Structured application log for Azure Spring Apps

Note

Azure Spring Apps is the new name for the Azure Spring Cloud service. Although the service has a new name, you'll see the old name in some places for a while as we work to update assets such as screenshots, videos, and diagrams.

This article applies to: ✔️ Basic/Standard tier ✔️ Enterprise tier

This article explains how to generate and collect structured application log data in Azure Spring Apps. With proper configuration, Azure Spring Apps provides useful application log query and analysis through Log Analytics.

Log schema requirements

To improve log query experience, an application log is required to be in JSON format and conform to a schema. Azure Spring Apps uses this schema to parse your application and stream to Log Analytics.

Note

Enabling the JSON log format makes it difficult to read the log streaming output from console. To get human readable output, append the --format-json argument to the az spring app logs CLI command. See Format JSON structured logs.

JSON schema requirements:

Json Key Json value Type Required Column in Log Analytics Description
timestamp string Yes AppTimestamp timestamp in UTC format
logger string No Logger logger
level string No CustomLevel log level
thread string No Thread thread
message string No Message log message
stackTrace string No StackTrace exception stack trace
exceptionClass string No ExceptionClass exception class name
mdc nested JSON No mapped diagnostic context
mdc.traceId string No TraceId trace ID for distributed tracing
mdc.spanId string No SpanId span ID for distributed tracing
  • The "timestamp" field is required, and should be in UTC format, all other fields are optional.
  • "traceId" and "spanId" in "mdc" field are used for tracing purpose.
  • Log each JSON record in one line.

Log record sample

{"timestamp":"2021-01-08T09:23:51.280Z","logger":"com.example.demo.HelloController","level":"ERROR","thread":"http-nio-1456-exec-4","mdc":{"traceId":"c84f8a897041f634","spanId":"c84f8a897041f634"},"stackTrace":"java.lang.RuntimeException: get an exception\r\n\tat com.example.demo.HelloController.throwEx(HelloController.java:54)\r\n\","message":"Got an exception","exceptionClass":"RuntimeException"}

Limitations

Each line of JSON logs may have at most 16K bytes. If the JSON output of a single log record exceeds this limit, it will be forcibly broken into multiple lines, and each raw line will be collected into the Log column, without being parsed structurally.

Generally, this happens on exception logging with deep stacktrace, especially when the AppInsights In-Process Agent is enabled. Apply limit settings to the stacktrace output (see the below configuration samples) to ensure the final output gets parsed properly.

Generate schema-compliant JSON log

For Spring applications, you can generate expected JSON log format using common logging frameworks, such as logback and log4j2.

Log with logback

When using Spring Boot starters, logback is used by default. For logback apps, use logstash-encoder to generate JSON formatted log. This method is supported in Spring Boot version 2.1+.

The procedure:

  1. Add logstash dependency in your pom.xml file.

    <dependency>
        <groupId>net.logstash.logback</groupId>
        <artifactId>logstash-logback-encoder</artifactId>
        <version>6.5</version>
    </dependency>
    
  2. Update your logback-spring.xml config file to set the JSON format.

    <configuration>
        <appender name="stdout" class="ch.qos.logback.core.ConsoleAppender">
            <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
                <providers>
                    <timestamp>
                        <fieldName>timestamp</fieldName>
                        <timeZone>UTC</timeZone>
                    </timestamp>
                    <loggerName>
                        <fieldName>logger</fieldName>
                    </loggerName>
                    <logLevel>
                        <fieldName>level</fieldName>
                    </logLevel>
                    <threadName>
                        <fieldName>thread</fieldName>
                    </threadName>
                    <nestedField>
                        <fieldName>mdc</fieldName>
                        <providers>
                            <mdc />
                        </providers>
                    </nestedField>
                    <stackTrace>
                        <fieldName>stackTrace</fieldName>
                        <!-- maxLength - limit the length of the stack trace -->
                        <throwableConverter class="net.logstash.logback.stacktrace.ShortenedThrowableConverter">
                            <maxDepthPerThrowable>200</maxDepthPerThrowable>
                            <maxLength>14000</maxLength>
                            <rootCauseFirst>true</rootCauseFirst>
                        </throwableConverter>
                    </stackTrace>
                    <message />
                    <throwableClassName>
                        <fieldName>exceptionClass</fieldName>
                    </throwableClassName>
                </providers>
            </encoder>
        </appender>
        <root level="info">
            <appender-ref ref="stdout" />
        </root>
    </configuration>
    
  3. When using the logging configuration file with -spring suffix like logback-spring.xml, you can set the logging configuration based on the Spring active profile.

