Here, you'll learn in detail about the virtual proxy. It is a placeholder for objects that are very heavy to instantiate. For example, you want to load a large image on your website. Now this request will take a long time to load. Typically, developers will create a placeholder icon on the web page suggesting that there's an image. However, the image will only be loaded when the user actually clicks on the icon thus saving the cost of loading a heavy image in memory. Thus, in virtual proxies, the real object is created when the client first requests or accesses the object.

A remote proxy can be defined in the following terms. It provides a local representation of a real object that resides on a remote server or different address space. For example, you want to build a monitoring system for your application that has multiple web servers, DB servers, celery task servers, caching servers, among others. If we want to monitor the CPU and disk utilization of these servers, we need to have an object that is available in the context of where the monitoring application runs but can perform remote commands to get the actual parameter values. In such cases, having a remote proxy object that is a local representation of the remote object would help.

You'll understand more about the protective proxy with the following points. This proxy controls access to the sensitive matter object of RealSubject. For example, in today's world of distributed systems, web applications have multiple services that work together to provide functionality. Now, in such systems, an authentication service acts as a protective proxy server that is responsible for authentication and authorization. In this case, Proxy internally helps in protecting the core functionality of the website for unrecognized or unauthorized agents. Thus, the surrogate object checks that the caller has access permissions required to forward the request.

Smart proxies interpose additional actions when an object is accessed. For example, consider that there's a core component in the system that stores states in a centralized location. Typically, such a component gets called by multiple different services to complete their tasks and can result in issues with shared resources. Instead of services directly invoking the core component, a smart proxy is built-in and checks whether the real object is locked before it is accessed in order to ensure that no other object can change it.