1. If no metrics provided, CB uses timeout mechanism to change its state. The circuit breaker uses fixed timeout value for each request on its own. If the external service call takes more time than the timeout value(example 100ms for each request) for responding, then it moves to OPEN after tripping.
 2. One possibility is if Retry is used in circuit breaker implementation. CB transition from CLOSED to HALF OPEN directly instead of OPEN due to retry logic to handle failure calls.

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