--- layout: "guides" page_title: "Workload Migration" sidebar_current: "guides-operations-decommissioning-nodes" description: |- Workload migration is a normal part of cluster operations for a variety of reasons: server maintenance, operating system upgrades, etc. Nomad offers a number of parameters for controlling how running jobs are migrated off of draining nodes. --- # Workload Migration Migrating workloads and decommissioning nodes are a normal part of cluster operations for a variety of reasons: server maintenance, operating system upgrades, etc. Nomad offers a number of parameters for controlling how running jobs are migrated off of draining nodes. ## Configuring How Jobs are Migrated In Nomad 0.8 a [`migrate`][migrate] stanza was added to jobs to allow control over how allocations for a job are migrated off of a draining node. Below is an example job that runs a web service and has a Consul health check: ```hcl job "webapp" { datacenters = ["dc1"] migrate { max_parallel = 2 health_check = "checks" min_healthy_time = "15s" healthy_deadline = "5m" } group "webapp" { count = 9 task "webapp" { driver = "docker" config { image = "hashicorp/http-echo:0.2.3" args = ["-text", "ok"] port_map { http = 5678 } } resources { network { mbits = 10 port "http" {} } } service { name = "webapp" port = "http" check { name = "http-ok" type = "http" path = "/" interval = "10s" timeout = "2s" } } } } } ``` The above `migrate` stanza ensures only 2 allocations are stopped at a time to migrate during node drains. Even if multiple nodes running allocations for this job were draining at the same time, only 2 allocations would be migrated at a time. When the job is run it may be placed on multiple nodes. In the following example the 9 `webapp` allocations are spread across 2 nodes: ```text $ nomad run webapp.nomad ==> Monitoring evaluation "5129bc74" Evaluation triggered by job "webapp" Allocation "5b4d6db5" created: node "46f1c6c4", group "webapp" Allocation "670a715f" created: node "f7476465", group "webapp" Allocation "78b6b393" created: node "46f1c6c4", group "webapp" Allocation "85743ff5" created: node "f7476465", group "webapp" Allocation "edf71a5d" created: node "f7476465", group "webapp" Allocation "56f770c0" created: node "46f1c6c4", group "webapp" Allocation "9a51a484" created: node "46f1c6c4", group "webapp" Allocation "f6f6e64c" created: node "f7476465", group "webapp" Allocation "fefe81d0" created: node "f7476465", group "webapp" Evaluation status changed: "pending" -> "complete" ==> Evaluation "5129bc74" finished with status "complete" ``` If one those nodes needed to be decommissioned, perhaps because of a hardware issue, then an operator would issue node drain to migrate the allocations off: ```text $ nomad node drain -enable -yes 46f1 2018-04-11T23:41:56Z: Ctrl-C to stop monitoring: will not cancel the node drain 2018-04-11T23:41:56Z: Node "46f1c6c4-a0e5-21f6-fd5c-d76c3d84e806" drain strategy set 2018-04-11T23:41:57Z: Alloc "5b4d6db5-3fcb-eb7d-0415-23eefcd78b6a" marked for migration 2018-04-11T23:41:57Z: Alloc "56f770c0-f8aa-4565-086d-01faa977f82d" marked for migration 2018-04-11T23:41:57Z: Alloc "56f770c0-f8aa-4565-086d-01faa977f82d" draining 2018-04-11T23:41:57Z: Alloc "5b4d6db5-3fcb-eb7d-0415-23eefcd78b6a" draining 2018-04-11T23:42:03Z: Alloc "56f770c0-f8aa-4565-086d-01faa977f82d" status running -> complete 2018-04-11T23:42:03Z: Alloc "5b4d6db5-3fcb-eb7d-0415-23eefcd78b6a" status running -> complete 2018-04-11T23:42:22Z: Alloc "78b6b393-d29c-d8f8-e8e8-28931c0013ee" marked for migration 2018-04-11T23:42:22Z: Alloc "78b6b393-d29c-d8f8-e8e8-28931c0013ee" draining 2018-04-11T23:42:27Z: Alloc "78b6b393-d29c-d8f8-e8e8-28931c0013ee" status running -> complete 2018-04-11T23:42:29Z: Alloc "9a51a484-8c43-aa4e-d60a-46cfd1450780" marked for migration 2018-04-11T23:42:29Z: Alloc "9a51a484-8c43-aa4e-d60a-46cfd1450780" draining 2018-04-11T23:42:29Z: Node "46f1c6c4-a0e5-21f6-fd5c-d76c3d84e806" has marked all allocations for migration 2018-04-11T23:42:34Z: Alloc "9a51a484-8c43-aa4e-d60a-46cfd1450780" status running -> complete 2018-04-11T23:42:34Z: All allocations on node "46f1c6c4-a0e5-21f6-fd5c-d76c3d84e806" have stopped. ``` There are a couple important events to notice in the output. First, only 2 allocations are migrated initially: ``` 2018-04-11T23:41:57Z: Alloc "5b4d6db5-3fcb-eb7d-0415-23eefcd78b6a" marked for migration 2018-04-11T23:41:57Z: Alloc "56f770c0-f8aa-4565-086d-01faa977f82d" marked for migration ``` This is because `max_parallel = 2` in the job specification. The next allocation on the draining node waits to be migrated: ``` 2018-04-11T23:42:22Z: Alloc "78b6b393-d29c-d8f8-e8e8-28931c0013ee" marked for migration ``` Note that this occurs 25 seconds after the initial