Top MySQL DBA interview questions (Part 2)

Also find Sean Hull’s ramblings on twitter @hullsean.

Continuing from our Top MySQL DBA interview questions (Part 1) here are five more questions that test a MySQL DBA’s knowledge, with two that will help suss out some personality traits.

  1. Disk I/O
  2. Disk performance should be an ever present concern to a DBA.  So although they don’t need to be a storage specialist, they should have a working knowledge.  Ask them about RAID versions, mirroring versus striping, and so forth.  Mirroring combines two disks as a unit.  Every write is duplicated on both disks.  If you lose one disk, you have an immediate copy.  Like a tandem truck that has spare tires running in parallel.  Lose one, and you don’t have to pull over immediately to replace it.  Striping spreads I/O over multiple disks so you on the one hand increase throughput linearly as you add disks.  That’s because you have more disks working for you.  At the same time you increase risk with each new disk you add, because the failure rate is then the sum total of all those disks.

    For relational databases the best RAID level is 10, which is striping over mirrored sets.  You use more disks, but disks are cheap compared to the hassle of any outage.

    If you’re deploying on Amazon, your candidate should be familiar with the Elastic Block Storage offering also known as EBS.  This is virtualized storage, so it introduces a whole world of operational flexibility.  No longer do you have to jump through hoops to attach, add or reconfigure storage on your servers.  It can all be done through command line API calls.  That said EBS suffers from variability problems as with any other shared resource.  Although Amazon guarantees your average throughput, the I/O you get at a given time can swing wildly from low to high.  Consider Linux software RAID across multiple EBS volumes to mitigate against this.

  3. How would you setup master/slave & master/master replication?
  4. A basic replication setup involves creating a full dump of the primary database, while it’s tables are locked.  The DBA should capture the master status, logfile & position at that time.  She should then copy the dump file to the secondary machine & import the full dump.  Finally the CHANGE MASTER TO statement should be run to point this database instance to it’s master.  Lastly START SLAVE should be issued.  If all goes well SHOW SLAVE STATUS should show YES for both of these status variables:

    Slave_IO_Running: Yes
    Slave_SQL_Running: Yes

    Master-master replication is similar, except one additional step.  After the above steps have run, you know that your application is not pointing at the slave database.  If you’re not sure, verify that fact first.  Now determine the logfile name & position on the slave with SHOW MASTER STATUS.  Return to the primary box, and run the CHANGE MASTER TO command to make it slave from the secondary box.  You’ve essentially asked MySQL to create a circular loop of replication.

    How does MySQL avoid getting into an infinite loop in this scenario?  The server_id variable must be set, and be unique for all MySQL instances in your replication topology.

    For extra credit, ask the candidate about replication integrity checking.  As important as this piece is to a solid reliable replication setup, many folks in the MySQL world are not aware of the necessity.  Though replication can be setup, and running properly, that does not mean it will keep your data clean and perfect.  Due to the nature of statement based replication, and non-deterministic functions and/or non-transactional tables, statements can make their way into the binary logs, without completing.  What this means is they may then complete on the slave, resulting in a different row set on the same table in master & slave instance.

    Percona’s pt-table-checksum is the preventative tool to use.  It can build checksums of all your tables, and then propagate those checksums through replication to the slave.  An additional check can then be run on the slave side to confirm consistency, or show which rows & data are different.

  5. How are Users & Grants different in MySQL than other DBs?
  6. Creating a grant in MySQL can effectively create the user as well.  MySQL users are implemented in a very rudimentary fashion.  The biggest misunderstanding in this area surrounds the idea of a user.  In most databases a username is unique by itself.  In MySQL it is the *combination* of user & hostname that must be unique.  So for example if I create user sean@localhost, sean@server2 and sean@server3, they are actually three distinct users, which can have distinct passwords, and privileges.  It can be very confusing that sean logging in from the local command line has different privileges or password than sean logging in from server2 and server3.  So that’s an important point.

