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Shlomo Swidler
Cloud Computing Consultant, Developer, EC2 Expert
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There's no reason to assume that the spot price is always set below some bid. It could be a pure function of the supply of spot instances. Just because the price was set to a certain level doesn't mean anyone bought at that price. In effect, when there are no spot instances left the spot price can be said to be infinite - so no bids would ever win.
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"There will always be a premium placed on guaranteed capacity." And when there are no more on-demand instances... the spot price will be higher than the on-demand price. If you really want to ensure you have access to the on-demand prices longer-term, you should buy reserved instances now. In less than six months (at current rates) the 1-year term breaks even.
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Looking at the historical graphs of spot instance prices you can see that the spot price occasionally spikes to above the on-demand price. http://cloudexchange.org/charts/us-east-1.linux.m1.large.html Presumably this occurs when a large batch of on-demand instances is provisioned. For example, the spike to 0.400 in the us-east region on 17 Dec is nicely correlated with this user's provisioning of 300 m1.large instances: http://twitter.com/Lounibos/status/6777451245 The fact that the price drops back down to its prior level after a few hours is likely an indication that Amazon's pricing algorithm is still being perfected. But the bottom line is: the spot price can exceed the on-demand price.
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One more Whoa Moment data point: As reported in this thread, http://developer.amazonwebservices.com/connect/thread.jspa?threadID=39924&tstart=0 the spot price for one instance type did already exceed the on-demand price. It was for a short time, but it happened.
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Hi Geva, The idea of a spot instance contest came from David Kavanagh (and includes the caveat of "for the least $"). Here's my tweet, which is a retweet of David's: http://twitter.com/ShlomoSwidler/status/6722022106 Regarding the availability of EC2 instances, don't assume they're always available. Here are a bunch of threads in the EC2 support forum where people complain of the InsufficientCapacity error. This error means there's no more on-demand instances available at the moment. These threads are from all over the calendar, so it's not a recent development. http://www.google.com/search?q=site%3Adeveloper.amazonwebservices.com+InsufficientCapacity Currently there are 270 results for this query. Whoa moment. IMO, Spot Instances should be a wake-up call for developers: Be careful wedding your code to EC2, lest you find yourself paying above the on-demand price when there are no more on-demand instances.
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Very nice to see the application handling its own operational events. A minor nitpick: Example 3 probably should say if (averageRequests > maxRequestsPerInstance) otherwise you're scaling up when you don't need to.
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