Architecture

Core Components of the MQTT Transport

MCP over MQTT introduces a centralized MQTT broker, while other components (Hosts, Clients, Servers) remain unchanged.

Host, Client, and Server

The Host, Client, and Server components remain unchanged:

• The host process acts as the container and coordinator of the clients.
• Each client is created by the host and maintains an isolated server connection.
• Servers provide specialized context and capabilities.

With the exception that the clients and servers communicate with the MQTT broker instead of directly with each other.

See Core Components

MQTT Broker

The MQTT broker acts as a centralized message router:

• Facilitates communication between clients and servers.
• Support service discovery and service registration (via retained messages).
• Authenticates and authorizes clients and servers.

Server Side Load Balancing and Scalability

To achieve MCP server-side load balancing and scalability, an MCP server can start multiple instances (processes), each using a unique server-id as the MQTT Client ID to establish an independent MQTT connection. All instances of an MCP server share the same server-name.

The client must first subscribe to the service discovery topic to obtain the list of server-ids for a specific server-name. Then, based on a client-defined server selection strategy (e.g., random selection or round-robin), it initiates an initialize request to one of the server-ids. After initialization is complete, the MCP client communicates with the selected MCP server instance on a specific RPC topic.

This allows us to achieve high availability and scalability on the MCP server side:

• When scaling up, existing MCP clients remain connected to the old server instances, while new MCP clients have the opportunity to initiate initialization requests to the new server instances.

• When scaling down, MCP clients could re-initiate initialization requests to the MCP server, thereby connecting to another server instance.