Schedulers

Nomad has three scheduler types that can be used when creating your job: service, batch and system. Here we will describe the differences between each of these schedulers.

Service

The 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 desirable.

Service jobs are intended to run until explicitly stopped by an operator. If a service task exits it is considered a failure and handled according to the job's restart and reschedule stanzas.

Batch

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 batch 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.

Batch jobs are intended to run until they exit successfully. Batch tasks that exit with an error are handled according to the job's restart and reschedule stanzas.

System

The 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, 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.