Abstract factory creates a base class with abstract methods defining methods for the objects that should be created. Each factory class which derives the base class can create their own implementation of each object type.
Factory method is just a simple method used to create objects in a class. It's usually added in the aggregate root (The Order class has a method called CreateOrderLine)
Abstract factory
In the example below we design an interface so that we can decouple queue creation from a messaging system and can therefore create implementations for different queue systems without having to change the code base.
interface IMessageQueueFactory{ IMessageQueue CreateOutboundQueue(string name); IMessageQueue CreateReplyQueue(string name);}public class AzureServiceBusQueueFactory : IMessageQueueFactory{ IMessageQueue CreateOutboundQueue(string name) { //init queue return new AzureMessageQueue(/*....*/); } IMessageQueue CreateReplyQueue(string name) { //init response queue return new AzureResponseMessageQueue(/*....*/); }}public class MsmqFactory : IMessageQueueFactory{ IMessageQueue CreateOutboundQueue(string name) { //init queue return new MsmqMessageQueue(/*....*/); } IMessageQueue CreateReplyQueue(string name) { //init response queue return new MsmqResponseMessageQueue(/*....*/); }}Factory method
The problem in HTTP servers is that we always need an response for every request.
public interface IHttpRequest{ // .. all other methods .. IHttpResponse CreateResponse(int httpStatusCode);}Without the factory method, the HTTP server users (i.e. programmers) would be forced to use implementation specific classes which defeat the purpose of the IHttpRequest interface.
Therefore we introduce the factory method so that the creation of the response class also is abstracted away.
Summary
The difference is that the intended purpose of the class containing a factory method is not to create objects, while an abstract factory should only be used to create objects.
One should take care when using factory methods since it's easy to break the LSP (Liskov Substitution principle) when creating objects.