Resilience is the ability of a system to recover quickly and effectively from failures, errors, or unexpected events. It is a critical aspect of software development which requires careful planning and the use of appropriate tools and patterns to ensure that the systems remain available and responsive. One such tool is the Resilience4j library.

Common problems and patterns

Resilience4j is a Java library that provides several fault tolerance strategies to help build resilient applications, able to withstand failures and errors. Resilience4j's fault tolerance strategies are implemented as patterns. They are useful in distributed systems where services communicate with each other over potentially unreliable networks such as microservices, database access, and REST APIs. Resilience4j offers several patterns, including circuit breaker, rate-limiting, retry, bulkhead, time limiter, and cache. These patterns can be applied to different components of a system to improve its reliability.

The circuit breaker, time limiter, and bulkhead patterns prevent the cascading failure problem that arises when a single malfunctioning component triggers a series of system-wide failures.
The rate-limiting pattern is created to restrict the amount of requests or calls made to a specific resource or service within a set time frame. This approach aids in preventing the overloading of the resource and guarantees its availability and responsiveness.
The retry pattern is a technique used to deal with transient errors by repeatedly attempting to execute a failed operation with a specified delay between each retry attempt.
The cache pattern helps handle overloaded or unavailable services, reducing the impact of service outages and improving application performance.

Overall, resilience patterns are the starting point for building reliable systems. They describe common strategies for achieving the increasing robustness goals and provide guidance on using Resilience4j's modules to implement those strategies effectively.

Setting up a project with Resilence4j

Resilience4j's modules are designed to be used in functional programming contexts. They provide specific capabilities that can be applied to functions and methods using higher-order functions. You can combine these higher-order functions to create complex modules that can handle a variety of resilience concerns.

One of the key benefits of Resilience4j is that it's highly configurable. You can fine-tune the system behaviors by using different core modules: Circuit Breaker, Rate Limiter, Bulkhead, Retry, TimeLimiter, and Cache. Feel free to use each of these modules independently or in combination to create powerful and flexible resilience mechanisms.

Before delving into the description of those modules, let’s see how you can use Resilience4j in your project.

To use Resilience4j in your Maven or Gradle project, you need to add the appropriate Resilience4j dependencies related to a specific module to your project's build file. Here's an example of how to add the Circuit Breaker module as a dependency in a Maven project:

<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-circuitbreaker</artifactId>
    <version>2.0.2</version>
</dependency>

And here's an example of how to add the Retry module as a dependency in a Gradle project:

implementation group: 'io.github.resilience4j', name: 'resilience4j-retry', version: '2.0.2'

Main modules

Here are the main modules of Resilience4j:

resilience4j-circuitbreaker provides the circuit breaker pattern implementation. It includes several circuit breaker types, such as the traditional circuit breaker, the sliding window circuit breaker, and the rate limiter circuit breaker.
resilience4j-ratelimiter provides the rate limiter pattern implementation. It includes several rate limiter types, such as the fixed rate limiter, the token bucket rate limiter, and the sliding window rate limiter.
resilience4j-bulkhead provides the bulkhead pattern implementation, which is used to limit the number of concurrent executions of a method call. It can be used to prevent resource exhaustion or contention.
resilience4j-retry provides the retry pattern implementation, which is used to retry failed method calls based on various retry strategies such as fixed, exponential, or jittered intervals.
resilience4j-timelimiter provides the time limiter pattern implementation. It includes several time limiter types, such as the fixed time limiter, the timeout time limiter, and the jitter time limiter.
resilience4j-cache module provides the cache decorator implementation, which is used to cache method calls and reduce the number of remote calls.

You can use each of these modules in your code independently or in combination with other modules to provide a reliable solution.

Conclusion

Resilience4j allows developers to build applications that can gracefully handle failures and recover from them quickly, improving the overall reliability and robustness of their systems.

Resilience patterns provide guidance on how to use Resilience4j's modules to achieve specific resilience goals, while the modules themselves are designed to be used in functional programming contexts and can be combined to create complex resilience behaviors.

Together, patterns and modules provide a powerful toolkit for building applications that can withstand the challenges of modern distributed systems.