Hire full-stack developers, from schema to deploy

1. Sketch the tables for one part of our product. What do you index, and what do you leave out of version one?

Strong: asks how the data will be read before drawing anything. Puts foreign keys and unique constraints in the database, not only in application code. Weak: starts from the screens and ends with one wide table.

2. A production table has tens of millions of rows. Add an index and rename a column with no downtime. How?

Strong: knows a plain index build in PostgreSQL blocks writes until it finishes, and that CREATE INDEX CONCURRENTLY avoids that at the price of a slower build (PostgreSQL documentation). Renames in steps: add the new column, write to both, backfill in batches, switch reads, drop the old one. Weak: "run the migration at night".

3. The front end and the API keep disagreeing about a field. How do you stop it happening again?

Strong: one source of truth for the contract, such as an OpenAPI file or shared TypeScript types, validated at the boundary. Additive changes only, or a version. One error format. Weak: "we talk more".

4. Where does the session live, and what happens when a user logs out on one device?

Strong: compares a server-side session with a signed token, and knows the token is harder to revoke. Sets the cookie HttpOnly, which MDN describes as forbidding JavaScript from accessing the cookie, plus Secure and SameSite. Checks permissions on the server for every request. Weak: hides the button and calls it access control.

5. Which of our pages would you build ahead of time, render per request, or render in the browser? What gets cached where?

Strong: decides page by page, by who sees it and how fresh it must be. Names the cache layers (browser, CDN, application, database) and how each is cleared. Points out unprompted that a per-user response in a shared cache leaks data. The Next.js page goes further.

6. Walk me from merge to production. Then undo a bad release at 6 pm on a Friday.

Strong: CI runs the tests. One Docker image is built once and promoted between environments. Migrations are backwards compatible, so the previous version still runs on the new schema. Rollback is redeploying the previous image, and they have done it for real. Weak: "revert the commit and wait for the build".

7. A page takes six seconds to load. Where do you look first?

Strong: measures first. Opens the browser's network waterfall and checks Largest Contentful Paint, where Google's guidance is 2.5 seconds or less at the 75th percentile of page loads. Then server timing, then the query log and EXPLAIN for a missing index or a query inside a loop, then payload, bundle and image sizes. Weak: reaches for "add Redis" or a rewrite before measuring anything.

8. Which side are you weaker on, and what did you last hand to a specialist?

Strong: a specific answer, such as complex animation, query tuning beyond indexes, or cluster networking, with a real example. Weak: "I am equally strong at everything". Nobody is.

Start with one. Add the second person when the work tells you to. The contract is month to month, so the team can grow or shrink with the roadmap. The notice a change needs is written in your engagement contract.

Four signs that one is no longer enough

• Pull requests wait days because nobody else can review them.
• The interface is always waiting for the API, or the other way round.
• The roadmap has two streams that do not depend on each other.
• One person is carrying every production alert alone.

Who to add second

Usually a specialist, not another generalist. Look at where the backlog piles up. If it is screens and interaction, add a React developer. If it is integrations, queues and data, add a Node.js developer. A new request goes through the same steps as the first, with a shortlist in about 48 hours.

At three or more

Past two developers, somebody has to run delivery. If one of your own engineering managers has the time, keep buying single developers and point them at the same backlog. If nobody on your side has the time, ask for a tech lead on the team: they run stand-ups, review the code and own the sprint, and you get one person to call instead of three. That changes the shape of the team, not the contract, which stays month to month.

For scale: Qilin Lab ran teams of 10 engineers on Canco Petroleum, 8 on Eduley, 7 on Kashti FinServ and 6 on EcoProcurer. Those were project teams, not staff placements.