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| layout | page_title | sidebar_current | description |
|---|---|---|---|
| intro | Running Nomad | getting-started-running | Learn about the Nomad agent, and the lifecycle of running and stopping. |
Running Nomad
Nomad relies on a long running agent on every machine in the cluster. The agent can run either in server or client mode. Each region must have at least one server, though a cluster of 3 or 5 servers is recommended. A single server deployment is highly discouraged as data loss is inevitable in a failure scenario.
All other agents run in client mode. A client is a very lightweight process that registers the host machine, performs heartbeating, and runs any tasks that are assigned to it by the servers. The agent must be run on every node that is part of the cluster so that the servers can assign work to those machines.
Starting the Agent
For simplicity, we will run a single Nomad agent in development mode. This mode is used to quickly start an agent that is acting as a client and server to test job configurations or prototype interactions. It should not be used in production as it does not persist state.
vagrant@nomad:~$ sudo nomad agent -dev
No configuration files loaded
==> Starting Nomad agent...
==> Nomad agent configuration:
Atlas: <disabled>
Client: true
Log Level: DEBUG
Region: global (DC: dc1)
Server: true
==> Nomad agent started! Log data will stream in below:
2016/08/23 21:26:03 [INFO] serf: EventMemberJoin: nomad.global 127.0.0.1
2016/08/23 21:26:03.853216 [INFO] nomad: starting 1 scheduling worker(s) for [service batch system _core]
2016/08/23 21:26:03.854711 [INFO] client: using state directory /tmp/NomadClient632906511
2016/08/23 21:26:03.854732 [INFO] client: using alloc directory /tmp/NomadClient174428962
2016/08/23 21:26:03.854761 [DEBUG] client: built-in fingerprints: [arch cgroup cpu env_aws env_gce host memory network nomad storage]
2016/08/23 21:26:03.854879 [INFO] fingerprint.cgroups: cgroups are available
2016/08/23 21:26:03.854951 [DEBUG] fingerprint.cpu: frequency: 2294 MHz
2016/08/23 21:26:03.854954 [DEBUG] fingerprint.cpu: core count: 1
2016/08/23 21:26:03 [INFO] raft: Node at 127.0.0.1:4647 [Follower] entering Follower state (Leader: "")
2016/08/23 21:26:03.861876 [INFO] nomad: adding server nomad.global (Addr: 127.0.0.1:4647) (DC: dc1)
2016/08/23 21:26:03.861924 [DEBUG] client: fingerprinting cgroup every 15s
2016/08/23 21:26:05 [WARN] raft: Heartbeat timeout from "" reached, starting election
2016/08/23 21:26:05 [INFO] raft: Node at 127.0.0.1:4647 [Candidate] entering Candidate state
2016/08/23 21:26:05 [DEBUG] raft: Votes needed: 1
2016/08/23 21:26:05 [DEBUG] raft: Vote granted from 127.0.0.1:4647. Tally: 1
2016/08/23 21:26:05 [INFO] raft: Election won. Tally: 1
2016/08/23 21:26:05 [INFO] raft: Node at 127.0.0.1:4647 [Leader] entering Leader state
2016/08/23 21:26:05 [INFO] raft: Disabling EnableSingleNode (bootstrap)
2016/08/23 21:26:05 [DEBUG] raft: Node 127.0.0.1:4647 updated peer set (2): [127.0.0.1:4647]
2016/08/23 21:26:05.685611 [INFO] nomad: cluster leadership acquired
2016/08/23 21:26:05.685685 [DEBUG] leader: reconciling job summaries at index: 0
2016/08/23 21:26:05.855402 [DEBUG] fingerprint.env_aws: Error querying AWS Metadata URL, skipping
2016/08/23 21:26:07.855934 [DEBUG] fingerprint.env_gce: Could not read value for attribute "machine-type"
2016/08/23 21:26:07.855942 [DEBUG] fingerprint.env_gce: Error querying GCE Metadata URL, skipping
2016/08/23 21:26:07.860576 [DEBUG] fingerprint.network: Detected interface lo with IP 127.0.0.1 during fingerprinting
2016/08/23 21:26:07.861368 [DEBUG] fingerprint.network: Unable to read link speed from /sys/class/net/lo/speed
2016/08/23 21:26:07.861376 [DEBUG] fingerprint.network: Unable to read link speed; setting to default 100
2016/08/23 21:26:07.863896 [DEBUG] client: applied fingerprints [arch cgroup cpu host memory network nomad storage]
2016/08/23 21:26:07.863914 [DEBUG] driver.exec: exec driver is enabled
