0), the
 167           migration continues.  Any other return code indicates failure, and
 168           the migration is aborted.<br/><br/></li>
 169       <li>The QEMU guest is then migrated to the destination host.<br/>
 170           <br/></li>
 171       <li>Unless an error occurs during the migration process, the <i>qemu</i>
 172           hook script on the <b>source</b> host is then executed with the "stopped"
 173           operation, to indicate it is no longer running on this
 174           host.<br/><br/>
 175           Regardless of the return code from this hook script, the migration
 176           is not aborted as it has already been performed.</li>
 177     </ol>
 178     <br/>
 179
 180     <h2><a name="recursive">Calling libvirt functions from within a hook script</a></h2>
 181     <p><b>DO NOT DO THIS!</b></p>
 182     <p>A hook script must not call back into libvirt, as the libvirt daemon
 183        is already waiting for the script to exit.</p>
 184     <p>A deadlock is likely to occur.</p>
 185     <br/>
 186
 187     <h2><a name="return_codes">Return codes and logging</a></h2>
 188     <p>If a hook script returns with an exit code of 0, the libvirt daemon
 189        regards this as successful and performs no logging of it.</p>
 190     <p>However, if a hook script returns with a non zero exit code, the libvirt
 191        daemon regards this as a failure, logs it with return code 256, and
 192        additionally logs anything on stderr the hook script returns.</p>
 193     <p>For example, a hook script might use this code to indicate failure,
 194        and send a text string to stderr:</p>
 195     <pre>echo "Could not find required XYZZY" &gt;&amp;2
 196 exit 1</pre>
 197     <p>The resulting entry in the libvirt log will appear as:</p>
 198     <pre>20:02:40.297: error : virHookCall:416 : Hook script execution failed: Hook script /etc/libvirt/hooks/qemu qemu failed with error code 256:Could not find required XYZZY</pre>
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