docs(blog): public graphql schema federation contracts

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+---
+title: Incrementally Exposing a Public GraphQL API with Federation Contracts
+authors: laurin
+tags: [federation, graphql, hive]
+date: 2025-09-22
+description:
+  'Learn how to safely expose a public GraphQL API from a monolith using federation contracts and
+  tags, enabling incremental and controlled schema evolution.'
+---
+
+Many teams start with a **single GraphQL monolith** powering their applications. Over time, the need
+arises to expose parts of that schema publicly - whether for partners, customers, or other external
+integrations.
+
+But here’s the problem:
+
+Your internal schema wasn’t designed for public consumption. It likely contains inconsistent naming,
+experimental fields, and sensitive operations you don’t want outsiders touching.
+
+At the same time, you don’t want to maintain multiple APIs - one for internal use and another for
+public users. That leads to duplication of business logic, increased maintenance burden, and the
+constant risk of the two drifting out of sync. Having **a single source of truth in one API**
+ensures consistency, reduces overhead, and allows you to evolve your system with confidence.
+
+So how do you **evolve a monolithic GraphQL schema into a safe, public API** while keeping
+everything unified?
+
+The answer: **GraphQL Federation** and **Schema Contracts**.
+
+## Step 1: Treat the Monolith as a Subgraph
+
+Before exposing your schema, the first step is to make your monolith federation-compatible.
+
+Federation is often associated with microservices, but you don’t need dozens of subgraphs to benefit
+from it. A monolithic schema can also be treated as a subgraph. All it takes is a few federation
+directives:
+
+```graphql
+extend schema
+  @link(url: "https://specs.apollo.dev/link/v1.0")
+  @link(url: "https://specs.apollo.dev/federation/v2.9", import: ["@tag"])
+```
+
+Now your monolith can participate in the same contract-based filtering that federated graphs use.
+
+## Step 2: Use Tags to Mark What’s Public
+
+Next, we need a way to label which parts of the schema are safe to expose. The `@tag` directive is a
+simple but powerful tool for this:
+
+```graphql
+type Query {
+  publicInfo: String @tag(name: "public")
+  privateInfo: String
+}
+```
+
+By tagging fields, you can later generate a **contract schema** that only includes the safe,
+public-facing parts of your internal API.
+
+---
+
+## Step 3: Define a Public Contract
+
+Once tagging is in place, you can generate a **contract schema**:
+
+```graphql
+type Query {
+  publicInfo: String
+}
+```
+
+This filtered contract becomes your **public API schema**, while your full internal schema continues
+to serve your own applications.
+
+There are a few ways to create a contract schema:
+
+**1. Using Hive Console or other schema registry**
+
+If you’re working with a hosted schema registry, like
+[**Hive Console**](https://the-guild.dev/graphql/hive), you can:
+
+- Define a new contract and select which tags (e.g., `public`) to include.
+- Automatically generate and validate the filtered schema whenever a new subgraph is published.
+- Take advantage of features like usage analytics and breaking change detection to **collaborate
+  safely** and ensure consistency across contributors.
+
+[Learn more in the Hive Console documentation](https://the-guild.dev/graphql/hive/docs/schema-registry/contracts)
+
+**2. Using a CLI or Library**
+
+If you not yet have adopted a schema registry, you can also use our
+[MIT licensed JavaScript library for Federation Composition](https://github.com/graphql-hive/federation-composition)
+to generate the contract programmatically from your monolith.
+
+```ts
+import { parse } from 'graphql'
+import { composeSchemaContract } from '@theguild/federation-composition'
+
+const result = composeSchemaContract(
+  [
+    {
+      name: 'monolith',
+      typeDefs: parse(/* GraphQL */ `
+        type Query {
+          publicInfo: String @tag(name: "public")
+          privateInfo: String
+        }
+      `)
+    }
+  ],
+  /** Tags to include and exclude */
+  {
+    include: new Set(['public']),
+    exclude: new Set()
+  },
+  /** Exclude unreachable types */
+  true
+)
+
+// This is the filtered schema!
