# ADR-002: Sub-Path Exports + Peer Deps for Adapter Isolation

**Status**: Accepted
**Date**: 2026-04-30

## Context

Each event target adapter has different peer dependencies (ioredis for Redis, @rayhanadev/iroh for Iroh). Consumers that only use in-process or one adapter should not be forced to install peer deps for adapters they don't use. Two approaches:

1. **Barrel + tree-shaking** — single entry point, rely on bundler to drop unused adapters
2. **Sub-path exports** — explicit per-adapter entry points in `package.json` exports map

Typemap uses barrel-only. Taskgraph uses plain barrel with no sub-path exports.

## Decision

Use sub-path exports with optional peer dependencies.

## Rationale

1. **Explicit dependency declaration** — `import { createRedisEventTarget } from '@alkdev/pubsub/event-target-redis'` makes it clear at the import site that this module needs ioredis. A barrel import doesn't.

2. **No bundler reliance** — sub-path exports don't depend on the consumer's bundler correctly tree-shaking. Not all consumers use bundlers (Deno, Node with `--experimental-strip-types`).

3. **peerDependenciesMeta** — npm treats optional peer deps as install warnings, not errors. `npm install @alkdev/pubsub` installs only the core. `npm install @alkdev/pubsub ioredis` gets Redis support.

4. **Consistent with typemap's philosophy** — typemap's peer deps (zod, valibot, typebox) are each their own module island. Sub-path exports make this explicit at the package boundary. We're adding the package.json entry points that typemap doesn't have.

5. **Incremental** — adapters can be added one at a time. Each new adapter adds one entry to the exports map and one entry point to tsup.

## Consequences

- More entries in `package.json` exports — maintenance burden scales with adapter count
- Both barrel and sub-path work — barrel re-exports everything for convenience, sub-path for explicitness
- tsup must list each adapter as a separate entry point
- Consumer docs should recommend sub-path imports for adapter-specific code