Overview
This article addresses a specific challenge with constraint warnings in Nomad system jobs when deploying workloads using either Podman or Docker drivers. In certain configurations, these warnings can generate non-zero exit codes, which may interfere with monitoring systems that rely on exit codes for health checks.
Context
Nomad supports the use of constraint blocks within job definitions, which control where specific jobs can be scheduled based on node attributes. System jobs, which are frequently used for services that should run on all eligible nodes, may encounter issues when these constraints cause allocation placement warnings. These warnings can result in non-zero exit codes during job plans or run commands, leading to potential monitoring discrepancies.
Note - This issue was identified in Nomad version 1.4.12 and later, but has now been resolved in Nomad v1.10.2+ent.
Problem Description
When running a nomad job plan or nomad job run on a Nomad system job without modifying the job specification, users may encounter constraint-related warnings that return a non-zero exit code, even if no changes were made to the job file. This behavior can interfere with monitoring tools that interpret non-zero exit codes as operational issues.
Updating metadata within the job definition, such as the meta parameter under the service block triggers a re-evaluation of the job. This metadata, which includes user-defined values for service registration in Consul, updates in place without restarting allocations. As a result, the allocation remains unchanged, and the constraint warning persists due to the lack of an allocation restart.
However, with the release of Nomad v1.10.2+ent, this issue has been resolved, and constraint warnings will no longer trigger non-zero exit codes in such scenarios.
Fix Details:
The fix ensures that Nomad properly handles constraint warnings in system jobs, preventing non-zero exit codes when no job changes are made and when only metadata is updated (as described in the previous behavior). Nomad now correctly re-evaluates the allocation placement even for metadata-only updates, which resolves the warnings and resets the exit code appropriately.
Reproduction Steps
To reproduce this behavior, use the following example job configuration and modify the metadata to observe the constraint warning.
Sample Job Configuration
job "example-job" {
datacenters = ["dc1"]
type = "system"
group "example-group" {
task "example-task" {
driver = "docker"
constraint {
attribute = "${attr.unique.hostname}"
value = "nomad-client"
}
config {
image = "nginx:latest"
}
service {
name = "example-job"
address = "${attr.unique.hostname}"
meta {
APP_VERSION = 1
}
}
}
}
}
Step-by-Step Reproduction
-
Deploy the Job: Run
nomad job run example-job.hclto deploy the job. It should successfully place the allocation. -
Modify Metadata in
metaBlock: ChangeAPP_VERSIONunder themetablock from1to2without altering other job components. -
Plan or Re-run the Job: Execute
nomad job plan example-job.hclornomad job run example-job.hclagain. The plan will generate a constraint warning, displaying an in-place allocation update and a non-zero exit code due to the existing constraint.
Sample Output
Upon re-running the job with an updated meta value, the following output may occur:
root@nomad-server:/home/ubuntu# nomad job plan test.hcl
+/- Job: "example-job"
+/- Task Group: "example-group" (1 in-place update)
+/- Task: "example-task" (forces in-place update)
+/- Service {
Address: "${attr.unique.hostname}"
AddressMode: "auto"
Cluster: "default"
EnableTagOverride: "false"
+/- Meta[APP_VERSION]: "1" => "2"
Name: "example-job"
Namespace: "default"
OnUpdate: "require_healthy"
PortLabel: ""
Provider: "consul"
TaskName: "example-task"
}
Scheduler dry-run:
- WARNING: Failed to place allocations on all nodes.
Task Group "example-group" (failed to place 1 allocation):
* Constraint "${attr.unique.hostname} = nomad-client": 1 nodes excluded by filter
Job Modify Index: 143
To submit the job with version verification run:
nomad job run -check-index 143 test.hcl
When running the job with the check-index flag, the job will only be run if the
job modify index given matches the server-side version. If the index has
changed, another user has modified the job and the plan's results are
potentially invalid.
root@nomad-server:/home/ubuntu# nomad job run -check-index 143 test.hcl
==> 2024-10-28T12:45:16Z: Monitoring evaluation "c1912c9e"
2024-10-28T12:45:16Z: Evaluation triggered by job "example-job"
2024-10-28T12:45:17Z: Allocation "259533f1" modified: node "9aa1f9eb", group "example-group"
2024-10-28T12:45:17Z: Evaluation status changed: "pending" -> "complete"
==> 2024-10-28T12:45:17Z: Evaluation "c1912c9e" finished with status "complete" but failed to place all allocations:
2024-10-28T12:45:17Z: Task Group "example-group" (failed to place 1 allocation):
* Constraint "${attr.unique.hostname} = nomad-client": 1 nodes excluded by filter
This output demonstrates an in-place update attempt for the allocation, where the constraint remains unfulfilled. The non-zero exit code from this warning can impact monitoring systems that depend on exit codes to evaluate job health.
Technical Explanation
Nomad’s in-place update mechanism for system jobs does not restart allocations if only metadata fields, such as those in the meta block, are updated. Constraint-based warnings thus persist if the allocation does not meet the specified conditions, resulting in a non-zero exit code during the planning or run stages. This is especially relevant when the allocation needs a full restart to reassess placement with the updated constraints.
In Nomad v1.10.2+ent, this behavior has been addressed, and the system job's allocation is now properly re-evaluated even when only metadata is updated, resolving the constraint warnings and ensuring that no non-zero exit codes are generated during the planning or running process.
Workaround
To temporarily resolve the constraint warning and reset the exit code, perform a full job stop and redeploy:
-
Stop the Job:
nomad job stop example-job -
Re-deploy the Job:
nomad job run example-job.hcl
This approach ensures that the allocation is restarted and allows Nomad to re-evaluate the constraint condition, clearing any warnings and resetting the exit code.
Expected Resolution
With Nomad v1.10.2+ent and later versions, the issue with constraint warnings and non-zero exit codes in system jobs has been fully resolved. Monitoring teams relying on exit codes should upgrade to the latest version to benefit from this fix.
Additional Considerations
Note on ${attr.unique.hostname} in Constraints
When defining constraints in Nomad job specifications, special attention must be paid to the use of ${attr.unique.hostname}. This variable interpolation is unique to each client node, and conflicting constraints involving this attribute can result in unplaceable jobs.
This warning about ${attr.unique.hostname} variable interpolation can be found in the official Nomad documentation under the constraint block section.
Additional Notes
For environments where system stability is essential, it is recommended to validate constraint conditions before updating the job definition.
References