With the news of impeding regulation and the stock crash in Japanese social game companies, Konpu gacha are getting a lot of attention at the moment. However, I've seen the konpu gacha mechanism explained incorrectly in the Western media again and again, including an news article here in Gamasutra that states:
"kompu gacha," a common online game practice that rewards players with a random virtual prize -- which can be a rare or ho-hum item -- after purchasing a set of goods
This is not completely accurate and with many people arguing about the legality and/or morality of the mechanism I thought it'd be important to get the facts straight before starting the discussion.
The first thing that needs to be clear is that konpu gacha is a special case of the more general gacha mechanic.
Gacha in social games are inspired by the lottery gift machines of the same name that are widespread in Japan. You pay a certain ammount in virtual or real currency for a try and get a random item in return. It is often the case that some prizes are much more valuable than the others. Sometimes the list of possible rewards is disclosed, sometimes not. The probabilities of getting each item are usually not disclosed.
While more tightly monetized, at their heart gacha are pretty similar to other variable-ratio reward schedule mechanisms such as loot drops.
It is important to note that basic gacha are not under consideration for regulation. To the best of my knowledge, the Consumer Affairs Agency has gone as far as stating that there is "no problem" with regular gacha.
Regarding konpu gacha, "konpu" is simply a shortened adaptation of the English word "complete". Another way to call them would be "completion gacha".
Completion gacha are like regular gacha with the added rule that you get an special reward (usually a rare, high-value item) if you obtain all the items from a set. Usually the special reward is not only disclosed but featured prominently.
Probably the closest analogue in Western games would be a bonus granted for obtaining all the pieces of equipment in a set.
It takes not more than a passing familiarity with combinatorial theory to know that, depending on the indivual parameters, it can take an inordinate number of tries to complete a set. However it should also be noted that in many cases, though by no means all, games contain trading mechanisms and ways to gather currency for gacha during gameplay.
As stated above, it is only completion gacha that are currently being put under the spotlight in Japan. It may be a subtle difference, but specially when bringing themes such as gambling and addiction into the discussion it is important to know the exact mechanisms at play.