Cristian Angulo

Posted on Mar 23

Microservices Federation (GraphQL, Python and Apollo)

#api #architecture #microservices #python

graphql #python #fastapi #docker

When building microservices, one of the hardest challenges is exposing a single, unified API to the client without coupling your services together.

Apollo Federation 2 solves this: each service owns its slice of the graph, and a Gateway composes them into one supergraph — transparently.

This is the architecture we'll be building:

Client
  └─► Gateway :4000
         ├─► User Service    :5001  /graphql
         ├─► Product Service :5002  /graphql
         └─► Order Service   :5003  /graphql
                                        └─► Postgres :5432
                                             ├── users DB
                                             ├── products DB
                                             └── orders DB

The full source code is here: https://github.com/nietzscheson/microservices-federation

Stack

Gateway: Node.js + @apollo/gateway / Apollo Router (Rust)
Services: Python 3.13 + FastAPI + Strawberry GraphQL
ORM: SQLAlchemy 2 + Alembic
DB: PostgreSQL 17
Packages: uv
DI: dependency-injector
Infra: Docker Compose

Database: one Postgres, three databases

We use the same trick from my previous article: a single Postgres container that initializes multiple databases via an entrypoint script.

### docker/postgres/multiple-databases.sh

#!/bin/bash

set -e
set -u

function create_user_and_database() {
    local database=$1
    echo "  Creating user and database '$database'"
    psql -v ON_ERROR_STOP=1 --username "$POSTGRES_USER" <<-EOSQL
        CREATE USER $database;
        CREATE DATABASE $database;
        GRANT ALL PRIVILEGES ON DATABASE $database TO $database;
EOSQL
}

if [ -n "$POSTGRES_MULTIPLE_DATABASES" ]; then
    echo "Multiple database creation requested: $POSTGRES_MULTIPLE_DATABASES"
    for db in $(echo $POSTGRES_MULTIPLE_DATABASES | tr ',' ' '); do
        create_user_and_database $db
    done
    echo "Multiple databases created"
fi

Docker Compose

Each service gets its own DATABASE_URL pointing to its isolated database inside the same Postgres instance. The services share a single multi-stage Dockerfile.

### docker-compose.yaml

services:
  postgres:
    image: postgres:17.4
    container_name: postgres
    ports:
      - "6543:5432"
    environment:
      POSTGRES_PASSWORD: postgres
      POSTGRES_MULTIPLE_DATABASES: users,products,orders
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 5s
      retries: 5
    volumes:
      - ./docker/postgres/multiple-databases.sh:/docker-entrypoint-initdb.d/multiple-databases.sh

  user:
    build:
      context: ./services
      dockerfile: ./Dockerfile
      target: user
    environment:
      DATABASE_URL: postgresql://postgres:postgres@postgres:5432/users
    ports:
      - 5001:5000
    depends_on:
      postgres:
        condition: service_healthy

  product:
    build:
      context: ./services
      dockerfile: ./Dockerfile
      target: product
    environment:
      DATABASE_URL: postgresql://postgres:postgres@postgres:5432/products
    ports:
      - 5002:5000
    depends_on:
      postgres:
        condition: service_healthy

  order:
    build:
      context: ./services
      dockerfile: ./Dockerfile
      target: order
    environment:
      DATABASE_URL: postgresql://postgres:postgres@postgres:5432/orders
    ports:
      - 5003:5000
    depends_on:
      postgres:
        condition: service_healthy

  gateway:
    build:
      context: ./gateway
    ports:
      - "4000:4000"
    depends_on:
      user:
        condition: service_healthy
      product:
        condition: service_healthy
      order:
        condition: service_healthy

Multi-stage Dockerfile

All three services share a single Dockerfile using multi-stage builds and uv for fast dependency installation.

### services/Dockerfile

FROM python:3.13 AS base

COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /usr/local/bin/

ENV UV_PROJECT_ENVIRONMENT=/opt/venv
ENV PATH="/opt/venv/bin:$PATH"
ENV PYTHONPATH=.

FROM base AS user
WORKDIR /services/user
COPY user/pyproject.toml ./
RUN uv sync --no-install-project
CMD ["uv", "run", "uvicorn", "src.app:app", "--host", "0.0.0.0", "--port", "5000"]

FROM base AS product
WORKDIR /services/product
COPY product/pyproject.toml ./
RUN uv sync --no-install-project
CMD ["uv", "run", "uvicorn", "src.app:app", "--host", "0.0.0.0", "--port", "5000"]

FROM base AS order
WORKDIR /services/order
COPY order/pyproject.toml ./
RUN uv sync --no-install-project
CMD ["uv", "run", "uvicorn", "src.app:app", "--host", "0.0.0.0", "--port", "5000"]

Dependency Injection

Each service uses the same DI pattern: pydantic-settings reads DATABASE_URL from the environment, and dependency-injector wires the SQLAlchemy engine and session as singletons.

### services/order/src/containers.py

from dependency_injector import containers, providers
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from src.settings import Settings


class MainContainer(containers.DeclarativeContainer):

    settings = providers.Configuration(pydantic_settings=[Settings()])

    engine = providers.Singleton(
        create_engine,
        settings.database_url,
    )

    session = providers.Singleton(
        sessionmaker,
        bind=engine,
        expire_on_commit=False,
    )

The Federation Pattern

This is the core of the project. Here's how the three services relate to each other without a single Python import between them.

User Service — owns the entity

The User service defines the full UserType with keys=["id"] and implements resolve_reference. The Gateway calls this whenever another service references a UserType by its ID.

