Chat: what you will build

A chat backend: rooms people join, message history that pages backwards, and live delivery over a socket that refuses anyone who is not a member.

By the end you will have:

• POST /rooms, POST /rooms/:id/join, and a membership check every room route shares
• GET /rooms/:id/messages with cursor pagination that does not break while people are typing
• POST /rooms/:id/messages, delivered to everyone connected in under a millisecond
• GET /rooms/:id/ws, guarded by the same auth as everything else and by room membership

The two things worth reading

Cursor pagination, not offset. LIMIT 20 OFFSET 40 is wrong on a feed people are writing to: between page one and page two, three new messages arrive, everything shifts, and the reader sees three they already saw. This is the single most common bug in a chat API and the fix is not difficult — it just has to be chosen deliberately.

The socket checks membership before it opens. auth: true proves who someone is. It does not prove they belong in room 4f2a. The check happens in upgrade, which is the last moment before a socket exists.

Every query here was run

The pagination on chapter 3 is real output from a table of 25 messages, not an illustration.

The shape

src/
  schema.ts              rooms, members, messages
  app.ts                 bricks, and the socket route
  rooms.ts               requireMember — the check every route shares
  routes/rooms/
    index.post.ts        create a room
    [id]/join.post.ts    join one
    [id]/messages.get.ts history, paged
    [id]/messages.post.ts send

Four route files, a schema, and one shared function. Auth, validation, error shapes, request parsing and graceful shutdown you get without writing them.

Before you start

bun create theoven chat --db sqlite --auth basic
cd chat && bun install

That gives you a working app with signup, login and password reset already mounted, which is what the rest of this assumes.

Set up the schema →