Logging

Log Aggregation

Grafana Loki

Loki - HTTP API Reference

Loki HTTP API (opens in a new tab)

Loki - Health check

GET :3100/ready

# e.g.
❯ http :3100/ready
HTTP/1.1 200 OK
Content-Length: 6
Content-Type: text/plain; charset=utf-8
Date: Fri, 03 Oct 2025 15:03:01 GMT
X-Content-Type-Options: nosniff
 
ready

Loki - Send log data to Loki

Send log data to Loki (opens in a new tab)

Tooling - LogQL simulator

• LogQL simulator (opens in a new tab)

Spring Boot - integration

Pusing logs directly over HTTP to Loki using a Logback Appender loki-logback-appender.

1. Add dependencies (Gradle Kotlin DSL)

implementation("com.github.loki4j:loki-logback-appender:2.0.0")

2. Configure Logback

<!-- logback-spring.xml -->
<configuration>
    <include resource="org/springframework/boot/logging/logback/defaults.xml"/>
    <include resource="org/springframework/boot/logging/logback/console-appender.xml"/>
 
    <contextName>playground.spring-data</contextName>
 
    <springProperty name="logging.loki.url" source="logging.loki.url"/>
 
    <appender name="LOKI" class="com.github.loki4j.logback.Loki4jAppender">
        <http>
            <url>${logging.loki.url}</url>
        </http>
    </appender>
 
    <root level="DEBUG">
        <appender-ref ref="CONSOLE"/>
        <appender-ref ref="LOKI"/>
    </root>
</configuration>

3. Configure Spring Boot application properties

logging.loki.url=http://${LOKI_HOST:localhost}:${LOKI_PORT:3100}/loki/api/v1/push

VictoriaLogs

• LogsQL examples (opens in a new tab)

VictoriaLogs datasource for Grafana

• Grafana Plugin - VictoriaLogs datasource (opens in a new tab)
• VictoriaMetrics Playground - k8s Logs Demo (opens in a new tab)

Log Format

logfmt

• Logfmt Documentation (opens in a new tab)

JSON Lines

• JSON Lines Documentation (opens in a new tab)
• JSONL Tools (opens in a new tab)

Logging API

SLF4J

SLF4J - Multiple bindings were found on the class path (opens in a new tab)

SLF4J - Modules

jul-to-slf4j

slf4j-jdk-platform-logging

Logback

Logback - Turn on debug flag

# System Property
-Dlogback.debug=true
# This will print out internal status messages of logback

Logback - MDC

• Store contextual information on a per-thread basis, such as a transaction ID.

Logback - Config

• By default, additivity="true" which sends logging output to any appender attached higher in the hierarchy.

  Try setting additivity="false" if duplicate logging output happens.

• Logback - Chapter 3 Configuration - Overriding the default cumulative behaviour (opens in a new tab)

Logback - Appenders

• immediateFlush

  By default, each log event is immediately flushed to the underlying output stream. This default approach is safer in the sense that logging events are not lost in case your application exits without properly closing appenders.

  Try using immediateFlush = true if logging events are lost in logs.

Logback - RollingFileAppender

• RollingPolicy

  • How to roll over log files

• TriggeringPolicy

  • When to roll over log files

Logback - PatternLayout

• Colored output

  • Jansi library is required for ANSI color code interpretation on Windows.
  • POSIX systems support ANSI color code interpretation natively.

• Logback - Chapter 6 Layouts - PatternLayout (opens in a new tab)

  Reference for conversion patterns

Logback - Spring Boot - integration

• Spring Boot by default uses programmatic configuration to improve startup time, with Spring config for customizations.

  Logback default config class: org.springframework.boot.logging.logback.DefaultLogbackConfiguration

Logback - Spring Boot - default config

org.springframework.boot.logging.logback