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This is a great interview question I don't want to be 'that guy' by picking at what may be a detail but there is another response to look for from a candidate.

This is a great interview question I don't want to be 'that guy' by picking at what may be a detail but there is another response to look for from a candidate. We should think very carefully about adding a multiplication command because it introduces a failure mode that may be unanticipated by the client. Code that previously worked could begin to fail after this command goes into use. Specifically, if the client needs to revert a series of operations on integers, and if the operations are transitive, there is no need for a mechanism to ensure they occur in any particular order (the usual caveat about working within the limits of precision applies.) This holds true for addition and for multiplication, in isolation, but is not true if they are combined. Change the order and the end result will change. Adding multiplication puts a burden on the client to understand this risk and be explicit in the ordering. Some people may argue that the client should already understand it and they have a point. We can't defend against every possible misunderstanding. I think there is a good discussion to be had on this question and if the candidate were to go there, it would be a favorable sign of experience on their part.

jameshart 4y ago
This is a REALLY SOLID response to the question, and I'd hope it would get some strong 'hire' points. But, to be fair, there's already technically this issue in the memcached API as described in the post, in that it supports both append and add, and "append 0" (on a value that add could also act upon) is effectively the same thing as "mult 10". If a field contains "1" and successive "add 1" and "append 0" commands come in, depending on what order they arrive in the result could be 20 or 11. So I think the interviewer would be justified in saying 'yeah, let's assume we've evaluated that risk and we plan on adding some really thorough documentation warning people about that risk, so can you just go ahead and try and implement it?' But absolutely, this was the thought that came into my mind when reading the spec too. Not enough developers think about APIs in terms of compositional, algebraic terms, and being able to see that adding 'multiply' and 'add' together at the same precedence in an API might cause trouble is a really valuable skill.
- akhmatova 4y ago
  This is a REALLY SOLID response to the question, and I'd hope it would get some strong 'hire' points. I agree -- but unfortunately the interview question doesn't filter for this level of thinking. If anything, it filters for the exact opposite: your ability and willingness to shove a random new feature into the codebase in 3 hours (or GTFO) -- stability and other consequences be damned.
  - db48x 4y ago
    You can’t expect every question to give you an opportunity to show off every skill that you have. If I were asked this question, I would certainly comment on whether it seemed like a smart thing to do, but I wouldn’t refuse to do it. I can demonstrate both my ability to do some software archeology, and do some coding, and figure out the design of memcached first, and then also mention to the interviewer that I had reservations about the soundness of the operation. The interviewer may even have more questions to ask along those lines, and might be disappointed in candidates who don’t bring it up. But they would certainly be disappointed in candidates who don’t take the opportunity to show off the programming skills that they have put on their resume.
    - akhmatova 4y ago
      As long as we agree that, at the end of the day -- it's basically a crapshoot, as to what you can really tell about the other person based on whether they answer these kinds of questions on time and in as much detail as you like. That is -- I'm not saying these questions don't tell you anything about the candidate. But at the end of the day ... I just don't think they tell you that much. Certainly not in the divining-rod, "finally I found a question that will sniff out the true h@ck3rz from the wannabe drudges" sense that people seem to think questions of this sort are imbued with.
      - db48x 4y ago
