How to build service discovery with Consul DNS

Consul is a service mesh solution providing a full-featured control plane with service discovery, configuration, and segmentation capabilities. These features can be used individually or collectively to build a complete service mesh. Consul requires a data plane and supports Envoy as the default proxy, though other proxies can also be integrated.

Consul provides a DNS API for service discovery. This means applications can discover service IP addresses and ports through standard DNS queries without modifying application logic.

The following are the basic steps to build service discovery using Consul DNS:

1. Install and Configure Consul:

   • First, install Consul and start a Consul agent on the server.
   • This agent can run in server mode or client mode.
   • Configure service definition files, typically in JSON or HCL format, containing service names, ports, health checks, and other information.

2. Register Services:

   • When the service starts, it registers its information with Consul.
   • This is typically achieved by modifying the service's startup script to automatically register the service upon startup.
   • After registration, Consul periodically performs health checks to ensure service status updates.

3. Service Discovery:

   • Applications can directly query services through Consul. For example, if you have a service named 'web', you can resolve its address via DNS query web.service.consul.
   • Consul's DNS service handles these requests and returns the IP address of the current healthy instance of the service.

Example Usage

Suppose we have a web service and a database service, and we want the web service to discover the location of the database service.

1. Database Service Registration:

   • The database service registers itself with Consul upon startup, including IP address, port, and health check configuration.

2. Web Service Querying Database:

   • When the web service needs to query the database service, it simply queries database.service.consul. Consul processes this DNS request and returns a list of healthy instances of the database service.

Benefits

Benefits of using Consul DNS for service discovery include:

• Decentralized: Each service is responsible for registering itself, reducing configuration complexity.
• Health Check Integration: Consul automatically handles health checks and service status updates, ensuring DNS records are always current.
• Ease of Use: Service discovery via DNS does not require modifying existing application code.

Through this approach, Consul DNS provides a simple and powerful method for implementing service discovery, which is crucial for building scalable and reliable microservice architectures.