Last month I was invited by Microsoft to an interview in their
largest european development center in Dublin. It was exciting trip and
real challenge to see the Microsoft development live. I was excited of
their professionalism in all directions:
Professional organization - phone interviews, invitation, flight, accomodation, etc.
Nearly perfect development process - like I read in the software engineering books
Very skillful developers and manager that interview me
Really professional way of conducting interviews
Challengeable product development
I was most excited on the people and process. I think this is what
makes Microsoft so successfull: brilliant people and solid engineering
Microsoft had a small fault. They didn’t ask me to sing any NDA
agreement, so I can share all the interview questions to help all other
candidates that want to join Microsoft.
Interview Questions @ Microsoft Dublin
I had 5 interviewers asking me lots of software engineering
questions. The questions were very adequate for the team leader
position that was my objective (”program manager” position in Microsoft
is senior technical position, like team leader in a typical software
development company). Interviewers was not only asking me to explain
some concept. They gave me a blackboard to write some code and to see
how I am attacking the problems, what types of pictures I draw, how my
thinking flows, etc.
I remember most of the questions and the answers I gave. I hope my
answers were good because I was approved and Microsoft sent me an offer
few days after the interview. Below are the questions with my answers:
Question 1: You need to architecture the security for a bank software. What shall you do?
There is not exact answer here. This is about thinking: follow the
exisitng standards in the banking sectors, establish global security
policy, secure the network infrastructure, secureg the application
servers, secure the database, establish auditing policy, securing the
operators workstations, secure the Internet and mobile banking, etc.
Think about authentication (smart cards), authorization, secure
Question 2: You are given a string. You want to
reverse the words inside it. Example “this is a string” –> “string a
is this”. Design an algorithm and implement it. You are not allowed to
use String.Split. After you are done with the code, test it. What will
you test? What tests you will write?
Elegant solution in 2 steps:
1) Reverse whole the string char by char.
2) Reverse again the characters in each word.
You need to write a method Reverse(string s, int startPos, int endPos).
Test normal cases first (middle of the word, beginning of the word, end
of the word, 1 character only, all leters in the string). Check bounds
(e.g. invalid range). Test it with Unicode symbols (consisting of
several chars). Perform stress test (50 MB string).
Write a method ReverseWords(string s). Test it with usual cases (few
words with single space between), with a single word, with an empty
string, with words with several separators between. Test it with string
containing words with capital letters.
Question 3: What is the difference between black box and white box testing?
Black box testing is testing without seeing the code. Just looking for incorrect bahaviour.
White box testing is about inspecting the code and guessing what can
go wrong with it. Look inside arrays (border problems), loops (off by 1
problems), pointers, memory management (allocate / free memory), etc.
Question 4: What is cross-site scripting (XSS)?
In Web application XSS is when text coming from the user is printed
in the HTMl document without being escaped. This can cause injecting
cookies, logging keyboard, and sensitive data (like credi card numbers).
Question 5: What is SQL injection?
SQL injection is vunerability comming from dynamic SQL created by
concatenating strings with an input comming from the user, e.g. string
cmd = “SELECT * from USERS where LOGIN=’” + login + “‘ and PASS=’” +
password + “‘”. if username has value “‘ OR 1=1 ‘;”, any login /
password will work. To avoid SQL injection use parametric commands or
at least SQL escaping.
Question 6: What is the most challengeable issue with multithreading?
Maybe this is the synchronization and avoiding deadlocks.
Question 7: Explain about deadlocks. How to avoid them.
Deadlock arise when 2 or more resources wait for each other. To
avoid it, be careful. Allocate resources always in the same order.
Question 8: Do you know some classical synchronization problem?
The most important classical problem is “producer-consumer”. You
have several producers and several consumers. Producers produce some
kinf of production from time to time and consumer consume the
production from time to time. We have limited buffer for the
production. When the buffer is full, producers wait until space is
freed. When the buffer is empty, the consumers wait until some
producers put something inside.
Practical use of the producer-consumer pattern is sending 1 000 000 emails (production) with 25 working threads (consumers).
Question 9: You need to design a large distributed
system system with Web and mobile interface. Through the Web customers
subscribe for stock quotes (choosing a ticker and time interval) and
get notified by SMS at their mobile phones about the price for given
tickers and the requested intervals. A web service for getting the
price for given ticker is considered already existing.
