A common question asked about Sentinel is how policies are processed in Terraform Cloud/Terraform Enterprise and how policy check concurrency is implemented.
Sentinel worker processes are invoked by Nomad in batches- each individual worker handles exactly one policy check and then exits. The result of the policy check is sent via HTTP back to the Rails application.
Because of this, there isn't a limit on the concurrency of policy checks from that perspective (Nomad may invoke however many it needs to in parallel). In a single server architecture, all of the Sentinel worker processes do compete for resources with everything else on the host.
When busy, Nomad may not immediately process the jobs if it can’t reasonably place them all. As other jobs in the system complete, Nomad re-evaluates and processes any remaining Sentinel policy checks sequentially.