2016/08/23 21:26:07.864114 [DEBUG] client: fingerprinting exec every 15s
2016/08/23 21:26:07.864966 [DEBUG] driver.docker: using client connection initialized from environment
2016/08/23 21:26:07.867270 [DEBUG] client: available drivers [exec raw_exec docker]
2016/08/23 21:26:07.868498 [DEBUG] client: fingerprinting docker every 15s
2016/08/23 21:26:07.872181 [DEBUG] client: node registration complete
2016/08/23 21:26:07.872209 [DEBUG] client: periodically checking for node changes at duration 5s
2016/08/23 21:26:07.872352 [DEBUG] client: updated allocations at index 1 (pulled 0) (filtered 0)
2016/08/23 21:26:07.872411 [DEBUG] client: allocs: (added 0) (removed 0) (updated 0) (ignore 0)
2016/08/23 21:26:07.879168 [DEBUG] client: state updated to ready
2016/08/23 21:26:07.879907 [DEBUG] consul.syncer: error in syncing: 1 error(s) occurred:
* Get http://127.0.0.1:8500/v1/agent/services: dial tcp 127.0.0.1:8500: getsockopt: connection refused
As you can see, the Nomad agent has started and has output some log data. From the log data, you can see that our agent is running in both client and server mode, and has claimed leadership of the cluster. Additionally, the local client has been registered and marked as ready.
-> Note: Typically any agent running in client mode must be run with root level privilege. Nomad makes use of operating system primitives for resource isolation which require elevated permissions. The agent will function as non-root, but certain task drivers will not be available.
Cluster Nodes
If you run nomad node-status in another terminal, you
can see the registered nodes of the Nomad cluster:
$ vagrant ssh
...
$ nomad node-status
ID Datacenter Name Class Drain Status
171a583b dc1 nomad <none> false ready
The output shows our Node ID, which is a randomly generated UUID, its datacenter, node name, node class, drain mode and current status. We can see that our node is in the ready state, and task draining is currently off.
The agent is also running in server mode, which means it is part of
the gossip protocol used to connect all
the server instances together. We can view the members of the gossip
ring using the server-members command:
$ nomad server-members
Name Address Port Status Leader Protocol Build Datacenter Region
nomad.global 127.0.0.1 4648 alive true 2 0.4.1 dc1 global
The output shows our own agent, the address it is running on, its
health state, some version information, and the datacenter and region.
Additional metadata can be viewed by providing the -detailed flag.
Stopping the Agent
You can use Ctrl-C (the interrupt signal) to halt the agent.
By default, all signals will cause the agent to forcefully shutdown.
The agent can be configured to gracefully
leave on either the interrupt or terminate signals.
After interrupting the agent, you should see it leave the cluster and shut down:
^C==> Caught signal: interrupt
[DEBUG] http: Shutting down http server
[INFO] agent: requesting shutdown
[INFO] client: shutting down
[INFO] nomad: shutting down server
[WARN] serf: Shutdown without a Leave
[INFO] agent: shutdown complete
By gracefully leaving, Nomad clients update their status to prevent further tasks from being scheduled and to start migrating any tasks that are already assigned. Nomad servers notify their peers they intend to leave. When a server leaves, replication to that server stops. If a server fails, replication continues to be attempted until the node recovers. Nomad will automatically try to reconnect to failed nodes, allowing it to recover from certain network conditions, while left nodes are no longer contacted.
If an agent is operating as a server, a graceful leave is important to avoid
causing a potential availability outage affecting the
consensus protocol. If a server does
forcefully exit and will not be returning into service, the
server-force-leave command should
be used to force the server from a failed to a left state.
Next Steps
If you shut down the development Nomad agent as instructed above, ensure that it is back up and running again and let's try to run a job!