MongoDB certification

Recently, I managed to get C100DEV: MongoDB Certified Developer Associate Exam and C100DBA: MongoDB Certified DBA Associate Exam certification. In this article I want to share my experience and say what you should pay attention to when preparing, and how the exam went. I will also give a few examples of questions.

Project website
university.mongodb.com
Cost of
Each exam costs $ 150.00. Each retake is paid . But there is a system of discounts. If forgive public traning, then a promo code is given for a 50% discount. I could not find information about this on the site until after the end of the training course I received a promo code on the mail.
Examination threshold
To pass, you need to score 490 points out of 600. If you consider that there are only 600 questions, and I scored 525 points, then, most likely, the partially correct answers also include points.
Passing the exam
Examination sessions are held for 2 weeks with breaks of 1-2 months. That is, at any time you will not pass the exam. The exam is an online test. GoToMeeting is installed for the exam. In it, through a video chat, you contact a company representative (I constantly came across Indians with a strange accent of English). He dictates to you the general rules (about the rules below). Next, you cut out access to your PC. He enters the password on the site to access the exam.
Rules for passing the exam (I will list only the most basic):
- you need to be in the room alone, no one should go in (it is better to warn loved ones if you are at home so that no one comes in)
- no phone
- do not talk to anyone (keep silent for the entire exam)
- do not turn off webcams
- you can not look around
- it is impossible to use third-party programs (console to test the request, documentation)
- you can use google translate
During the entire exam, you sit opposite a webcam with expanded access to a PC and the verifier is watching you. Once I thought about it, looked away and immediately received a comment from the reviewer, I had to turn the webcam away to show that there was nothing there. By the way, before the exam through the webcam we show that there is a clean table, no one is in the room. I was even asked to show what was under the table.
results
Results are available 3 weeks after the exam. A count of points with detail in sections is sent to the post office. For example CRUD 85%, aggregation 90%, indexing 75%, sharding: 50%. According to the results of the exams, you get into the MongoDB Certified Professional Finder database (similar to yellow pages zend certification)
Training
For preparation, I used docs.mongodb.org. I just took and re-read all sections. It turned out very useful. You can also take any free public training (https://university.mongodb.com/training). There came across questions from the exam. From the fact that it did not like it - this is a boring video where a lot of water.
Sample Questions
For the C100DEV exam, the following sections:
-Philosophy & Features (general concept)
- CRUD (work with data)
- Indexing
- Aggregation
- Data Modeling
- Replication
- Sharding
I will give a few examples of questions from the exam:
The index is built on the collection {a: 1, b: 1}
What queries will use it:
- db.data.find ({a: {$ gt: 4}}) .sort ({a: 1, b: 1})
- db.data.find ({a: 5, b: {$ lt: 3}}) .sort ({b: 1})
- db.data.find ({b: 5}) .sort ({a: 1})
- db.data.find ({a: 1})
- db.data.find ({b: 2})

Which of the following will return a query db.records.find ({a: {$ exists: true}})
- {_id: 0, a: 1}
- {_id: 1, a: {b: 1}}
- {_id: 2, a: [1,2,4]}
- {_id: 3, a: null}
- {_id: 4}

Which of the following is true about the capped collection
- you can not change the size
- documents are stored sorted on the hard disk
- documents cannot be moved
- you can not change the size of the documents (to do an update with an increase in the size of the document)
- you can not delete documents
- cannot be renamed
')
Given the composite index {a: 1, b: 1, c: 1, d: 1}. What queries will not use in-memory sorting
- db.test.find ({a: 5, b: 3}) .sort ({a: 1, b: 1, c: 1})
- db.test.find ({a: 5, b: 3}) .sort ({a: 1})
- db.test.find ({a: 5, b: 3}) .sort ({c: 1})
- db.test.find ({a: 5, b: 3}) .sort ({c: 1, d: 1})

The collection contains the following data:
{a: [5]},
{a: [5,3]},
{a: [3]},
{a: [3, 'dsfasf']}
How to get only one document {a: [5,3]}
- find ({a: [5,3]})
- find ({a: {$ and: [3,5]}})
- find ({a: {$ all: [3,5]}})

What is stored in the first 4 bytes of ObjectId?
- identifier of the machine on which the ObjectId was generated
- process ID
- timestamp time

How many maximum nodes are permissible in the replica set
- 27
- 50
- 100

It is necessary to look at the work of the $ all, $ addToset, $ pull, $ push, $ slice, $ in, $ text operators
How to create a user, create index syntax, get a list of indexes. What are mongofiles, mongosniff, mongotop, mongostat, bsondump, mongorestore, mongodump used for? Where explain () works. How many maximum voting nodes in the replica set. What is the arbitrator. Syntax of basic operations (insert, distinct, remove, update etc.)

The configuration for sharding and the questions of the type are given: on which nodes the query will be executed.
For example, sharding is organized by {a: 1, b: 1}
What requests give the opportunity to uniquely determine on which balls the request will work (instead of driving at all at once)
a> 1
a = 1 and b> 2
b> a

For exam C100DBA more questions on administration and is ready to tell in a separate article.