APIs and data

Question 1

A bike-share app sends POST /rentals and the server creates the rental. Which response best follows HTTP conventions?

Accepted Answer

201 Created with a Location header for the new rental

Question 2

What is the difference between 401 and 403 in an HTTP API?

Accepted Answer

401 means the caller is not authenticated; 403 means it is but lacks permission

Question 3

A county library API exposes PUT /books/42. A client sends a PUT body that contains only a new title. Under standard PUT semantics, what should happen?

Accepted Answer

The body replaces the whole book, so omitted fields may be cleared

Question 4

A payroll API answers bad input with 200 OK and a body like {"error": "invalid tax code"}. What is the main problem?

Accepted Answer

Clients, proxies, and monitors will count the failure as a success

Question 5

Which set of HTTP methods is defined as idempotent by the HTTP specification?

Accepted Answer

GET, PUT, and DELETE

Question 6

A clinic scheduler needs a way for patients to cancel an appointment while keeping a record of it. Which API design fits best?

Accepted Answer

Set the appointment's status to cancelled on its own resource

Question 7

Two admins edit the same product record. The second admin's PUT carries an If-Match ETag that no longer matches the stored version. What status should the server return?

Accepted Answer

412 Precondition Failed

Question 8

A tax office API lets users request a yearly report that takes several minutes to build. How should the API respond to the request?

Accepted Answer

Return 202 Accepted with a status URL the client can poll

Question 9

A recipe site's feed uses ?page=3 with OFFSET pagination. New recipes are added at the top all day, and users report seeing the same recipe on two pages. What is the best fix?

Accepted Answer

Switch to cursor pagination keyed on a stable sort order

Question 10

An API pages through events with a cursor that holds only the last created_at value. Some events are silently skipped between pages. What is the most likely cause?

Accepted Answer

Rows with equal timestamps need a unique tiebreaker such as id

Question 11

A Postgres query with OFFSET 500000 LIMIT 20 is very slow even though it uses an index on the sort column. Why?

Accepted Answer

The database still reads and discards every skipped row

Question 12

A public API used by mobile apps needs to rename the field "zip" to "postal_code". Many app versions stay installed for months. What is the safest approach?

Accepted Answer

Add the new field, keep the old one, and deprecate it on a timeline

Question 13

Which change to a JSON API is backward compatible for existing clients?

Accepted Answer

Adding a new optional field to a response

Question 14

What should a well-designed API error body contain?

Accepted Answer

A stable error code, a readable message, and a request id

Question 15

A grain co-op API adds a new shipment status "on_hold". An older phone app crashes when it sees it. Which client-side practice would have prevented the crash?

Accepted Answer

Handling unknown enum values as a generic state and carrying on

Question 16

A checkout service's POST /payments sometimes times out. The client retries, and some customers are charged twice. What is the right fix?

Accepted Answer

Have clients send an idempotency key the server stores with its result

Question 17

Two requests with the same idempotency key reach a ticketing API at the same moment. How should the server make sure only one is processed?

Accepted Answer

Insert the key under a unique constraint so only one request can claim it

Question 18

A client reuses an idempotency key from an earlier request, but this time the request body is different. What should the server do?

Accepted Answer

Reject it with an error, since the key belongs to another request

Question 19

Why do retry policies add random jitter to exponential backoff?

Accepted Answer

So many clients do not retry at the same instant

Question 20

Which failures should an API client usually retry automatically, with backoff?

Accepted Answer

503 responses and connection timeouts

Question 21

A user request flows through services A, B, and C, then to a database. Each service makes up to 3 attempts on its downstream call. If the database is down, how many calls can it receive for one user request?

Accepted Answer

Up to 27 calls

Question 22

A message consumer runs UPDATE accounts SET balance = balance - 10 WHERE id = 5 for every debit message it receives. Is this safe under redelivery?

Accepted Answer

No, a repeat subtracts again, so dedupe by message id

Question 23

During an incident, a gateway's retries double the traffic to an already struggling inventory service. Which control most directly limits this?