  7. How might you hack a MySQL server?
  8. This is a good opportunity for the candidate to show some creativity with respect to operations and Linux servers.  There are all sorts of ways into a database server:

    a.bad, weak or unset passwords
    b.files with incorrect permissions – modifying or deleting filesystem files can take a database down or corrupt data
    c.intercepting packets – could reveal unencrypted data inside the database
    d.unpatched software – bugs often reveal vulnerabilities that allow unauthorized entry
    e.moving, disabling or interrupting the backup scripts – a possible timebomb until you need to restore
    f.DNS spoofing, could allow login as a different user
    g.generous permissions – may allow an unprivileged user access to protected data

    There are endless possibilities here.  Listening for creative thinking here, reveals how much that person will think thoroughly and effectively about protecting your systems from those same threats.

  9. Brain teasers, riddles and coding problems
  10. Google for a long time was a fan of these types of tests at interviews, but I’m not at all.  For one thing you filter for good test takers, and for another the candidate has no resources – either books or the internet at their disposal.

    Why not instead ask them to tell a story.  Storytelling conveys a lot of things.  It conveys a bit of teaching ability, which extends far beyond internalizing some multiple choice questions.  It tells you more about their personality, which as I’ve said is very important.  It shows how they solve problems, as they’ll take you through their process.  And gives them an opportunity to tell you a real world triumph they presided over.

Personality Questions

In my experience, some of the most important traits of a new hire center around personality traits, and how they might mix with your existing team.  Being punctual for an interview, for instance sets a precedent for many things.  But that door swings both ways, so if you want to hire these types of folks, don’t keep them waiting either!

Pay attention to whether or not the candidate takes some lead in the conversation at all.  This can indicate the person is a self starter.  Obviously a great candidate will also listen carefully and patiently to what you have to say, but may then take the ball and run with it somewhat.

Listen for signals that the person is active in the field, posting on forums, and attending conferences, meetups and forums on technology topics.  You might also ask them if they blog, and what topics interest them.

Top MySQL DBA interview questions (Part 1)

MySQL DBA interview questions

Join 38000 others & follow Sean Hull on twitter @hullsean.

MySQL DBAs are in greater demand now than they’ve ever been. While some firms are losing the fight for talent, promising startups with a progressive bent are getting first dibs with the best applicants. Whatever the case, interviewing for a MySQL DBA is a skill in itself so I thought I’d share a guide of top MySQL DBA interview questions to help with your screening process.

It’s long and detailed with some background to give context so I will be publishing this in two parts.

The history of the DBA as a career

In the Oracle world of enterprise applications, the DBA has long been a strong career path. Companies building their sales staff required Peoplesoft or SAP, and those deploying the financial applications or e-business suite needed operations teams to manage those systems. At the heart of that operations team were database administrators or DBAs, a catchall title that included the responsibility of guarding your businesses crown jewels. Security of those data assets, backups, management and performance were all entrusted to the DBA.

In the world of web applications, things have evolved a bit differently. Many a startup are driven only by developers. In those smaller shops, Operations tasks are designated to one developer who takes on the additional responsibility of managing systems. In that scenario, Operations or DBA duties becomes a sort of secondary role to the primary one of building the application. Even in cases where the startup creates a specific operations role with one person managing systems administration, chances are they don’t also have DBA experience. Instead, these startups are more likely to manage the database as a typical Linux application.

When I grow up I (don’t) want to be a MySQL DBA

Where do they come from, and why don’t a lot of computer science folks gravitate towards operations, and DBA? This may be in part due to and the romance of certain job roles which we discussed in a past article, The Mythical MySQL DBA. This pattern appeared a lot in the Oracle world as well. Many folks who were career DBAs actually moved to that role from the business side. In fact you’d find that many didn’t have a computer science or engineering background in the first place. In my experience I saw many Linux and Unix administrators with a stronger foundation who would fit into the DBA role but were simply not interested in it. The same can be said of the MySQL side of the house. Computer science grads don’t get out of school aiming for a career in ops or as a DBA because it has never been regarded as the pinnacle. It’s typically the PROGRAMMERS who become the rockstars in a cool startup.

But as the Internet grows into a richer and more complex medium, things are changing. People talk about scalability, high availability, zero downtime and performance tuning. When brief outages cost millions in losses expectations are very high and that requires skilled, experienced DBAs.

