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Question 6 is a trap, a very useful one

In my interview questions, I give candidates a list of 6 questions. They can either solve 3 questions from 1 to 5, or they can solve question 6.

Stop for a moment and ponder that. What do you assume that relative complexity of those questions?

Questions 1 –5 should take anything between 10 – 15  minutes to an hour & a half, max. Question 6 took me about 8 hours to do, although that included some blogging time about it.

Question 6 requires that you create an index for a 15 TB CSV file, and allow efficient searching on it.

While questions 1 – 5 are basically gate keeper questions. If you answer them correctly, we’ve a high view of you and you get an interview, answering question 6 correctly pretty much say that we past the “do we want you?” and into the “how do we get you?”.

But people don’t answer question 6 correctly. In fact, by this time, if you answer question 6, you have pretty much ruled yourself out, because you are going to show that you don’t understand something pretty fundamental.

Here are a couple of examples from the current crop of candidates. Remember, we are talking about a 15 TB CSV file here, containing about 400 billion records.

Candidate 1’s code looked something like this:

foreach(var line in File.EnumerateAllLines("big_data.csv"))
{
       var fields = line.Split(',');
       var email = line[2]
       File.WriteAllText(Md5(email), line);
}

Plus side, this doesn’t load the entire data set to memory, and you can sort of do quick searches. Of course, this does generate 400 billion files, and takes more than 100% as much space as the original file. Also, on NTFS, you have a max of 4 billion files per volume, and other FS has similar limitations.

So that isn’t going to work, but at least he had some idea about what is going on.

Candidate 2’s code, however, was:

// prepare
string[] allData = File.ReadAllLines("big_data.csv");
var users = new List<User>();
foreach(var line in allData)
{
     users.Add(User.Parse(line));
}
new XmlSerializer().Serialize(users, "big_data.xml");

// search by:

var users = new XmlSerialize().Deserialize("big_data.xml") as List<User>()
users.AsParallel().Where(x=>x.Email == "the@email.wtf");

So take the 15 TB file, load it all to memory (fail #1), convert all 400 billion records to entity instances (fail #2), write it back as xml (fail #3,#4,#5). Read the entire (greater than) 15 TB XML file to memory (fail #6), try to do a parallel brute force search on a dataset of 400 billion records (fail #7 – #400,000,000,000).

So, dear candidates 1 & 2, thank you for making it easy to say, thanks, but no thanks.

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