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s2-lite is an open source server implementation of core S2 APIs. It’s a single binary designed to run as a single instance, making self-hosting straightforward. Thanks to its SlateDB engine, s2-lite provides the same durability guarantee as the managed service — every write is durable on object storage before acknowledgment. See the README for more on internals, and quickstart for running either directly from s2 CLI or Docer. The SDKs and CLI can connect to s2-lite by overriding the default endpoints. See SDK: Endpoints and CLI: Configuration for details.

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Use for testing

Even if you plan to exclusively use the managed service, s2-lite is useful as a local service emulator for integration tests and CI/CD pipelines. It implements the same API surface, so your tests run against a real S2 instance without needing network access or credentials.

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Init file

s2-lite can pre-create basins and streams at startup from a JSON spec file using the same format used by s2 apply. 
s2 lite --init-file init.json
Or set the environment variable: 
S2LITE_INIT_FILE=init.json s2 lite
The spec is applied with create-or-reconfigure semantics. Resources that already exist are reconfigured to match the spec, and only the fields present in the spec are updated. Example init.json: 
{
  "$schema": "https://raw.githubusercontent.com/s2-streamstore/s2/main/cli/schema.json",
  "basins": [
    {
      "name": "my-basin",
      "config": {
        "create_stream_on_append": true
      },
      "streams": [
        { "name": "events" }
      ]
    }
  ]
}
See CLI: Apply for the full spec file format reference.