CSC/ECE 517 Fall 2013/ch1 1w44 s

Non-relational Databases in Rails Applications

Active Record is basic to Rails and it is responsible to for allowing programs to treat records in relational dbs as objects. It employs Object-Relational Mapping, commonly referred to as its abbreviation ORM, is a technique that connects the rich objects of an application to tables in a relational database management system. While Active Record is efficient in what it does, the complexities of ORM make way for a new approach for databases with rails, that is to move to different Database Models namely Object-Oriented Databases and No-SQL Databases.

Overview

Rails, as a web development framework, and Ruby, as a programming language, can be used with a choice of operating systems (Linux, Mac OS X, Windows), databases (SQLite, MySQL, PostgreSQL, and others), and web servers (Apache, Nginx, and others). The Rails stack can also include a variety of software libraries (gems) that add features to a website or make development easier. Sometimes the choice of components in a stack is driven by the requirements of an application

Rails uses the Active Record pattern which describes the mapping of an object instance to a row in a relational database table, using accessor methods to retrieve columns/properties, and the ability to create, update, read, and delete entities from the database.

Active Record is the layer of the system responsible for representing business data and logic. It facilitates the creation and use of business objects whose data requires persistent storage to a database. It employs Object-Relational Mapping, commonly referred to as its abbreviation ORM, is a technique that connects the rich objects of an application to tables in a relational database management system. Using ORM, the properties and relationships of the objects in an application can be easily stored and retrieved from a database without writing SQL statements directly and with less overall database access code.

Active Record gives us several mechanisms, the most important being the ability to:

Represent models and their data.

Represent associations between these models.

Represent inheritance hierarchies through related models.

Validate models before they get persisted to the database.

Perform database operations in an object-oriented fashion.

Object-relational impedance mismatch

The relational model on which Active Record relies on takes data and separates it into many interrelated tables that contain rows and columns. Tables reference each other through foreign keys that are stored in columns as well. When looking up data, the desired information needs to be collected from many tables (often hundreds in today’s enterprise applications) and combined before it can be provided to the application. Similarly, when writing data, the write needs to be coordinated and performed on many tables.

The Active Record pattern describes the mapping of an object instance to a row in a relational database table, using accessor methods to retrieve columns/properties, and the ability to create, update, read, and delete entities from the database. The pattern has numerous limitations referred to as the Object-relational impedance mismatch. Some of these technical difficulties are structural. In OO programming, objects may be composed of other objects. The Active Record pattern maps these sub-objects to separate tables, thus introducing issues concerning the representation of relationships and encapsulated data. Active Record uses macros to create relationships between objects and single table inheritance to represent inheritance.

Active Record is a solution for the Object-relational impedance mismatch, but this is assuming the datastore is relational. It’s also, fundamentally, a hack. The obvious cleaner solution being using a Object Oriented datastore.

Object Database

An object database (also object-oriented database management system). Object-Oriented Database Management System (OODBMS) is a Database Management System (DBMS) which allows information to be represented in the form of objects as used in object-oriented programming. OODBMSs provide an integrated application development environment by joining object-oriented programming with database technology. OODBMSs enforce object oriented programming concepts such as encapsulation, polymorphism and inheritance as well as database management concepts such as Atomicity, Consistency, Isolation and Durability. Since both the programming language and OODBMS use the same object-oriented model, the programmers can maintain the consistency easily between the two environments.

Comparison with RDBMS

Relational and Object database models are very different. For example, a document-oriented OODBMS takes the data you want to store and aggregates it into documents using the JSON format. Each JSON document can be thought of as an object to be used by your application. A JSON document might, for example, take all the data stored in a row that spans 20 tables of a relational database and aggregate it into a single document/object. Aggregating this information may lead to duplication of  information, but since storage is no longer cost prohibitive, the resulting data model flexibility, ease of efficiently distributing the resulting documents and read and write performance improvements make it an easy trade-off for web-based applications.

Document Object Model equivalent to Relational tables

Another major difference is that relational technologies have rigid schemas. Relational technology requires strict definition of a schema prior to storing any data into a database. Changing the schema once data is inserted is a big deal, extremely disruptive and frequently avoided. In comparison, document databases are schemaless, allowing you to freely add fields to JSON documents without having to first define changes. The format of the data being inserted can be changed at any time, without application disruption.

