Improving Resilience Using Resilience4j

In this chapter, we will learn how to use Resilience4j to make our microservices more resilient, that is, how to mitigate and recover from errors. As we already discussed in Chapter 1, Introduction to Microservices, in the Circuit breaker section, and Chapter 8, Introduction to Spring Cloud, in the Using Resilience4j for improved resilience section, a circuit breaker can be used to minimize the damage that a slow or unresponsive downstream microservice can cause in a large-scale system landscape of synchronously communicating microservices. We will see how the circuit breaker in Resilience4j can be used together with a time limiter and retry mechanism to prevent two of the most common error situations:

• Microservices that start to respond slowly or not at all
• Requests that randomly fail from time to time, for example, due to temporary network problems

The following topics will be covered in this chapter:

• Introducing...

Technical requirements

For instructions on how to install the tools used in this book and how to access the source code for this book, see:

• Chapter 21, Installation Instructions for macOS
• Chapter 22, Installation Instructions for Microsoft Windows with WSL 2 and Ubuntu

The code examples in this chapter all come from the source code in $BOOK_HOME/Chapter13.

If you want to view the changes applied to the source code in this chapter, that is, see what it took to add resilience using Resilience4j, you can compare it with the source code for Chapter 12, Centralized Configuration. You can use your favorite diff tool and compare the two folders, $BOOK_HOME/Chapter12 and $BOOK_HOME/Chapter13.

Introducing the Resilience4j resilience mechanisms

The circuit breaker, time limiter, and retry mechanisms are potentially useful in any synchronous communication between two software components, for example, microservices. In this chapter, we will apply these mechanisms in one place, in calls from the product-composite service to the product service. This is illustrated in the following figure:

Figure 13.1: Adding resilience capabilities to the system landscape

Note that the synchronous calls to the discovery and config servers from the other microservices are not shown in the preceding diagram (to make it easier to read).

With the Spring Cloud Hoxton release in November 2019, the Spring Cloud Circuit Breaker project was added. It provides an abstraction layer for circuit breakers. Resilience4j can be configured to be used under the hood. This project does not provide other resilience mechanisms such as retries, time limiters, bulkheads, or rate limiters in...

Adding the resilience mechanisms to the source code

Before we add the resilience mechanisms to the source code, we will add code that makes it possible to force an error to occur, as a delay and/or as a random fault. Next, we will add a circuit breaker together with a time limiter to handle slow or unresponsive APIs, as well as a retry mechanism that can handle faults that happen randomly. Adding these features from Resilience4j follows the Spring Boot way, which we have been using in the previous chapters:

• Add a starter dependency on Resilience4j in the build file
• Add annotations in the source code where the resilience mechanisms will be applied
• Add a configuration that controls the behavior of the resilience mechanisms

Handling resilience challenges is a responsibility of the integration layer; therefore, the resilience mechanisms will be placed in the ProductCompositeIntegration class. The source code in the business logic, implemented in the ProductCompositeServiceImpl...

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In this chapter, we will learn how to use the Spring Cloud Configuration server to centralize managing the configuration of our microservices. As already described in Chapter 1, Introduction to Microservices, an increasing number of microservices typically come with an increasing number of configuration files that need to be managed and updated.With the Spring Cloud Configuration server, we can place the configuration files for all our microservices in a central configuration repository that will make it much easier to handle them. Our microservices will be updated to retrieve their configuration from the configuration server at startup.The following topics will be covered in this chapter:

• Introduction to the Spring Cloud Configuration server
• Setting up a config server
• Configuring clients of a config server
• Structuring the configuration repository
• Trying out the Spring Cloud Configuration server

Technical requirements

For instructions on how to install tools used in this book and how to access the source code for this book, see:

• Chapter 21 for macOS
• Chapter 22 for Windows

The code examples in this chapter all come from the source code in $BOOK_HOME/Chapter12.If you want to view the changes applied to the source code in this chapter, that is, see what it took to add a configuration server to the microservice landscape, you can compare it with the source code for Chapter 11, Securing Access to APIs. You can use your favorite diff tool and compare the two folders, $BOOK_HOME/Chapter11 and $BOOK_HOME/Chapter12.

Introduction to the Spring Cloud Configuration server

The Spring Cloud Configuration server (shortened to config server) will be added to the existing microservice landscape behind the edge server, in the same way as for the other microservices:

Figure 12.1: Adding a config server to the system landscape

When it comes to setting up a config server, there are a number of options to consider:

• Selecting a storage type for the configuration repository
• Deciding on the initial client connection, either to the config server or to the discovery server
• Securing the configuration, both against unauthorized access to the API and by avoiding storing sensitive information in plain text in the configuration repository

Let's go through each option one by one and also introduce the API exposed by the config server.

Selecting the storage type of the configuration repository

As already described in Chapter 8, Introduction to Spring Cloud, the config server supports the storing of configuration...