Amazon Web Services has altered its Reserved Instance (RI) terms to enable customers to make certain changes after the contract is signed. Even with new powers to modify a contract, setting initial terms is not as simple as it sounds. Cloudability is stepping in to try to make it easier.

AWS Reserved Instance customers may now change the availability zone in which a RI runs or the network attributes assigned to it, but they can't vary the lifespan set forth in the contract. RIs come with one-year or three-year subscriptions. In the past, customers were stuck with whatever RI attributes they signed up for, over the duration of the contract. But the RIs also come with a significant discount over Amazon's on-demand server prices.

Availability zones may be changed, provided customers stay in the same region, such as U.S. West in Oregon or Northern California, or U.S. East in Ashburn, Va. The data center complex at U.S. East contains five availability zones, and RI users may move among them. Availability zones have their own power and communications links within the data center complex. Amazon urges customers to deploy to more than one zone to guarantee uptime.

RI users may also move them from what AWS calls "EC2 classic" to Amazon Virtual Private Cloud, with the VLAN networking attributes of the private cloud service, and back again. Customers may move as they choose, provided they continue paying for the complete term of the instance.

[ Want to learn more about how to best make use of Reserved Instances? See 5 Ways To Avoid Costly Cloud Surprises. ]

Amazon keeps trying to make it easier to adopt its RIs because they give it a more predictable demand curve, upfront payments and steady income for its AWS infrastructure.

However, RI users must make predictions and decisions about what sort of plan will work best for them. For example, consider that RIs come in Light, Medium and Heavy configurations of the various instance types. As the number of virtual servers needed increases, the calculations become more complex.

Knowing there are high hurdles associated with RI decision-making, AWS has made its Trusted Advisor available to customers as a beta service. It incorporates 100 best practices in the use of RIs and makes recommendations to customers, based on their stated needs, "when opportunities exist to save money, improve system performance or close security gaps."

Amazon acknowledges there are also "equally capable third-party tools (with which) you can lower your compute costs while also reserving capacity."

One of those third-party tools is Cloudability, which launched its Reserved Instance Planner last week, at the same time AWS changed its rules governing RI use. It charges $49 a month for its entry-level, supported service. Other third parties in the Amazon-engendered growth field are Cloudyn, Uptime Software, CloudCruiser, 6Fusion and Apptio.

Cloudability's Reserved Instance Planner assesses a customer's cloud virtual machine needs and recommends RI contract terms. It also displays the upfront cost. Reserved instances require a down payment upon order, with lower hourly rates to follow.

The planner provides a break-even point, where the customer starts to save money on the RI over what he would have paid for on-demand instances. The break-even point is usually reached well before the end of the RI term, but to do so, the customer must make use of Light, Medium and Heavy RIs in the right proportion.

The goal of the planner's authors, said the Cloudability announcement, was to enable AWS users to decide on RIs "without spending days in a spreadsheet."

Learn more about cloud contracts by attending the Interop conference track on Cloud Computing and Virtualization in New York from Sept. 30 to Oct. 4.