Communications - July 2011

the relational and key-value models are equivalent, but rather dual. The issue of weakening consistency checking goes to the heart of the interest in NoSQL systems and is beyond the scope of our article.

Erik Meijer, redmond, WA Gavin Bierman, Cambridge, u.K.

Financial Incentives vs.
Algorithms in Social networks

I thank John C. Tang et al. for their analysis of the crowdsourcing strategies of three successful teams in their article “Reflecting on the DARPA Red Balloon Challenge” (Apr. 2011). Though the iSchools team might have had better data-mining algorithms, it was the MIT team that recognized and exploited financial incentives as the most effective way to be first to identify the 10 red balloons DARPA scattered across the U.S. last year.

In retrospect, the recursive incentive strategy adopted by the MIT team is used in many network-marketing situations worldwide. I first came across it almost 20 years ago when trying to sell a database management system to one of India’s oldest non-banking finance companies, which happened to employ a motivated network of insurance agents throughout India. These agents were required to recruit other agents, with the initial premium for the first few months from each new account they signed up distributed hierarchically, though not in the precise geometric progression the MIT team used in the DARPA Challenge. This way, the company’s senior agents, having recruited a large network, could virtually sit back and watch as the money poured in. I suppose this, too, is how most Ponzi schemes work, though, in this case, nothing illegal was involved, as is generally implied by the term.

The important takeaway from the Tang et al. analysis is that motivating people is the key to success and that money is often the most effective motivation in any given social network. Whether that is good or bad is a question that needs a totally different kind of analysis.

Prithwis Mukerjee, Kharagpur, india

Let Leap Seconds Sync

Poul-Henning Kamp’s article “The
One-Second War” (May 2011) was en-
lightening and, from the perspective
of an old-time (ex)hardware engineer,
entertaining. The reason solder jock-
eys (hardware engineers) don’t see leap
seconds as a problem is they presume
computers know only what they’ve
been told; if the system clock slows by
1/86,400th of a second per second, the
system’s software won’t have the slight-
est idea it happened, nor will it care.

Author’s Response:

Byrd proposes a number of additional ways
we might paper over the fact that the planet
is itself an unpredictable and unstable clock.
There is no shortage of such ideas, and
all are bad hacks. If computers were still
huge boxes with a few attached terminals
and printer, all these ideas would work,
as indeed a number of them did, from the
invention of the computer to the mid-1980s.
Like today’s deployed bad hack—leap
seconds—all the schemes Byrd proposes
rely on somebody measuring what the
planet does and everybody else reacting to
it on short notice. His ideas do not improve
the current situation in any way but do
reintroduce at least one bad idea already
discarded—variable-length seconds.

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