Mode determines if the request is processed in real time or is executed at a future time. Cloud Search accepts higher throughput for asynchronous updates, but these updates also have higher latency from acceptance to serving.

UNSPECIFIED The priority is not specified in the update request. Leaving priority unspecified results in an update failure.
SYNCHRONOUS For real-time updates.
ASYNCHRONOUS For changes that are executed after the response is sent back to the caller.