Question 1

What is a WebSocket, and when do you actually need one?

Accepted Answer

A protocol providing a **persistent, bidirectional connection** between client and server over a single TCP connection — so the server can send data **without being asked**, which ordinary HTTP request-response cannot do.

**How it works.** The connection begins as an HTTP request carrying an `Upgrade` header. If the server agrees, the same TCP connection switches to the WebSocket protocol and both sides can send **frames** at any time until either closes. Because it starts as HTTP, it traverses most firewalls and proxies that would block an arbitrary protocol — which was much of the design intent.

**What it solves.** Before it, the options were **polling** — asking repeatedly, wasting requests and adding latency — or **long polling**, holding a request open until data arrives, then immediately reopening. Both are workarounds for HTTP being client-initiated.

**When you genuinely need one:**

**Bidirectional, low-latency communication** — collaborative editing, multiplayer games, trading interfaces, live chat.

**High message frequency**, where per-request overhead dominates.

**When you probably do not:**

**Server-to-client updates only** — notifications, live feeds, progress indicators. **Server-Sent Events are simpler**, run over plain HTTP, reconnect automatically, and are frequently the better answer. They are consistently underused because WebSockets are better known.

**Infrequent updates**, where polling is simpler and adequate.

**Anything cacheable**, since WebSocket traffic bypasses HTTP caching entirely.

**What it costs operationally:**

**Connections are stateful**, so load balancers need sticky routing or a shared backplane, and **a deploy disconnects everyone at once** — reconnection with jittered backoff is mandatory, or you get a thundering herd.

**Each connection holds server resources**, which caps concurrency per instance.

**No automatic reconnection**, unlike SSE — you implement it.

**Proxies and mobile networks kill idle connections**, so heartbeats are required.

**Authentication is awkward**, since custom headers are unavailable in browsers — authenticate on the upgrade request and re-verify periodically.

Question 2

What is a webhook?

Accepted Answer

An HTTP request sent by a service to you when something happens — the inverse of an API call, and the reason it is sometimes described as a reverse API or a push notification for servers. The problem

Question 3

What is a message queue and when do you need one?

Accepted Answer

A message queue sits between a producer of work and a consumer of it, holding messages until they can be processed. It converts a synchronous call into an asynchronous one. What it gives you:

Question 4

What is idempotency and why does it matter for APIs?

Accepted Answer

An operation is idempotent when performing it more than once has the same effect as performing it once. Setting a value to ten is idempotent; adding ten is not. The property matters because networks

Question 5

What is the event loop in Node.js?

Accepted Answer

Node runs your JavaScript on a single thread. It achieves concurrency not by running code in parallel but by never waiting: operations that would block, such as reading a file or making a network

Question 6

What is a deadlock, and how do you avoid one?

Accepted Answer

A deadlock is a state in which two or more processes are each waiting for a resource held by another, so none can proceed and none will ever proceed without intervention. The classic shape. Thread A

Question 7

What is eventual consistency?

Accepted Answer

Eventual consistency is a guarantee that, if no new updates are made, all replicas of a piece of data will eventually converge on the same value. It does not guarantee when, and it permits reads that

Related Questions