Use 3-tier architecture (ASP.NET, C# business tier, SQL Server
database). Use a queue of tasks in the business tier and a pool of
working threads (e.g. 100 threads) that execute the tasks. A task has 2
steps (query for the ticker price and send SMS). These steps are
executed synchronously (with reasonable timeout).
We have another thread that performs SQL query in the database to
get the subscriptions matching the current time and appending tasks for
We consider the SMS gateway is an external system.
Question 10: How you secure the stock quote notification system?
We need to secure all its parts:
1) The user registration process - need to verify phone number with
confirmation code sent by SMS. Need to keep the password with salted
hash. Need to keep the communication through HTTPs / SSL.
2) The application server with business logic. Secure the host, put reasonable limitations to avoid flooding the server.
3) Secure the database (e.g. Windows authentication without using passwords).
4) Secure the network (e.g. use IPSEC)
5) Secure the access to the Web service (WS Security).
6) Secure the mobile phone (e.g. sending encrypted SMS messages and
decrypt them with a proprietary software running on the phone).
Question 11: How you write a distributed Web crawler (Web spider)? Think about Windows Live Search which crawls the Internet every day.
You have a queue of URLs to be processed and asynchronous sockets
that process the URLs in the queue. Each processing has several states
and you describe them in a state machine. Using threads with blocking
sockets will not scale. You can still use multiple threads if you have
multiple CPUs. The Web crawler should be stateleass and keep its state
in the DB. This will allow good scalability.
A big problem is how to distribute the database. It is very, very
large database. The key here is to use partitioning, e.g. by site
domain. Take the site domain, compute a hash code and distribute the
data between the DB nodes based on the hash code. No database server
can store all the pages in Internet, so you should use thousands of DB
servers and partitioning.
Question 12: You have a set of pairs of characters
that gives a partial ordering between the characters. Each pair (a, b)
means that a should be before b. Write a program that displays a
sequence of characters that keeps the partial ordering (topolocial
We have 2 algorithms:
1) Calculate the number of the direct predecessors for each character.
Find a character with no predecessors, print it and remove it. Removing
reduces the number of predecessors for all its children. Repeat until
all characters are printed. If you find a situation where every
character has at least 1 predecessor, this means a loop inside the
graph (print “no solution”). Use Dictionary<string, int> for
keeping the number of predecessors for each character. Use a
Dictionary<string, List<char>> to keep the children for
each character. Use PriorityQueue<char, int> to keep the
characters by usign their number of predecessors as priority. The
running time will be O(max(N*log N, M)) where N is the number of
characters and M is the number of pairs.
2) Create a graph from the pairs. Use recursive DFS traversal starting
from a random vertice and print the vertices when returning from the
recursion. Repeat the above until finished. The topological sorting
will be printed in reversed order. The running time is O(N + M).
Question 13: You are given a coconut. You have
large building (n floors). If you throw the coconut from the first
floor, if can be croken or not. If not you can throw it from the second
floor. With n attempts you can find the maximal floor keeping the
Now you have 2 coconuts. How many attempts you will need to find the maximal floor?
It is a puzzle-like problem. You can use the first coconut and throw
it from floors: sqrt(n), 2*sqrt(n), …, sqrt(n) * sqrt(n). This will
take sqrt(n) attempts. After that you will have an interval of sqrt(n)
floors that can be attempted sequentially with the second coconut. It
takes totally 2*sqrt(n) attempts.
Question 14: You have 1000 campaigns for
advertisments. For each of them you have the returns of investments for
every day in a fixed period of time in the past (e.g. 1 year). The goal
is to visualize all the campaigns in a single graphics or different UI
form so the user can easily see which campaigns are most effective.
If you visualize only one campaign, you can use a classical
bar-chart or pie-chart to show the efficiency at weekly or monthly
basis. If you visualize all campaigns for a fixed date, week or month,
you can also use classical bar-chart or pie-chart. The problem is how
to combine the above.
One solution is to use a bar for each campaign and use different
colors for each week in each bar. For example the first week is black,
followed by the second week, which is 90% black, followed by the third
week which is 80% black, etc. Finally we will have a sequence of bars
and the most dark bars will shows best campaigns while the most bright
bars will show the worst campaingns.
I knew that I was approved even at the interview. It was a good sign
that the manager of the development in Microsoft Dublin for the Windows
Live platform Dan Teodosiu
personally invited me in his office at the end of the interview day to
give me few additional questions and to present me the projects in his
department. Dan is extremely smart person - PhD from Stanford
University, technical director and co-owner of a company acquired by
Microsoft few years ago. It was really pleasure for me to meet him.