Accepted Answer

A retry budget that caps retries as a share of requests

Question 24

A ride-hailing service calls a pricing service with no timeout. When pricing slows down, every worker thread ends up waiting and the whole service stops responding. What is the first fix?

Accepted Answer

Set a timeout on the pricing call and fail fast when it expires

Question 25

A web frontend gives a request 2 seconds. The backend it calls then makes inner calls, each with its own 5 second timeout. What is the better design?

Accepted Answer

Pass the remaining deadline down so inner calls stop with the caller

Question 26

What does a circuit breaker do in a service that calls a flaky dependency?

Accepted Answer

Stops calling the dependency for a while after repeated failures

Question 27

A photo-resize worker reads jobs into an unbounded in-memory queue. During traffic spikes its memory grows until the process is killed. What is the right fix?

Accepted Answer

Bound the queue and push back on producers when it is full

Question 28

A search API is at full capacity, and more requests keep arriving. What should it do with the excess?

Accepted Answer

Reject it early with 503 or 429 so admitted requests finish in time

Question 29

A ticketing API hangs. All threads are waiting for one of 20 database connections. A new endpoint holds its connection while it makes a slow HTTP call to a partner. What is the best fix?

Accepted Answer

Release the connection before calling the partner

Question 30

Which anomaly can still happen under the read committed isolation level?

Accepted Answer

A row read twice in one transaction returns different values

Question 31

A cafe loyalty app reads a customer's points, adds 10 in application code, and writes the total back. Under read committed, some points go missing. What is the fix?

Accepted Answer

Use an atomic UPDATE points = points + 10, or lock the row first

Question 32

A warehouse rule says at least one forklift driver must be on shift. Two drivers each check that two are on shift, then clock out in separate transactions. Both commit and nobody is left. Which isolation level prevents this?

Accepted Answer

Serializable

Question 33

A Postgres transaction at serializable isolation fails with a serialization error. What should the application do?

Accepted Answer

Retry the whole transaction from the beginning

Question 34

A web form opens a database transaction when a user starts editing and commits when they click Save, sometimes minutes later. What is the main problem?

Accepted Answer

Locks and old row versions are held for minutes, blocking others

Question 35

An order service writes to its own database and then calls a separate inventory service over HTTP. A teammate wants both steps to be atomic. What is the realistic approach?

Accepted Answer

One local transaction cannot span both, so use a saga with compensations

Question 36

In Postgres, what does a transaction at repeatable read isolation see?

Accepted Answer

A consistent snapshot taken at its first query

Question 37

What is a phantom read?

Accepted Answer

A repeated range query returns rows another transaction inserted

Question 38

A wiki stores a version number on each page. Saves run UPDATE pages SET body = ?, version = version + 1 WHERE id = ? AND version = ?. How does the app detect that someone else saved first?

Accepted Answer

The update matches zero rows because the version changed

Question 39

Transaction A updates row 1 then row 2. Transaction B updates row 2 then row 1. They deadlock often. What is the best fix?

Accepted Answer

Always lock rows in the same order, such as by primary key

Question 40

A forum's signup code checks whether a username exists and inserts it if not. Under load, two accounts end up with the same username. What is the right fix?

Accepted Answer

Add a unique constraint and handle the violation error

Question 41

Ten workers poll a jobs table with SELECT ... FOR UPDATE LIMIT 1 and keep blocking on the same row. Which Postgres clause lets each worker grab a different job?

Accepted Answer

FOR UPDATE SKIP LOCKED

Question 42

A worker holds a lock with a 10 second lease. A long garbage collection pause freezes it for 15 seconds, another worker takes the lock, and then the first wakes and writes. What prevents the bad write?

Accepted Answer

A fencing token that storage checks on every write

Question 43

A metrics service increments a shared counter from many threads without synchronization. The final total is lower than the number of events. Why?

