The White House billed the Obamacare portal as the Kayak.com of health coverage — only instead of plane rides, it was selling health insurance. It was supposed to be a one-stop click and shop place to compare and buy health plans in the 36 states not running their own insurance exchanges — and for the millions who are eligible, get federal subsidies.

Hundreds of millions of dollars later, it’s fundamentally broken and a frantic fix is underway.

( PHOTOS: House hearing on Obamacare)

What made it so complicated?

The Obama administration has done little to give outsiders a peek under the hood — although it promises daily briefings from the Department of Health and Human Services starting Thursday.

But it’s way more complicated than tweaking a simple commercial site.

“This is one of the most complex IT projects the federal government has ever undertaken,” said Dan Schuyler, senior technology expert at Leavitt Partners.

( POLITICO’s full Obamacare coverage)

With the contractors who built the system preparing for a grilling from House Republicans today, here’s a quick guide to all the things that have to go right for HealthCare.gov to start humming.

( WATCH: 7 quotes on Obamacare glitches)

The code

By all accounts, HealthCare.gov and the programs it’s built upon include tens of millions — if not hundreds of millions — of lines of code. When major portions are rewritten, it usually takes time and testing to ensure that the fixes worked. But in the political pressure-cooker surrounding the health law, time isn’t really an option. Fixes are happening on the fly.

“It’s almost like test riding a 747 while writing a manual from the cockpit seat,” said Bryce Williams, managing director of exchange solutions for Tower Watson.

The code directs all the big parts of the enrollment process — creating online accounts, managing reams of personal and demographic data, determining whether someone get a tax credit, or qualifies for Medicaid.

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“If you take a look at the work involved in this particular project, it’s at least five different transactional functions,” said Aneesh Chopra, the Obama administration’s former chief technology officer. “What that really means is it’s performing a series of application functions. Each of those functions has a level of complexity and stitching them together has a level of complexity.”

Not only does everything have to work, it has to work well with thousands — if not millions — of people interacting with the site simultaneously, putting pressure on different aspects of the system. That’s a situation that’s barely come up yet because most visitors to the site can’t get past the early stages. More problems may lurk once the first wave is solved.

The data hub

The hub is what a lot of people worried about, but the early indications are that it actually works quite well. The hub can be thought of as the heart of the system, linking five federal agencies to process information about people who sign up for health insurance through HealthCare.gov as well as in the states running their own enrollment systems.

( Also on POLITICO: Obamacare hearings: Oversight or heckling?)

When a person seeks coverage through the new Obamacare marketplaces, the hub draws data from the Department of Health and Human Services, Social Security, the Department of Homeland Security, the IRS and the U.S. Treasury to determine eligibility for subsidies.

“The accuracy and the effectiveness and the quality with which it is pulling information from various systems appears to be working,” Chopra said.

More tests will come as more applicants reach this point in the signup process.

The federal-state connection

Not only do federal agencies have to work in concert, the enrollment system has to talk to 50 states with 50 diverse Medicaid programs. Each state has different eligibility rules and years of regulation, so connecting them all to the same system has been a challenge.

( Understanding Obamacare: A guide to the ACA)

The National Association of Medicaid Directors said Monday that states are reporting vastly different experiences. Some have experienced seamless connectivity to the federal system. Others — not so good. “Communication between states and marketplaces continue to contain challenges,” the association said.

“With this many agencies, this many states, there are a whole lot of cooks in the kitchen,” said Williams of Towers Watson.

Talking — and listening — to insurers

The crux of the new enrollment system is the marketplace — getting health plans sold by private insurers. But to complete that process, the system has to be able to transfer massive amounts of data — subsidy calculations, personal information of enrollees and application files.

( PHOTOS: Obamacare online glitches: 25 great quotes)

Some applications have made it to insurers without a hitch. But sometimes the system is spewing out files that are garbled, incorrect or repetitive.

And for people who wish to bypass HealthCare.gov and purchase plans directly from an insurer, they’re running into the same brick wall. Some insurers are directing people back to HealthCare.gov because of their own problems calculating subsidies and interacting with the federal system.

The rules

Sites like Kayak.com don’t have to deal with an Obamacare-sized thicket of federal rules and regulations. The health law passed in March 2010, but the final regulatory framework for HealthCare.gov wasn’t really in place until earlier this year.

“HHS was slow in distributing and promulgating the necessary rules and regulation,” Dan Schuyler of Leavitt Partners said. “Had the federal government initiated the legislation in March 2010 … we wouldn’t be having these problems today. The rules, the regs, the guidance are, in essence the business processes, if you will, of how the exchanges are supposed to function.”

Some allies of the administration point back to the way the law was passed — after Republican Scott Brown was elected to the Senate in early 2010, the Democrats couldn’t merge the House and Senate bills, a process that could have purged some of the kinks. They ended up enacting the Senate bill — and regulators had more work to fill in some gaps.

The threat of the Supreme Court overturning the law and the 2012 election also may have slowed some of the implementation.

But Schuyler has a simpler theory: “Poor design and poor planning. There is no other reason.”