We’ve made a list of comprised of skill questions, general questions and ‘good-to-know’ questions. Have fun grilling your candidate with them, although bear in mind that with interviews it’s not about knowing it all, rather how the person demonstrates critical thinking skills.

Skills Questions

  1. Why are SQL queries so fundamental to database performance?
  2. This is the one question which a DBA should have an answer to. If they can’t answer this question, they’re unlikely to be a good candidate.

    After a MySQL server is setup and running, with many of the switches and dials set to use memory, and play well with other services on the Linux server, queries remain an everyday challenge. Why is this?

    SQL queries are like little programs in and of themselves. They ask the database server to collect selections of records, cross tabulate them with other collections of records, then sort them, and slice and dice them. All of this requires MySQL to build temporary tables, perform resource intensive sorts and then organize the output in nice bite size chunks.

    Unfortunately there are many ways to get the syntax and the results right, yet not do so efficiently. This might sound like a moot point, but with modern websites you may have 5000 concurrent users on your site, each hitting pages that have multiple queries inside them.

    What makes this an ongoing challenge is that websites are typically a moving target, with business requirements pushing new code changes all the time. New code means new queries, which pose ongoing risks to application stability.

  3. Indexes – too many, too few; what’s the difference?
  4. Indexes are very important to the smooth functioning of a relational database. Imagine your telephone book of yore. I can look up all the people with last name of “Hull” in Manhattan because I have the proper index. But most yellow pages don’t include an index for *first* names even though they might occaisionally come in handy, for example with the names “Star” or “Persephone”.

    You can imagine that, if you had a phone book which you maintain and update, everytime you add or remove a name you also have to update the index. That’s right, and the same goes for your relational database.

    So therein lies the trade off, and it’s an important one. When you are *modifying* your data, adding, updating or removing records, you must do work to keep the index up to date. More indexes mean more work. However when you’re looking up data or *querying* in SQL speak, more indexes mean more ways of looking up data fast. One more trade off is that indexes take up more pages in your phonebook, and so too they take up more space on disk.

  5. Backup & Recovery – explain various types & scenarios for restore
  6. Backups come in a few different flavors that the DBA should be familiar with.

    Cold backups involve shutdown down the database server (mysqld) and then backing up all the data files by making a copy of them to another directory. To be really thorough, the entire datadir including binlogs, log files, /etc/my.cnf config file should also be backed up. The cold backup is a database in itself, and can be copied to an alternate server and mounted as-is.

    Logical backups involve using the mysqldump tool. This locks tables while it runs to maintain consistency of changing data, and can cause downtime. The resulting dump file contains CREATE DATABASE, CREATE TABLE & CREATE INDEX statements to rebuild the database. Note the file itself is not a database, but rather a set of instructions which can tell a MySQL server *HOW* to reconstruct the database. Important distinction here.

    Hot backups are a great addition to the mix as they allow the physical database data files to be backed up *WHILE* the server is up and running. In MySQL this can be achieved with the xtrabackup tool, available from Percona. Despite the name, it works very well with MyISAM and InnoDB tables too, so don’t worry if you’re not using xtradb tables.

    There are a few different restore scenarios, and the candidate should be able to describe how these various backups can be restored, and what the steps to do so would be. In addition they should understand what point-in-time recovery is, and how to perform that as well. After restoring one of the above three backup types, the DBA would use the mysqlbinlog utility to apply any subsequent transactions from the binary logs. So if the backup was made at 2am last night, and you restore that backup, the mysqlbinlog tool would be used to dig up transactions since 2am, and apply them to that restored database.

  7. Troubleshooting Performance
  8. Since this is an ongoing challenge with relational databases, a good grasp of it is crucial. One way to challenge the candidate would be to describe a recent performance problem you experienced with your infrastructure, and ask them how they would go about resolving it.

    If they struggle with the particulars of what you ran into, ask them to describe a big performance challenge they solved, what the cause was, and how they performed analysis.

