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.
• Forked from the git repository.

Enjoy, Gray Watson

Maven Configuration

<dependencies>
	<dependency>
		<groupId>com.flo354.cloudwatchlogbackappender</groupId>
		<artifactId>cloudwatchlogbackappender</artifactId>
		<version>2.0.0</version>
	</dependency>
</dependencies>

Dependencies

By default the appender has dependencies on version 2.10.19 of 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>2.10.19</version>
</dependency>
<dependency>
	<groupId>com.amazonaws</groupId>
	<artifactId>aws-java-sdk-ec2</artifactId>
	<version>2.10.19</version>
</dependency>

logback.xml Configuration

Minimal logback appender configuration:

<appender name="CLOUDWATCH" class="com.flo354.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 supports the following tokens which expand into the instance-name: %{instanceName}, %instanceName, %{instance}, %instance, %{in}, and %in. These expand into the instance-id: %{instanceId}, %instanceId, %{iid}, and %iid. If the name is not available then the id will be used instead. So you might want to use something like:

	<logGroup>your-log-group-name-here</logGroup>
	<logStream>general-%instance</logStream>

Similar patterns are available in the log entries themselves if you use our Ec2PatternLayout class which adds support for the ec2 instance-name tag from the tokens %instance, %instanceName, and %in. It also supports %instanceId and %iid for the instance-id as well. If the instance-name is not available then the instance-id will be used for the name instead.

<appender name="CLOUDWATCH" class="com.flo354.cloudwatchlogbackappender.CloudWatchAppender">
	...
	<layout class="com.flo354.cloudwatchlogbackappender.Ec2PatternLayout">
		<pattern>\[%instance\] \[%thread\] %level %logger{20} - %msg%n%xThrowable</pattern>
	</layout>

Here is the complete list of the appender properties.

Property	Type	Description
region	string	AWS region needed by CloudWatch API
logGroup	string	Log group name
logStream	string	Log stream name
accessKeyId	string	Default: none, code will use DefaultCredentialsProvider AWS API access key ID, see AWS Permissions below
secretKey	string	Default: none, code will use DefaultCredentialsProvider AWS API secret key, see AWS Permissions below
maxBatchSize	int	Default: 128 Maximum number of log events put into CloudWatch in single request.
maxBatchTimeMillis	long	Default: 5000 Maximum time in milliseconds to collect log events to submit batch.
maxQueueWaitTimeMillis	long	Default: 100 Maximum time in milliseconds to wait if internal queue is full before using the emergency appender (see below).
initialWaitTimeMillis	long	Default: 0 Initial wait time before logging messages. Helps if server needs to configure itself initially.
internalQueueSize	int	Default: 8192 Size of the internal log event queue.
createLogDests	boolean	Default: true Create the CloudWatch log and stream if they don't exist.

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.flo354.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 DefaultCredentialsProvider 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:*:*:*.