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| <h5 class="page-header text-uppercase">Documentation |
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| <h1 id="scheduling-constraints">Scheduling Constraints</h1> |
| |
| <p>By default, Aurora will pick any random agent with sufficient resources |
| in order to schedule a task. This scheduling choice can be further |
| restricted with the help of constraints.</p> |
| |
| <h2 id="mesos-attributes">Mesos Attributes</h2> |
| |
| <p>Data centers are often organized with hierarchical failure domains. Common failure domains |
| include hosts, racks, rows, and PDUs. If you have this information available, it is wise to tag |
| the Mesos agent with them as |
| <a href="https://mesos.apache.org/documentation/attributes-resources/">attributes</a>.</p> |
| |
| <p>The Mesos agent <code>--attributes</code> command line argument can be used to mark agents with |
| static key/value pairs, so called attributes (not to be confused with <code>--resources</code>, which are |
| dynamic and accounted).</p> |
| |
| <p>For example, consider the host <code>cluster1-aaa-03-sr2</code> and its following attributes (given in |
| key:value format): <code>host:cluster1-aaa-03-sr2</code> and <code>rack:aaa</code>.</p> |
| |
| <p>Aurora makes these attributes available for matching with scheduling constraints.</p> |
| |
| <h2 id="limit-constraints">Limit Constraints</h2> |
| |
| <p>Limit constraints allow to control machine diversity using constraints. The below |
| constraint ensures that no more than two instances of your job may run on a single host. |
| Think of this as a “group by” limit.</p> |
| <pre class="highlight plaintext"><code>Service( |
| name = 'webservice', |
| role = 'www-data', |
| constraints = { |
| 'host': 'limit:2', |
| } |
| ... |
| ) |
| </code></pre> |
| |
| <p>Likewise, you can use constraints to control rack diversity, e.g. at |
| most one task per rack:</p> |
| <pre class="highlight plaintext"><code>constraints = { |
| 'rack': 'limit:1', |
| } |
| </code></pre> |
| |
| <p>Use these constraints sparingly as they can dramatically reduce Tasks’ schedulability. |
| Further details are available in the reference documentation on |
| <a href="../../reference/configuration/#specifying-scheduling-constraints">Scheduling Constraints</a>.</p> |
| |
| <h2 id="value-constraints">Value Constraints</h2> |
| |
| <p>Value constraints can be used to express that a certain attribute with a certain value |
| should be present on a Mesos agent. For example, the following job would only be |
| scheduled on nodes that claim to have an <code>SSD</code> as their disk.</p> |
| <pre class="highlight plaintext"><code>Service( |
| name = 'webservice', |
| role = 'www-data', |
| constraints = { |
| 'disk': 'SSD', |
| } |
| ... |
| ) |
| </code></pre> |
| |
| <p>Further details are available in the reference documentation on |
| <a href="../../reference/configuration/#specifying-scheduling-constraints">Scheduling Constraints</a>.</p> |
| |
| <h2 id="running-stateful-services">Running stateful services</h2> |
| |
| <p>Aurora is best suited to run stateless applications, but it also accommodates for stateful services |
| like databases, or services that otherwise need to always run on the same machines.</p> |
| |
| <h3 id="dedicated-attribute">Dedicated attribute</h3> |
| |
| <p>Most of the Mesos attributes arbitrary and available for custom use. There is one exception, |
| though: the <code>dedicated</code> attribute. Aurora treats this specially, and only allows matching jobs to |
| run on these machines, and will only schedule matching jobs on these machines.</p> |
| |
| <h4 id="syntax">Syntax</h4> |
| |
| <p>The dedicated attribute has semantic meaning. The format is <code>$role(/.*)?</code>. When a job is created, |
| the scheduler requires that the <code>$role</code> component matches the <code>role</code> field in the job |
| configuration, and will reject the job creation otherwise. The remainder of the attribute is |
| free-form. We’ve developed the idiom of formatting this attribute as <code>$role/$job</code>, but do not |
| enforce this. For example: a job <code>devcluster/www-data/prod/hello</code> with a dedicated constraint set as |
| <code>www-data/web.multi</code> will have its tasks scheduled only on Mesos agents configured with: |
| <code>--attributes=dedicated:www-data/web.multi</code>.</p> |
| |
| <p>A wildcard (<code>*</code>) may be used for the role portion of the dedicated attribute, which will allow any |
| owner to elect for a job to run on the host(s). For example: tasks from both |
| <code>devcluster/www-data/prod/hello</code> and <code>devcluster/vagrant/test/hello</code> with a dedicated constraint |
| formatted as <code>*/web.multi</code> will be scheduled only on Mesos agents configured with |
| <code>--attributes=dedicated:*/web.multi</code>. This may be useful when assembling a virtual cluster of |
| machines sharing the same set of traits or requirements.</p> |
| |
| <h5 id="example">Example</h5> |
| |
| <p>Consider the following agent command line:</p> |
| <pre class="highlight plaintext"><code>mesos-slave --attributes="dedicated:db_team/redis" ... |
| </code></pre> |
| |
| <p>And this job configuration:</p> |
| <pre class="highlight plaintext"><code>Service( |
| name = 'redis', |
| role = 'db_team', |
| constraints = { |
| 'dedicated': 'db_team/redis' |
| } |
| ... |
| ) |
| </code></pre> |
| |
| <p>The job configuration is indicating that it should only be scheduled on agents with the attribute |
| <code>dedicated:db_team/redis</code>. Additionally, Aurora will prevent any tasks that do <em>not</em> have that |
| constraint from running on those agents.</p> |
| |
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