I want to use Amazon ELB plus EC2 for fault tolerance (high availability)
In particular, it is not clear how it supports the following high availability features:
Does ELB have..
1) preemptive migration I suppose it could be configured to detect early failure somehow by hooking the health checks up to something within your application that could detect an early failure. But it's not part of the design strategy. Nodes are marked as bad and new nodes are brought on, this method isn't part of how AWS is supposed to work. The nodes are thought of as immutable
2) Checkpointing The idea of duplicating data across nodes as part of a regular process isn't part of the AWS high availability vision. The HA of data tends to take place at a database layer, not as data on nodes
3) Job Migration The use of "sticky sessions" allows users to continue with the same data even in the event of a system failure. How the job data is exactly persisted isn't controlled by the ELB.
4) Self Detection In the context of an ELB this is pretty much what the health checks do. But the health checks detect failure in the downstream nodes, one has to imagine the system as being ELB+nodes
5) Fault Mask This is more of a low-level thing, I don't see how it applies to ELB
I suppose that many of your questions would be better addressed as queries about the database layer. AWS RDS has an interesting set of HA capabilites