Nomad

Nomad

Sep 28 2015    Armon Dadgar

Today we announce Nomad, a cluster manager and scheduler designed for microservices and batch workloads. Nomad is distributed, highly available, and scales to thousands of nodes spanning multiple datacenters and regions.

Nomad provides a common workflow to deploy applications across an infrastructure. Developers use a declarative job specification to define how an application should be deployed and the resources it requires (CPU, memory, disk). Nomad accepts these jobs and finds available resources to run them. The scheduling algorithm ensures all constraints are satisfied, and packs as many applications on a host as possible to optimize resource utilization. Additionally, Nomad supports virtualized, containerized, or standalone applications running on all major operating systems giving it the flexibility to support a broad range of workloads.

Nomad is now being deployed in production environments and we are proud to share it publicly. We are excited about the future of the project, and are just beginning to build on the foundation that it provides.

Read on to learn more.

Features

Nomad combines the features of a resource manager and a scheduler. As a resource manager, Nomad collects information on the available resources and capabilities per host. As a scheduler, the central Nomad servers use this information to make optimal placement decisions when jobs are submitted. These capabilities give Nomad a lot of possibilities, but here are a few highlighted features:

  • Docker: Nomad supports Docker as a first-class workload type. Jobs submitted to Nomad can use the docker task driver to easily deploy containerized applications. Jobs can specify the number of instances required and Nomad will handle placement and recover from failures automatically.

  • Operationally Simple: Nomad ships as a single binary, both for clients and servers, and requires no external services for coordination or storage. Nomad is distributed and highly available, and combines resource management and scheduling into a single system for simplicity.

  • Multi-Datacenter and Multi-Region: Nomad models infrastructure as groups of datacenters which form a larger region. Scheduling operates at the region level allowing for cross-datacenter scheduling. Multiple regions federate together allowing jobs to be registered globally.

  • Flexible Workloads: Nomad has extensible support for task drivers, allowing it to run containerized, virtualized, and standalone applications. Users can easily start Docker containers, VMs, or application runtimes like Java. Nomad supports Linux, Windows, BSD and OSX, providing the flexibility to run any workload.

  • Built for Scale: Nomad was designed from the ground up to support global scale infrastructure. Nomad is distributed and highly available, using both leader election and state replication to provide availability in the face of failures. Nomad is optimistically concurrent, enabling all servers to participate in scheduling decisions which increases the total throughput and reduces latency to support demanding workloads.

Nomad Jobs

Jobs are the heart of Nomad as they describe the tasks that Nomad should run. The HCL language is used to specify jobs, which makes them easy to read and friendly for version control. Jobs are declarative and specify what should be run, leaving the details of how and where to Nomad.

Here is an example Nomad job specification to run web servers with Docker:

# Define the hashicorp/web/frontend job job "hashicorp/web/frontend" {

» Run in two datacenters

datacenters = ["us-west-1", "us-east-1"]

» Only run our workload on linux

constraint { attribute = "$attr.kernel.name" value = "linux" }

» Configure the job to do rolling updates

update { # Stagger updates every 30 seconds stagger = "30s"

# Update a single task at a time
max_parallel = 1

}

» Define the task group

group "frontend" { # Ensure we have enough servers to handle traffic count = 10

task "web" {
    # Use Docker to run our server
    driver = "docker"
    config {
        image = "hashicorp/web-frontend:latest"
    }

    # Ask for some resources
    resources {
        cpu = 500
        memory = 128
        network {
            mbits = 10
            dynamic_ports = ["http"]
        }
    }
}

} }

This is a very simple example, but shows some of the capabilities of Nomad. Nomad jobs are composed of task groups which are a collection of tasks that must run together. Each task is an individual application that is executed by a driver. Drivers give Nomad the flexibility to run virtualized, containerized or standalone applications.

At the time of release, Nomad can use Qemu to run full virtual machines, Docker to run containers, Java to run standalone Jars, and the exec driver to run any pre-installed application. These drivers are meant to demonstrate the spectrum of workloads that are possible, and support will be expanded over time.

This example job also shows the constraint enforcement abilities of Nomad. Constraints can be used to restrict the hosts that are eligible to run a task and can be expressed at the job, group or individual task level. They can be imposed for technical reasons of the application requiring a certain kernel version, or for business reasons such as PCI compliance.

We can see that an update block has been specified in the job, which informs the update strategy used when a job definition changes. This gives developers a simple way to do rolling updates to an application to minimize service disruptions. In addition to updates, the application can be easily scaled by changing the desired count in the task group.

Lastly, we can see that the task specifies the resources it needs. This is used by the scheduler to pack many applications onto a machine to maximize utilization. Resource isolation uses operating system controls like cgroups, namespaces and chroots to prevent runaway usage.

State of the Art

Nomad is built to be a general purpose scheduler that is simple to use but powerful enough to handle complex workload at enormous scale. We have designed it after carefully studying the prior art and leveraging mature and production-hardened libraries from our portfolio of distributed systems tools.

Cluster management is based on Google Borg and Omega which have been refined over a decade of production usage. Omega introduces novel optimistic concurrency mechanisms that we've adopted to support the most demanding of workloads across multiple scheduler implementations.

Leader election uses Raft, a consensus algorithm out of Stanford. Our implementation was developed for Consul and powers service discovery and configuration for thousands of organizations and millions of servers. Nomad uses Raft to avoid any external coordination or storage making it operationally simple.

Federation is done via gossip based on Serf, built on top of SWIM. Serf is deployed around the world, including the San Diego Supercomputer Center, and powers the membership and coordination of extremely high scale infrastructures. Gossip is used because it is lightweight and enables Nomad to federate multiple regions together into a single global cluster.

Data storage uses BoltDB, an embedded database that is fully ACID compliant and provides Multi-Version Concurrency Control (MVCC). These properties keep the data Nomad stores safe while also being extremely performant.

State Indexing uses MemDB, an in-memory index built on immutable data structures. MemDB provides concurrent access to state for scheduling decisions without lock contention, allowing Nomad to efficiently scale.

HashiCorp Built

At HashiCorp, we build solutions to DevOps problems that are technically sound and are a joy to use. We do not take shortcuts with the technologies we choose, and just as importantly we don't take shortcuts in the experience of using and operating our tools. As a result, HashiCorp-made tools are stable, scalable, and easy to use and operate.

Nomad is the eighth such tool we've built. We've also built Vagrant, Packer, Serf, Consul, Terraform, Vault, and Otto. Nomad works with our ecosystem of tooling, but doesn't require any of them. We have plans to integrate Nomad more closely with our existing tooling to provide richer functionality for service discovery, security, and capacity management. Otto will be able to transparently use Nomad, providing the simplest user experience possible. We are proud of Nomad and are excited to see the community learn about it and begin using it.

Learn More

To learn more about Nomad, please visit the Nomad website. The following pages in particular are good next steps:

  • Intro - The intro section explains in more detail what Nomad is, how it works, and includes a brief getting started guide so you can play with Nomad on your own machine and start scheduling jobs.

  • Internals - The internals section is an advanced topic but covers details about all the internals of Nomad for those that are interested. It isn't required reading to use Nomad, but is recommended if you want to learn about the technologies behind Nomad.

  • Comparison to other software - If you'd like to know how Nomad is different from other options, take a look at this page where we go into detail on the differences.

  • GitHub - The source code for Nomad is hosted on GitHub if you want to dive right in. We recommend reading the documentation first, since an understanding of how Nomad works will help greatly in understanding the implementation.

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