Embodiments for providing compare and swap (CAS) functionality to key value storage to allow multi-threaded applications to share storage devices and synchronize multiple concurrent threads or processes. A key-value application programming interface (API) is modified to include a CAS API in addition to the standard Put and Get APIs. The CAS function uses a key, expected old value, and new value to compare and swap an existing key value only if its current value equals the expected old value. Hash values of the key value and expected old value may be used by the CAS function to improve performance and reduce bandwidth.