There were 2 teams in Dublin that wanted to have me onboard: the
edge computing team working on Windows Live and the Office Tube team
working on video recording and sharing functionality for the Microsoft
Office. I met the manager of the Office Tube team at the end of the
interview day to discuss their products and development process.
I was Offered a Senior Position @ Microsoft Dublin
Few days later I was offered senior software engineering position at
Microsoft in Dublin. I was approved and the recruiters started to talk
with me about my rellocation in Dublin. Few days later I received the
official offer from Microsoft. It was good enough for the average
Dublin citizen but not good enough for me.
I Rejected Working at Microsoft Dublin
Yes, I rejected the Microsoft’s offer to work in their development
center in Dublin. The reason was that their offer was not good enough.
The offer was better than the avegare for the IT industry in Dublin. It
was good offer for a software engineer and if I got it 5-6 years ago I
would probably accept it.
I was working as software engineer for more than 12 years. I am
currentlty CTO and co-owner of a small software development, training
and consulting company and I am a team leader of 3 software projects in
the same time (two Java and one .NET project). In the same time I am a
head of the training activities and I manage directlty more than 10
engineers and of course I am paid several times better than the average
for the industry. In the same time I am part-time professor in Sofia
University. I am about to finish my PhD in computational linguistics. I
have share in few other software companies. All of this was a result of
many years of hard working @ 12+ hours / day.
In Bulgaria I was famous, very well paid, working at own company
with no boss, managing development teams and having very good
perspective for development. To leave my current position, I needed
really amazingly good offer. I got good offer, but not amazingly good.
That’s why I rejected it.
My Experience at Interviews with Microsoft and Google
Few months ago I was interviewed for a software engineer in Google
Zurich. If I need to compare Microsoft and Google, I should tell it in
short: Google sux! Here are my reasons for this:
1) Google interview were not professional. It was like Olympiad in
Informatics. Google asked me only about algorithms and data structures,
nothing about software technologies and software engineering. It was
obvious that they do not care that I had 12 years software engineering
experience. They just ignored this. The only think Google wants to know
about their candidates are their algorithms and analytical thinking
skills. Nothing about technology, nothing about engineering.
2) Google employ everybody as junior developer, ignoring the
existing experience. It is nice to work in Google if it is your first
job, really nice, but if you have 12 years of experience with lots of
languages, technologies and platforms, at lots of senior positions, you
should expect higher position in Google, right?
3) Microsoft have really good interview process. People working in
Microsoft are relly very smart and skillful. Their process is far ahead
of Google. Their quality of development is far ahead of Google. Their
management is ahead of Google and their recruitment is ahead of Google.
Microsoft is Better Place to Work than Google
At my interviews I was asking my interviewers in both Microsoft and
Google a lot about the development process, engineering and
technologies. I was asking also my colleagues working in these
companies. I found for myself that Microsoft is better organized,
managed and structured. Microsoft do software development in more
professional way than Google. Their engineers are better. Their
development process is better. Their products are better. Their
technologies are better. Their interviews are better. Google was like a
kindergarden - young and not experienced enough people, an office full
of fun and entertainment, interviews typical for junior people and lack
of traditions in development of high quality software products.
promised, here follows what I hope will be an interesting overview of
my interview experience as an intern candidate at Google, Microsoft and
My interview at Google was probably the most unusual of the bunch. A
long time ago (Almost a year), one of my friends in the fellow
reverse-engineering community contacted me about a job opporunity at a
Google office in Montreal, working on a top-secret project, but which
was related to my knowledge. I got to the phone screens, and had a
great first interview. My second interview however didn’t go so well.
It was the first time my interviewer was ever screening a candidate,
and he kept me stuck on a single question. The question was related to
a low-level structure change in a private datatype used in Vista’s
kernel; this change was documented in a patent, which I always found
fishy as an interview question. Nevertheless, I believe I answered
correctly some of the more generic implementation details, but the
interviewer kept coming back on the same question and seemed like he
wanted to hear a precise answer. Additionally, it didn’t seem like the
project was fully related to my field of experise; unsurprinsingly, I
got a refusal letter two weeks later.
