MongoDB for DBA's 1/7. Introduction

Chapter 1. Introduction to MongoDB, key concepts

Concepts

What were the big differences in hardware over the last few decades that MongoDB attempted to address?

Parallelism of cores

Parallelism of servers

Quantum computers

Faster clock speeds

More memory

Scaling

Q: When scaling out horizontally (adding more servers to contain your data), what are problems that arise as you go from, say, 1 commodity server to a few dozen?

Any one server is likelier to fail per given unit of time.

The servers must communicate with one another eating up network bandwidth

The need for redundancy increases as the likelihood of some failure in the system per unit of time increases.

Hardware cost per server is likely to increase.

SQL and Complex Transactions

What causes significant problems for SQL when you attempt to scale horizontally (to multiple servers)?

Joins Queries Transactions Inserts Indexes

Documents Overview

What are some advantages of representing our data using a JSON-like format?

JSON presents a flexible and concise framework for specifying queries as well as storing records.

JSON syntax is similar to that of common data structures used in many programming languages and is, therefore, familiar to developers.

JSON is language independent.

JSON is optimized for use with JavaScript.

JSON Types

How many data types are there in JSON?

1 (just strings) 2 4 6

1. String
2. Integer
3. Boolean
4. Null
5. Array
6. Object or documents (subdocuments)

JSON Syntax

What is the corresponding JSON for the following XML document?

<person>
  <name>John</name>
  <age>25</age>
  <address>
    <city>New York</city>
    <postalCode>10021</postalCode>
  </address>
  <phones>
    <phone type="home">212-555-1234</phone>
    <phone type="mobile">646-555-1234</phone>
  </phones>
</person>

{"name":"John",
  "age":25,
  "address":{"city":"New York","postalCode":"10021"},
  "phones":[
                {"phone":"212-555-1234","type":"home"},
                {"phone":"646-555-1234","type":"mobile"}
                ]
}

JSON Syntax 2

For the following XML, Is the corresponding JSON example legal json?

<things>
  <hat>one</hat>
  <coat>z</coat>
  <hat>two</hat>
</things>

{
  "hat" : "one",
  "coat" : "z",
  "hat" : "two"
}

Yes No Maybe

Binary JSON

Why do we represent our data as BSON rather than JSON in the system?

Fast machine scanability.

Human readability.

Stronger typing (and more types) than JSON

BSON and applications

For a typical client (a python client, for example) that is receiving the results of a query in BSON, would we convert from BSON to JSON to the client's native data structures (for example, nested dictionaries and lists in Python), or would we convert from BSON straight to those native data structures?

BSON -> Native data structures

BSON -> JSON -> Native data structures

Dynamic Schema

True or False: MongoDB is schemaless because a schema isn't very important in MongoDB

True False

What is the MongoDB shell?

Q: By default, which database does the mongo shell connect to?

test localhost mongo help default

Cursors Introduction

In order to query a collection in the mongo shell, we can type which of the following?

db.collection.find()

db.find()

db.collection.get()

db.collection.select(*)

db.collection.get(true)

Query Language: Basic concepts

 

Mongo querys are represented as BSON (JSON)

You have a collection where every document has the same fields, and you want to look at the value of the “_id”, "name", and “email” fields in order to see their format. Furthermore, you want to eliminate all other fields from the query results. What query might you write?

db.collection.find({ name : 1, email: 1 }, {} )

db.collection.find( { } , { name: 1 , email: 1 } )

db.collection.find( { name: 1 } , { email : 1 } )

db.collection.find( {name: 1 , email: 1 } )

Query Language: Projection

 

You want to query the “people” collection, you want the results of the query to include only documents where age is 50, and you want to look at all fields except “email”. What query should you write?

db.people.find( { email : 0, age : 50 } , { } )

db.people.find( { email : 0 }, { age : 50 } )

db.people.find( { age : 50 } , {email: 0 } )

db.people.find( { } , { email : 0 , age : 50 } )

db.people.find( { age : 50 }, {email : 1} )

Query Language: Advantages of a Dynamic Schema

If you want to add a new key: value pair to the documents in the “shapes” collection, what methods could you use?

db.shapes.alterCollection()

db.shapes.update()

db.shapes.resizeObject()

db.shapes.find()

db.shapes.save()

Shell: Queries

We have sample documents in our products collection such as:

{
  name: "AC1 Case Green",
  color: "green",
  price: 12.00,
  for: "ac1",
  type: ["accessory", "case"],
  available: true
}

How would we query in the shell for all products that are cases for an ac9 phone? That is, where type contains the value "case" and for equals "ac9"?

db.products.find({"for":"ac9","type":"case"})

Sorting

If you want to run a query on the collection, “books,” and sort ASCIIbetically by title on the query, which of the following will work?

db.books.find( { } , { sort : { title : 1 } } )

db.books.find( { } , { title : "$sort" } )

db.books.find( { } ).sort( { title : -1 } )

db.books.sort( { title : 1 } ).find( { } )

db.books.find().sort( { title : 1 } )

Query Language: Cursors

Recall the documents in the scores collection:

{
 "_id" : ObjectId("50844162cb4cf4564b4694f8"),
 "student" : 0,
 "type" : "exam",
 "score" : 75
}

Write a query that retrieves documents of type "exam", sorted by score in descending order, skipping the first 50 and showing only the next 20.

db.scores.find({"type":"exam"}).sort({"score":-1}).skip(50).limit(20)