Basic Consul Architecture

In this lesson, we’ll explore the core technical components of Consul: agent deployment patterns, operational modes, data center concepts, and the underlying protocols.

Consul Agent Deployment

The Consul agent is a lightweight, long-running daemon that enables service discovery, health checking, and key/value storage on any host. Thanks to its platform-agnostic design, you can run the agent in various environments:

Running consistent agent binaries across your infrastructure provides a unified control plane for service registration and health monitoring.

Agent Operational Modes

A single Consul binary supports three modes—server, client, and dev—configured via JSON files or CLI flags at startup.

Server Mode

Consul servers form the cluster’s control plane. Their key responsibilities include:

• Cluster state & membership: Track all nodes and maintain a consistent cluster view.
• Service registry: Store service entries and respond to DNS/HTTP API queries.
• Quorum management: Elect a leader and provide high availability via Raft consensus.
• Data center gateway: Forward queries between DCs in multi-DC deployments.

Without a quorum of servers, the Consul cluster cannot operate.

Client Mode

Client agents run alongside application services (e.g., web servers, databases) and handle:

• Service registration: Register local services with the server cluster.
• Health checks: Execute and report checks, influencing traffic routing.
• API forwarding: Relay DNS/RPC requests to servers.
• LAN gossip: Participate in local failure detection and membership updates.
• Stateless operation: Join or leave the cluster without persistent state.

Dev Mode

Dev mode is intended for local testing and demonstrations—never production.

Consul Data Centers

A data center in Consul represents a logical cluster co-located in one physical region or network. All members within a DC use the LAN gossip pool for low-latency communication.

Single Data Center

Key traits:

• One cluster in a single region
• Private network (no public internet)
• LAN gossip only
• Low-latency, high-bandwidth communication

Multi-Data Center

Federated Consul DCs allow cross-DC service discovery and failover. Each DC remains an independent cluster, communicating over WAN gossip or mesh gateways:

• Multi-cloud / multi-region deployments
• Physical DCs in different locations
• Multiple clusters within one site

Typical scenarios:

• On-prem primary DC with a cloud-based failover DC
• Federated clusters across AWS, GCP, and Azure

Key Protocols

Consul’s distributed architecture relies on two core protocols:

Understanding these components is critical for designing, deploying, and managing a resilient Consul architecture optimized for service discovery and health monitoring.

Links and References

• Consul Documentation
• Raft Consensus Algorithm
• Serf (Gossip Protocol)