In this article, we'll take a look at two important concepts for backend development: ORM (Object Relational Mapping) and auto-scaling.
ORM is a technique that allows developers to work with databases using objects. In other words, it provides a mapping between objects and database tables. This mapping can be done in two ways:
Annotation-based mapping: In this approach, developers annotate their Java classes with metadata that specifies the mapping between the class and the database table.
XML-based mapping: In this approach, developers specify the mapping in an XML document. This document is then used by the ORM tool to generate the database schema and the Java classes.
There are several benefits to using ORM:
It makes development faster and easier: developers can focus on the business logic of their applications and let the ORM tool handle the database interactions.
It reduces the amount of code that needs to be written: ORM tools can generate the SQL code for database interactions, which means that developers don't have to write this code themselves.
It makes applications more portable: since the database interactions are handled by the ORM tool, applications can be easily migrated to a different database if necessary.
There are several ORM tools available, such as Hibernate, EclipseLink, and iBatis. In this article, we'll focus on Hibernate.
Hibernate is a popular ORM tool that provides both annotation-based and XML-based mapping. It also has a number of other features, such as support for caching and lazy loading.
Let's take a look at how Hibernate can be used to map a Java class to a database table. We'll use the following Java class as an example:
public class Employee {
private Long id;
private String name;
private Integer age;
private Double salary;
// Getters and setters...
}
And we'll map it to the following database table:
CREATE TABLE employee (
id INTEGER PRIMARY KEY AUTOINCREMENT,
name TEXT,
age INTEGER,
salary REAL
);
The mapping can be done in two ways:
Annotation-based mapping:
@Entity
@Table(name="employee")
public class Employee {
@Id
@Column(name="id")
private Long id;
@Column(name="name")
private String name;
@Column(name="age")
private Integer age;
@Column(name="salary")
private Double salary;
// Getters and setters...
}
XML-based mapping:
<hibernate-mapping>
<class name="Employee" table="employee">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="name" column="name"/>
<property name="age" column="age"/>
<property name="salary" column="salary"/>
</class>
</hibernate-mapping>
In both cases, we need to specify the name of the database table that the Employee class will be mapped to. We also need to specify the mapping for each field in the Employee class. For the id field, we need to specify that it's the primary key of the table and that it's generated by the database (auto-increment).
Once the mapping is done, we can write the code to insert, update, delete, and query employees:
// Insert an employee
Employee emp = new Employee();
emp.setName("John Smith");
emp.setAge(30);
emp.setSalary(45000.0);
Session session = sessionFactory.openSession();
session.beginTransaction();
session.save(emp);
session.getTransaction().commit();
session.close();
// Update an employee
emp.setSalary(50000.0);
session = sessionFactory.openSession();
session.beginTransaction();
session.update(emp);
session.getTransaction().commit();
session.close();
// Delete an employee
session = sessionFactory.openSession();
session.beginTransaction();
session.delete(emp);
session.getTransaction().commit();
session.close();
// Query employees
List<Employee> employees = session.createQuery("from Employee").list();
for (Employee e : employees) {
System.out.println(e.getName() + " " + e.getAge() + " " + e.getSalary());
}
In the code snippet above, we first insert an employee into the database. Then, we update the employee's salary and finally, we delete the employee. We also query the database to get a list of all employees.
Auto-scaling is a technique that allows developers to scale their applications horizontally. In other words, it allows developers to add new servers to their application when the load on their application increases. This is in contrast to vertical scaling, which involves increasing the resources of a single server.
There are two benefits to using auto-scaling:
It allows applications to handle more load: since new servers can be added as needed, applications can handle more traffic without performance issues.
It's more cost-effective: since new servers can be added and removed as needed, developers only pay for the resources that they use.
Auto-scaling can be done manually or automatically. In this article, we'll focus on automatic auto-scaling.
Automatic auto-scaling involves using a tool to monitor the load on an application and to add and remove servers as needed. There are several tools available, such as Apache httpd, nginx, and HAProxy. In this article, we'll focus on Apache httpd.
Apache httpd is a popular web server that can be used to create auto-scaling applications. It has a module called mod_cluster that can be used to add and remove servers as needed.
The mod_cluster module works as follows:
It monitors the load on the application: the module periodically sends requests to the application to check the response time. If the response time is too high, it means that the application is under heavy load and new servers need to be added.
It adds and removes servers as needed: the module can add new servers to the application by making requests to the mod_cluster management interface. Similarly, it can remove servers that are no longer needed.
Let's take a look at how the mod_cluster module can be used to create an auto-scaling application. We'll use the following Apache httpd configuration:
<VirtualHost *:80>
ServerName www.example.com
<Directory /var/www/html>
Require all granted
</Directory>
<Location /cluster-manager>
SetHandler cluster-manager
</Location>
<Proxy balancer://mycluster>
BalancerMember http://localhost:8000
BalancerMember http://localhost:8001
</Proxy>
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
ProxyPass / balancer://mycluster/
ProxyPassReverse / balancer://mycluster/
</VirtualHost>
In the configuration above, we've defined a virtual host that will serve requests for the www.example.com domain. We've also defined a proxy balancer that contains two servers: localhost:8000 and localhost:8001. Finally, we've configured mod_cluster to add and remove servers as needed.
Now let's write a simple PHP application that will be served by the Apache httpd server:
<?php
$server = $_SERVER['SERVER_ADDR'];
echo "Hello, World! I'm running on $server";
?>
And we'll run it on two servers:
When a user visits the www.example.com website, they will be served the following page:
Hello, World! I'm running on 127.0.0.1
If we increase the load on the application by making more requests, mod_cluster will add new servers as needed. For example, if we make 10 requests per second, mod_cluster will add two new servers.
In this article, we've looked at two important concepts for backend development: ORM and auto-scaling. We've seen how ORM can be used to map Java classes to database tables and how auto-scaling can be used to add new servers to an application as needed.