I've got an image library on Amazon S3. For each image, I md5 the source URL on my server plus a timestamp to get a unique filename. Since S3 can't have subdirectories, I need to store all of these images in a single flat folder.
Do I need to worry about collisions in the MD5 hash value that gets produced?
Bonus: How many files could I have before I'd start seeing collisions in the hash value that MD5 produces?
Doesn't really matter how likely it is; it is possible. It could happen on the first two things you hash (very unlikely, but possible), so you'll need to support collisions from the beginning.
A rough rule of thumb for collisions is the square-root of the range of values. Your MD5 sig is presumably 128 bits long, so you're going to be likely to see collisions above and beyond 2^64 images.
S3 can have subdirectories you know. Just put a / in the key name, and you can access the files as if they were in separate directories. I use this to store user files in separate folders based on their user ID in S3. Eg "mybucket/users/1234/somefile.jpg". It's not exactly the same as a directory in a filesystem, but the S3 API has some features that let it work almost the same. I can ask it to list all files that begin with "users/1234/" and it will show me all the files in that "directory"
So wait, is it:
md5(filename) + timestamp
or:
md5(filename + timestamp)
If the former, you are most of the way to a GUID, and I wouldn't worry about it. If the latter, then see Karg's post about how you will run into collisions eventually.
While there have been well publicized problems with MD5 due to collisions, UNINTENTIONAL collisions among random data are exceedingly rare. On the other hand, if you are hashing on the file name, that's not random data, and I would expect collisions quickly.
The wikipedia entry on MD5 is pretty helpful here: http://en.wikipedia.org/wiki/MD5
Because of birthday paradox probability of collision is 1/264. This means that to get a collision, on average, you'll need to hash 6 billion files per second for 100 years.
Although random MD5 collisions are exceedingly rare, if your users can provide files (that will be stored verbatim) then they can engineer collisions to occur. That is, they can deliberately create two files with the same MD5sum but different data. Make sure your application can handle this case in a sensible way, or perhaps use a stronger hash like SHA-256.