Showing posts with label Read-Write Lock. Show all posts

Thursday, 15 August 2013

Scheduler Design Pattern

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What is the Scheduler Design Pattern?

In concurrent programming, managing the execution timing and coordination among threads accessing shared resources is crucial. The Scheduler Pattern is a specialized concurrency design pattern that provides explicit control over thread scheduling.

Core Definition: The Scheduler design pattern is used to explicitly control when threads may execute single-threaded code blocks. It uses an explicit scheduler object that coordinates and sequences waiting threads.

Real-World Example: File Read & Write Operations

Consider a shared text file on the file system named names.txt containing employee names (such as "David" and "Steve"). In our system, we have three threads interacting with this file:

Two Reader Threads: Read contents from names.txt and print them to the console.
One Writer Thread: Appends new names into names.txt.

If both the writer and readers access the file at the exact same moment without coordination, data corruption, file lock errors, or stale reads occur.

How the Scheduler Object Coordinates Threads

Instead of allowing threads to access the file arbitrarily, a Scheduler object explicitly sequences and coordinates when each thread gets access:

The Scheduler can schedule the reader threads to read from names.txt at fixed intervals (for instance, every 30 minutes).
The Scheduler can schedule the writer thread to execute and write updates at designated intervals (for instance, every 45 minutes).
It ensures write operations execute in isolation without clashing with ongoing reads, keeping thread functionalities well-ordered and conflict-free.

Relationship with the Read-Write Lock Pattern

The Read-Write Lock Pattern is frequently implemented using the Scheduler Pattern. By placing a scheduler in charge of lock acquisition, the system can enforce fairness—ensuring writer threads are not starved by a constant stream of reader threads, and vice-versa.

Key Benefits of the Scheduler Pattern

Explicit Execution Sequencing: Gives application logic complete control over when and in what order waiting threads run.
Prevents Thread Starvation: Ensures fair access to shared resources across competing tasks.
Separates Scheduling from Execution: Keeps business task logic clean and decoupled from thread timing and synchronization rules.