migrations. The 25 second delay is because a replacement allocation took 10 seconds to become healthy and then the `min_healthy_time = "15s"` meant node draining waited an additional 15 seconds. If the replacement allocation had failed within that time the node drain would not have continued until a replacement could be successfully made. ### Scheduling Eligibility Now that the example drain has finished we can inspect the state of the drained node: ```text $ nomad node status ID DC Name Class Drain Eligibility Status f7476465 dc1 nomad-1 false eligible ready 96b52ad8 dc1 nomad-2 false eligible ready 46f1c6c4 dc1 nomad-3 false ineligible ready ``` While node `46f1c6c4` has `Drain = false`, notice that its `Eligibility = ineligible`. Node scheduling eligibility is a new field in Nomad 0.8. When a node is ineligible for scheduling the scheduler will not consider it for new placements. While draining, a node will always be ineligible for scheduling. Once draining completes it will remain ineligible to prevent refilling a newly drained node. However, by default canceling a drain with the `-disable` option will reset a node to be eligible for scheduling. To cancel a drain and preserving the node's ineligible status use the `-keep-ineligible` option. Scheduling eligibility can be toggled independently of node drains by using the [`nomad node eligibility`][eligibility] command: ```text $ nomad node eligibility -disable 46f1 Node "46f1c6c4-a0e5-21f6-fd5c-d76c3d84e806" scheduling eligibility set: ineligible for scheduling ``` ### Node Drain Deadline Sometimes a drain is unable to proceed and complete normally. This could be caused by not enough capacity existing in the cluster to replace the drained allocations or by replacement allocations failing to start successfully in a timely fashion. Operators may specify a deadline when enabling a node drain to prevent drains from not finishing. Once the deadline is reached, all remaining allocations on the node are stopped regardless of `migrate` stanza parameters. The default deadline is 1 hour and may be changed with the [`-deadline`][deadline] command line option. The [`-force`][force] option is an instant deadline: all allocations are immediately stopped. The [`-no-deadline`][no-deadline] option disables the deadline so a drain may continue indefinitely. Like all other drain parameters, a drain's deadline can be updated by making subsequent `nomad node drain ...` calls with updated values. ## Node Drains and Non-Service Jobs So far we have only seen how draining works with service jobs. Both batch and system jobs are have different behaviors during node drains. ### Draining Batch Jobs Node drains only migrate batch jobs once the drain's deadline has been reached. For node drains without a deadline the drain will not complete until all batch jobs on the node have completed (or failed). The goal of this behavior is to avoid losing progress a batch job has made by forcing it to exit early. ### Keeping System Jobs Running Node drains only stop system jobs once all other allocations have exited. This way if a node is running a log shipping daemon or metrics collector as a system job, it will continue to run as long as there are other allocations running. The [`-ignore-system`][ignore-system] option leaves system jobs running even after all other allocations have exited. This is useful when system jobs are used to monitor Nomad or the node itself. ## Draining Multiple Nodes A common operation is to decommission an entire class of nodes at once. Prior to Nomad 0.7 this was a problematic operation as the first node to begin draining may migrate all of their allocations to the next node about to be drained. In pathological cases this could repeat on each node to be drained and cause allocations to be rescheduled repeatedly. As of Nomad 0.8 an operator can avoid this churn by marking nodes ineligible for scheduling before draining them using the [`nomad node eligibility`][eligibility] command: ```text $ nomad node eligibility -disable 46f1 Node "46f1c6c4-a0e5-21f6-fd5c-d76c3d84e806" scheduling eligibility set: ineligible for scheduling $ nomad node eligibility -disable 96b5 Node "96b52ad8-e9ad-1084-c14f-0e11f10772e4" scheduling eligibility set: ineligible for scheduling $ nomad node status ID DC Name Class Drain Eligibility Status f7476465 dc1 nomad-1 false eligible ready 46f1c6c4 dc1 nomad-2 false ineligible ready 96b52ad8 dc1 nomad-3 false ineligible ready ``` Now that both `nomad-2` and `nomad-3` are ineligible for scheduling, they can be drained without risking placing allocations on an _about-to-be-drained_ node. Toggling scheduling eligibility can be done totally independently of draining. For example when an operator wants to inspect the