Balking

The Balking pattern is a behavioral pattern that allows an object to postpone the execution of a request until a specific condition is met. It essentially prevents an object from performing an action if it’s already in a particular state, often a “busy” or “occupied” state. This is achieved by delegating the responsibility of handling the request to a state object, which determines whether the request can be fulfilled immediately or needs to be deferred.

This pattern is useful when an operation is resource-intensive or requires exclusive access to a resource, and you want to avoid contention or errors that might occur if multiple requests are attempted concurrently. It provides a clean way to manage state and control access to critical sections of code, ensuring that operations are performed only when the object is ready.

Usage

The Balking pattern is commonly used in scenarios like:

• Asynchronous Task Queues: Preventing multiple submissions of the same task when the queue is already processing one.
• Resource Management: Controlling access to a limited number of resources (e.g., database connections, file handles) by delaying requests until a resource becomes available.
• Event Handling: Deferring the processing of events when the system is overloaded or in a critical state.
• UI Interactions: Disabling buttons or input fields while a long-running operation is in progress to prevent multiple triggers.

Examples

1. Java’s java.util.concurrent.BlockingQueue: Implementations like LinkedBlockingQueue internally use a mechanism similar to Balking. When a producer attempts to add an element to a full queue (a “busy” state), the put() method blocks until space becomes available, effectively “balking” the immediate addition.

2. Thread Pools: Most thread pool implementations employ a Balking-like strategy. When all threads in the pool are occupied, incoming tasks are queued. The task submission “balks” – it doesn’t immediately start executing – until a thread becomes free to pick it up from the queue. Libraries like java.util.concurrent provide ExecutorService which manages this behavior.

3. Asynchronous JavaScript and Promises: While not a direct implementation, the concept of awaiting a Promise before performing an action embodies the Balking pattern. If a resource is not yet available (the Promise is not yet resolved), the execution “balks” until the resource is ready.