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Plan rejections occur when the scheduler work and the leader plan applier disagree on the feasibility of a plan. This may happen for valid reasons: since Nomad does parallel scheduling, it is expected that different workers will have a different state when computing placements. As the final plan reaches the leader plan applier, it may no longer be valid due to a concurrent scheduling taking up intended resources. In these situations the plan applier will notify the worker that the plan was rejected and that they should refresh their state before trying again. In some rare and unexpected circumstances it has been observed that workers will repeatedly submit the same plan, even if they are always rejected. While the root cause is still unknown this mitigation has been put in place. The plan applier will now track the history of plan rejections per client and include in the plan result a list of node IDs that should be set as ineligible if the number of rejections in a given time window crosses a certain threshold. The window size and threshold value can be adjusted in the server configuration. To avoid marking several nodes as ineligible at one, the operation is rate limited to 5 nodes every 30min, with an initial burst of 10 operations.
Nomad 
Nomad is a simple and flexible workload orchestrator to deploy and manage containers (docker, podman), non-containerized applications (executable, Java), and virtual machines (qemu) across on-prem and clouds at scale.
Nomad is supported on Linux, Windows, and macOS. A commercial version of Nomad, Nomad Enterprise, is also available.
- Website: https://nomadproject.io
- Tutorials: HashiCorp Learn
- Forum: Discuss
- Mailing List: Google Groups
- Gitter: hashicorp-nomad
Nomad provides several key features:
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Deploy Containers and Legacy Applications: Nomad’s flexibility as an orchestrator enables an organization to run containers, legacy, and batch applications together on the same infrastructure. Nomad brings core orchestration benefits to legacy applications without needing to containerize via pluggable task drivers.
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Simple & Reliable: Nomad runs as a single binary and is entirely self contained - combining resource management and scheduling into a single system. Nomad does not require any external services for storage or coordination. Nomad automatically handles application, node, and driver failures. Nomad is distributed and resilient, using leader election and state replication to provide high availability in the event of failures.
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Device Plugins & GPU Support: Nomad offers built-in support for GPU workloads such as machine learning (ML) and artificial intelligence (AI). Nomad uses device plugins to automatically detect and utilize resources from hardware devices such as GPU, FPGAs, and TPUs.
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Federation for Multi-Region, Multi-Cloud: Nomad was designed to support infrastructure at a global scale. Nomad supports federation out-of-the-box and can deploy applications across multiple regions and clouds.
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Proven Scalability: Nomad is optimistically concurrent, which increases throughput and reduces latency for workloads. Nomad has been proven to scale to clusters of 10K+ nodes in real-world production environments.
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HashiCorp Ecosystem: Nomad integrates seamlessly with Terraform, Consul, Vault for provisioning, service discovery, and secrets management.
Quick Start
Testing
See Learn: Getting Started for instructions on setting up a local Nomad cluster for non-production use.
Optionally, find Terraform manifests for bringing up a development Nomad cluster on a public cloud in the terraform directory.
Production
See Learn: Nomad Reference Architecture for recommended practices and a reference architecture for production deployments.
Documentation
Full, comprehensive documentation is available on the Nomad website: https://www.nomadproject.io/docs
Guides are available on HashiCorp Learn.
Contributing
See the contributing directory for more developer documentation.