There are following below difference between Offline Shipping Method and Online Shipping Method
The Online Shipping methods are those which actively consume the API of a 3rd party system owned by a shipping services provider in order to fetch the shipping cost and the information regarding the parcel’s delivery tracking
While
The Offline Shipping methods implements the shipping methods which do not involve a direct interaction with shipping carriers, so called offline shipping methods.
[2] sales_order_item :: Store [Product Details, Order ID(order_id)]
[3] sales_order_grid :: Store [Billing/Shipping Address Grand Item Price,Customer ID(customer_id), Order ID as increment_id]
[4] sales_order_address :: Store [Shipping & Billing Address, , Customer ID(customer_id)], each time store two row one for Billing address and another For shipping address while ordering any product
[5] sales_order_payment :: Store [payment details on the bases of Order ID]
[6] sales_order_status :: Store [Sales Order Status like canceled, closed, complete, fraud, holded, payment_review, Paypal Canceled Reversal, Paypal Reversed, Pending, Pending Payment, Pending Paypal, Processing]
sales_order_grid :: Store [Billing/Shipping Address Grand Item Price,Customer ID(customer_id), Order ID as increment_id]
order_id of sales_order_item Become increment_id of sales_order_grid increment_id display in
Magento 2 virtual type allows user to change the arguments of a specific injectable dependency and change the behavior of a particular class. This allows user to use a customized class without affecting other classes that have a dependency on the original.
Preference is used to override or rewrite existing / custom classes (Controller, Block & Helper). Preference using Dependency Injection to override or rewrite classes.
How To Define Preference : Preference is defined inside di.xml
Path : app/code/VendorName/ModuleName/etc/di.xml
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="urn:magento:framework:Acl/etc/acl.xsd">
<preference for="Class Which Override" type="Custom Class By Which Override" />
</config>
Overriding (By Preference in di.xml) a method is not recommended because it can cause conflicts in the system and increase the complexity to upgrade.
Always try to user other extensibility options, such as event observers and plugins, when possible.
“A plugin or interceptor is a class that modifies or expand or edit the behavior of public class functions by intercepting a function call or set of code before, after or around that function call”
“Inserting code dynamically without changing original class behavior is called Plugin”
Plugins using design pattern “Interception”
Plugins Limitation : There are following below criteria where plugins can not used.
final classes
non public methods
static methods
constructor
virtual types
objects that are initialized before plugins loading