Fast forward eight months later, and the DRM hacking news appears on
the Internet. I get a call from Google the day after about setting up
some interviews. My interviews get cancelled a couple of days later,
then rescheduled for Monday, after my return from the SCALE 5X talk. I
have a short (and very interesting) conversation with someone at Google
would probably end up being my boss/mentor, and I get news a couple of
days later that I got the job. And that’s about it.
Job Description: Security/Software Engineer/Developer. Code Auditing plus Windows Internals consulting/special projects.
Phone Screens: 1
Campus Visit: No
My path to Apple was a long and ultimately rewarding one. I attended
CUTC last January, already with knowledge that I would be interviewing
with Microsoft later. Therefore, I avoided most of the smaller booths,
avoided Microsoft since I already had an interview, as well as Google
since, at the time, I had not received the phone call about a new
opportunity. The only company that I still had some interest in during
the job fair was Apple. This is mostly because during the day, I
attended two sessions on Apple Development Tools. The first one was on
Shark, which completely amazed me. There were lots of technical
questions during the presentation, and I was always the only one
answering them correctly, so the Apple people noticed me and asked me
to come for a chat. I went to see them, and handed in my CV. The Apple
recruiter was mostly looking for people to work on the Ipod or Mac
stuff, so my Windows Kernel experience didn’t seem relevant at first.
My friends got calls from Apple during the days after, I didn’t. I
gave up on the opporunity since I thought they wouldn’t be interested.
Two weeks later, I get a call from the recruiter saying she passed on
my information to the OS X Kernel Team. After the DRM news, the
Security Team gets interested as well. What followed after was the most
exhausting interviewing process I’ve been through. Because Apple
couldn’t fly me in (I don’t think they do that for non-local
candidates), I had to go through the equivalent of the Microsoft
interview process, but over the phone. Since I was actually
interviewing with two teams, double the amount of time and people for
an accurate depiction. In total, I believe I spoke with 9 or 10 Apple
developers, managers and testers on both teams.
The questions were very technical, but not in the “optimize this
algorithm” way. The engineers there seemed to be genuinely interested
in my ideas, thought process, and solutions/problems I could find to
various designs. One question I was asked, which I think I can share,
is how a Hypervisor Rootkit would be more dangerous then a normal
Kernel Rootkit, how to protect against that, as well as how to create a
workable Hypervisor Patchguard-like system, what to look for, how to
discriminate against the OS touching critical data, and malware, etc.
There also of course the general ReactOS/TinyKRNL questions as well as
questions on my interest for the job/company.
I felt exhausted at the end of about the 1-2 weeks this process
took, but I thought I had done very well on all the interviews. During
my talk in Waterloo, I got a call saying I got offers from both teams,
and had to choose one. I chose the Core OS kernel team, and received my
offer in the mail a couple of days later.
Job Description: Kernel Developer. Working on various Darwin/OS X related undisclosed projects.
Phone Screens: 6, some were conference calls with multiple people on the line.
Campus Visit: No
My path at Microsoft started through various friends and contacts
that I’ve made a the company in the last few months thanks to my
security-related research and presentations/papers. They saw in me a
really good candidate for the various security groups at Mircrosoft,
and also on the actual NT Kernel Team. The interview process at
Microsoft was both disappointing and amazing. First off, it started
with a pretty technical phone screen. Unforunately, my screen was on
SQL, which I knew absolutely nothing about. However, my interviewer was
very understanding, gave me a couple of hints, and I was able to
identify and solve issues with “cursors”, something I had never even
heard of. I was also asked some more generic and personality questions,
and I my opinion of the interview was that I did decently. I was also
interviewed by one of the most prominent figures in SQL, working on the
Core SQL Engine Group at Microsoft, and someone I deeply respect.
This interview let me to an actual invitation for a campus
interview. This is where the disappointing part starts. My phone screen
was sometime in October or November. It took about two months, by
email, to get an actual interview date, and it ended up being in March.
Therefore, even though Microsoft was my first confirmed interview, in
the time frame that it took the mto set something up for me, Apple and
Google had the chance to hear about me, contact me, interview me and
both send me offers. This created a very difficult problem for me in
terms of various deadlines that the other offers had to meet. All in
all, I didn’t feel that my RC (Recruiter Coordinator) was very
communicative with me, and I had to rely on my connections inside the
company to figure out what was going on. Contrast this with Apple which
had everyone on their team calling me (which greatly raised my interest
in the company) and even Google, who had one of their top engineers
chat with me on the phone, and both companies kept in touch by email
and phone relating my status, offers, interviews, etc. Microsoft’s
replies, when available, were always robotic and template files.
