Configuring Logback for Asynchronous Logging in Spring Boot and Performance Comparison

The article introduces a step‑by‑step guide for Spring Boot developers to configure Logback so that logs are written to different files based on their LEVEL , and to enable asynchronous logging to reduce disk I/O overhead.

What you will learn

Log output to files categorized by level.

How asynchronous logging improves performance by avoiding blocking I/O.

The underlying mechanism of asynchronous logging in Logback.

Logback configuration (logback‑spring.xml)

[%d{yyyy-MM-dd' HH:mm:ss.SSS}] [%C] [%t] [%L] [%-5p] %m%n
ERROR
DENY
ACCEPT
[%d{yyyy-MM-dd' HH:mm:ss.SSS}] [%C] [%t] [%L] [%-5p] %m%n
${LOG_INFO_HOME}//%d.log
30
ERROR
[%d{yyyy-MM-dd' HH:mm:ss.SSS}] [%C] [%t] [%L] [%-5p] %m%n
${LOG_ERROR_HOME}//%d.log
30

Advanced feature: Asynchronous output

0
256
0
256

Performance test

Using Apache JMeter with 100 concurrent threads, the synchronous configuration achieved a throughput of 44.2 requests per second, while the asynchronous configuration reached 497.5 requests per second – more than a ten‑fold increase.

How asynchronous logging works

When Logger.info(...) is called, Logback’s AsyncAppenderBase places the event into an ArrayBlockingQueue . A dedicated worker thread takes events from the queue and invokes AppenderAttachableImpl.appendLoopOnAppenders(e) , which writes the log to the underlying file appender. The key methods involved are encode and write .

protected void append(E eventObject) {
    if (!this.isQueueBelowDiscardingThreshold() || !this.isDiscardable(eventObject)) {
        this.preprocess(eventObject);
        this.put(eventObject);
    }
}

E e = parent.blockingQueue.take();
aai.appendLoopOnAppenders(e);

The article concludes with a summary of the configuration steps, a reminder to use the async appender for high‑throughput services, and links to additional resources and interview questions.