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Logback log appender for AWS CloudWatch

Background

This package provides a logback appender that writes its log events to Cloudwatch. Before you say it, there seem to be many projects like this out there but I could find none of them that were self-contained and that were published to the central Maven repo.

  • Code available from the git repository. CircleCI CodeCov
  • Maven packages are published via Maven Central javadoc

Enjoy. Gray Watson

Maven Configuration

<dependencies>
	<dependency>
		<groupId>com.j256.cloudwatchlogbackappender</groupId>
		<artifactId>cloudwatchlogbackappender</artifactId>
		<!-- NOTE: change the version to the most recent release version from the repo -->
		<version>2.0</version>
	</dependency>
</dependencies>

Dependencies

By default the appender has dependencies on logback (duh) but also the log (cloudwatch) and ec2 AWS SDK packages. You can add a exclusion for these packages if you want to depend on different versions.

<dependency>
	<groupId>com.amazonaws</groupId>
	<artifactId>aws-java-sdk-logs</artifactId>
	<version>1.11.914</version>
</dependency>
<dependency>
	<groupId>com.amazonaws</groupId>
	<artifactId>aws-java-sdk-ec2</artifactId>
	<version>1.11.914</version>
</dependency>

logback.xml Configuration

Minimal logback appender configuration:

<appender name="CLOUDWATCH" class="com.j256.cloudwatchlogbackappender.CloudWatchAppender">
	<region>us-east-1</region>
	<logGroup>your-log-group-name-here</logGroup>
	<logStream>your-log-stream-name-here</logStream>
	<layout>
		<!-- possible layout pattern -->
		<pattern>[%thread] %level %logger{20} - %msg%n%xThrowable</pattern>
	</layout>
</appender>

Cloudwatch unfortunately does not allow multiple hosts to write to the same log-stream. If multiple servers are writing logs, you should configure the log-stream name with an instance-name suffix or something. The logStream name setting uses the Ec2PatternLayout to generate the name, which can also be used to format your log lines. This allows you to use the standard %token such as %date in the name of the log-stream – see the logback documentation. The Ec2PatternLayout class also adds support for additional tokens:

Property Description
instanceName Name of the EC2 instance or ID if the name is not available.
instance Same as instanceName.
in Same as instanceName.
instanceId ID of the EC2 instance.
iid Same as instanceId.
uuid Random UUID as a string
hostName Name of the host from InetAddress.getLocalHost().
host Same as hostName.
hostAddress IP address of the host from InetAddress.getLocalHost().
address Same as hostAddress.
addr Same as hostAddress.
systemProperty Value of a system-property whose name is set as an {option}. Ex: %systemProperty{os.version}`.
property Same as systemProperty.
prop Same as systemProperty.
systemEnviron Value of a environmental variable whose name is set as an {option}. Ex: %systemEnviron{SHELL}`.
environ Same as systemEnviron.
env Same as systemEnviron.

For example:

	<logGroup>your-log-group-name-here</logGroup>
	<logStream>general-%instance-%date{yyyyMMdd,UTC}-%uuid</logStream>

This will generate a log-stream name with the prefix "general-" and then with the instance-name, date in UTC timezone, and a random UUID.

NOTE: The instance-name and instance-id tokens will only work when running on an EC2 instance that supports the EC2MetadataUtils methods for looking up the information. You can call Ec2InstanceNameConverter.setInstanceName(...) or Ec2InstanceIdConverter.setInstanceId(...) early in your program if you want to set them yourself.

NOTE: logGroup must match the regex pattern [\.\-_/#A-Za-z0-9]+. logStream cannot contain the ':' character which will be replaced by '_'.

The appender also adds the support for the previous list of % tokens to be expanded on each log line:

<appender name="CLOUDWATCH" class="com.j256.cloudwatchlogbackappender.CloudWatchAppender">
	...
	<layout class="com.j256.cloudwatchlogbackappender.Ec2PatternLayout">
		<pattern>\[%instance\] \[%thread\] %level %logger{20} - %msg%n%xThrowable</pattern>
	</layout>

Here is the complete list of the appender properties.

