Most ArcGIS functionality is available on Amazon Web Services; however, this topic lists some features and configurations that are not supported.
Server clusters are not supported
ArcGIS Server clusters are not supported on Amazon Web Services. ArcGIS Server sites that you create in Amazon EC2 use the default cluster for all GIS servers. You should not attempt to create additional clusters.
Limitations related to the use of the Elastic Load Balancer (ELB) and the Web ADF
The use of Elastic Load Balancers (ELBs) for scaling in the Amazon cloud constrains a few specific features of ArcGIS Server. ELBs are configured by default so that incoming requests are brokered across any of your instances participating in the GIS server site; that is, there is no guarantee that requests from a particular client (web browser user session, desktop client, or mobile application) will be handled by a particular EC2 instance. This limits the use of stateful web applications.
ArcGIS Server Web Application Developer Framework (ADF) applications rely on user session information that is stored at the web tier (in Internet Information Services). For the Web ADF application to work, incoming requests from a user session (web browser) have to hit the EC2 instance in which the session information is being stored. As described previously, an ELB does not guarantee that.
Advanced users can accommodate this limitation by storing Web ADF session information in SQL Server. Amazon also provides several scripts for configuring sticky sessions in the ELB; however, Esri has not certified these configurations in Amazon EC2.
Amazon enforces some limitations on resources you can create, such as the size of EBS volumes, the number of EBS volumes you can attach to an instance, the number of instances you can run at once, and the number of Elastic IPs you can allocate in one region. Exceptions to the limitations are sometimes available upon request to Amazon.
For the most up-to-date and official information about Amazon limitations, see the AWS Documentation.