    <configuration>
        <springProfile name="dev">
            <!-- JSON appender definitions for local development, in human readable format -->
            <include resource="org/springframework/boot/logging/logback/defaults.xml" />
            <include resource="org/springframework/boot/logging/logback/console-appender.xml" />
            <root level="info">
                <appender-ref ref="CONSOLE" />
            </root>
        </springProfile>
    
        <springProfile name="!dev">
            <!-- JSON appender configuration from previous step, used for staging / production -->
            ...
        </springProfile>
    </configuration>
    

    For local development, run the Spring application with JVM argument -Dspring.profiles.active=dev, then you can see human readable logs instead of JSON formatted lines.

Log with log4j2

For log4j2 apps, use json-template-layout to generate JSON formatted log. This method is supported in Spring Boot version 2.1+.

The procedure:

  1. Exclude spring-boot-starter-logging from spring-boot-starter, add dependencies spring-boot-starter-log4j2, log4j-layout-template-json in your pom.xml file.

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <exclusions>
            <exclusion>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-logging</artifactId>
            </exclusion>
        </exclusions>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-log4j2</artifactId>
    </dependency>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-layout-template-json</artifactId>
        <version>2.14.0</version>
    </dependency>
    
  2. Prepare a JSON layout template file jsonTemplate.json in your class path.

    {
        "mdc": {
            "$resolver": "mdc"
        },
        "exceptionClass": {
            "$resolver": "exception",
            "field": "className"
        },
        "stackTrace": {
            "$resolver": "exception",
            "field": "stackTrace",
            "stringified": true
        },
        "message": {
            "$resolver": "message",
            "stringified": true
        },
        "thread": {
            "$resolver": "thread",
            "field": "name"
        },
        "timestamp": {
            "$resolver": "timestamp",
            "pattern": {
                "format": "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'",
                "timeZone": "UTC"
            }
        },
        "level": {
            "$resolver": "level",
            "field": "name"
        },
        "logger": {
            "$resolver": "logger",
            "field": "name"
        }
    }
    
  3. Use this JSON layout template in your log4j2-spring.xml config file.

    <configuration>
        <appenders>
            <console name="Console" target="SYSTEM_OUT">
                <!-- maxStringLength - limit the length of the stack trace -->
                <JsonTemplateLayout eventTemplateUri="classpath:jsonTemplate.json" maxStringLength="14000" />
            </console>
        </appenders>
        <loggers>
            <root level="info">
                <appender-ref ref="Console" />
            </root>
        </loggers>
    </configuration>
    

Analyze the logs in Log Analytics

After your application is properly set up, your application console log will be streamed to Log Analytics. The structure enables efficient query in Log Analytics.

Check log structure in Log Analytics

Use the following procedure:

  1. Go to service overview page of your service instance.

  2. Select the Logs entry in the Monitoring section.

  3. Run this query.

    AppPlatformLogsforSpring
    | where TimeGenerated > ago(1h)
    | project AppTimestamp, Logger, CustomLevel, Thread, Message, ExceptionClass, StackTrace, TraceId, SpanId
    
  4. Application logs return as shown in the following image:

    Json Log show

Show log entries containing errors

To review log entries that have an error, run the following query:

AppPlatformLogsforSpring
| where TimeGenerated > ago(1h) and CustomLevel == "ERROR"
| project AppTimestamp, Logger, ExceptionClass, StackTrace, Message, AppName
| sort by AppTimestamp

Use this query to find errors, or modify the query terms to find specific exception class or error code.

Show log entries for a specific traceId

To review log entries for a specific tracing ID "trace_id", run the following query:

AppPlatformLogsforSpring
| where TimeGenerated > ago(1h)
| where TraceId == "trace_id"
| project AppTimestamp, Logger, TraceId, SpanId, StackTrace, Message, AppName
| sort by AppTimestamp

Next steps