Spring WebFlux, a reactive framework built on Project Reactor, is a powerful tool for building non-blocking, asynchronous applications. However blocking operations can inadvertently creep in, degrading performance and responsiveness. BlockHound is a library designed to detect and potentially mitigate blocking operations in the application, making it a valuable tool for optimizing Spring WebFlux applications.

Understanding Blocking Operations in Reactive Systems

In a reactive programming, blocking operations are those that halt the thread of execution until a task is completed. This can significantly impact performance, especially in high-throughput scenarios. Common blocking operations in Spring WebFlux application includes:

Database Access Synchronous database operations will block threads, especially while using traditional JDBC or ORM frameworks.
Network I/O Blocking network calls, such as traditional HTTP requests, can block the thread and downgrade performance.
File I/O Synchronous file operations also leads to blocking the thread.
Thread sleep If you have added thread sleep in your code, it will also result to blocking the thread.
External library Some of your external libraries also may add blocking nature

How BlockHound Works

BlockHound operates by instrumenting the JVM at runtime to detect blocking operations. It can be configured to either throw an exception when a blocking operation is detected. By identifying and addressing these blocking points, developers can significantly improve the performance and scalability of their Spring WebFlux applications.

Enabling BlockHound

Step 1 Add maven dependency.

<dependency>
    <groupId>io.projectreactor.tools</groupId>
    <artifactId>blockhound</artifactId>
    <version>MENTION_LATEST_VERSION</version>
</dependency>

Step 2 Pass -XX:+AllowRedefinitionToAddDeleteMethods JVM option if using Java 13+. Here i passing args through file.

Step 3 Instrument BlockHound to the application

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import reactor.blockhound.BlockHound;

@SpringBootApplication
public class WebfluxExApplication {

    static {
        BlockHound.install();
    }

    public static void main(String[] args) {
        SpringApplication.run(WebfluxExApplication.class, args);
    }

}

Now you will see an exception logged into console and request will be stopped further processing if any blocking calls present in your request flow.

Points to remember

It is not a code scan tool which scans and tell you where blocking call present.
It will throw exception if it find any blocking call in your request flow.
It is advised to not to deploy the code with BlockHound to production as it will result into throwing an error if blocking call is found.

By understanding the potential pitfalls of blocking operations and leveraging tools like BlockHound, you can build highly responsive and efficient Spring WebFlux applications.

Please leave a comment if you have any question.

detect blocking calls using BlockHound in Spring Webflux