Nomad has three scheduler types that can be used when creating your job:
system. Here we will describe the differences between
each of these schedulers.
service scheduler is designed for scheduling long lived services that
should never go down. As such, the
service scheduler ranks a large portion
of the nodes that meet the job's constraints and selects the optimal node to
place a task group on. The
service scheduler uses a best fit scoring algorithm
influenced by Google's work on Borg. Ranking this larger set of candidate
nodes increases scheduling time but provides greater guarantees about the
optimality of a job placement, which given the service workload is highly
Batch jobs are much less sensitive to short term performance fluctuations and
are short lived, finishing in a few minutes to a few days. Although the
scheduler is very similar to the
service scheduler, it makes certain
optimizations for the batch workload. The main distinction is that after finding
the set of nodes that meet the job's constraints it uses the power of two
choices described in Berkeley's Sparrow scheduler to limit the number of nodes
that are ranked.
system scheduler is used to register jobs that should be run on all
clients that meet the job's constraints. The
system scheduler is also invoked
when clients join the cluster or transition into the ready state. This means
that all registered
system jobs will be re-evaluated and their tasks will be
placed on the newly available nodes if the constraints are met.
This scheduler type is extremely useful for deploying and managing tasks that should be present on every node in the cluster. Since these tasks are managed by Nomad, they can take advantage of job updating, rolling deploys, service discovery and more.
Since Nomad 0.9, the system scheduler will preempt eligible lower priority tasks running on a node if there isn't enough capacity to place a system job. See preemption for details on how tasks that get preempted are chosen.
Systems jobs are intended to run until explicitly stopped either by an operator or preemption. If a system task exits it is considered a failure and handled according to the job's restart stanza; system jobs do not have rescheduling.