Property Type Default Description
region string none AWS region needed by CloudWatch API
logGroup string none Log group name
logStream string none Log stream name
accessKeyId string none AWS API access key ID, see AWS Permissions below.
Code will use DefaultAWSCredentialsProviderChain by default.
secretKey string none AWS API secret key, see AWS Permissions below.
Code will use DefaultAWSCredentialsProviderChain by default.
maxBatchSize int 128 Maximum number of log events put into CloudWatch in single request.
maxBatchTimeMillis long 5000 Maximum time in milliseconds to collect log events to submit batch.
maxQueueWaitTimeMillis long 100 Maximum time in milliseconds to wait if internal queue is full before using the emergency appender (see below).
initialWaitTimeMillis long 0 Initial wait time before logging messages. Helps if server needs to configure itself initially.
internalQueueSize int 8192 Size of the internal log event queue.
createLogDests boolean true Create the CloudWatch log and stream if they don't exist.
maxEventMessageSize int 256k Maximum size of event message before it is truncated or sent to emergency appender.
truncateEventMessages boolean true If an event it too large, should the message be truncated. If false then it will be sent to emergency appender.
copyEvents boolean true Copies the event for logging by the background thread.
printRejectedEvents boolean false Print any rejected events to stderr if the emergency appender doesn't work.

Emergency Appender

Since this appender is queuing up log events and then writing them remotely, there are a number of situations which might result in log events not getting remoted correctly. To protect against this, you can add in an "emergency" appender to write events to the console or a file by adding the following to your CLOUDWATCH appender stanza:

<appender name="CLOUDWATCH" class="com.j256.cloudwatchlogbackappender.CloudWatchAppender">
	...
	<appender-ref ref="EMERGENCY_FILE" />

This appender will be used if:

  • there was some problem configuring the CloudWatch or other AWS APIs
  • the internal queue fills up and messages can't be written remotely fast enough
  • there was some problem with the actual put events CloudWatch call – maybe a transient network failure

If no emergency appender is configured and a problem does happen then the log messages will be not be persisted.

AWS Permissions

You can specify the AWS CloudWatch permissions in a number of ways. If you use the accessKeyId and secretKey settings in the logback.xml file then the appender will use those credentials directly. You can also set the cloudwatchappender.aws.accessKeyId and cloudwatchappender.aws.secretKey Java System properties which will be used. If neither of those are specified then the appender will use the DefaultAWSCredentialsProviderChain which looks for the access and secret keys in:

  • Environment Variables: AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY or AWS_ACCESS_KEY and AWS_SECRET_KEY
  • Java System Properties: aws.accessKeyId and aws.secretKey
  • Credential file at the default location (~/.aws/credentials) shared by all AWS SDKs and the AWS CLI
  • Instance profile credentials delivered through the Amazon EC2 metadata service

IAM Permissions

When making any AWS API calls, we typically create a IAM user with specific permissions so if any API keys are stolen, the hacker only have limited access to our AWS services. To get the appender to be able to publish to CloudWatch, the following IAM policy is required to create the log group and put log events to CloudWatch.

The logs:CreateLogGroup and logs:CreateLogStream actions are only required if the appender is creating the log-group and stream itself (see createLogDests option above). The ec2:DescribeTags action is only required if you want the appender to query for the ec2 instance name it is on – see Ec2PatternLayout above.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "logs:CreateLogGroup",
                "logs:CreateLogStream",
                "logs:DescribeLogGroups",
                "logs:DescribeLogStreams",
                "logs:PutLogEvents",
                "ec2:DescribeTags"
            ],
            "Resource": [
                "*"
            ]
        }
    ]
}

I couldn't figure out how to restrict to all ec2 instances. If you are only doing log requests then you should be able to limit it to the resource arn:aws:logs:*:*:*.

ChangeLog Release Notes

See the ChangeLog.txt file.