My first trail

SaaS - 3.10. - Databases and migrations

A Database<ref>http://en.wikipedia.org/wiki/Database</ref> is a coherent collection of data with inherent meaning. Random assortment of data is not a database. Data is organized in order to model relevant aspects of reality, so that it supports processes requiring this data. Data Migration is related to transfer of data between storage types, formats, or computer systems. This is performed programmatically to accomplish an automated migration so that humans are free from doing this repetitive task.

Database

A database<ref>http://en.wikipedia.org/wiki/Database</ref> is nothing more than a collection of meaningful information. Databases can be of multiple types, for example Distributed Databases, Relational Databases, Flat File Databases. A database could be as simple as a text file with a list of names or it can even be very complex as a large relational database management system
Examples:

1. Banking Systems where accounts are maintained and it is made sure that money does not disappear as a result of system failure.
2. Airline Reservation Systems where the plane details, the airport details and the customer details are maintained.
3. Hotel Management Systems where the availability of rooms, the rates and the customer details are maintained.

Data Migration

Every application in reality has a database in the backend to store all the relevant data of the application. We should not test the application on database because it may contain valuable data like customer information in a banking system. So editing that data present in the database is not a good idea. The solution Rails provides for above problem is Defining three different environments: Development, Production and Testing each of which has a separate database and appropriate database types.

The Development Environment is what we use while developing the application. The production environment refers to the database that is used when the application is published in the real world. The testing environment is meant for testing tools. Testing of the application is done using Testing tools which automates the entire testing procedure. Since there are different databases in each environment, the problem that occurs is changes made in one Database do not reflect in the other. A programmer is also responsible to tell the other developers what changes have been made in the database. Also one has to keep track of which changes need to be run against production machines during deployment. The solution for the above problem that rails offers is data migration. Databases of different types can also be migrated between the three different environments. For example, we may use SQLlite in Development environment, but we can still migrate into production environment where heroku may have been used. The source is portable and the backend understands what operations to do on the database.

In reality Rails migrations are similar to version control of databases. Rails migrations is actually used since databases change requires modifications to both code and data. Hence we cannot use a source code version control system like Subversion or Sourcesafe.

In a nutshell, DSL<ref>DSL examples[1]</ref> is a language that is specific to a particular domain. DSLs are different from general purpose programming languages like C++ and Java. They take advantage of the fact that there are groups of users who rather than communicating every single thing, are able to implicitly understand certain terms and notations. This makes communication much more efficient for such a group. Eg: 2 basketball fans will talk in basketball jargon. Not everyone will understand what zone defence, 3 pointers and jump ball mean. The jargon serves the specific purpose of easing communication between 2 basketball fans. Some would argue that DSLs have been around for a far longer time dating back to even before the advent of computers. Subway maps, electronics circuit diagrams all demonstrate properties of a DSL. Since then, DSLs have come a long way. Domain Specific Languages (DSLs), as they are spoken about now, have been around since the 1970's and their origin can be traced back to a tool called Draco. Since Draco, computers have made it possible to design a whole host of other useful DSLs. Some of the more popular ones among them are SQL, Prolog, Verilog, HTML etc.

Object-Oriented Programming (OO) is a programming paradigm that tries to apply real world principles to make programming easier. OO has been around since the 1950's. Starting with Simula and Smalltalk, OO languages have come a long way since then, and has grown to become one of the most popular programming paradigms employed worldwide. Some of the modern object oriented languages include static languages like C++, Java, C#, and dynamic languages like Perl, Ruby etc.

OODSLs, that bring out the features of both DSLs as well as OO languages are a fairly recent invention. OODSLs have been around since the 1980's/1990's. They are now primarily used in the realm of web development, game development, and are also used in certain specialized industries like music.

Examples

PHP

PHP is an OODSL that is commonly used for web development. It is an open source web development framework that is designed to make web development simpler and easier. The language can be perceived as a general purpose language, but it also has domain specific components designed for the web domain alone. PHP is in general embedded into HTML to perform server side computations before the page is sent to the client.

OODSL compiler for FPGA

An object oriented domain specific compiler was developed for programming Field Programmable Gate Arrays.<ref>Object-Oriented Domain Specific Compilers for Programming FPGAs[2]</ref> The compiler allows the hardware programmer to make use of object-oriented style while programming. The research paper claims that this way of programming the hardware brings down the time taken for design by several notches and still results in good optimal performance.

