LESS than a year ago Babbage wrote an article, entitled “Clouds under the hammer”, in The Economist discussing whether computing capacity would become a tradable commodity. In the conclusion he sided with the sceptics, arguing that “virtual machines”—the main unit of measurement in cloud computing—would increasingly move about, but mostly within clouds controlled by a single company (“private clouds”) or trusted federations of “public clouds” (where anyone can buy capacity).

One of the people quoted in the article, Reuven Cohen, the founder of Enomaly, a maker of software that allows firms to build public clouds, was more optimistic. He has now proven that he was right: today Enomaly will launch SpotCloud, the world’s first spot market for cloud computing.

Fundamentally, SpotCloud works like other spot markets (for more screen shots click here). Firms with excess computing capacity—operators of data centres, cloud providers, hosting firms—put it up for sale. Others, who have a short-term need for some number-crunching, can bid for it. Enomaly takes a cut of between 10% and 30% depending on the size of the deal. But there is an important difference: SpotCloud is what Enomaly calls an “opaque market”, meaning that the firms offering capacity do not have to reveal their identity. Thus selling computing services for cheap on SpotCloud does not cannibalise regular offerings.

Technically, the service is also not entirely what one would expect. Enomaly did not build a big central infrastructure—because the bandwidth demands “would have killed us”, in the words of Mr Cohen: files of virtual machines are very large. Instead, the firm works with Google App Engine, itself a cloud-computing provider, which gives Enomaly access to a decentralised global system. The buyer’s virtual machines files are parked on App Engine before being send to a seller’s servers. Buyers can also define in which country or even city they want their virtual machines to run.

SpotCloud has been up and running since November in a closed trial—and has already attracted a lot of supply. It often comes from unexpected corners of the computing universe. For instance, an entertainment company has offered capacity on 4,000 servers that would otherwise sit unused (probably in a lull between making animated movies). In other cases, says Mr Reuven, firms offer the capacity of old servers, which otherwise would be thrown away at some point.

The big question is whether there is also enough demand. Again, Mr Cohen is optimistic, and sees many ways in which Enomaly’s market place could be used: getting non-critical computing tasks done quickly and cheaply, testing new websites and quickly adding computing capacity in certain regions. But he also warns that SpotCloud is not for those who want long-term computing needs satisfied with the service-level guaranteed. Instead, it is for those who need capacity quickly and do not much care if the computing task has to be restarted if something goes wrong.