Q1. What is the difference between RPO and RTO (from a backup perspective)?
A1. The recovery point objective (RPO) and the recovery time objective (RTO) are two very specific parameters that are closely associated with recovery. The RTO is how long you can basically go without a specific application. This is often associated with your maximum allowable or maximum tolerable outage.
The RTO is really used to dictate your use of replication or backup to tape or disk. That also dictates what you will put together for an infrastructure whether it's a high-availability cluster for seamless failover or something more modest. If your RTO is zero (I cannot go down) then you may opt to have a completely redundant infrastructure with replicated data off-site and so on. If your RTO is 48 hours or 72 hours then maybe tape backup is OK for that specific application. That's the RTO.
The RPO is slightly different. This dictates the allowable data loss -- how much data can I afford to lose? In other words, if I do a nightly backup at 7:00 p.m. and my system goes up in flames at 4:00 p.m. the following day, everything that was changed since my last backup is lost. My RPO in this particular context is the previous day's backup. If I'm a company that does online transaction processing -- American Express for example -- well maybe my RPO is down to the last, latest transaction, the latest bits of information that came in. Again, that dictates the kind of data protection solution you want in place.
So both of them, RTO and RPO, really influence the kind of redundancy or backup infrastructure you will put together. The tighter the RTO, and the tighter the RPO, the more money you will spend on your infrastructure.
Q2. What is the difference between SaaS and Cloud.
A2. SaaS is any software application that you run that is not located on your premises. It is a full-blown application, not a component part of something else. It is not a way to build applications. It is not a plug-in to other applications. It is never something that sits on your machines.
Instead of having the application running on servers and data storage in your data center, it is running in the vendor's data center.
The way SaaS applications are licensed is different from on-premise applications. Instead of buying the license to use the application, and then paying for software maintenance to support it and keep it current, you "rent" the software over a period of time - usually monthly or yearly. Instead of buying and installing infrastructure and then paying ongoing operating and maintenance costs, the vendor runs the application on their infrastructure. The cost of the SaaS application covers the costs of the software itself and the ongoing operations and infrastructure costs.
When you run a SaaS application, you generally log into your vendor's web site and you are on. You can say that SaaS applications are running "in the cloud," and you would be correct. But SaaS applications are not the Cloud.
So what is "the Cloud?" Cloud Computing provides computing resources that are not tied to any specific location. Cloud Computing basically consists of:
1. Virtual computers/servers.
2. Data storage capacity.
3. Communications and messaging capacity.
4. Network capacity.
5. Development environments
In other words, Cloud Computing is for software developers, application vendors, savvy computer users, and corporate IT departments, not for people who use computer applications.
Take, for example, virtual computers. A virtual computer acts like a physical server, but is actually a program that runs on a much larger machine. It acts exactly like a physical computer - you can reboot it, load software on it - except that there is no actual hardware. To take advantage of Cloud Computing, you can go to Amazon and use their Amazon Web Services (AWS) Elastic Compute Cloud (EC2) service. What are you doing, exactly? You are essentially creating and renting one or more virtual servers that are running on Amazon's infrastructure. Instead of buying and installing, say, an IBM System x or HP ProLiant server, you tell Amazon to "instantiate" a virtual server for you. You pay by the hour, and by the amount of data stored.
Unless you work directly with your company's servers, you won't be the person actually working directly with Cloud Computing. Instead, your IT department may decide to use cloud services as the infrastructure to run applications on or to store data. Or, your software vendor could use cloud services like Google AppEngine or SalesForce.com's Force.com to build applications that then become "SaaS" applications.
That's primarily the difference: SaaS offerings are applications that are fully formed end-user applications. Cloud Computing is computing infrastructure and services that you can rent.
External Links :
1. Saas V/s Cloud Computing : http://www.itworld.com/software/109287/saas-and-cloud-whats-difference