Tools for Creating OODSL

When creating an OODSL, one can either choose to start from scratch or use a tool that is already available. Creating a language from scratch gives the programmer a certain amount of flexibility to define it the way he wants to, but it takes a lot of time. Therefore, in general, software tools are used to aid in the process of creating Domain Specific Languages. This is because it not only saves time, but also helps to make the language easy to use due to familiarity with the syntax of the underlying language.

Boo

Boo<ref>Boo Home[3]</ref> is an open source object oriented language that is used to create Domain Specific languages. Defining basic data structures like array, hash, variables etc are inherently supported in Boo and it makes their creation easier. Although Boo is a statically typed language, these restrictions can be bent once in a while so that the type of objects can be found and manipulated in run time as well (Duck typing).

Visual Studio

Visual Studio has Domain-specific language tools that are designed to help the designer to build a language focusing on the language grammar and characteristics rather than paying much attention to the internal details of the compiler.<ref>Domain specific development[4]</ref> Visual tools are used that help focus on the business requirements of the OODSL rather than worrying about how it works internally.

Groovy

Groovy <ref>Groovy Home[5]</ref> is a dynamic language for the Java Virtual Machine. Also, the syntax is groovy is similar to that of Java, making designing of new OODSLs easier and similar to programming in Java. An example for developing an OODSL in Groovy.

OODSL vs. non-OODSL

When defining a DSL, choosing whether to go with an OODSL or a non-OODSL depends on several factors like the domain for which it is defined, existing DSLs for that domain etc. Non-OODSLs are domain specific languages that do not necessarily embrace object oriented concepts like abstraction, polymorphism etc. OODSLs can be well suited for some situations whereas in other situations, they might be an overkill. Defining a new OODSL when there is an already widely accepted non-OODSL may not be very useful. For example, HTML is the standard method for displaying web pages and it is non-OODSL. Defining a new non-OODSL that replaces HTML may not prove to be significantly advantageous.

Advantages and Disadvantages

Some of the advantages of OODSLs would be:

• Since OODSLs, by their name, are object oriented, they give the developer an easier alternative to model their applications based on business goals and using real world paradigms.
• Because of their object oriented nature, the code is generally simple to understand and therefore code maintenance would be easier in OODSLs than in non-OODSLs.
• Object oriented concepts like encapsulation and abstraction give the code better security when they are modified in the future.

Though OODSLs in general have several advantages over other languages, one must also consider their disadvantages:

• Defining an object oriented language in general takes more time than defining a normal language because we have to properly define it to include OO concepts.
• They may not be well-suited for all domains and environments. Sometimes there may be a simpler non-OODSL way to achieve the task.

A Running Example

Given here is a simple example <ref>Example DSL in groovy[6]</ref> for creating an OODSL. The DSL defined here simply prints the text we have specified in the program. This is defined in Groovy.

The definition of our new OODSL is as follows:

package com.solutionsfit.dsl.memotemplate
import groovy.xml.MarkupBuilder
class MemoDsl {
	String toString
	String fromString
	String messageString
	def sections = []
	/**
	* Make a closure. Main definition of the DSL program
	*/
	def static make(closure) {
		MemoDsl memoDsl = new MemoDsl()
		closure.delegate = memoDsl
		closure()
	}
	/**
	* Store the values in the local variables
	*/
	def to(String toString){
		this.toString = toString
	}
	def from(String fromText){
		this.fromString = bodyString
	}
	def message(String messageString){
		this.messageString = messageString
	}
	/**
	* The getText method gets called by the DSL by delegation
	*/
	def getText(){
		doText(this)
	}
	private static doText(MemoDsl memodsl){
		String template = "Memo\nTo: ${memoDsl.toText}\nFrom: 	${memoDsl.fromText}\n${memoDsl.body}\n"
		def sectionStrings =""
		for (s in memoDsl.sections) {
			sectionStrings += s.title.toUpperCase() + "\n" + s.body + "\n"
		}
		template += sectionStrings
		println template
	}
}

A sample program in our newly developed OODSL would be:

package com.solutionsfit.dsl.memotemplate

class MemolDslTest extends GroovyTestCase {
	void testDslUsage_outputText() {
		MemoDsl.make {
			to "Nirav Assar"
			from "Barack Obama"
			message "How are things? We are doing well. Take care"
			text
		}
	}
}

References

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