# Inter-app communication design (`@wrnexus/rpc`)

Date: 2026-08-05
Status: phase 1 implemented (2026-08-05); phases 2–4 remain deferred
Depends on: the permissions system (`@wrnexus/authz`), merged 2026-08-05

## Problem

Workspace apps are separate OS processes behind a Host-routed gateway — full isolation, own
database registry, own memory. They can currently reach each other in exactly two ways:

- `@wrnexus/helpers` — `appOrigin(name)` / `appUrl(name, path)`, string builders over the
  `WRNEXUS_WORKSPACE_ORIGINS` environment map.
- `@wrnexus/pubsub` — fire-and-forget events, cross-process via the Redis, NATS or Kafka
  drivers.

There is no request/response layer. An app that needs an answer from another hand-writes
`fetch(appUrl("billing", "/api/invoice"))`: no shared contract, no type safety, no timeout,
no retry policy, and no way for the callee to learn who the end user is. The permissions
system reserved a seam for that last part — `exportSubjectContext` / `importSubjectContext` —
and left it unbuilt.

## Scope

Of the four cross-app call shapes, three already have implementations:

| Shape                  | Status                                                            |
| ---------------------- | ----------------------------------------------------------------- |
| Request/response       | Missing. This is the gap.                                         |
| Fire-and-forget events | `@wrnexus/pubsub`                                                 |
| Scheduled / deferred   | `@wrnexus/queue` — `defineJob`, durable store, workflows, cron    |
| Streaming              | `@wrnexus/realtime` covers browser↔server; app↔app is not covered |

This design builds request/response and app-to-app streaming. It does **not** wrap `pubsub`
or `queue` in a new facade: they work, and an abstraction over working code leaks and needs
keeping in sync. They instead gain identity propagation, and all four shapes are documented
in one place so a developer knows what to reach for.

## Approach

Service contracts live in the workspace's shared package and are imported by both sides. The
callee `implement`s a contract; the caller gets a typed proxy client. Types flow through a
normal TypeScript import — no generator, no generated file to go stale, no build step. This
matches the idiom the framework already uses: `defineAuthz`, `defineJob`, `defineRbac`,
`defineWorkflow`.

Rejected alternatives:

- **Callee exports types, caller imports them.** No new abstraction, but the callee still
  hand-writes and hand-mounts every route and nothing keeps the route path and the type in
  agreement.
- **Generate a client from the callee's `app/api` routes.** Codegen over hand-written routes
  is brittle, and the permissions work has already demonstrated that a generated file with no
  consumer becomes dead weight.

## Module layout

```
@wrnexus/rpc
  contract.ts    defineService(), the procedure builder
  server.ts      implement(); routes discovered from app/services/*.ts
  client.ts      serviceClient(contract, { as: ctx }) -> typed proxy
  transport.ts   Transport interface, httpTransport(), inProcessTransport()
  identity.ts    exportSubjectContext / importSubjectContext
  errors.ts      ServiceError and retryability classification
```

## Contracts

```ts
// packages/shared/src/services/billing.ts — imported by both apps
import { defineService, procedure } from "@wrnexus/rpc";
import { v } from "@wrnexus/validation";

export const billing = defineService({
  name: "billing",
  procedures: {
    createInvoice: procedure
      .input(v.object({ userId: v.string(), amountCents: v.number().integer() }))
      .output<{ invoiceId: string }>()
      .permission("invoice:create"),
  },
});
```

Input validation reuses `@wrnexus/validation`'s `v`, which already returns
`{ ok, value, errors }`. No new validator is introduced.

`.permission(id)` is **enforced**, not merely declarative: `implement` checks it against the
callee's own catalog before invoking the handler, and a denial never reaches the handler. A
handler may additionally perform finer-grained, resource-level checks — `.permission()` is
the coarse gate, not the whole authorization story. Declaring it on the contract also means
the caller can see, at the type level, what the call requires.

## Server side

```ts
// app/services/billing.ts
export default implement(billing, {
  async createInvoice({ userId, amountCents }, ctx) {
    /* ... */
  },
});
```

`buildRouter` discovers `app/services/*.ts` exactly as it discovers `app/authz` and
`app/schemas`, skipping `*.gen.ts`. Procedures mount at
`/__wrnexus/rpc/<service>/<procedure>`.