+console.log(result.publicSdl)
+```
+
+Then you can simply create a private schema, similar to the following
+
+```ts
+import { createSchema } from 'graphql-yoga'
+import { composeSchemaContract } from '@theguild/federation-composition'
+import { resolvers } from './resolvers'
+
+// ...
+
+const publicSchema = createSchema({
+  typeDefs: parse(result.publicSdl),
+  resolvers,
+  resolverValidationOptions: {
+    // The resolvers still contain the ones of the public schema
+    // Instead of filtering them out ignoring it is good enough.
+    requireResolversToMatchSchema: 'ignore'
+  }
+})
+```
+
+## Step 4: Serve the Public Schema Contract
+
+Creating a filtered schema is only useful if clients can actually query it. Once you have your
+contract, you need to **serve it as your public API**.
+
+The good news: **any federation-compatible router that supports supergraphs can serve a federation
+contract**. Popular choices include Apollo Gateway,
+[Hive Gateway](https://the-guild.dev/graphql/hive/gateway), or
+[Hive Router](https://github.com/graphql-hive/router).
+
+If using Hive Console as a schema registry, point your gateway to
+[the contract supergraph endpoint](https://the-guild.dev/graphql/hive/docs/schema-registry/contracts#access-contract-cdn-artifacts)
+to have it expose the public API.
+
+Additionally, you can then configure things like authentication, rate limiting, and access policies.
+
+Clients can now consume the public API fields by pointing to the gateway, while the internal schema
+remains private.
+
+As a additional security measure you should leverage
+[persisted documents to avoid execution of arbitary GraphQL operations](https://the-guild.dev/graphql/hive/docs/gateway/persisted-documents)
+against the private schema.
+
+For more guidance on choosing a gateway for your project, refer to the
+[Federation Gateway Audit](https://the-guild.dev/graphql/hive/federation-gateway-audit) for feature
+compatibility and the
+[Federation Gateway Performance Benchmark](https://the-guild.dev/graphql/hive/federation-gateway-performance)
+for performance considerations.
+
+As mentioned before, if you are not relying on a schema registry you can simply use and GraphQL
+server for serving the public schema.
+
+```ts filename="Example GraphQL Yoga"
+import { createServer } from 'node:http'
+import { createSchema, createYoga } from 'graphql-yoga'
+import { publicSchema } from './public-schema'
+
+const server = createServer(
+  createYoga({
+    schema: publicSchema
+  })
+)
+
+server.listen(8080)
+```
+
+## Step 5: Evolve the Public Schema Incrementally
+
+Federation contracts let you add fields to the public schema **at your own pace**.
+
+For example, when you decide to open up a mutation:
+
+```graphql
+input PublishInput @tag(name: "public") {
+  data: String!
+}
+
+type Mutation {
+  publishData(input: PublishInput!): PublishResult! @tag(name: "public")
+}
+```
+
+Tag it, release the new version of your GraphQL schema, regenerate the contract, and the public
+schema expands automatically.
+
+Iterate and refactor your schema internally, then make it public when you are ready.
+
+No risky schema forks, no duplication, just maintain a **single, unified GraphQL API** while safely
+evolving your public interface.
+
+## Conclusion
+
+GraphQL Federation isn’t just for distributed architectures. It’s also a powerful tool for
+**partitioning access within a monolith**.
+
+By combining federation contracts with tagging, you can safely evolve a private schema into a public
+one, while only exposing the parts you want today, and leaving the door open for more tomorrow.
+
+This approach provides a clean, incremental path to offering a public GraphQL API without
+compromising the flexibility of your internal schema.
+
+[Learn more on schema contracts with Hive
+Console]([Learn more in the Hive Console documentation](https://the-guild.dev/graphql/hive/docs/schema-registry/contracts).
+).