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Amazon Elastic MapReduce (Amazon EMR) uses Amazon Machine Images (AMIs) to initialize the EC2 instances it launches to run a cluster. The AMIs contain the Linux operating system, Hadoop, and other software used to run the cluster. These AMIs are specific to Amazon EMR and can be used only in the context of running a cluster. Periodically, Amazon EMR updates these AMIs with new versions of Hadoop and other software, so users can take advantage of improvements and new features.
For general information about AMIs, go to Amazon Machine Images in the Amazon Elastic Compute Cloud User Guide for Linux. For more information about the software versions included in the Amazon EMR AMIs, see AMI Versions Supported in Amazon EMR.
If your application depends on a specific version or configuration of Hadoop, you might want delay upgrading to the new AMI until you have tested your application on it. AMI versioning gives you the option to specify which AMI version your cluster uses to launch EC2 instances.
Specifying the AMI version during cluster creation is optional; if you do not provide an AMI-version parameter, and you are using the CLI, your clusters will run on the most recent AMI version. This means you always have the latest software running on your clusters, but you must ensure that your application will work with new changes as they are released.
If you specify an AMI version when you create a cluster, your instances will be created using that AMI. This provides stability for long-running or mission-critical applications. The trade-off is that your application will not have access to new features on more up-to-date AMI versions.