Amazon held its fourth annual public sector cloud summit Tuesday, marketing its Internet-based cloud offerings to federal and public sector clients.

Attendance for the conference, this year held at the Walter E. Washington Convention Center, has been growing — this year about 2,500 public and private sector representatives registered. In 2010, attendance was about 100.

Patrick Gallagher, director of the Commerce Department’s National Institute of Standards and Technology, and Frank Baitman, the Health and Human Services Department’s chief information officer, delivered keynote speeches. In break-out sessions, representatives from Amazon, smaller tech companies that deliver Amazon’s services, and officials from the public sector discussed cloud applications.

More than 600 government agencies and 2,400 educational institutions have implemented Amazon Web Services, according to the company. Amazon doesn’t release revenue figures for its cloud offerings, but it’s certainly the market leader in federal cloud computing, Gartner analyst Lydia Leong said. The company isn’t alone in the space — competitors include Verizon’s Terremark — but it is one of few with a product designed specifically for federal clients, she said.

Here are a few take-aways from the summit:

1. Not everything belongs in the cloud

In his keynote address, Baitman described the Health and Human Service Department’s adoption of cloud technologies. For instance, HHS currently uses AWS to power, a public online health data repository for entrepreneurs and scientists.

But Baitman noted that adoption of new technology must be strategic and cost-effective, especially in the public sector. Agencies will likely discover that not all operations necessarily work better in the cloud, though some might.

“The cloud isn’t the answer for everything,” he said.

2. One cloud provider won’t necessarily rule the rest

For instance, Health and Human Services encourages many cloud providers to meet federal technology standards, Baitman said. An ultimate cloud solution might involve services from various providers such as Amazon and its competitors, Salesforce and Verizon’s Terremark.

3. A “cloud broker” model is emerging

Tech companies can generate revenue by helping clients integrate cloud technology, instead of actually providing the cloud technology itself, Baitman explained. Many of Amazon’s tech partners, including SRA and Aquilent, offer AWS and other cloud solutions, often serving as “cloud brokers”.

4. There are barriers to adoption

Transitioning entire server systems to the cloud can be challenging for government clients with complex databases, said Max Peterson, Amazon Web Services’ director of partners and contracts: “Customers sometimes don’t know where to begin.”

Peterson noted that the federal security accreditation system has improved in the past two years, but federal clients are sometimes still concerned about the risks of new technology.

5. Public education may benefit from the cloud

Cloud adoption could help public schools process large volumes of learning analytics, helping educators improve material, said Karen Cator, chief executive of Digital Promise, an educational nonprofit.

School systems “can’t afford the capacity required to manage a server-based farm,” she said.

Some states school systems — North Carolina’s, for instance — are already transitioning data to the cloud, she said.

Disclosure: Amazon founder and chief executive Jeff Bezos has reached a deal to buy The Washington Post.