Develop, test, debug, and deploy KurrentDB projections.

KurrentDB projections are server-side JavaScript that derive new streams and state from existing events. Gaffer uses the same JS engine as KurrentDB itself, so what you debug locally behaves like what runs in production.

CLI:

npm i -g @kurrent/gaffer

VS Code extension - install KurrentDB Gaffer for run/debug from gaffer.toml, breakpoint debugging, and an auto-registered MCP server.

Test library:

npm i -D @kurrent/projections-testing

Run a projection from the CLI:

gaffer dev order-count --fixture happy

Or test it from your existing test suite:

import { createProjection } from "@kurrent/projections-testing";

const projection = createProjection<{ count: number }>(`
  fromAll().when({
    $init: () => ({ count: 0 }),
    OrderPlaced: (s) => ({ count: s.count + 1 }),
  });
`, { engineVersion: 2 });

for (const { state } of projection.run(events)) {
  // assert on state at each step
}

See the demo project for a complete example with fixtures, errors, partitioned state, and bi-state projections.

Per-component LICENSE files live in each directory. See LICENSE_CONTRIBUTIONS.md for the license map and NOTICE.md for third-party attribution.

• Kurrent community
• Discuss
• Discord (Kurrent)
• Discord (ddd-cqrs-es)

See CONTRIBUTING.md for development setup and conventions. Bugs go to Issues; feature requests and questions to Discussions.