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Logging

Logging provides an opinionated logger with output structured as JSON.

Key features

  • Capture key fields from Lambda context, cold start and structures logging output as JSON
  • Log Lambda event when instructed, disabled by default, can be enabled explicitly via annotation param
  • Append additional keys to structured log at any point in time

Initialization

Powertools extends the functionality of Log4J. Below is an example log4j2.xml file, with the LambdaJsonLayout configured.

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<?xml version="1.0" encoding="UTF-8"?>
<Configuration packages="com.amazonaws.services.lambda.runtime.log4j2">
    <Appenders>
        <Console name="JsonAppender" target="SYSTEM_OUT">
            <LambdaJsonLayout compact="true" eventEol="true"/>
        </Console>
    </Appenders>
    <Loggers>
        <Logger name="JsonLogger" level="INFO" additivity="false">
            <AppenderRef ref="JsonAppender"/>
        </Logger>
        <Root level="info">
            <AppenderRef ref="JsonAppender"/>
        </Root>
    </Loggers>
</Configuration>

You can also override log level by setting POWERTOOLS_LOG_LEVEL env var. Here is an example using AWS Serverless Application Model (SAM)

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Resources:
    HelloWorldFunction:
        Type: AWS::Serverless::Function
        Properties:
        ...
        Runtime: java8
        Environment:
            Variables:
                POWERTOOLS_LOG_LEVEL: DEBUG
                POWERTOOLS_SERVICE_NAME: example

You can also explicitly set a service name via POWERTOOLS_SERVICE_NAME env var. This sets service key that will be present across all log statements.

Standard structured keys

Your logs will always include the following keys to your structured logging:

Key Type Example Description
timestamp String "2020-05-24 18:17:33,774" Timestamp of actual log statement
level String "INFO" Logging level
coldStart Boolean true ColdStart value.
service String "payment" Service name defined. "service_undefined" will be used if unknown
samplingRate int 0.1 Debug logging sampling rate in percentage e.g. 10% in this case
message String "Collecting payment" Log statement value. Unserializable JSON values will be casted to string
functionName String "example-powertools-HelloWorldFunction-1P1Z6B39FLU73"
functionVersion String "12"
functionMemorySize String "128"
functionArn String "arn:aws:lambda:eu-west-1:012345678910:function:example-powertools-HelloWorldFunction-1P1Z6B39FLU73"
xray_trace_id String "1-5759e988-bd862e3fe1be46a994272793" X-Ray Trace ID when Lambda function has enabled Tracing
function_request_id String "899856cb-83d1-40d7-8611-9e78f15f32f4"" AWS Request ID from lambda context

Capturing context Lambda info

You can enrich your structured logs with key Lambda context information via logEvent annotation parameter. You can also explicitly log any incoming event using logEvent param. Refer Override default object mapper to customise what is logged.

Warning

Log event is disabled by default to prevent sensitive info being logged.

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import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import software.amazon.lambda.logging.LoggingUtils;
import software.amazon.lambda.logging.Logging;
...

/**
 * Handler for requests to Lambda function.
 */
public class App implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    Logger log = LogManager.getLogger();

    @Logging
    public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent input, final Context context) {
     ...
    }
}
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/**
 * Handler for requests to Lambda function.
 */
public class AppLogEvent implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    Logger log = LogManager.getLogger();

    @Logging(logEvent = true)
    public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent input, final Context context) {
     ...
    }
}

Appending additional keys

You can append your own keys to your existing logs via appendKey.

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/**
 * Handler for requests to Lambda function.
 */
public class App implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    Logger log = LogManager.getLogger();

    @Logging(logEvent = true)
    public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent input, final Context context) {
        ...
        LoggingUtils.appendKey("test", "willBeLogged");
        ...

        ...
         Map<String, String> customKeys = new HashMap<>();
         customKeys.put("test", "value");
         customKeys.put("test1", "value1");

         LoggingUtils.appendKeys(customKeys);
        ...
    }
}

Override default object mapper

You can optionally choose to override default object mapper which is used to serialize lambda function events. You might want to supply custom object mapper in order to control how serialisation is done, for example, when you want to log only specific fields from received event due to security.

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/**
 * Handler for requests to Lambda function.
 */
public class App implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    Logger log = LogManager.getLogger();

    static {
        ObjectMapper objectMapper = new ObjectMapper();
        LoggingUtils.defaultObjectMapper(objectMapper);
    }

    @Logging(logEvent = true)
    public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent input, final Context context) {
        ...
    }
}

Sampling debug logs

You can dynamically set a percentage of your logs to DEBUG level via env var POWERTOOLS_LOGGER_SAMPLE_RATE or via samplingRate attribute on annotation.

Info

Configuration on environment variable is given precedence over sampling rate configuration on annotation, provided it's in valid value range.

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/**
 * Handler for requests to Lambda function.
 */
public class App implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

    Logger log = LogManager.getLogger();

    @Logging(samplingRate = 0.5)
    public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent input, final Context context) {
     ...
    }
}
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Resources:
    HelloWorldFunction:
        Type: AWS::Serverless::Function
        Properties:
        ...
        Runtime: java8
        Environment:
            Variables:
                POWERTOOLS_LOGGER_SAMPLE_RATE: 0.5

Last update: 2021-03-05