Naming Guidelines<ref>http://itsignals.cascadia.com.au/?p=7</ref>

---

Ruby uses the first character of the name to help it determine it’s intended use. The standard Ruby file extension is .rb, although many people working on UNIX-like systems don't bother with it for stand-alone scripts. Whether or not one uses it for scripts is up to them, but they will need to use it for library files or they will not be found by the interpreter.

• Local Variables

Should use lowercase letter followed by other characters. Naming convention states that it is better to use underscores rather than camelBack for multiple word names, e.g. average, variable_xyz

• Instance Variables

Instance variables are defined using the single "at" sign (@) followed by a name. It is suggested that a lowercase letter should succeed @, e.g. @color

• Instance Methods

Method names should start with a lowercase letter, and may be followed by digits, underscores, and letters. The name should possibly be a verb e.g. move, display_details

• Class Variables

Class variable names start with a double "at" sign (@@) and may be followed by digits, underscores, and letters, e.g. @@color

• Constant

Constant names usually start with an uppercase letter followed by other characters. Constant objects are by convention named using all uppercase letters and underscores between words, e.g. THIS_IS_A_CONSTANT

• Class and Module

Class names are recommended to be be nouns. In the case of modules, it's harder to make a clear recommendation. The names of mix-ins (which are just modules), however, should probably be adjectives, such as the standard Enumerable and Comparable modules. Class and module names starts with an uppercase letter, by convention they are named using MixedCase, e.g. module Encryption, class MixedCase

• Global Variables

Starts with a dollar ($) sign followed by other characters, e.g. $global

Considering customer order information as the data being used for an application, below naming guidelines give an idea of good class/table/file names.

• Model Naming Convention

Table: orders
Class: Order
File: /app/models/order.rb
Primary Key: id
Foreign Key: customer_id
Link Tables: items_orders

• Controller Naming Convention

Class: OrdersController
File: /app/controllers/orders_controller.rb
Layout: /app/layouts/orders.html.erb

• View Naming Convention

Helper: /app/helpers/orders_helper.rb
Helper Module: OrdersHelper
Views: /app/views/orders/… (list.html.erb for example)

• Tests Naming Convention

Unit: /test/unit/order_test.rb
Functional: /test/functional/orders_controller_test.rb
Fixtures: /test/fixtures/orders.yml

Class Design Guidelines

---

A class in Ruby always starts with the keyword class followed by the name of the class. The name should always be in initial capitals. Class Customer can be displayed as:

class Customer
end

A class is terminated by using the keyword end. All the data members in the class are between the class definition and the end keyword.

Ruby blurs the distinction between design and implementation. Ideas that have to be expressed at the design level in other languages can be implemented directly in Ruby. To help in this process, Ruby has support for some design-level strategies. These strategies can be used to design classes accordingly as suitable for different types of applications.

• The Visitor pattern is a way of traversing a collection without having to know the internal organization of that collection.
• Delegation is a way of composing classes more flexibly and dynamically than can be done using standard inheritance.
• The Singleton pattern is a way of ensuring that only one instantiation of a particular class exists at a time.
• The Observer pattern implements a protocol allowing one object to notify a set of interested objects when certain changes have occurred.

Normally, all four of these strategies require explicit code each time they're implemented. With Ruby, they can be abstracted into a library and reused freely and transparently.

Member Design Guidelines

---

While defining class members, it is very important to keep in mind the access restrictions. Scope and life-time of class members are different for different restrictions, as illustrated below.

public
totally accessible.
protected
accessible only by instances of class and direct descendants. Even through hasA relationships. (see below)
private
accessible only by instances of class (must be called nekkid no “self.” or anything else).
class A
  # Restriction used w/o arguments set the default access control.
  protected

  def protected_method
    # nothing
  end
end

class B < A
  def test_protected
    myA = A.new
    myA.protected_method
  end

  # Used with arguments, sets the access of the named methods and constants.
  public :test_protected
end

b = B.new.test_protected

Maintainability Guidelines

---

Maintainability guidelines are important to programmers for a number of reasons:

• 40%-80% of the lifetime cost of a piece of software goes to maintenance.
• Hardly any software is maintained for its whole life by the original author.
• Code conventions improve the readability of the software, allowing engineers to understand new code more quickly and thoroughly.
• If you ship your source code as a product, you need to make sure it is as well packaged and clean as any other product you create.

