Fail fast requests allow terminating requests (with status UNAVAILABLE) prior to the deadline of the request being met.
gRPC implementations of fail fast can terminate requests whenever a channel is in the TRANSIENT_FAILURE or SHUTDOWN states. If the channel is in any other state (CONNECTING, READY, or IDLE) the request should not be terminated.
Fail fast SHOULD be the default for gRPC implementations, with an option to switch to non fail fast.
The opposite of fail fast is 'ignore connectivity'.