Pattern: Client-side service discovery

pattern   inter-service communication   service discovery  


Services typically need to call one another. In a monolithic application, services invoke one another through language-level method or procedure calls. In a traditional distributed system deployment, services run at fixed, well known locations (hosts and ports) and so can easily call one another using HTTP/REST or some RPC mechanism. However, a modern microservice-based application typically runs in a virtualized or containerized environments where the number of instances of a service and their locations changes dynamically.

Consequently, you must implement a mechanism for that enables the clients of service to make requests to a dynamically changing set of ephemeral service instances.


How does the client of a service - the API gateway or another service - discover the location of a service instance?


  • Each instance of a service exposes a remote API such as HTTP/REST, or Thrift etc. at a particular location (host and port)
  • The number of services instances and their locations changes dynamically.
  • Virtual machines and containers are usually assigned dynamic IP addresses.
  • The number of services instances might vary dynamically. For example, an EC2 Autoscaling Group adjusts the number of instances based on load.


When making a request to a service, the client obtains the location of a service instance by querying a Service Registry, which knows the locations of all service instances.

The following diagram shows the structure of this pattern.

This is typically handled by a Microservice chassis framework


The Microservices Example application is an example of an application that uses client-side service discovery. It is written in Scala and uses Spring Boot and Spring Cloud as the Microservice chassis. They provide various capabilities including client-side discovery.

RegistrationServiceProxy is a component of that application. In order to register a user, it invokes another service using the Spring Framework’s RestTemplate:

class RegistrationServiceProxy @Autowired()(restTemplate: RestTemplate) extends RegistrationService {

  var userRegistrationUrl: String = _

  override def registerUser(emailAddress: String, password: String): Either[RegistrationError, String] = {

      val response = restTemplate.postForEntity(userRegistrationUrl,
        RegistrationBackendRequest(emailAddress, password),

It is injected with the RestTemplate and the user_registration_url, which specifies the REST endpoint.

When the application is deployed user_registration_url is set to this URL http://REGISTRATION-SERVICE/user - see the docker-compose.yml file. REGISTRATION-SERVICE is the logical service name that is resolved to a network location using client-side service discovery. The service discovery is implemented using Netflix OSS components. It provides Eureka, which is a Service Registry, and Ribbon, which is an HTTP client that queries Eureka in order to route HTTP requests to an available service instance.

Client-side service discovery is configured using various Spring Cloud annotations:

class EurekaClientConfiguration {

  def restTemplate(scalaObjectMapper : ScalaObjectMapper) : RestTemplate = {
    val restTemplate = new RestTemplate()
    restTemplate.getMessageConverters foreach {
      case mc: MappingJackson2HttpMessageConverter =>
      case _ =>

The @EnableEurekaClient annotation enables the Eureka client. The @LoadBalanced annotation configures the RestTemplate to use Ribbon, which has been configured to use the Eureka client to do service discovery. As a result, the RestTemplate will handle requests to the http://REGISTRATION-SERVICE/user endpoint by querying Eureka to find the network locations of available service instances.

Resulting context

Client-side discovery has the following benefits:

Client-side discovery also has the following drawbacks:

  • This pattern couples the client to the Service Registry
  • You need to implement client-side service discovery logic for each programming language/framework used by your application, e.g Java/Scala, JavaScript/NodeJS. For example, Netflix Prana provides an HTTP proxy-based approach to service discovery for non-JVM clients.

pattern   inter-service communication   service discovery  

Copyright © 2024 Chris Richardson • All rights reserved • Supported by Kong.

About is brought to you by Chris Richardson. Experienced software architect, author of POJOs in Action, the creator of the original, and the author of Microservices patterns.

New workshop: Architecting for fast, sustainable flow

Enabling DevOps and Team Topologies thru architecture

DevOps and Team topologies are vital for delivering the fast flow of changes that modern businesses need.

But they are insufficient. You also need an application architecture that supports fast, sustainable flow.

Learn more and register for my June 2024 online workshops....


I help organizations improve agility and competitiveness through better software architecture.

Learn more about my consulting engagements, and training workshops.

LEARN about microservices

Chris offers numerous other resources for learning the microservice architecture.

Get the book: Microservices Patterns

Read Chris Richardson's book:

Example microservices applications

Want to see an example? Check out Chris Richardson's example applications. See code

Virtual bootcamp: Distributed data patterns in a microservice architecture

My virtual bootcamp, distributed data patterns in a microservice architecture, is now open for enrollment!

It covers the key distributed data management patterns including Saga, API Composition, and CQRS.

It consists of video lectures, code labs, and a weekly ask-me-anything video conference repeated in multiple timezones.

The regular price is $395/person but use coupon GIVLKECM to sign up for $145 (valid until June 19th, 2024). There are deeper discounts for buying multiple seats.

Learn more

Learn how to create a service template and microservice chassis

Take a look at my Manning LiveProject that teaches you how to develop a service template and microservice chassis.

Signup for the newsletter

BUILD microservices

Ready to start using the microservice architecture?

Consulting services

Engage Chris to create a microservices adoption roadmap and help you define your microservice architecture,

The Eventuate platform

Use the platform to tackle distributed data management challenges in your microservices architecture.

Eventuate is Chris's latest startup. It makes it easy to use the Saga pattern to manage transactions and the CQRS pattern to implement queries.

Join the microservices google group