Objects, Services, and Dependencies

In this article we’ll discuss all the relevant aspects of instantiating a service. You’ll learn how to deal with its dependencies, what you can and can’t do inside its constructor, and you should be able to instantiate it once and make it reusable many times.


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Two types of objects

In an application there are typically two types of objects:

  1. Service objects which either perform a task, or return a piece of information.
  2. Objects that hold some data, and optionally expose some behavior for manipulating or retrieving that data.

Objects of the first type are created once, then used any number of times, but nothing can be changed about them. They have a simple lifecycle. Once they’ve been created, they can run forever, like little machines with specific tasks. These objects are called services.

The second type of object is used by the first type to complete tasks. These objects are the materials which the services work with. For instance, a service may retrieve such an object from another service, and it manipulates the object and hands it over to another service for further processing (Figure 1). The lifecycle of a material object may be more complicated than that of a service: after it has been created, it could optionally be manipulated, and it may even keep an internal event log of everything which has happened to it.

Image for post

Figure 1. This UML-style sequence diagram shows how services call other services, passing along other types of objects as method arguments or return values. Inside a service method, such an object may be manipulated, or a service may retrieve data from it.

Objects that perform a task are often called “services”. These objects are doers, and they often have names which indicate that: controller, renderer, calculator, etc. Service objects can be constructed by using the new keyword to instantiate their class, e.g. new FileLogger().

Inject dependencies and configuration values as constructor arguments

Services usually need other services to do their job, which are its dependencies, and they should be injected as constructor arguments. An example of a service with its dependency is the FileLogger class in Listing 1.

Listing 1. The **FileLogger** service.

interface Logger
 {
     public function log(string message): void;
 }

 final class FileLogger implements Logger
 {
     private Formatter formatter;

     public function __construct(Formatter formatter)           ❶
     {
         this.formatter = formatter;
     }

     public function log(string message): void
     {
         formattedMessage = this.formatter.format(message);

         // ...
     }
 }

 logger = new FileLogger(new DefaultFormatter());
 logger.log('A message');

❶ Formatter** is a dependency of FileLogger.**

Making every dependency available as a constructor argument makes the service ready for use immediately after instantiation. No further setup is required, and no mistakes can be made with that.

Sometimes a service needs some configuration values, like a location for storing files, or credentials for connecting to an external service. Inject such configuration values as constructor arguments too, as is done in Listing 2.

Listing 2. Besides a dependency**,**``**FileLogger**** also requires a configuration value.**

final class FileLogger implements Logger
 {
     // ...

     private string logFilePath;

     public function __construct(                       ❶
         Formatter formatter,
         string logFilePath
     ) {
         // ...

         this.logFilePath = logFilePath;
     }

     public function log(string message): void
     {
         // ...

         file_put_contents(
             this.logFilePath,
             formattedMessage,
             FILE_APPEND
         );
     }
 }

❶ logFilePath** is a configuration value which tells the FileLogger to which file the messages should be written.**

These configuration values may be globally available in your application, in some kind of a parameter bag, settings object, or otherwise large data structure containing all the other configuration values too. Instead of injecting the whole configuration object, make sure you only inject the values which the service should have access to. In fact, only inject the values it needs.

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Objects, Services, and Dependencies
Arvel  Parker

Arvel Parker

1591611780

How to Find Ulimit For user on Linux

How can I find the correct ulimit values for a user account or process on Linux systems?

For proper operation, we must ensure that the correct ulimit values set after installing various software. The Linux system provides means of restricting the number of resources that can be used. Limits set for each Linux user account. However, system limits are applied separately to each process that is running for that user too. For example, if certain thresholds are too low, the system might not be able to server web pages using Nginx/Apache or PHP/Python app. System resource limits viewed or set with the NA command. Let us see how to use the ulimit that provides control over the resources available to the shell and processes.

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MEAN Stack Tutorial MongoDB ExpressJS AngularJS NodeJS

We are going to build a full stack Todo App using the MEAN (MongoDB, ExpressJS, AngularJS and NodeJS). This is the last part of three-post series tutorial.

MEAN Stack tutorial series:

AngularJS tutorial for beginners (Part I)
Creating RESTful APIs with NodeJS and MongoDB Tutorial (Part II)
MEAN Stack Tutorial: MongoDB, ExpressJS, AngularJS and NodeJS (Part III) 👈 you are here
Before completing the app, let’s cover some background about the this stack. If you rather jump to the hands-on part click here to get started.

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systemctl List All Failed Units/Services on Linux

Is there is a command to print list all failed units or services when using systemd on Linux? Can you tell me the systemctl command to list all failed services on Linux?

This quick tutorial explains how to find/list all failed systemd services/units on Linux operating systems using the systemctl command.

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Creating RESTful APIs with NodeJS and MongoDB Tutorial

Welcome to this tutorial about RESTful API using Node.js (Express.js) and MongoDB (mongoose)! We are going to learn how to install and use each component individually and then proceed to create a RESTful API.

MEAN Stack tutorial series:

AngularJS tutorial for beginners (Part I)
Creating RESTful APIs with NodeJS and MongoDB Tutorial (Part II) 👈 you are here
MEAN Stack Tutorial: MongoDB, ExpressJS, AngularJS and NodeJS (Part III)

#[object object] #[object object] #[object object] #[object object] #[object object] #[object object] #[object object]

Yoshiko  Jones

Yoshiko Jones

1598195340

How to configure AWS SES with Postfix MTA

How do I configure Amazon SES With Postfix mail server to send email under a CentOS/RHEL/Fedora/Ubuntu/Debian Linux server?

Amazon Simple Email Service (SES) is a hosted email service for you to send and receive email using your email addresses and domains. Typically SES used for sending bulk email or routing emails without hosting MTA. We can use Perl/Python/PHP APIs to send an email via SES. Another option is to configure Linux or Unix box running Postfix to route all outgoing emails via SES.

  • » Remove sendmail
  • » Install postfix
  • » Configuring postfix for SES
  • » Test postfix

Procedure to configure AWS SES with Postfix

Before getting started with Amazon SES and Postfix, you need to sign up for AWS, including SES. You need to verify your email address and other settings. Make sure you create a user for SES access and download credentials too.

Step 1 – Uninstall Sendmail if installed

If sendmail installed remove it. Debian/Ubuntu Linux user type the following apt command/apt-get command:

$`` sudo apt --purge remove sendmail

CentOS/RHEL user type the following yum command or dnf command on Fedora/CentOS/RHEL 8.x:

$`` sudo yum remove sendmail

$`` sudo dnf remove sendmail

Sample outputs from CentOS 8 server:

Dependencies resolved.
===============================================================================
 Package           Architecture  Version               Repository         Size
===============================================================================
Removing:
 sendmail          x86_64        8.15.2-32.el8         @AppStream        2.4 M
Removing unused dependencies:
 cyrus-sasl        x86_64        2.1.27-1.el8          @BaseOS           160 k
 procmail          x86_64        3.22-47.el8           @AppStream        369 k

Transaction Summary
===============================================================================
Remove  3 Packages

Freed space: 2.9 M
Is this ok [y/N]: y

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