combd

Self-hosted Durable Objects, built from scratch in Rust. Tiny sealed cells, packed tight. Light, but very strong.

combd runs apps made of small WebAssembly programs, called cells. A cell can be a stateless HTTP handler, a durable object, or one process per WebSocket. A durable object is one per id, each with its own SQLite database. You say which in the app’s manifest, combd.toml, and combd routes requests to them.

It is one binary. It runs on one machine, or on a fleet that shares a bucket you own on S3 or R2. The bucket decides who owns each object. There is no control plane or consensus cluster to run.

What it does:

• Every cell is sandboxed. A cell has no system calls. It can’t read files or open sockets. It gets only what the host hands it: its own SQLite, a clock, calls to other processes in its app, and Postgres through a pool the host runs.
• Cells take turns. combd runs each cell a short slice at a time. A cell stuck in a loop can’t starve its neighbors.
• Let it crash. A cell that crashes takes only its own request down. For a durable object, that request’s writes roll back, and the next request gets a fresh cell.
• An answer means it’s saved. A durable object answers only once its writes are safe. That is on disk on one node, and with its peers or the bucket in a fleet. See Storage and durability.
• Idle things are cheap. Idle objects leave memory, and their state stays in SQLite. An idle WebSocket session sleeps, and its socket stays open.
• Two engines. combd has its own WebAssembly interpreter, and a compiler that turns a module into safe Rust, then machine code. Each module says which it runs on. See Compiled modules and builders.

Apps are written in Rust today, with combd-sdk. Other languages that compile to Wasm work at the host’s ABI level, without an SDK. See Other languages.

Start with the Quickstart, a page and a counter on your machine, then on a server.

How combd is built and tested, and the numbers behind it, are in the repo: plan.md and combd/RESULTS.md. combd is open source, under Apache-2.0.

Every page is also plain markdown, at its name with .md. llms.txt lists them, and llms-full.txt has them all in one file, for tools that read docs.