Per-account task concurrency without a lock service

Many background jobs call an external system on behalf of separate accounts, tenants, or installations. The external system allows parallel calls across different accounts, but it does not allow two calls for the same account to run at the same time. That is not a global rate limit. It is concurrency by key: the key might be account_id , tenant_id , or another argument that identifies the shared quota or state boundary. You want the worker pool busy across many accounts, while each account stays