    Typically, first steps involve mitigating the immediate problem by finding out what changed in the environment either operationally or code changes. If there is a bug that was hit, or other strange performance anomaly, the first stop is usually looking at log files. MySQL server error logs, and the slow query log are key files. From there, analyzing those files during the timeframe where problems occurred should yield some clues.

    You might also hope to hear some comment about metrics collection in this discussion. Tools such as cacti, munin, opennms, or ganglia are invaluable tools for drilling down on a past event or outage, and sifting through server stats to find trouble.

  9. Joins – describe a few kinds and how the server performs them
  10. A basic understanding of INNER JOIN and OUTER JOIN would be a great start. A simple example might be employees and departments. If you have four employees and two departments, an INNER JOIN of these tables together will give you the departments employees belong to. Add another employee without assigning her to a department, and the inner join won’t display her. Further adding a new department which doesn’t yet contain employees won’t display either. However performing an OUTER JOIN will give you those matches with null in the department field, and null in the employee field respectively.

    Thought of with another example, take a credit card company. One tables contains cardholders identity, their number, address, and other personal information. A second table contains their account activity. When they first join, they don’t have any monthly statements, so an INNER JOIN of cardholders with statements will yield no rows. However an OUTER JOIN on those two tables will yield a record, with a null for the statements columns.

Feeling like a MySQL expert yet? In Part 2 of Top MySQL DBA Interview Questions we’ll walkthrough four more questions plus a bonus.

The Problem with Startup Bootcamps

instant startups

Scanning Crains NY Business recently, I saw an article on ‘starting up’ in 54 hours.  It’s the brainchild of Marc Nager, Clint Nelsen and Franck Nouyrigat called Startup Weekend. Startup bootcamps seem to be the current extra-curricular activity of choice these days. Wharton is also getting in on it with their Innovation Tournament. Then there is the 48 Hour Startup  and of course let’s not forget the 3 Day Startup.

So what’s my beef?  Truth be told I admire the ambition, the optimism, and the openness of these efforts.  And for sure these bootstrapping marathons do introduce entrepreneurs to future colleagues and partners, get them asking the right questions about financing, customers, revenue, competition and so forth.

My problem with these events is they frame startups as something you *can* do quickly. As if it were a Lego set or pop-up book that gives instant results and gratification. Sure startups are 21st century tech-driven business that provide innovative products in a very short development cycle but a lot of the day-to-day running of the business are still very mundane 20th century sensibilities; not unlike running a mom and pop store, a laundromat, deli or sandwich shop.

Continue reading “The Problem with Startup Bootcamps”

The Mythical MySQL DBA

I’ve  been getting more than my fair share of calls from recruiters of late. Even in this depressed economic climate where jobs are rarer than a cab at rush-hour, it’s heartening to know that tech engineers are in great demand. And it’s even more heartening to think that demand for MySQL DBAs has never been better.

My reckoning was confirmed by a Bloomberg news report about stalwart retailers suffering from a dearth of talented engineers. Bloomberg cited Target’s outage-prone e-commerce site as a symptom of, among other things the market’s shortage. One of the challenges old-timers like Target face is having to compete with Silicon Valley startups as a fulfilling and ultimately, financially rewarding place to work. Continue reading “The Mythical MySQL DBA”

How to hire a developer that doesn’t suck

Strip by Randall Munroe;

First things first. This is not meant to be a beef against developers. But let’s not ignore the elephant in the living room that is the divide between brilliant code writers and the risk averse operations team.

By the way we also have a MySQL DBA Interview Questions article which is quite popular.

Also take a look at our AWS & EC2 Interview questions piece.

Lastly we have a great Oracle DBA Hiring Guide.

It is almost by default that developers are disruptive with their creative coding while the guys in operations, those who deploy the code, constantly cross their fingers in the hope that application changes won’t tilt the machine. And when you’re woken up at 4am to deal with an outage or your sluggish site is costing millions in losses, the blame game and finger-pointing starts.

Also: Does Amazon have a dirty little secret?

If you manage a startup you may be faced with this problem all the time. You know your business, you know what you’re trying to build but how do you find people who can help you build and execute your ideas with minimal risk?

Join 38,000 others and follow Sean Hull on twitter @hullsean.

