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This seems a bit hard to understand, until you remember that a job has preferential priority on a Worker whose cluster matches the job's cluster, but the job is free to run on any Worker in other clusters. The job's restriction value can be used to limit what other clusters the job could possibly run on.
Restrictions defined for jobs
When a job has a restriction defined, it means only run on hosts that satisfy the restriction expression. Hosts that don't satisfy the restriction expression won't be considered as dispatch candidates (the job will never be sent to that Worker).
Restrictions defined for Workers
When a Worker has a restriction defined via its worker_restrictions value, it means only run jobs whose cluster value matches one of the clusters in that worker's restriction expression. The worker won't accept jobs whose cluster doesn't match one of the clusters in the worker's restriction expression.
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