        > “question that will sniff out the true h@ck3rz from the wannabe drudges” No one ever said that it would. No single question can tell you everything you want to know about a candidate. This question is designed to weed out candidates who talk well but can’t actually do the work. It won’t tell you which of those passing candidates are great and which are merely good; that’s what the rest of the interview is for. I agree that interviewing is, unfortunately, a “crapshoot” for the candidates. As a candidate you are going to interact with dozens or hundreds of companies, and most of them won’t do a good job of interviewing you. Most of them end up with more candidates than they can really handle, so they end up passing up plenty of good prospects. But I disagree that this question is a “crapshoot”; it gives you specific information that you really want about each candidate, and it does so without a lot of the irritating artificiality we often take for granted in interview situations.
bombcar 4y ago
There's another potential issue that may or may not matter - since the value is stored as text it probably can't overflow in memcached - but the multiplication COULD overflow internally in the C code; addition could do this also - I would wonder how it's implemented.
jodrellblank 4y ago
Having just gone through this question for fun, the codebase DECR command has this clipping code in it to avoid the value going negative: if(delta > value) { value = 0; } else { value -= delta; } A caller reverting operations by sending the same values with the operations flipped around must already keep track that they didn't ask to drop below 0, or they may not get back to the original value. Plus, the atomic update happens with a lock/release between each operation, so while you might get the same result at the end of your rearranged ordering, clients may see intermediate results and changing the order would change which values they see, which may or may not matter.
jseban 4y ago
This is what experience and skill looks like, careful consideration and really trying to understand what you are doing. Too bad it's so difficult to market this in a superhero genius package that managers will buy
geocar 4y ago
I agree it's a great interview question, but I would never ask anyone to spend three hours doing this for free. Someone comes to me and says they can program, and that's that. They have other employers and coworkers in their life, or just teachers and other students -- people I can ask what it's like to work with them if I want, but at the end of the day, I'll know if I've been lied to in the first 30 days and neither of us want that, so I think all I'm trying to find out at the interview-stage is whether I want to spend 30 days with this person, and I don't need to study the outcome of three-hours of them guessing at what I want to help me do that. But I like to talk about programming, and this question just creates so many different kinds of ideas in my head too. I like how you immediately think about ordering of events and the implications of that. Maybe memcached needs a division operator too. I think that's fun to talk about. Maybe overflows are important; Maybe the task is to add some new types to memcached to protect against that. Maybe we should add some stats to track the number of overflows (or otherwise give some estimate of the accuracy). Now I am looking at the slide-rule on my desk and wondering what the required precision for the use-case is; That is, is it possible that instead of modifying memcached and having to support your freaky-version forever that you can simply instruct the application to increment by log multiplied out by the desired precision, then reverse with division+exp on output? And now I am thinking about supportability: Once we've decided what we want out of memcached, is it worth trying to get those changes into the upstream memcached so we don't have to worry so much about that feature disappearing (or becoming difficult in the future)? Talking to people about the social aspects of programming with open-source can be important too. So yeah, lots of reasons to like this question. I'm probably going to use some variant of it myself, because wherever the candidate goes with it is going to be informative, but I'm extremely disappointed by the rest of the process (and all of part 2); it's definitely not for me.
- mchusma 4y ago
  I don't know the rest of the process, but 3 hours for an interview seems 100% fine. And practical tests are so much better than random abstract questions IMO. I think your solution is to watch and plan on firing many people within 30 days, which seems worse for everyone.
  - 4y ago
red_trumpet 4y ago
Do you mean "associative" instead of "transitive"?
- davidst 4y ago
  You are right! This is why I appreciate HN so much. I don't see how to edit my comment but thank you for the correction.
  - baud147258 4y ago
    you can only edit your comments for a limited duration, like one hour. Otherwise open the link for that comment (on the date) and there should be an edit link next to the parent link.