## Client side

```ts
const client = serviceClient(billing, { as: ctx });
const { invoiceId } = await client.createInvoice({ userId, amountCents });
```

`{ as: ctx }` carries identity: it calls `exportSubjectContext(ctx)` to mint a short-lived
signed token.

## Identity

The token carries **`sub` and `tenant` only — never roles.** Every app shares the
`PermissionStore`, so the callee resolves roles itself. A stale or forged roles claim is
therefore impossible by construction, the token stays small, and there is no path to
injecting privileges through a claim.

Built on `@wrnexus/jwt`: `alg` pinned to HS256, signature verified before the payload is
parsed, `exp`/`nbf`/`aud`/`iss` enforced.

| Claim    | Purpose                                                                        |
| -------- | ------------------------------------------------------------------------------ |
| `iss`    | calling app name                                                               |
| `aud`    | target app name, so B cannot replay A's token to C                             |
| `sub`    | subject id — a non-empty string, matching the permissions system's requirement |
| `tenant` | tenant id, absent for global                                                   |
| `exp`    | 60 seconds                                                                     |

**The token authenticates; it never authorizes.** The callee runs its own permission checks
against its own catalog. An app cannot grant access the callee's catalog does not allow.

### Trust boundary

With a workspace-wide signing secret, any app can mint a token claiming to be any user. All
workspace apps are therefore one trust boundary: compromising the lowest-privilege app
compromises user identity across all of them. This is the standard trade-off and is
acceptable for apps in one repository under one operator, but it must be documented rather
than discovered. Per-app signing keys are the hardening path if apps ever run at differing
trust levels.

The RPC secret MUST NOT be the session secret. Reusing it would make a leaked RPC token a
session-forgery primitive.

### The RPC path is a new attack surface

`/__wrnexus/rpc/*` must be unreachable from the public internet — blocked at the gateway
**and** verified at the app, because either alone is a single point of failure. If that
endpoint is reachable with a forged token, every permission check in the workspace is
bypassable. This is the single most important property in the design.

## Transport

```ts
interface Transport {
  call(
    target: { app: string; service: string; procedure: string },
    payload: unknown,
    options: { signal: AbortSignal; identity?: string },
  ): Promise<ServiceResult>;
}
```

`httpTransport()` is the default and the only production transport in v1. A gRPC transport,
if ever needed, implements this interface and nothing else changes.

`inProcessTransport()` implements the same interface by calling the implementation directly
with no network. It exists for tests.

## Errors and retries

Errors classify into three kinds, because the classification decides whether a retry is safe:

| Kind                                                                  | Retryable |
| --------------------------------------------------------------------- | --------- |
| Transport failure — connection refused, timeout, 5xx                  | Yes       |
| Application error — the procedure threw or returned a failure         | No        |
| Rejected — validation failed, authorization denied, unknown procedure | No        |

Only procedures explicitly marked `.idempotent()` are retried at all. Retrying a
non-idempotent `createInvoice` because a response was slow is how a customer gets billed
twice: the default is no retry, and opting in is a deliberate act recorded in the contract.

`ServiceError` crossing an app boundary is **opaque by default** — code and message only, no
stack, no internal detail. An error that explains the callee's internals is an information
leak, the same defect class as `authorizeDecision`'s original 403 body.

## Testing

- **In-process transport.** Contract, identity round-trip and error classification are all
  testable without standing up two servers.
- **Contract conformance suite.** An implementation must satisfy its contract at runtime, not
  only at compile time.
- **Security regressions**, each proven by reverting the guard and confirming the test fails:
  the RPC path rejects an external request; a forged signature is rejected; a token with the
  wrong `aud` is rejected; an expired token is rejected; a missing token is rejected.

## Build order

1. Contract, server discovery, typed client, HTTP transport, identity — the usable core.
2. Failure handling: timeouts, idempotent retry, circuit breaking.
3. App-to-app streaming.
4. Identity propagation for `pubsub` and `queue`, and the unified "how apps talk" guide.

Phase 1 is what makes the system usable; everything after is additive.

## Out of scope

- gRPC and Protocol Buffers. The `Transport` seam exists so this stays additive. gRPC's value
  is cross-language interop and streaming; workspace apps are all TypeScript in one
  repository sharing types through workspace packages, so a typed HTTP transport delivers
  end-to-end type safety without a code generator.
- Wrapping `pubsub` or `queue` behind a unified facade.
- Service discovery beyond the existing `WRNEXUS_WORKSPACE_ORIGINS` map.
- Cross-workspace federation.
