Techniques

Move the shelter's name and capacity out of the code and into .env: load it with ConfigModule, shape it with a configuration factory, validate it before the app boots, and read it through ConfigService.

Enforce the API's contract on every route: a global ValidationPipe that strips and refuses unknown properties and transforms payloads, an update DTO derived from the create DTO, and validated arrays in the body and the query string.

Decide once, on the class, what leaves the server: exclude the microchip, expose a computed label, flatten the owner to a name, and apply the same rules to plain objects with SerializeOptions.

Move the cats into a database: connect with TypeOrmModule, turn the Cat class into an entity, register it so a repository can be injected, and write the service against find, save, delete and the query builder.

Give cats owners: a one-to-many and its many-to-one, typed with Relation<> so the two entity files can import each other under ES modules, loaded on request and set by assignment.

Make an adoption all-or-nothing with dataSource.transaction() and its explicit QueryRunner form, and tidy every cat's name before it is written with an entity subscriber.

Unit-test a service that talks to the database without a database: provide a mock under the repository's token with getRepositoryToken, script its answers, and prove the service's rules from what it asked the repository for.

Ship version 2 of the cats API without breaking version 1: URI versioning with a default for the controllers that existed before, a version 2 controller on the same path, a route shared by both versions, and a health check outside them.

Remember a visitor between requests: parse cookies with cookie-parser, read them through custom decorators, set a plain cookie for a theme and a signed one for a favourite cat that nobody can forge.

Keep each visitor's data on the server behind a signed session cookie: count visits, remember the cats they looked at, and forget them on request, with express-session and @Session().

Accept a photo for a cat: parse multipart with FileInterceptor, validate the file's size and real type with ParseFilePipe, read the fields beside it, and take several documents at once with FilesInterceptor.

Send bytes, not JSON: stream a file from disk with StreamableFile, declare its type and download name three ways, refuse a missing file before streaming, and export the cats as a CSV built in memory.

Push adoptions to clients as they happen: a replaying stream of events in the service, an @Sse() route that merges it with a heartbeat, and the text/event-stream wire format Nest writes for you.

Call another service from the cats API: HttpModule with a timeout, HttpService's observables turned into promises, and every way the upstream can fail translated into the right status for your own client.

Compute once, serve many times: CacheModule with a default lifetime, a route cached automatically with CacheInterceptor and invalidated on writes, and a cache-aside lookup by hand with the cache manager.

Log through Nest's Logger with a class context, then make the application use a logger of your own that extends ConsoleLogger and keeps what it prints, with the boot log buffered until it exists.

Decouple what happens after an adoption from the code that adopts: CatsService emits namespaced events with typed payloads, and an audit module that never imports it listens, with a wildcard for the whole namespace.

Work on a clock rather than on a request: feed the cats on an interval, open the shelter once after boot, report on a cron schedule, and pause and resume a named task through the SchedulerRegistry.