# Evolve contracts without breaking clients

Provide versioned services, translate earlier contracts and validate inputs and results.

This page describes WU Framework `4.0.0`. Read the [Framework 4.0 and CLI 1.2 migration guide](/docs/guides/releases/) before upgrading an existing project.

`wu-framework/services` defines schema-based contracts, validates calls and generates clients. `provideService` returns `{ dispose }`; `consumeService` returns asynchronous methods. Inside an app use `context.wu` and grant `rpc.provide` or `rpc.call` for the service channels. This example uses the host runtime to focus on translation.

```js
import { wu } from 'wu-framework';
import { schema, defineService, provideService, consumeService,
  checkServiceCompatibility } from 'wu-framework/services';

const v1 = defineService({ name: 'price', version: '1.0.0', methods: {
  quote: { input: schema.number(), output: schema.number() },
} });
const v2 = defineService({ name: 'price', version: '2.0.0', methods: {
  quote: { input: schema.object({ quantity: schema.number() }),
    output: schema.object({ total: schema.number() }) },
} });
const translators = [{ definition: v1, methods: {
  quote: { input: quantity => ({ quantity }), output: result => result.total },
} }];
const provider = provideService(wu, v2, {
  quote: ({ quantity }) => ({ total: quantity * 12 }),
}, { translators });
try {
  const oldClient = consumeService(wu, v1);
  const newClient = consumeService(wu, v2);
  console.log(await oldClient.quote(2)); // 24
  console.log(await newClient.quote({ quantity: 2 })); // { total: 24 }
  console.log(checkServiceCompatibility(v1, v2, { translators }));
} finally {
  provider.dispose();
}
```

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## What is validated

Each translator declares `definition` and per-method adaptations: `input`, `output` or `implement` to fully serve a removed method. Unchanged methods need no conversion. WU checks coverage at registration and validates each input and output against its version schema. Incomplete coverage produces `WU_SERVICE_TRANSLATOR`; structural checking does not prove that translation functions preserve business meaning.

`checkServiceCompatibility(previous, next, { translators })` returns `compatible`, `changes` and `translated`. `generateServiceTypes` produces client declarations and `sampleValue(schema)` creates deterministic fixture data. See [CLI contracts](/docs/cli/contracts/) and [tests and mocks](/docs/cli/testing-mocks/) to use the same contract in CI and development.

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<span class="section-anchor" id="section-2"></span>

## Versioned RPC and objects

`wu.handle(channel, handler, { version, translators })` also supports direct translation. Each `translators` key is a served version with `request` and `response`. Context identifies `version`, `servedVersion` and `translated`; `WU_RPC_TRANSLATION_FAILED` identifies the failing stage. For shared services, `wu.provide` accepts translators that build an older object view; `wu.consume` exposes the served version through `__wuVersion`.