The following guidelines are to be followed to improve the software maintainability

• Profile your code regularly

If you profile your code regularly you’ll be able to tell if the latest change to the code will have an adverse effect on performance. Integrate profiling into your testing process and make it automated to ensure that it’s not forgotten. Like unit testing and BDD(Behavior-driven development) profiling goes a long way.

Performance Guidelines<ref>http://blog.monitis.com/index.php/2012/02/08/20-ruby-performance-tips/</ref>

---

The key parameter on which a software application is rated is by its performance. It is the time taken by the application to respond to a user's request. Performance of ruby can be improved significantly by following certain coding guidelines, as below.

• Avoid nesting loops more than three levels deep

Nesting affects the performance of the code proportionally with the increasing levels in loops. Limiting nesting to three levels is one good practice to keep the code's performance well.

• Avoid unnecessary variable assignments

New programmers, often use unwanted variables in code. A great example is when someone defines a variable to store a return value and then returns that variable; just return the value directly.

• Reduce usage of disk I/O

Disk I/O is a very costly operation as far as computing is concerned. Keeping it to the minimal improves the application response time drastically. Using disk I/O, makes the application extremely slow. Using storage systems such as memcached reduces disk I/O operations to a great extent as lot of data is kept in memory. The speed improvement while using a memory caching system is tremendous.

• Use Ruby Enterprise Edition

Ruby Enterprise edition provides up to 33% lower memory usage. Though, to get benefited by this performance one needs to follow their guidelines.

• Avoid method calls as much as possible

Method calls are expensive operations in ruby. They should be avoided to keep up the performance of the application.

• Use interpolated strings instead of concatenated strings

Concatenated strings calls a method to get executed. So, it affects the performance as it is one of the most frequently used operation. Its better to replace them with Interpolated strings which runs faster as it doesn't invoke a method call.

put “Hello there, #{name}!”vs.puts “Hello there, ” << name = “!”

• Destructive operations are faster

Ruby’s in-place methods that which modifies the actual value than working on a copy of the data are much faster. But this should be handled carefully as original data gets disturbed.

• Avoid unnecessary calls to uniq on arrays

In many cases methods are already calling uniq on an array and there’s no need for you to call it yet again.

• For loops are faster than .each

.each uses an enumeration object behind the scene which adds a delay in execution. Using for instead of .each would improve performance but for short loops only.<ref>http://stackoverflow.com/questions/6406112/why-are-ruby-method-calls-particularly-slow-in-comparison-to-other-languages</ref>

• Use x.blank? over x.nil? || x.empty?

When using ActionPack there’s no need for x.nil? or x.empty?; x.blank? checks for both of these.

• Avoid calls to parse_date and strftime

Both these calls are very expensive. using regular expressions would improve the time.

• Know your gems

All libraries are not efficient. Many gems may need to be removed to problem to fix a problem with the code. This is a common scenario when many gems are installed. So performing bench marking and testing a gem with others that perform same task would help in picking the right gem.

• Improve your algorithms before you try to improve your code

Algorithmic improvements results in evident improvements in performance of the code. It is always ideal to design an efficient algorithm without unwanted method calls.

• Test the most frequently occurring case first

While using if statements or a case statement always test the cases that occur most frequently. This allows less code to run before a decision is made. It may not seem like much but over several hundred or thousand runs through the decision logic you’ll notice a definite performance gain.

• Optimize the way you access global constants

While accessing the global constants one should use namespace before the constants to avoid entire library search for the constant.

• Use explicit returns

Even though the result of last operation is returned for a method, using explicit returns would speed up the code. Explicit returns are faster, especially in older Ruby versions such as 1.8.x.

Documentation Guidelines<ref>http://guides.rubyonrails.org/api_documentation_guidelines.html</ref>

---

Documentation comments<ref>http://www.jetbrains.com/ruby/webhelp/documenting-source-code-in-rubymine.html</ref> can be created in accordance with RDoc and YARD syntax. Note that RDoc highlighting in documentation comments can be turned enabled or disabled in the Appearance page of the editor settings.

