Data replication and synchronization are crucial aspects of building scalable and resilient applications. In this blog post, we will discuss how to implement data replication and synchronization in a JavaScript GraphQL server using the Apollo Server and GraphQL subscriptions.

Why is data replication and synchronization important?

Data replication involves copying data from one database or server to another, ensuring that both systems remain in sync. Synchronization, on the other hand, ensures that any updates made to the data are propagated across the replica servers in real-time. These techniques are essential for ensuring high availability, fault tolerance, and performance in distributed systems.

Choosing the right tools

To implement data replication and synchronization in a JavaScript GraphQL server, we will utilize the following tools:

1. Apollo Server: A community-driven, open-source GraphQL server that provides a robust foundation for building GraphQL APIs. Apollo Server allows us to easily integrate data replication and synchronization capabilities.

2. GraphQL subscriptions: A feature of the GraphQL specification that enables real-time, bidirectional communication between clients and servers. GraphQL subscriptions provide the mechanism for keeping replicas in sync with the master data source.

Setting up data replication

To set up data replication, we need to have a master database or server and one or more replica servers. The master server will be responsible for handling write operations, while the replica servers will mirror the data from the master.

1. Install the required dependencies by running the following command:

npm install apollo-server graphql graphql-subscriptions

1. Define the GraphQL schema that represents your data models:

// schema.js
const { gql } = require('apollo-server');

const typeDefs = gql`
  type User {
    id: ID!
    name: String!
    email: String!
  }

  type Query {
    getUser(id: ID!): User
  }

  type Mutation {
    createUser(name: String!, email: String!): User
  }

  type Subscription {
    userAdded: User
  }
`;

module.exports = typeDefs;

1. Implement the resolvers for the schema, including the logic for data replication:

// resolvers.js
const { PubSub } = require('graphql-subscriptions');

const pubsub = new PubSub();
const USER_ADDED = 'USER_ADDED';

const resolvers = {
  Query: {
    getUser: (_, { id }) => {
      // Implement logic to retrieve user from the master data source
    },
  },
  Mutation: {
    createUser: (_, { name, email }) => {
      // Implement logic to create user in the master data source
      const user = { id: '1', name, email };

      // Publish the userAdded event to notify the replicas
      pubsub.publish(USER_ADDED, { userAdded: user });

      return user;
    },
  },
  Subscription: {
    userAdded: {
      subscribe: () => pubsub.asyncIterator(USER_ADDED),
    },
  },
};

module.exports = resolvers;

1. Instantiate the Apollo Server with the defined schema and resolvers:

// server.js
const { ApolloServer } = require('apollo-server');
const typeDefs = require('./schema');
const resolvers = require('./resolvers');

const server = new ApolloServer({ typeDefs, resolvers });

server.listen().then(({ url }) => {
  console.log(`Server ready at ${url}`);
});

Synchronizing replica servers

To synchronize the replica servers with the master data source, we will utilize GraphQL subscriptions.

1. Connect the replica servers to the master server’s GraphQL subscriptions:

// replicaServer.js
const { ApolloServer, makeRemoteExecutableSchema, introspectSchema, mergeSchemas } = require('apollo-server');
const { HttpLink } = require('apollo-link-http');
const { WebSocketLink } = require('apollo-link-ws');
const { split } = require('apollo-link');
const { getMainDefinition } = require('apollo-utilities');

(async () => {
  // Introspect the master server's schema
  const httpLink = new HttpLink({ uri: 'http://localhost:4000', fetch });
  const wsLink = new WebSocketLink({ uri: 'ws://localhost:4000/graphql', options: { reconnect: true } });
  const link = split(
    ({ query }) => {
      const { kind, operation } = getMainDefinition(query);
      return kind === 'OperationDefinition' && operation === 'subscription';
    },
    wsLink,
    httpLink
  );

  const schema = await introspectSchema(httpLink);
  const executableSchema = makeRemoteExecutableSchema({ schema, link });

  // Create the replica server with the merged schema
  const server = new ApolloServer({
    schema: mergeSchemas({ schemas: [executableSchema] }),
  });

  server.listen().then(({ url }) => {
    console.log(`Replica server ready at ${url}`);
  });
})();

1. Start the master server and the replica servers:

node server.js
node replicaServer.js

Conclusion

Implementing data replication and synchronization in a JavaScript GraphQL server is crucial for building scalable, resilient, and real-time applications. By utilizing Apollo Server and GraphQL subscriptions, we can easily replicate data and keep replicas synchronized with the master data source.

Remember to handle conflicts and implement a robust error-handling mechanism to ensure data consistency and integrity.