MID Server
A MID Server is a lightweight Java application installed inside the customer network that lets the ServiceNow instance reach systems it cannot contact directly, used by Discovery, Orchestration and internal integrations.
Also searched as: management instrumentation and discovery server, ECC queue.
In practice
Communication is pull-based through the ECC Queue: output records are work for the MID Server, input records are results. Add a second MID Server for network segmentation, for throughput once the queue backs up, or for high availability using a MID Server cluster.
Example
Discovery of a segregated DMZ subnet runs through a MID Server placed in that segment rather than the corporate one.
What interviewers check
Interviewers probe why you would add a second MID Server, and expect ecc_queue as your first stop when a probe never returns.