{"id":6540,"date":"2017-03-10T11:49:17","date_gmt":"2017-03-10T03:49:17","guid":{"rendered":"http:\/\/rmohan.com\/?p=6540"},"modified":"2017-03-10T11:49:17","modified_gmt":"2017-03-10T03:49:17","slug":"for-loops-in-chef","status":"publish","type":"post","link":"https:\/\/mohan.sg\/?p=6540","title":{"rendered":"for loops in Chef"},"content":{"rendered":"
for<\/code> config <\/code>in<\/code> [ <\/code>\"contacts.cfg\"<\/code>, <\/code>\"contactgroups.cfg\"<\/code> ] <\/code>do<\/code><\/div>\n
\u00a0\u00a0<\/code>remote_file <\/code>\"\/etc\/nagios3\/#{config}\"<\/code> do<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>source <\/code>\"#{config}\"<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>owner <\/code>\"root\"<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>group <\/code>\"root\"<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>mode <\/code>0644<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>notifies <\/code>:restart<\/code>, resources(<\/code>:service<\/code> => <\/code>\"nagios\"<\/code>), <\/code>:delayed<\/code><\/div>\n
\u00a0\u00a0<\/code>end<\/code><\/div>\n
end<\/code><\/div>\n
<\/div>\n
\n

Write it in the order you want it to run<\/h2>\n

Chef executes resources in the order they appear in a recipe. Coming to Chef from Puppet<\/a>, I found this to be a welcome surprise. Need to install the NTP daemon and then make sure the service is started? No problem. Just put your service resource<\/a> after your package resource<\/a>.<\/p>\n

package 'ntp' do\r\n  action :install\r\nend\r\n\r\nservice 'ntp' do\r\n  action :start\r\nend\r\n<\/code><\/pre>\n
This is the most common way to sequence events. You can tell what the control flow is just by reading a cookbook from top to bottom.<\/p>\n
Notify resources about changes<\/h2>\n
Chef provides a notification mechanism for signaling when things change. This lets you do things like restart a service when a configuration file changes. Want to lay down your own NTP configuration file and bounce the service when it\u2019s updated? Easy. Have your template resource<\/a> notify your service resource<\/a> of the change.<\/p>\n
template '\/etc\/ntp.conf' do\r\n  notifies :restart, 'service[ntp]'\r\nend\r\n\r\nservice 'ntp' do\r\n  action :start\r\nend\r\n<\/code><\/pre>\n
Something that surprised me when I first started working with Chef, was that notifications on resources queue, then trigger at the end of the run. So the recipe above takes the following actions:<\/p>\n
    \n
  1. Updates the template.<\/li>\n
  2. Queues a restart of the service.<\/li>\n
  3. Starts the service.<\/li>\n
  4. Restarts the service.<\/li>\n<\/ol>\n
    Sometimes you want a notification to trigger right after something changes, instead of waiting until the end of the run. In that case, you can use the :immediately<\/code> flag.<\/p>\n
    template '\/etc\/ntp.conf' do\r\n  notifies :restart, 'service[ntp]', :immediately\r\nend\r\n\r\nservice 'ntp' do\r\n  action :start\r\nend\r\n<\/code><\/pre>\n
    Check if resources have changed<\/h2>\n
    Chef resources are first class objects, which means you can ask them about their state during the run. Every resource has an updated_by_last_action?<\/code> method which returns true<\/code> if the resource changed. Combining this with sequential execution lets you build robust cookbooks.<\/p>\n
    Suppose you want to download and unpack a tarball. If the folder you\u2019re unpacking into exists, you\u2019ll want to remove it first. But if the tarball hasn\u2019t changed, you\u2019ll want to skip that step. Download the package with a remote file resource<\/a> resource, cache the result, and use the only_if<\/code>modifier on a directory resource<\/a> to skip the removal if the tarball\u2019s unchanged.<\/p>\n
    tarball = remote_file '~\/node-v0.8.20.tar.gz' do\r\n  source 'http:\/\/nodejs.org\/dist\/v0.8.20\/node-v0.8.20.tar.gz'\r\nend\r\n\r\nfolder = directory '~\/node-v0.8.20' do\r\n  action :remove\r\n  only_if { tarball.updated_by_last_action? }\r\nend<\/code><\/pre>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
    for config in [ “contacts.cfg”, “contactgroups.cfg” ] do remote_file “\/etc\/nagios3\/#{config}” do source “#{config}” owner “root” group “root” mode 0644 notifies :restart, resources(:service => “nagios”), :delayed end end Write it in the order you want it to run <\/p>\n
    Chef executes resources in the order they appear in a recipe. Coming to Chef from Puppet, I found […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[80],"tags":[],"_links":{"self":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts\/6540"}],"collection":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=6540"}],"version-history":[{"count":1,"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts\/6540\/revisions"}],"predecessor-version":[{"id":6541,"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts\/6540\/revisions\/6541"}],"wp:attachment":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6540"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6540"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6540"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}