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Home/Blog/define proxy, how to represent the proxy structure in a UML diagram?
define proxy, how to represent the proxy structure in a UML diagram?
PYPROXY·Jun 09, 2025
The proxy pattern is a design pattern commonly used in object-oriented software development. It involves providing an object that acts as a substitute for another object. In this pattern, the proxy class controls access to the real object, often adding an extra layer of control, such as lazy initialization, access control, logging, or caching. A proxy can serve different purposes, including remote proxies, virtual proxies, and protective proxies.
UML diagrams are a powerful tool for visualizing the structure and behavior of software systems. In the case of the proxy pattern, UML class and sequence diagrams are typically used to depict the relationships and interactions between the proxy, the real object, and the client. The proxy pattern can be effectively represented in UML through well-defined class diagrams, providing clarity on the roles and responsibilities of the proxy and the real object.
Understanding the Proxy Pattern
The proxy pattern is a structural design pattern that provides an object representing another object. This intermediary object (the proxy) controls access to the real object (also called the subject). The proxy can be used to add additional functionality, such as controlling access to the real object, caching, or logging. There are several types of proxies, including:
1. Virtual Proxy: This type of proxy delays the creation of the real object until it is actually needed. For example, in an image viewer application, a virtual proxy could be used to load high-resolution images only when they are viewed.
2. Remote Proxy: Used for objects that are in a different address space, often over a network. This proxy handles the communication between the client and the remote object.
3. Protection Proxy: This type of proxy controls access to the real object based on permissions. It is commonly used in scenarios where an object’s operations need to be controlled by security policies.
4. Cache Proxy: This proxy maintains a cache of previously accessed objects to improve performance. It is often used when fetching data from a remote or slow data source.
UML Class Diagram Representation of Proxy Pattern
To represent the proxy pattern using UML, the class diagram is an essential tool. The class diagram will typically consist of the following main components:
1. Client: This is the object that interacts with the proxy. The client does not directly interact with the real object but instead communicates with the proxy.
2. Proxy: The proxy is a class that implements the same interface as the real object. It may hold a reference to the real object and control access to it.
3. RealSubject: This is the actual object that the proxy represents. The real subject implements the same interface as the proxy, ensuring the proxy can act in the same way.
In the UML class diagram, the Client will have a relationship with the Proxy, and the Proxy will have a relationship with the RealSubject. This shows that the Client does not interact directly with the RealSubject but instead interacts with the Proxy.
UML Sequence Diagram for Proxy Pattern
While class diagrams provide the static structure, sequence diagrams help in understanding the dynamic behavior of the system. In a sequence diagram for the proxy pattern, the flow of interaction between the client, proxy, and real subject is represented.
1. Client: The client sends a request to the Proxy.
2. Proxy: The proxy decides whether it should forward the request to the real subject or handle the request itself. For example, the proxy could check if the object needs to be created or if it can use a cached value.
3. RealSubject: If necessary, the proxy forwards the request to the real object (the RealSubject), which then performs the required operation.
4. Return: The real object sends a response back to the proxy, which then returns the response to the client.
The sequence diagram illustrates the message passing between these components and helps to clarify the responsibilities of each object in the proxy pattern.
Advantages of Using the Proxy Pattern
The proxy pattern offers several key advantages:
1. Lazy Initialization: A virtual proxy allows for delaying the creation of an expensive object until it is actually needed. This improves the system’s startup performance and resource utilization.
2. Access Control: A proxy can be used to add security measures by controlling access to the real object. For instance, a protection proxy can restrict access based on the user's permissions.
3. Performance Optimization: Cache proxies improve system performance by storing frequently accessed data and reducing the need to repeatedly fetch it from a slower source.
4. Decoupling: The proxy pattern decouples the client from the real object, which can help in scenarios where the real object might change, or in remote communication, where direct access to the object is not possible.
Challenges and Considerations
While the proxy pattern offers various benefits, it also comes with some challenges:
1. Complexity: Adding proxy layers can introduce unnecessary complexity. If the proxy does not provide significant value (such as added security or performance improvements), it can make the code harder to maintain.
2. Performance Overhead: In some cases, using a proxy might introduce additional overhead, especially if the proxy is performing complex operations before forwarding the request to the real object.
3. Compatibility Issues: Ensuring that the proxy maintains compatibility with the real object’s interface can sometimes be difficult, particularly when dealing with more complex objects or legacy systems.
Real-World Applications of Proxy Pattern
The proxy pattern is widely used in various domains, including:
1. Virtual Proxies in Image Loading: In applications that deal with large media files (e.g., image or video streaming services), virtual proxies can be used to delay loading of the actual media files until they are requested by the user.
2. Remote Proxies in Distributed Systems: In distributed systems, such as microservices or client-server architectures, remote proxies are commonly used to represent objects that reside in a different physical location.
3. Protection Proxies in Security Systems: Protection proxies are often used in applications where users need different levels of access to resources. For example, in a banking application, a proxy could be used to ensure that only authorized users can access sensitive account information.
4. Cache Proxies in Web Services: Cache proxies are used in web services to store frequently accessed data, reducing the need to access slower databases or external APIs repeatedly.
The proxy pattern is a versatile and powerful design pattern that helps developers address various challenges, such as controlling access to resources, improving performance, and adding security measures. Using UML diagrams to represent the proxy structure provides clarity in understanding the roles and responsibilities of the components involved. While the proxy pattern offers many advantages, it is essential to consider the potential complexity and performance overhead that may arise from adding proxy layers to a system. In the end, when used appropriately, the proxy pattern can significantly enhance the flexibility and scalability of software systems.
