Software // Social
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2/8/2011
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Facebook Opens Live Commenting To All

A company engineer details the behind-the-scenes development challenges in creating the new instant messaging-like feature.

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Facebook on Monday confirmed all users now have access to live commenting, a capability that allows accountholders to communicate in real-time.

First unveiled about two weeks ago, live commenting now gives Facebook users access to a more instant message-like experience within the social network. Users no longer must refresh their page to see if a friend has responded to a post; instead, comments appear as other users post them. The idea sounds simple: Execution, however, was not as easy as it appears, said Ken Deeter, a Facebook software engineer, in a company blog.

"This wasn't a small challenge: every minute, we serve over 100 million pieces of content that may receive comments. In that same minute, users submit around 650,000 comments that need to get routed to the correct viewers," he wrote. "To make this feature work, we needed to invent new systems to handle load patterns that we had never dealt with before."

At first, Facebook considered a poll-based approach that would periodically send a request to check whether new comments had arrived on pages with content that could be commented on, said Deeter. Although the social media giant could increase the polling frequency to approximate a real-time feel, the approach would not scale, he said.

"Because humans are so sensitive to latency in real-time communications, creating a truly serendipitous commenting experience requires comments to arrive as quickly as humanly and electronically possible," wrote Deeter. "In a poll-based approach this would mean a polling interval of less than five seconds (and that would still feel slow!), which would very easily overload our servers."

Instead, Facebook opted for a push-based approach, one that would let it know who was viewing each of the 100 million pieces of content per minute that the company serves, he said. And Facebook required a system that also could manage the rate at which this data changed -- up to 16 million new associations per second, Deeter noted.

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