Cloud Architect interview questionsMigration review — the data centre exit round

Purpose

Sets the round as a review of a plan that can still change, so the candidate's job is to find what it gets wrong rather than to present a migration methodology.

Purpose

Removes politeness as a reason to soften findings. A candidate who still hedges every point here will hedge in a steering meeting too.

Purpose

The load-bearing question. Most candidates can describe rehost, replatform and refactor; the separator is whether they read the dependency map and the outside lead times before the labels, and whether they do the arithmetic that turns "wave 1" into a batch that cannot finish.

Signals to score

• Reads the dependency map before the inventory's labels, and groups workloads by how they talk rather than by team or size
• Finds the finance batch split from its database and works out the effect: 9 million queries at 25 ms is about 62 hours against a six-hour window
• Makes the batch server and the ERP database one move group, or names the change to the batch that would let them move apart
• Finds the bank's allow-list rule and works backwards from a quarter's first week to the date the payments cutover can happen
• Asks who owns the data centre's public address range, and keeps payment files leaving through it until the bank has changed its list
• Flags the licence tied to named physical hosts as something to settle with the vendor this week, before the wave that depends on it is booked
• Agrees with rehosting the 31 internal applications unchanged, and says why a fixed deadline makes that right
• Does the arithmetic on the file server and drops the appliance
• Orders the work by lead time, starting the outside conversations first even where the move itself comes later
• Names what would make them ask for the data centre contract to be extended, and by when they would need to know

Follow-up questions

• The batch server moves in wave 1 and its database in wave 3. What happens on the first night?
• The bank changes its allow-list once a quarter, on eight weeks' notice. When does that conversation have to start?
• How long does 40 TB take over the link you already have?
• Which workloads would you move without changing anything, and why?
• What would make you ask for more time on the contract, and when would you have to know?

Purpose

Tests whether the candidate sees the address overlap before designing on top of it, chooses what to renumber by what is being kept, and treats names, the link and the end state as part of the boundary.

Signals to score

• Sees that cloud production and staging overlap the data centre's server and storage ranges, so the draft route cannot work
• Rejects renumbering the data centre, which is being emptied, and is cautious about renumbering cloud production during the migration
• Proposes new cloud networks for migrated workloads from a range that overlaps nothing, taken from one address plan for the whole organisation
• Limits what cloud production reaches in the data centre to a short list of flows, uses translation only for those if unavoidable, and says what translation breaks
• Advertises specific routes instead of 10.0.0.0/8, so each side reaches only what it needs
• Designs name resolution in both directions so migrated systems find each other by name, and treats the 14 rows with written-in addresses as a list to fix
• Checks the 1 Gbps link against migration copies and coexistence traffic, and asks the lead time for a dedicated connection
• Treats the link as a single point of failure for the finance move group and the payment files while both sides run, and says what happens when it drops
• Describes the network on the day the data centre closes, so nothing temporary stays by accident

Follow-up questions

• Cloud production is 10.0.0.0/16, and so are the data centre's servers. Where does a packet for 10.0.3.20 go?
• Which side would you renumber, if either?
• A migrated application has 10.0.1.14 written in its configuration. What happens to it after the move?
• The link drops for an hour during wave 2. What stops?
• What is left of this design the day after the data centre closes?

Purpose

Tests whether the candidate prices people as well as instances, turns an unknown into a break-even instead of a guess, and notices that the version, not only the extension, is part of the decision.

Signals to score

• Notices that the managed service loads only extensions it supports, so an in-house C extension blocks the plan as written
• Notes that PostgreSQL 14 leaves community support in November 2026, so either path includes a major-version upgrade, and asks what it costs on each
• Prices both options over three years with people included: about $140,400 managed before any extension work, about $244,800 self-run
• Turns the unknown into a break-even: removing the extension can cost up to about $104,000 — roughly seven engineer-months at the sheet's rate — before self-run is cheaper
• Questions the 0.3 of an engineer, given that the extension's author has left and nobody is named as running PostgreSQL today
• Proposes a time-boxed estimate of removing the extension before deciding, with a date that still fits the contract
• Considers moving self-run first to meet the deadline and to managed later, and prices doing the move twice
• Asks what the extension does for customers, since replacing the ranking may be cheaper than preserving it
• States the decision and the evidence that would reverse it

Follow-up questions

• Will the managed service load this extension?
• PostgreSQL 14 leaves community support in November 2026. Which option does that hurt more?
• How much could removing the extension cost before self-run wins?
• The extension's author has left. What does that do to the 0.3 of an engineer?
• If the rewrite cannot be estimated for six weeks, what do you do on Monday?

Purpose

A candidate who has run a migration asks about the contract, the owners and the dependencies nobody has mapped; one who has only planned one asks which migration tooling we use. The topics offered make their choice the signal.

Purpose

A specific, unflattering fact is what a candidate who does this work wants to know, and it discourages one who expected the hard part to be finished. Check that it is still true before you say it.