jameshart 4y ago

This is a REALLY SOLID response to the question, and I'd hope it would get some strong 'hire' points. But, to be fair, there's already technically this issue in the memcached API as described in the post, in that it supports both append and add, and "append 0" (on a value that add could also act upon) is effectively the same thing as "mult 10". If a field contains "1" and successive "add 1" and "append 0" commands come in, depending on what order they arrive in the result could be 20 or 11. So I think the interviewer would be justified in saying 'yeah, let's assume we've evaluated that risk and we plan on adding some really thorough documentation warning people about that risk, so can you just go ahead and try and implement it?' But absolutely, this was the thought that came into my mind when reading the spec too. Not enough developers think about APIs in terms of compositional, algebraic terms, and being able to see that adding 'multiply' and 'add' together at the same precedence in an API might cause trouble is a really valuable skill.

akhmatova 4y ago
This is a REALLY SOLID response to the question, and I'd hope it would get some strong 'hire' points. I agree -- but unfortunately the interview question doesn't filter for this level of thinking. If anything, it filters for the exact opposite: your ability and willingness to shove a random new feature into the codebase in 3 hours (or GTFO) -- stability and other consequences be damned.
- db48x 4y ago
  You can’t expect every question to give you an opportunity to show off every skill that you have. If I were asked this question, I would certainly comment on whether it seemed like a smart thing to do, but I wouldn’t refuse to do it. I can demonstrate both my ability to do some software archeology, and do some coding, and figure out the design of memcached first, and then also mention to the interviewer that I had reservations about the soundness of the operation. The interviewer may even have more questions to ask along those lines, and might be disappointed in candidates who don’t bring it up. But they would certainly be disappointed in candidates who don’t take the opportunity to show off the programming skills that they have put on their resume.
  - akhmatova 4y ago
    As long as we agree that, at the end of the day -- it's basically a crapshoot, as to what you can really tell about the other person based on whether they answer these kinds of questions on time and in as much detail as you like. That is -- I'm not saying these questions don't tell you anything about the candidate. But at the end of the day ... I just don't think they tell you that much. Certainly not in the divining-rod, "finally I found a question that will sniff out the true h@ck3rz from the wannabe drudges" sense that people seem to think questions of this sort are imbued with.
    - db48x 4y ago
      > “question that will sniff out the true h@ck3rz from the wannabe drudges” No one ever said that it would. No single question can tell you everything you want to know about a candidate. This question is designed to weed out candidates who talk well but can’t actually do the work. It won’t tell you which of those passing candidates are great and which are merely good; that’s what the rest of the interview is for. I agree that interviewing is, unfortunately, a “crapshoot” for the candidates. As a candidate you are going to interact with dozens or hundreds of companies, and most of them won’t do a good job of interviewing you. Most of them end up with more candidates than they can really handle, so they end up passing up plenty of good prospects. But I disagree that this question is a “crapshoot”; it gives you specific information that you really want about each candidate, and it does so without a lot of the irritating artificiality we often take for granted in interview situations.

bombcar 4y ago

There's another potential issue that may or may not matter - since the value is stored as text it probably can't overflow in memcached - but the multiplication COULD overflow internally in the C code; addition could do this also - I would wonder how it's implemented.

jodrellblank 4y ago

Having just gone through this question for fun, the codebase DECR command has this clipping code in it to avoid the value going negative: if(delta > value) { value = 0; } else { value -= delta; } A caller reverting operations by sending the same values with the operations flipped around must already keep track that they didn't ask to drop below 0, or they may not get back to the original value. Plus, the atomic update happens with a lock/release between each operation, so while you might get the same result at the end of your rearranged ordering, clients may see intermediate results and changing the order would change which values they see, which may or may not matter.

jseban 4y ago

This is what experience and skill looks like, careful consideration and really trying to understand what you are doing. Too bad it's so difficult to market this in a superhero genius package that managers will buy

geocar 4y ago

I agree it's a great interview question, but I would never ask anyone to spend three hours doing this for free. Someone comes to me and says they can program, and that's that. They have other employers and coworkers in their life, or just teachers and other students -- people I can ask what it's like to work with them if I want, but at the end of the day, I'll know if I've been lied to in the first 30 days and neither of us want that, so I think all I'm trying to find out at the interview-stage is whether I want to spend 30 days with this person, and I don't need to study the outcome of three-hours of them guessing at what I want to help me do that. But I like to talk about programming, and this question just creates so many different kinds of ideas in my head too. I like how you immediately think about ordering of events and the implications of that. Maybe memcached needs a division operator too. I think that's fun to talk about. Maybe overflows are important; Maybe the task is to add some new types to memcached to protect against that. Maybe we should add some stats to track the number of overflows (or otherwise give some estimate of the accuracy). Now I am looking at the slide-rule on my desk and wondering what the required precision for the use-case is; That is, is it possible that instead of modifying memcached and having to support your freaky-version forever that you can simply instruct the application to increment by log multiplied out by the desired precision, then reverse with division+exp on output? And now I am thinking about supportability: Once we've decided what we want out of memcached, is it worth trying to get those changes into the upstream memcached so we don't have to worry so much about that feature disappearing (or becoming difficult in the future)? Talking to people about the social aspects of programming with open-source can be important too. So yeah, lots of reasons to like this question. I'm probably going to use some variant of it myself, because wherever the candidate goes with it is going to be informative, but I'm extremely disappointed by the rest of the process (and all of part 2); it's definitely not for me.

mchusma 4y ago
I don't know the rest of the process, but 3 hours for an interview seems 100% fine. And practical tests are so much better than random abstract questions IMO. I think your solution is to watch and plan on firing many people within 30 days, which seems worse for everyone.
- 4y ago

red_trumpet 4y ago

Do you mean "associative" instead of "transitive"?

davidst 4y ago
You are right! This is why I appreciate HN so much. I don't see how to edit my comment but thank you for the correction.
- baud147258 4y ago
  you can only edit your comments for a limited duration, like one hour. Otherwise open the link for that comment (on the date) and there should be an edit link next to the parent link.