客户既需要PaaS服务，也需要IaaS服务
由于IaaS最成熟，已在很多企业部署，基于IaaS构建PaaS可保护用户已有投资
缺点参考:
http://blog.afkham.org/2011/09/is-paas-on-top-of-iaas-good-idea-after.html
So, traditionally people think about building a PaaS on top of an IaaS. Samisa recently blogged about the practical
problems we have been facing with IaaS,
in particular, Amazon Web Services (AWS). To be honest, working with
AWS has been a big challenge since we experienced very bad performance
when it comes to IO, as well as the infrastructure itself behaving in a
very unpredictable manner from time to time. Every so often, we would
lose the network, and we would not even be able to connect to 127.0.0.1
from our software, forcing restarts. So much for SLAs & high
availability! It is also well known that virtualization leads to IO
performance degradation, so if your PaaS or SaaS is IO intensive, you
may see a considerable drop in performance.
Coming to think about it, running a PaaS on top of an IaaS such as AWS
could be overkill. In such a setup, we have two levels of multi-tenancy;
one at the IaaS layer, where the PaaS service provider is a tenant, the
other level is at the PaaS layer itself. One level of multi-tenancy at
the PaaS level is what is actually required. When it comes to elasticity
in a PaaS, what we actually need is a new process; in the case of Java
PaaS, a new JVM, but what we do in a setup such as the one shown in the
above figure is, we spin up a new image instance (in the case of AWS, a
new EC2 instance), and then start a process in that new instance.
Spinning up a new instance can take up to 15 minutes, so by the time a
new instance boots up & is able to perform some work, the need for
starting up that instance may have passed due to the traffic dropping
back to normal levels.
Perhaps, the proper model would be to run your PaaS on the
infrastructure (hardware+networking+OS) directly without virtualization
as shown in the above diagram, and have a few cold standby EC2 instances
for
Cloud bursting.
This is the model we will have to go with, at least until the time
where we have IaaSs that are much more stable. Another advantage is, the
cost of having your own hardware will be very much less than the
accumulated amount you would be paying the IaaS provider, since you will
be running your PaaS 24x7.
The challenges of going for such a setup include, having to implement
alternatives to many of the functionality that is already provided by
the IaaS. This includes geographically distributed deployments (AWS
provides this through availability zones & regions), firewall
functionality (AWS provide this using security groups), public IP
address assignment (AWS provides this through Elastic IPs), and so on.
However, the benefits of implementing such functionality at the
infrastructure level will yield huge benefits for large scale
Platforms-as-a-Service such as
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