However, this disappointement quickly faded away once I got on
campus. Microsoft has the most amazing interviewing experience. First
of all, not only do they pay all your expenses, you also get a generous
amount of money to spend during your daily activites, and you’re
encourage to stay more then one day. Taxis are included, up to 75$ of
food per day is included, museum visits, sightseeing, long-distance
calls, Internet access and more are all free perks you get.
Additionally, before your rounds start, Building 19 has various
computers, big-screen TVs and XBOXes to fill up your time. You can also
visit the campus, and even go see the Microsoft Museum, which has some
unique artifacts you’re likely not to see anywhere else.
Once your interviews start, you’ll meet with a variety of people on
the teams that are interviewing you. There are all very smart people
and each of them has his or her own interviewing style. You’ll probably
start out with coding questions/tricks, and move up to more high-level
implementation/architecture stuff. My final interview was with a hiring
manager, which consisted a lot more of personality and profesional
questions related to work habits, ethics, etc. I like the fact that the
interviews seemed to test every part of the candidate, from your
typical algorithm questions down to your pattern of thinking and
answering hard business problems.
I had a serious issue with my work at Microsoft however. First of
all, the deadlines for my other offers were Monday (and my interviews
on a Friday). Secondly, I needed to know if I could ever work on
ReactOS/TinyKRNL after my internship was over. The only peopel who
could answer this were LCA, the Law and Corporate Affairs deparment of
Microsoft, who are usually pretty hard to get by, especially on a
weekend. I made it clear that two things were critical to me: being
allowed to work on ReactOS after my internship was over, and working on
the Base or Virdian Team.
It turns out I passed my interviews, and my understanding was that
an offer could’ve been offered to me. Unfortunately, I was qualified as
a “legal risk”, and they did not want to go forward with it, due to my
work on ReactOS. It was made clear to me that I would have to choose
between the two. Since this wasn’t a full time employement, and only my
first internship in many to come, I didn’t want to sacrifice the
project for an internship. Who knows if I didn’t like the Base Team? Or
maybe I wanted to work in some other company later on, or maybe
Microsoft would not want me anymore. The restriction of never having to
work on ReactOS again seemed way too harsh — not even non-compete
agreements are this permanent, but regardless, I can understand why
Microsoft chose to do this. I am still very greatful for meeting all
those smart people, and will be keeping in touch with them in the
Job Description: Kernel Developer. Working on the base kernel and/or Virdian.
Phone Screens: 1
Campus Visit: Yes. Five Interviews.
Ultimately, because of the Microsoft situation, my choice became
Apple vs Google. Both companies are dream companies to work for, and it
wasn’t easy choosing between the two. Ultimately however, the work I
would be doing at Apple was a lot more related to my core competencies
(kernel development), and gave me the chance to discover a new
architecture and OS design. I felt like some of my work at Google might
be hindered by their requirement for computational/algorithm experience
and my lack of a formal training in the matter (which won’t come until
my next semesters). Also, Apple’s details about my work (which I can’t
mention) clearly became the defining factor in my decision. The team
size, which is extremly small, meant that my work would have a real
impact on the products/services/etc I’d be working on, and that was
also another great opporunity that I think an internship is good for.
Another important factor was ReactOS, which didn’t seem to hinder at
all my work at Apple, as well as the friendlyness of all the people at
Apple. I am trying to bring my girlfriend over with me for the summer,
and Apple was very forthcoming in helping with this. In the end, the
Apple offer was the most interesting, and the culture/ethics and work
seemed the most adapted to me, as did the helpfulness of everyone
involved in my interview process and offer. I felt like I was really
needed, a truly unique candidate, and that was indeed a great feeling
Please remember that this experience was unique to me; do not
attempt to generalize or make any employement choices based on this
experience, since you will most likely have a different one. I have
however tried my best to avoid giving away any confidential or private
information, so please do not ask/make comments on my offer, and perks
offered, et caetera, because I will not discuss them.
I strongly recommend anyone with the opportunity to work at any of
these three companies to take it, if their interest in the work they’ll
be doing is high. They are all amazing companies that I’d love to work
for during my life.
If there’s one lesson that I want to share from my experience it’s
this: go with your interests. Don’t be amazed by perks, salaries, or
other material things. Does the campus/team seem a good match? Does the
work interest you? If yes, everything else should be secondary.