### services/user/src/app.py

@strawberry.federation.type(keys=["id"])
class UserType:
    id: strawberry.ID
    name: str

    @classmethod
    def resolve_reference(cls, **representation) -> "UserType":
        with Session() as session:
            user = session.get(User, representation["id"])
            return cls(id=user.id, name=user.name)

Product Service — stubs UserType

The Product service doesn't import anything from the User service. It declares a stub UserType with only id. The Gateway knows it needs to resolve the rest from the User service.

strawberry.Private is the key pattern here: it stores the raw foreign key integer inside the Python object without exposing it to the GraphQL schema.

### services/product/src/app.py

@strawberry.federation.type(keys=["id"])
class UserType:
    id: strawberry.ID = strawberry.federation.field  # stub

    @classmethod
    def resolve_reference(cls, id: strawberry.ID):
        return UserType(id)


@strawberry.federation.type(keys=["id"])
class ProductType:
    id: strawberry.ID
    name: str
    _created_by: strawberry.Private[typing.Optional[int]] = None

    @strawberry.field
    def created_by(self) -> typing.Optional[UserType]:
        if self._created_by is not None:
            return UserType(id=self._created_by)
        return None

Order Service — stubs both

The Order service stubs both UserType and ProductType. An OrderType holds both foreign keys privately and exposes them as federated references.

### services/order/src/app.py

@strawberry.federation.type(keys=["id"])
class UserType:
    id: strawberry.ID = strawberry.federation.field

    @classmethod
    def resolve_reference(cls, id: strawberry.ID):
        return UserType(id)


@strawberry.federation.type(keys=["id"])
class ProductType:
    id: strawberry.ID = strawberry.federation.field

    @classmethod
    def resolve_reference(cls, id: strawberry.ID):
        return ProductType(id)


@strawberry.federation.type(keys=["id"])
class OrderType:
    id: strawberry.ID
    name: str
    _created_by: strawberry.Private[typing.Optional[int]] = None
    _product: strawberry.Private[typing.Optional[int]] = None

    @strawberry.field
    def created_by(self) -> typing.Optional[UserType]:
        if self._created_by is not None:
            return UserType(id=self._created_by)
        return None

    @strawberry.field
    def product(self) -> typing.Optional[ProductType]:
        if self._product is not None:
            return ProductType(id=self._product)
        return None

So a query like this, sent to the Gateway on port 4000:

query {
  orders {
    id
    name
    createdBy {
      name
    }
    product {
      name
    }
  }
}

Is resolved by the Gateway in three steps:

Fetch orders from the Order service → gets createdBy: {id} and product: {id}
Send an _entities lookup to the User service to hydrate the user
Send an _entities lookup to the Product service to hydrate the product

The services are completely decoupled at the code level. The contract is purely a runtime GraphQL protocol.

The Gateway

We ship two gateway implementations.

Apollo Gateway (Node.js)

Simple, no compile step. Introspects each subgraph at startup.

### gateway/server.js

const { ApolloGateway, IntrospectAndCompose } = require("@apollo/gateway");
const { ApolloServer } = require("@apollo/server");

const gateway = new ApolloGateway({
    supergraphSdl: new IntrospectAndCompose({
        subgraphs: [
            { name: "users", url: "http://user:5000/graphql" },
            { name: "products", url: "http://product:5000/graphql" },
            { name: "orders", url: "http://order:5000/graphql" },
        ],
    }),
});

const server = new ApolloServer({ gateway });

Apollo Router (Rust)

The Dockerfile for the gateway uses this approach. It composes a static SDL file using the rover CLI (retrying for 30s while services come up), then starts the high-performance Apollo Router binary.

### gateway/entrypoint.sh

for i in $(seq 1 30); do
    if rover supergraph compose --config /app/supergraph.yaml 2>/dev/null > /tmp/supergraph.graphql \
        && [ -s /tmp/supergraph.graphql ]; then
        cp /tmp/supergraph.graphql /app/supergraph.graphql
        echo "Supergraph composed successfully"
        break
    fi
    echo "Waiting for subgraphs... (attempt $i/30)"
    sleep 3
done

exec router --dev --config /app/router.yaml --supergraph /app/supergraph.graphql --log info

The supergraph.yaml tells rover where to find each subgraph:

### gateway/supergraph.yaml

federation_version: =2.9.0
subgraphs:
  users:
    routing_url: http://user:5000/graphql
    schema:
      subgraph_url: http://user:5000/graphql
  products:
    routing_url: http://product:5000/graphql
    schema:
      subgraph_url: http://product:5000/graphql
  orders:
    routing_url: http://order:5000/graphql
    schema:
      subgraph_url: http://order:5000/graphql

Running it

docker compose up --build

Open the Apollo Router sandbox at http://localhost:4000 and run:

mutation {
  userCreate(name: "Alice") { id name }
}

mutation {
  productCreate(name: "Widget", createdBy: 1) { id name }
}

mutation {
  orderCreate(name: "Order #1", createdBy: 1, product: 1) { id name }
}

query {
  orders {
    id
    name
    createdBy { name }
    product { name }
  }
}

Key Takeaways

resolve_reference is the federation contract — any federated entity must implement it so the Gateway can hydrate it from just an id.
strawberry.Private lets you store raw foreign keys in your Python object without leaking them into the GraphQL schema.
Services are truly decoupled — zero Python imports across service boundaries. The only contract is the GraphQL protocol at runtime.
One Dockerfile, three services — multi-stage builds keep your infra DRY.
uv over pip/poetry — significantly faster installs, especially in Docker layer caching.

DEV Community