The most common Documentation guidelines are listed below.

• Write simple, declarative sentences. Brevity is a plus: get to the point.
• Write in present tense: "Returns a hash that...", rather than "Returned a hash that..." or "Will return a hash that...".
• Start comments in upper case. Follow regular punctuation rules:

# Declares an attribute reader backed by an internally-named 
# instance variable.
def attr_internal_reader(*attrs)
  ...
end

• Communicate to the reader the current way of doing things, both explicitly and implicitly. Use the idioms recommended in edge. Reorder sections to emphasize favored approaches if needed, etc. The documentation should be a model for best practices and canonical, modern Rails usage.

Documentation has to be concise but comprehensive. Explore and document edge cases.

Layout Guidelines<ref>http://www.caliban.org/ruby/rubyguide.shtml</ref>

---

Designing the layout of any application determines the readability factor for other developers. Most followed order of code is as follows:

header block with author's name, Perforce Id tag and a brief description of what the program or library is for.
require statements
include statements
class and module definitions
main program section

• Spreading Code Out and Lining it Up

This is very important for readability. Basically the principle is to:

• separate each component part by white space.
• align everything meaningfully.

As such one can easily scan up and down the code and see the patterns. This is very important not only for understanding the code, but also for looking for anomalies and as a tool for rationalizing and consolidating the code.

Code that has a lot of 'noise' - a lot of unnecessary variation and untidiness - is code that one can waste a lot of time working on. Well written and formatted code is code that is easy and quick to work with. It is code that allows one to easily 'see the wood from the trees'.

Code Analysis Tools

Ruby itself goes a long way towards helping developers write clear code.

• The Ruby debugger is a library loaded into Ruby at run-time.

This is done as follows:

ruby -r debug [
            options
            ] [
            programfile
            ] [
            arguments
            ]

The debugger can do all the usual things one would expect it to, such as set breakpoints, step into and over code, print out the call stack, etc.

While tools for the mainstream languages such as Java and C++ have reached a certain maturity, tools for Ruby are still growing. And they might be needed more and more as Ruby's usage spreads from early adopters to the early majority, and SLOC (Source Lines Of Code) continues to increase. Automatic tools<ref>http://www.ruby-toolbox.com/categories/code_metrics</ref> can be used to detect several types of problems including inconsistent style, long methods and repeated code.

• Roodi (Ruby Object Oriented Design Inferometer) - this parses Ruby code and warns about design issues from the list configured, ie: Class line count check, for loop check, parameter number check, cyclomatic checks and 10 other checks
• Reek - similar in concept to Roodi
• Saikuro - designed to check cyclomatic complexity
• Flog - created by Ryan Davis, this computes a score of code written: the higher the score is, the worse your code is. ABC metrics (Assignments, Branches and Calls) are taken into account to compute the score
• Simian - a similarity analyzer, this can be used for duplication identification (a $99 license is needed for commercial use)
• Flay - this is another free tool from Ryan Davis that finds structural similarities in code

Code Analysis Tool Results

nosqlvsrdbms.jpg

Summary

Ruby developers should follow a certain criteria or guidelines during software development. Coding standards are set of rules, guidelines and regulations on the manner of writing a code that helps programmers and developers read and understand quickly the source code that conforms to style and help avoid introducing misunderstanding and faults.

Particularly in Ruby development, coding standards are extremely important; therefore Ruby developers should put an importance to them. This is because these standards offer higher uniformity and consistency when writing code by different programmers. This could result in a code that's simple to know and preserve, thus reducing the project’s overall expenses.

Some of the benefits of using coding standards are:

• Easy to understand and maintained
• Boost the code’s readability
• Maintainable applications
• Eradicates complexity
• Separate documents look more appropriate

See Also

• http://www.caliban.org/ruby/rubyguide.shtml
• http://github.com/styleguide/ruby
• http://www.ruby-doc.org/docs/ProgrammingRuby/
• http://www.ruby-lang.org/en/documentation/
• http://en.wikibooks.org/wiki/Ruby_programming_language

References

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