Oracle RAC on AWS

Oracle Real Application Clusters (RAC) is not natively supported on AWS. The word natively is used because it is possible to run Oracle RAC in an AWS Direct Connect facility http://aws.amazon.com/directconnect/.

There are a number of options when migrating an Oracle RAC database to AWS.  The option you use depends upon the reason RAC is being used.  For HA and fail over, AWS offers multi-AZ capabilities which can provide the same level of service.  For very large databases that require high transaction through put that can not be achieved on a single instance database, Direct Connect would be the solution.  Details on these options are as follows:
1. RDS with multi-AZ :  Oracle RDS is the managed database service from AWS.  Oracle RDS has builtin multi-AZ capabilities.  Because RDS is a managed service, AWS takes care of installation, configuration and management of the secondary database, the replication between AZs, and the fail over and fail back of the database instance.
2. EC2 with multi-AZ :  Running on EC2 requires the customer or partner to install, configure, manage, and take care of the replication.    Oracle Data Guard or GoldenGate can be used for replication.
3. Direct Connect : The AWS partner Datapipe runs RAC in a managed service model using Direct Connect.

Note: Remember that AWS RDS only supports databases up to 6 TB in size. Note this number changed to 6 TB in June 2015 after being 2 TB. https://aws.amazon.com/about-aws/whats-new/2015/06/amazon-rds-increases-storage-limits-to-6TB-for-piops-and-gp2/. Check the whats new web page for any updates related to Oracle on RDS.

Note:  The reason Oracle Real Application Clusters (RAC) is not supported on AWS is:
1. Multicast is not supported on the AWS network. An overlay network is possible on AWS: http://cloudconclave.blogspot.com/2013/06/overlay-networks-on-aws.html
2. AWS EBS is not a shared disk / clustered file system.
So, even if you use a solution such as Amazon EFS, GlusterFS, Zadara, SoftNAS, or custom NFS for shared disk you can not use RAC on AWS as you need multicast support.  More on Amazon Elastic File System (EFS): https://aws.amazon.com/blogs/aws/amazon-elastic-file-system-shared-file-storage-for-amazon-ec2/ 

Overlay networks on AWS

Software Defined Networks (SDN) are all the rage and for good reason.  A couple of very good reasons when running on AWS are:
1. Supporting multi-cast on AWS which does not natively support multi-cast
2. Dealing with network address overlapping when using AWS VPC

There are a number of alternatives.  https://code.google.com/p/openpgm/
This is one that is certified to run on AWS:
http://www.cohesiveft.com/products/vns3

Here is a very comprehensive post on multicast on AWS:

 https://www.buckhill.co.uk/blog/how-to-enable-broadcast-and-multicast-on-amazon-aws-ec2/2#.VZP_MbzI-lO

AWS RDS cross region replication

RDS offers a simple way to replicate across AZs (Multi-AZ feature) for fault tolerance and high availability.  RDS cross region synchronization / replication provides additional fault tolerance.  This is an interesting situation because RDS is a 'black box'/PaaS/DaaS environment. Therefore, no access to the OS level is provided. This means this replication solutions like MySQL Replication or Oracle Goldengate can not be used.  Until RDS offers snapshots like the new EBS region snapshotting, you will have to use tools like BitTorrent or CloudBeam to move database dump files across regions.You could use DB links with materialized views as this article mentions: http://aws.amazon.com/articles/4173109646282306. DB Links are slow but if you don't have a low RTO and RPO you could use this...Or come up with your own creative solution.