MongoDB University Course M102: MongoDB for DBAs

My first introduction to MongoDB was to sign up to university.mongodb.com and take the course M102: MongoDB for DBAs. This is the basic course for DBAs and while I wouldn't say it made me feel production-ready, it demystified MongoDB and gave me a good overview of JSON and the various MongoDB concepts.

The course lectures are provided via YouTube videos as normal with MongoDB University and the practical side is performed on a personal MongoDB installation.

I installed the latest version of MongodDB version 3.2.6 for Windows although the course recommended 3.2.2. However, I didn't have any problems related to the version.

I did encounter some problems due to starting MongoDB without the necessary permissions. This comes from how you start the Windows command line interface, cmd.exe. Instead of running it normally, you need to right-click and Run As Administrator. Then when you start your MongoDB processes they will function correctly. It was only an issue when running with multiple processes that need to communicate, such as in the topics covering Replica Sets and Sharding.

I took the May 2016 release of the course and passed with a 100% grade.

The questions were all quite straightforward and covered in the online course material.

Chapter 1: Introduction 
Introduction to MongoDB, key concepts and installing Mongo

Homework 1.1

What do you get as a result?

16777216

Homework 1.2

What's the result?

3

Homework 1.3

Now, what query would you run to get all the products where brand equals the string "ACME"?

db.products.find({brand:"ACME"})

Homework 1.4

Check all that apply:

 var c = db.products.find( { }, { name : 1, _id : 0 } ).sort( { name : 1 } ); while( c.hasNext() ) { print( c.next().name); }

 var c = db.products.find( { } ).sort( { name : 1 } ); c.forEach( function( doc ) { print( doc.name ) } );

Chapter 2: CRUD and Administrative Commands 
Creating, reading and updating data

Homework 2.1

What is the output? (The above will check that products_bak is populated.)

3.05

Homework 2.2

What is the output?

0.050.019031

Homework 2.3

How many products have a voice limit? (That is, have a voice field present in the limits subdocument.)

3


Chapter 3: Performance 
Indexing and monitoring

Homework 3.1

When you are done, run:

homework.a()

and enter the numeric result below (no spaces).

6

Homework 3.2

Once you have eliminated the slow operation, run (on your second tab):

homework.c()

and enter the output below. Once you have it right and are ready to move on, ctrl-c (terminate) the shell that is still running the homework.b() function.

12

Homework 3.3

• Q1: How many products match this query?
• Q2: Run the same query, but this time do an explain(). How many documents were examined?
• Q3: Does the explain() output indicate that an index was used?

Check all that apply:

Q1 : 4

Q2 : 4

Q3 : Yes

Homework 3.4

Which of the following are available in WiredTiger but not in MMAPv1? Check all that apply.

Check all that apply:

Document level locking

Data compression

Chapter 4: Replication 
Replication, Failover, Recovery

Homework 4.1

Now run:

homework.a()

and enter the result. This will simply confirm all the above happened ok.

5001

Homework 4.2

Once done with that, run

homework.b()

in the mongo shell and enter that result below.

5002

Homework 4.3

Once you have two secondary servers, both of which have sync'd with the primary and are caught up, run (on your primary):

homework.c()

and enter the result below.

5

Homework 4.4

When done, run:

> homework.d()

and enter the result.

6

Homework 4.5

What result does this expression give when evaluated?

db.oplog.rs.find( { } ).sort( { $natural : 1 } ).limit( 1 ).next( ).o.msg[0]

R

Chapter 5: Replication Part 2 
Optimizing and monitoring your Replica Sets

Homework 5.1

what is the text in the "state" field for the arbiter when you run rs.status()?

7

Homework 5.2

Which of the following options will allow you to ensure that a primary is available during server maintenance, and that any writes it receives will replicate during this time?

Add two data bearing members plus one arbiter.

Add another data bearing node.

Homework 5.3

You only have two data centers available. Which arrangement(s) of servers will allow you to be stay up (as in, still able to elect a primary) in the event of a failure of either data center (but not both at once)? Check all that apply.

None of the above

Homework 5.4
Find out the optional parameter that you'll need, and input it into the box below for your rs.reconfig(new_cfg, OPTIONAL PARAMETER).

{ force : true }

Chapter 6: Scalability 
Sharding setup, sharding monitoring, shard key selection, inserting large amounts of data

Homework 6.1

Run homework.a() and enter the result below. This method will simply verify that this simple cluster is up and running and return a result key.

1000001

Homework 6.2

Run homework.b() to verify the above and enter the return value below.

3

Homework 6.3

When done, run homework.c() and enter the result value.

2

Chapter 7: Backup and Recovery 
Security, backups and restoring for backups

Final Exam:

Question 1:

How many documents do you have?

6

Question 2:

Question: Which of the following are true about mongodb's operation in these scenarios? Check all that apply.

Check all that apply.

MongoDB preserves the order of writes in a collection in its consistency model. In this problem, 27003's oplog was effectively a "fork" and to preserve write ordering a rollback was necessary during 27003's recovery phase.

Question 3:

Choose the best answer:

It contains 3 documents.

Question 4:

Reconfigure the replica set so that the third member can never be primary. Then run:

$ mongo --shell a.js --port 27003

And run:

> part4()

And enter the result in the text box below (with no spaces or line feeds just the exact value returned).

233

Question 5:

Which of these statements is true?

Check all that apply:

We can create an index to make the following query fast/faster:

db.postings.find( { "comments.flagged" : true } )

One way to assure people vote at most once per posting is to use this form of update:

db.postings.update(
  { _id: . . . , voters:{$ne:'joe'} },
  { $inc : {votes:1}, $push : {voters:'joe'} } );

Question 6:

Which of these statements is true?

Check all that apply:

MongoDB (v3.0) supports transactions spanning multiple documents, if those documents all reside on the same shard.

MongoDB allows you to choose the storage engine separately for each collection on your mongod.

MongoDB has a data type for binary data

Question 7:

Which of these statements is true?

Check all that apply:

MongoDB is "multi-master" -- you can write anywhere, anytime.

Most MongoDB queries involve javascript execution on the database server(s).

Question 8:

Once you have the config server running, confirm the restore of the config server data by running the last javascript line below in the mongo shell, and entering the 5 character result it returns.

39:15

Question 9:

Connect to the mongos with a mongo shell. Run this:

use snps
var x = db.elegans.aggregate( [ { $match : { N2 : "T" } } , { $group : { _id:"$N2" , n : { $sum : 1 } } } ] ).next(); print( x.n )

Enter the number output for n.

47664

Question 10:

Based on the explain output, which of the following statements below are true?

Check all that apply:

2 shards are queried.

8 documents are examined.