Proxy design pattern

• allow us to control access to an object
• original definition
  • to provide a surrogate or placeholder for another object to control access to it
    • the surrogate / placeholder is used to access the original object
      • in doing so, the placeholder is able to augment and change the contents of the original object
        • this is often needed to introduce a virtualization layer between the object and the consumer
          • when the consumer requests access to the object, it does so through the proxy
          • the proxy then governs and modifies the data as needed by acting as a wrapper
• important thing is that the interface offered to the consumer remains inline and consistent with the original object
  • can apply a proxy-like behavior by 2 ways
    1. directly augmenting an object during access
    2. through composable wrappers
       • this is better and safer because it does not modify the original object

typically, this pattern is used when

• need to optimize or preprocess data when the consumer accesses it
• caching remotely access data
• logging
• encryption
• simulating private and inaccessible properties
• data validation

Why use a proxy

• use an extra level of indirection to support distributed, controlled, or conditional access
• add a wrapper and delegation to protect the real component from undue complexity

Structure

1. The Service Interface declares the interface of the Service.

   • The proxy must follow this interface to be able to disguise itself as a service object.

2. The Service is a class that provides some useful business logic.

3. The Proxy class has a reference field that points to a service object.

   • After the proxy finishes its processing (e.g., lazy initialization, logging, access control, caching, etc.), it passes the request to the service object.
   • Usually, proxies manage the full lifecycle of their service objects.

4. The Client should work with both services and proxies via the same interface.

   • This way you can pass a proxy into any code that expects a service object.

Summary

• the proxy pattern allows us to create placeholder wrappers for objects
• a proxy object allows external access control to the object
• implements the same interface as the original object
• the proxy api (ES6, ES2015) enables the creation of proxy wrappers for objects
• proxy implements traps for various kinds of access augmentation
• great for implementing middleware mechanism for caching, logging, encryption, and other kinds of augmented functionality as a virtualized proxy