ChallengeGenerate matching shard id–answer

time to read 2 min | 371 words

I posed a potential problem for a job interview. Given the following function, generate another key that has the same shard:

In other words, give “users/71” and a prefix of “orders/654”, generate a key that would be placed on the same shard as “users/71”. The answer in this case can be: “orders/654-vaueaa”.

In order to answer the question, we need to understand what is going on here. The function above is a fancy way to extract 16 bits of information from the key using a cryptographically hash function. MD5 is no longer considered secured, but given the fact that I’m needing just 16 bits, that is not an issue. The code above is slightly more complex than needed, I could simplify it to this and have the same effect (but not the same result, mind):

The need to generate a matching shard id is another way to say that we need a hash collision. Given that the key space is 2^16, and that we can assume that any mutation to the key will result effectively random changes to the result, we can simply generate different keys and try to see if they match. Here is a simple way to do so:

We are effectively throwing a dice and seeing if this match. So it is a probability game to wait until we have a collision. The actual implementation isn’t that important, what is interesting is to talk about the implications here:

  • Are there better ways to go about doing something like this? Not really, given that MD5 isn’t that broken.
  • How much time will it take to generate a shard id match? The answer, usually around 64K tries. But why is interesting. The birthday attack issues don’t play here, because we don’t need to match to multiple items, just one. So we role the dice and see if we match on the value.
  • Can we speed this up? Using a different hash function would probably help, yes.
  • What other ways do we have to handle this? Different shard id generation would allow much better alternative.

The last question is where we get into more interesting details about system design, ergonomics of the choices we make and get to see how the candidate actually thinks.

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