axolotl-federation

name	axolotl-federation
description	Axolotl micro-federation architecture - config, schema merging, mergeAxolotls, cross-module dependencies, best practices, and troubleshooting

Micro-Federation

• Each domain has its own schema.graphql, models.ts, axolotl.ts, and resolvers/
• axolotl build merges schemas into a single supergraph
• Each module has its own generated models.ts for local type safety
• All modules built and deployed together (not distributed microservices)

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Module Adapter

All modules use graphqlYogaWithContextAdapter<AppContext>() for typed context. Only root passes a context builder.

// every module's axolotl.ts — type-only, no context builder
import { graphqlYogaWithContextAdapter } from '@aexol/axolotl-graphql-yoga';
import type { AppContext } from '@/src/context.js';
const yogaAdapter = graphqlYogaWithContextAdapter<AppContext>();
export const { createResolvers } = Axolotl(yogaAdapter)<Models, unknown>();

// root src/axolotl.ts — WITH context builder (runs once at startup)
const yogaAdapter = graphqlYogaWithContextAdapter<AppContext>(async (initial) => {
  // verifyAuth here — runs per-request
  return { ...initial, authUser };
});
export const { createResolvers, adapter } = Axolotl(yogaAdapter)<Models, Scalars>();

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Schema Merging Rules

• Types merged by name — fields from all modules combined; conflicting field types cause build failure
• Root types auto-merged — Query, Mutation, Subscription fields combined across modules
• Shared field definitions must match exactly

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No extend type — Use type Declarations

Never use extend type. Axolotl merges by name, not SDL extension.

# ✅ CORRECT — plain type in each module
# src/modules/users/schema.graphql
type AuthorizedUserQuery {
  me: User @resolver
}

# src/modules/posts/schema.graphql
type AuthorizedUserQuery {
  posts: [Post!]! @resolver
}
# → merged: AuthorizedUserQuery has both `me` and `posts`

# ❌ WRONG
extend type AuthorizedUserQuery {
  posts: [Post!]! @resolver
}

Applies to ALL shared types: Query, Mutation, AuthorizedUserQuery, AuthorizedUserMutation, and any shared domain types.

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Sharing Types Across Modules

Modules can declare the same type name — shared fields must match exactly, unique fields are merged:

# users/schema.graphql         # posts/schema.graphql
type User {                     type User {
  _id: String!                    _id: String!  # must match
  email: String!                }
}                               type Post { owner: User! }
# → merged User has _id + email

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mergeAxolotls Behavior

• Non-overlapping resolvers — combined into one map
• Overlapping resolvers — executed in parallel, results deep-merged
• Subscriptions — first-wins; define each in exactly ONE module

import { mergeAxolotls } from '@aexol/axolotl-core';
export default mergeAxolotls(authResolvers, usersResolvers /*, postsResolvers */);

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Auth Gateway & Cross-Module Access

• Auth module exclusively owns Query.user / Mutation.user — never duplicate in domain modules
• Domain modules add fields to AuthorizedUserQuery / AuthorizedUserMutation
• Domain resolvers access auth via context.authUser (set by context builder):

export default createResolvers({
  AuthorizedUserQuery: {
    posts: async ([, , context]) => {
      return prisma.post.findMany({ where: { authorId: context.authUser!._id } });
    },
  },
});

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When to Run axolotl build

Run cd backend && npx @aexol/axolotl build after any schema/type/field change or federation config change.

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Custom Scalars

Declare scalar Secret in each module schema that uses it. Map once in root axolotl.ts:

Axolotl(yogaAdapter)<Models<{ Secret: number }>, Scalars>();

See axolotl-server skill for createScalars implementation.