allocations currently running on a node without risking new allocations being scheduled and changing the node's state: ```text $ nomad node eligibility -self -disable Node "96b52ad8-e9ad-1084-c14f-0e11f10772e4" scheduling eligibility set: ineligible for scheduling $ # ...inspect node state... $ nomad node eligibility -self -enable Node "96b52ad8-e9ad-1084-c14f-0e11f10772e4" scheduling eligibility set: eligible for scheduling ``` ### Example: Migrating Datacenters A more complete example of draining multiple nodes would be when migrating from an old datacenter (`dc1`) to a new datacenter (`dc2`): ```text $ nomad node status -allocs ID DC Name Class Drain Eligibility Status Running Allocs f7476465 dc1 nomad-1 false eligible ready 4 46f1c6c4 dc1 nomad-2 false eligible ready 1 96b52ad8 dc1 nomad-3 false eligible ready 4 168bdd03 dc2 nomad-4 false eligible ready 0 9ccb3306 dc2 nomad-5 false eligible ready 0 7a7f9a37 dc2 nomad-6 false eligible ready 0 ``` Before migrating ensure that all jobs in `dc1` have `datacenters = ["dc1", "dc2"]`. Then before draining, mark all nodes in `dc1` as ineligible for scheduling. Shell scripting can help automate manipulating multiple nodes at once: ```text $ nomad node status | awk '{ print $2 " " $1 }' | grep ^dc1 | awk '{ system("nomad node eligibility -disable "$2) }' Node "f7476465-4d6e-c0de-26d0-e383c49be941" scheduling eligibility set: ineligible for scheduling Node "46f1c6c4-a0e5-21f6-fd5c-d76c3d84e806" scheduling eligibility set: ineligible for scheduling Node "96b52ad8-e9ad-1084-c14f-0e11f10772e4" scheduling eligibility set: ineligible for scheduling $ nomad node status ID DC Name Class Drain Eligibility Status f7476465 dc1 nomad-1 false ineligible ready 46f1c6c4 dc1 nomad-2 false ineligible ready 96b52ad8 dc1 nomad-3 false ineligible ready 168bdd03 dc2 nomad-4 false eligible ready 9ccb3306 dc2 nomad-5 false eligible ready 7a7f9a37 dc2 nomad-6 false eligible ready ``` Then drain each node in `dc1`. For this example we will only monitor the final node that is draining. Watching `nomad node status -allocs` is also a good way to monitor the status of drains. ```text $ nomad node drain -enable -yes -detach f7476465 Node "f7476465-4d6e-c0de-26d0-e383c49be941" drain strategy set $ nomad node drain -enable -yes -detach 46f1c6c4 Node "46f1c6c4-a0e5-21f6-fd5c-d76c3d84e806" drain strategy set $ nomad node drain -enable -yes 9ccb3306 2018-04-12T22:08:00Z: Ctrl-C to stop monitoring: will not cancel the node drain 2018-04-12T22:08:00Z: Node "96b52ad8-e9ad-1084-c14f-0e11f10772e4" drain strategy set 2018-04-12T22:08:15Z: Alloc "392ee2ec-d517-c170-e7b1-d93b2d44642c" marked for migration 2018-04-12T22:08:16Z: Alloc "392ee2ec-d517-c170-e7b1-d93b2d44642c" draining 2018-04-12T22:08:17Z: Alloc "6a833b3b-c062-1f5e-8dc2-8b6af18a5b94" marked for migration 2018-04-12T22:08:17Z: Alloc "6a833b3b-c062-1f5e-8dc2-8b6af18a5b94" draining 2018-04-12T22:08:21Z: Alloc "392ee2ec-d517-c170-e7b1-d93b2d44642c" status running -> complete 2018-04-12T22:08:22Z: Alloc "6a833b3b-c062-1f5e-8dc2-8b6af18a5b94" status running -> complete 2018-04-12T22:09:08Z: Alloc "d572d7a3-024b-fcb7-128b-1932a49c8d79" marked for migration 2018-04-12T22:09:09Z: Alloc "d572d7a3-024b-fcb7-128b-1932a49c8d79" draining 2018-04-12T22:09:14Z: Alloc "d572d7a3-024b-fcb7-128b-1932a49c8d79" status running -> complete 2018-04-12T22:09:33Z: Alloc "f3f24277-4435-56a3-7ee1-1b1eff5e3aa1" marked for migration 2018-04-12T22:09:33Z: Alloc "f3f24277-4435-56a3-7ee1-1b1eff5e3aa1" draining 2018-04-12T22:09:33Z: Node "96b52ad8-e9ad-1084-c14f-0e11f10772e4" has marked all allocations for migration 2018-04-12T22:09:39Z: Alloc "f3f24277-4435-56a3-7ee1-1b1eff5e3aa1" status running -> complete 2018-04-12T22:09:39Z: All allocations on node "96b52ad8-e9ad-1084-c14f-0e11f10772e4" have stopped. ``` Note that there was a 15 second delay between node `96b52ad8` starting to drain and having its first allocation migrated. The delay was due to 2 other allocations for the same job already being migrated from the other nodes. Once at least 8 out of the 9 allocations are running for the job, another allocation could begin draining. The final node drain command did not exit until 6 seconds after the `drain complete` message because the command line tool blocks until all allocations on the node have stopped. This allows operators to script shutting down a node once a drain command exits and know all services have already exited. [deadline]: /docs/commands/node/drain.html#deadline [eligibility]: /docs/commands/node/eligibility.html [force]: /docs/commands/node/drain.html#force [ignore-system]: /docs/commands/node/drain.html#ignore-system [migrate]: /docs/job-specification/migrate.html [no-deadline]: /docs/commands/node/drain.html#no-deadline