Top 3 Zero downtime Interview Questions

Browse 3 real-world technical and behavioral interview questions about Zero downtime. Review scenarios, edge cases, and architectural best practices.

You are moving a sharded datastore to Cassandra with no maintenance window. How do you cut over, and how do you know both copies agree before you do?

Replicate from the source's change log rather than dual-writing from application code, backfill with each row's original write timestamp so history can never overwrite live data, then run continuous range checksums and shadow reads until divergence is understood rather than merely small. The read flip and the write flip are separate steps, each reversible.

hardscenario

You need to reindex a ten-million-chunk corpus with a new embedding model. How do you do it without downtime?

Build a shadow index beside the live one, dual-write from a recorded change-feed watermark so it never falls behind, verify on reconciliation counts plus a gold set and shadowed production queries, then cut over by moving an alias and keep the old index writable long enough to roll back.

harddesign

You need to migrate the schema of a 400-million-row table on a live system. Talk me through the plan, the backup you would want first, and how the connection pool affects it.

Every step is chosen to hold weak locks briefly - add the constraint invalid and validate separately, index concurrently, backfill in committed batches - behind a restore you have rehearsed and timed, and a pool small enough that one blocked DDL statement does not exhaust it.

hardscenario