Ideally, you want people who can bridge the mentality divide between the programmers eager to see feature changes, the business units pushing for them, and the operations team resistant to changes for the sake of stability.

DevOps – Why can’t we all live together?

The DevOps movement is an attempt to bring all these folks together. For instance by providing insight to developers about the implications of their work on performance and availability, they can better balance the onslaught of feature demands from users with the business’ need for up-time.

Also: The art of resistence or when you have to be the bad guy

Operations teams can work to expose operational data to the development teams.  Metrics collection and analytics aren’t just for the business units anymore. Employing tools like Cacti, OpenNMS or Ganglia allow you to communicate with developers and other business units alike about up-time, and the impact of deployments on site availability, and ultimately the bottom-line.

Above all, business goals and customer needs should underscore everything the engineering team is doing. Bringing all three to the table makes for a more cohesive approach that will carry everyone forward.

How to spot a DevOps person – Finding the sweet spot

The DevOps person is someone with the right combination of skill, knowledge and experience that places him or her in the sweet spot where quality assurance, programming skills and operations overlap.
There are also a few distinguishing characteristics that will help identify such an ideal candidate.

We also wrote a more general piece – What is Devops and why is it important.

Look for good writers and communicators

Imagine the beads of sweat forming when a developer tells you: “We’ve made the changes. Nothing is broken yet.”
This is like stepping on glass because it implies something will actually break. The point is savvy developers should be aware that the majority of people do not think along the same lines as they do.
Assuming your candidate has all the required technical skills, a programmer with writing skills tends to be better at articulating ideas and methods coherently. He or she would also be less resistant to documentation and be able to step back somewhat from the itty-bitty details. Communication, afterall is at the core of the DevOps culture where different sides attempt to understand each other.

Related: What’s the 4 letter word dividing dev & ops?

Pick good listeners

Even rarer than good writers are good listeners. Being able to hear what someone else is saying, and reiterate it in their own terms is a key important quality. In our example, the good listener would probably have translated ‘nothing is broken yet’ into “the app is running smoothly. We didn’t encounter any interruptions but we’ll keep watch on things.”

Lean towards pragmatists and avoid the fanatics

We all want people who are passionate about something but when that passion morphs into fanaticism it can be unpleasant. Fanaticism suggests a lower propensity to compromise. Such characters are very difficult to negotiate with. Analogously in tech, we see people latch on to a certain standard with unquestioning loyalty that’s bafflingly irrational. Someone who has had their hand in many different technologies is more likely to be technology agnostic, or rather, pragmatic. They’ll also have a broader perspective, and are able to anticipate how those technologies will play together.  Furthermore a good sense of where things will run smoothly and where there will be friction is vital.

Related: When you have to take the fall

Pay attention to extra-curricular activities

Look at technology interests, areas of study, or even outside interests. Does the person have varying interests and can converse about different topics?  Do they tell stories, and make analogies from other disciplines to make a point?  Do they communicate in jargon-free language you can understand?

Sniff out those hungry for success

As with any role, finding someone who is passionate and driven is important.  Are they on-time for appointments? Did they email you the information you requested? Are they prepared and communicative?  Are they eager to get started?

Hiring usually focuses on skills and very well-crafted resumés but why do you still find some duds now and then? By emphasizing personality, work ethics, and the ability to work with others, you can sift through the deluge of candidates and separate the wheat from the chaff for qualities that will surely serve your business better in the long run.

Related: When clients don’t pay


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Silicon Alley is blooming again

A very nice article just appeared in Crains New York Business this past week covering interesting people to watch in Gotham’s tech scene.

With such a long list of tech startups, Silicon Alley is nearly bursting at the seams!

  • Huffington Post
  • Foursquare
  • Rent the Runway
  • BankSimple
  • Knewton Inc.
  • GroupMe
  • Kickstarter
  • Tumblr
  • Tremor Media Inc.
  • MyCityWay
  • Bitly
  • Yodle
  • Boxee Inc.
  • Tabula Digita
  • AppNexus
  • Yext
  • Lot18
  • Tapad
  • Etsy
  • SecondMarket
  • GawkerMedia