Accepted Answer

The increment is a read, add, and write that is not atomic

Question 44

A nightly billing job runs from cron on three app instances, but it must run only once. What is a sound approach?

Accepted Answer

Have each instance try a shared advisory lock and skip if it fails

Question 45

When is optimistic locking a better fit than pessimistic locking?

Accepted Answer

When conflicts are rare and retrying a write is cheap

Question 46

A Node.js API stores the current user in a module-level variable at the start of each request handler. Under load, some users see other users' data. What is the cause?

Accepted Answer

Concurrent requests overwrite the shared variable mid-request

Question 47

A concert site reads a seat's status, sees it is free, and then marks it as sold. Some seats are sold twice. Which single statement fixes the race?

Accepted Answer

UPDATE seats SET buyer = ? WHERE id = ? AND buyer IS NULL, checking the row count

Question 48

A table has a composite B-tree index on (region, created_at). Which query can use it most effectively?

Accepted Answer

WHERE region = 'north' AND created_at > '2026-01-01'

Question 49

A users table has an index on email, but WHERE lower(email) = 'ann@example.com' does a full scan. What fixes it?

Accepted Answer

Add an expression index on lower(email)

Question 50

EXPLAIN ANALYZE shows the planner expected 10 rows from a filter, but the filter actually returned 2 million. The plan chose a nested loop. What do you check first?

Accepted Answer

Whether the table statistics are stale

Question 51

A table has an index on is_active, and 95 percent of rows are active. WHERE is_active = true uses a sequential scan. Why?

Accepted Answer

Reading most of the table in order beats going through the index

Question 52

A support desk table has 50 million tickets. Only about 1 percent are open, and the main dashboard queries WHERE status = 'open'. Which index fits best?

Accepted Answer

A partial index that covers only rows where status is open

Question 53

What does it mean for an index to cover a query?

Accepted Answer

The index holds every column the query needs, so the table is skipped

Question 54

A high-volume events table has 14 indexes, and inserts have become slow. What is the most likely reason?

Accepted Answer

Every insert must also update all 14 indexes

Question 55

An order history page loads 50 orders, then runs a separate query for each order's customer. The page is slow. What is the fix?

Accepted Answer

Load the customers in one batched query or a join

Question 56

A genealogy site searches names with WHERE surname LIKE '%sen'. There is a B-tree index on surname, but the query scans the whole table. Why?

Accepted Answer

A B-tree cannot use a leading wildcard; a trigram index can

Question 57

A multi-tenant notes app runs WHERE tenant_id = ? ORDER BY created_at DESC LIMIT 20. Which index serves this best?

Accepted Answer

(tenant_id, created_at)

Question 58

A pet adoption site stores each animal's tag ids as a comma-separated string, like "3,17,42". What is the main problem?

Accepted Answer

It is hard to query, index, and validate compared with a join table

Question 59

An online store copies the shipping address into each order row instead of only pointing to the customer's address. When is that the right call?

Accepted Answer

When the order must keep the address as it was at purchase

Question 60

A budgeting app stores amounts in a floating-point column. Monthly totals sometimes end in values like 19.999999. What should it use instead?

Accepted Answer

A fixed-point decimal type or integer cents

Question 61

An audit log collects events from servers in three time zones, each writing its local time. Events sort in the wrong order. What is the fix?

Accepted Answer

Store instants in UTC with a time zone-aware type

Question 62

A gym membership system uses email as the primary key of the members table, and other tables reference it. Members often change their email. What is the better design?

Accepted Answer

Use a surrogate id as the key and a unique index on email

Question 63

An orders table has columns order_id, customer_id, and customer_name. What normalization problem does this create?

Accepted Answer

Renaming a customer means updating many order rows

Question 64

A ride app soft-deletes users with a deleted_at column and has a unique index on email. A deleted user tries to sign up again and fails. What is the fix?

Accepted Answer

Make the unique index partial, covering only rows where deleted_at is null