Technologies are described herein for a health monitor that establishes and monitors a set of alive endpoints to be used as substitutes for a dead working window endpoints. The health monitor periodically monitors substitution endpoints. When a request manager determines that a working window endpoint is dead, the request manager sends a request to the health monitor for a substitute endpoint. The health monitor receives the request and identifies an appropriate substitution endpoint from the health monitor endpoints to provide to the request manager to substitute for the dead endpoint. The health monitor releases the identified substitution endpoint to the request manager.