Proxy

Also known as¶

Surrogate

Intent¶

Provide a surrogate or placeholder for another object to control access to it.

Explanation¶

Real-world example¶

Imagine a tower where the local wizards go to study their spells. The ivory tower can only be accessed through a proxy which ensures that only the first three wizards can enter. Here the proxy represents the functionality of the tower and adds access control to it.

In plain words¶

Using the proxy pattern, a class represents the functionality of another class.

Wikipedia says¶

A proxy, in its most general form, is a class functioning as an interface to something else. A proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked.

sequenceDiagram
    participant Client
    participant WizardTowerProxy
    participant IvoryTower

    Client->>WizardTowerProxy: enter(wizard)
    alt numWizards < limit
        WizardTowerProxy->>IvoryTower: enter(wizard)
        IvoryTower-->>WizardTowerProxy: done
        WizardTowerProxy-->>Client: wizard enters
    else limit reached
        WizardTowerProxy-->>Client: wizard denied
    end

Programmatic Example¶

Taking our wizard tower example from above. Firstly we have the WizardTower interface and the IvoryTower class.

internal interface WizardTower {
    fun enter(wizard: Wizard)
}

internal class IvoryTower : WizardTower {
    override fun enter(wizard: Wizard) =
        logger.info("$wizard enters the tower.")
}

Then a simple Wizard class.

internal class Wizard(private val name: String) {
    override fun toString() = name
}

Then we have the WizardTowerProxy to add access control to WizardTower.

internal class WizardTowerProxy(
    private val tower: WizardTower,
) : WizardTower {
    private var numWizards = 0

    override fun enter(wizard: Wizard) {
        if (numWizards < NUM_WIZARDS_ALLOWED) {
            tower.enter(wizard)
            numWizards++
        } else {
            logger.info(
                "$wizard is not allowed to enter!"
            )
        }
    }

    companion object {
        private const val NUM_WIZARDS_ALLOWED = 3
    }
}

And here is the tower entering scenario.

val proxy = WizardTowerProxy(IvoryTower())
proxy.enter(Wizard("Red wizard"))
proxy.enter(Wizard("White wizard"))
proxy.enter(Wizard("Black wizard"))
proxy.enter(Wizard("Green wizard"))
proxy.enter(Wizard("Brown wizard"))

Program output:

Red wizard enters the tower.
White wizard enters the tower.
Black wizard enters the tower.
Green wizard is not allowed to enter!
Brown wizard is not allowed to enter!

Class diagram¶

classDiagram
    class WizardTower {
        <<interface>>
        +enter(Wizard)
    }
    class IvoryTower {
        +enter(Wizard)
    }
    class Wizard {
        -name String
        +toString() String
    }
    class WizardTowerProxy {
        -NUM_WIZARDS_ALLOWED Int
        -numWizards Int
        -tower WizardTower
        +enter(Wizard)
    }
    WizardTowerProxy --> WizardTower : tower
    IvoryTower ..|> WizardTower
    WizardTowerProxy ..|> WizardTower

Applicability¶

Use the Proxy pattern when:

A remote proxy provides a local representative for an object in a different address space.
A virtual proxy creates expensive objects on demand.
A protection proxy controls access to the original object when objects should have different access rights.

Consequences¶

Benefits:

Controls access to the real object without clients knowing.
Can add functionality (logging, caching, access control) transparently.

Trade-offs:

Introduces an additional layer of indirection.
May add latency to object access.

Decorator: Decorator adds responsibilities to objects, while Proxy controls access to them.
Adapter: Adapter provides a different interface, while Proxy provides the same interface with controlled access.