Boost Spring Boot Performance with Asynchronous Logback Logging

01 What you will learn

Log output to files and classify logs by LEVEL into different files.

Reduce disk IO by asynchronous logging to improve performance.

Principles of asynchronous log output.

02 Logback configuration file (logback-spring.xml)

Spring Boot includes logback and slf4j dependencies, so you only need to write the configuration file. The framework automatically loads a file named logback-spring or logback from the classpath. Instead of storing all logs in a single growing file, separate error logs from other levels and rotate logs by time.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <property resource="logback.properties"/>
    <appender name="CONSOLE-LOG" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
            <pattern>[%d{yyyy-MM-dd' 'HH:mm:ss.SSS}] [%C] [%t] [%L] [%-5p] %m%n</pattern>
        </layout>
    </appender>
    <!-- INFO and higher (excluding ERROR) -->
    <appender name="INFO-LOG" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
            <level>ERROR</level>
            <onMatch>DENY</onMatch>
            <onMismatch>ACCEPT</onMismatch>
        </filter>
        <encoder>
            <pattern>[%d{yyyy-MM-dd' 'HH:mm:ss.SSS}] [%C] [%t] [%L] [%-5p] %m%n</pattern>
        </encoder>
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <fileNamePattern>${LOG_INFO_HOME}/%d.log</fileNamePattern>
            <maxHistory>30</maxHistory>
        </rollingPolicy>
    </appender>
    <appender name="ERROR-LOG" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>ERROR</level>
        </filter>
        <encoder>
            <pattern>[%d{yyyy-MM-dd' 'HH:mm:ss.SSS}] [%C] [%t] [%L] [%-5p] %m%n</pattern>
        </encoder>
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <fileNamePattern>${LOG_ERROR_HOME}/%d.log</fileNamePattern>
            <maxHistory>30</maxHistory>
        </rollingPolicy>
    </appender>
    <root level="info">
        <appender-ref ref="CONSOLE-LOG"/>
        <appender-ref ref="INFO-LOG"/>
        <appender-ref ref="ERROR-LOG"/>
    </root>
</configuration>

03 Asynchronous logging with Logback

Replace the synchronous configuration with an AsyncAppender that references the original file appender. This offloads the actual I/O to a separate thread, eliminating the blocking disk write during the logging call.

<!-- Asynchronous output -->
<appender name="ASYNC-INFO" class="ch.qos.logback.classic.AsyncAppender">
    <!-- Do not discard logs; default discardingThreshold is 0 -->
    <discardingThreshold>0</discardingThreshold>
    <!-- Queue depth (default 256) can be tuned -->
    <queueSize>256</queueSize>
    <appender-ref ref="INFO-LOG"/>
</appender>

<appender name="ASYNC-ERROR" class="ch.qos.logback.classic.AsyncAppender">
    <discardingThreshold>0</discardingThreshold>
    <queueSize>256</queueSize>
    <appender-ref ref="ERROR-LOG"/>
</appender>

04 Performance test of asynchronous logging

Using Apache JMeter with 100 threads and a ramp‑up of 0, the synchronous configuration achieved a throughput of 44.2 requests per second (TPS). The asynchronous configuration reached 497.5 TPS, an improvement of more than ten times.

05 Asynchronous logging principle

When Logger.info is called, Logback creates a logging event that is placed into an ArrayBlockingQueue (default size 256). A dedicated worker thread in AsyncAppenderBase takes events from the queue and invokes appendLoopOnAppenders on each configured appender.

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

The worker thread repeatedly executes:

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

Finally, the encode and write methods format the log message and write it to the underlying file or console.