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Consider jsfiddle.net:

When you press "save" they generate uid for your jsfiddle like vzBZB.

How do they do it? Why only 5 characters?

And first of all - How do they make sure there's never a collision? (same uid generated for 2 different jsfiddles)

Can they run out of UIDs some day?

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closed as off topic by slhck, Simon Sheehan, 8088, Sathya Dec 14 '11 at 7:15

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well if it is just letters and it is case sensitive that would give you 3.8E8 IDs. To put that into perspective, they could give out a new ID every second for 12 years –  EBGreen Dec 13 '11 at 20:43
    
As for avoiding collisions, I have no idea what that service is, but in theory they could just hand them out sequentially. –  EBGreen Dec 13 '11 at 20:44
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How do they do it? Why only 5 characters?

The only sure way to know is ask jsFiddle, so this part of your question is not really constructive.

The UIDs are not generates sequentially, which is probably to prevent someone from browsing thorugh everybody else's fiddles.

How do they make sure there's never a collision?

That part is simple: You can just generate one at random and check if it does not exists already.

If it doesn't, you are done. If it does, repeat.

As long as there at least as much unused UIDs as used ones, you don't have to repeat more than once most times.

Can they run out of UIDs some day?

With a 52 letter alphabet (A-Z and a-z), you have 52 ^ 5 = 380,204,032 different UIDs available.

That already is a lot of fiddles, but if they ever run out, they could just add a sixth letter.

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