WCF Waiting for Database Change

Introduction

In this article, we will explore a common issue in WCF (Windows Communication Foundation) applications that involves waiting for changes to a database. Specifically, we will delve into the scenario where a client application sends a request to a WCF service, which then saves the task in a database and waits for it to be completed. We will examine how this can be achieved without polling the database repeatedly.

Background

WCF is a framework provided by Microsoft for building communication between applications. It supports various protocols such as WS-* (Windows Communication Foundation) and HTTP/HTTPS. WCF services are typically self-hosted, meaning they run on the same machine as the application that generates them.

Database Changes

In this scenario, we have two clients: a PHP website running on Client 1 and a WCF client running on Client 2. The database in question is an SQL Server 2008 R2 instance.

The flow of events can be summarized as follows:

Client 1 sends a request to the WCF service (Client 2).
The WCF service saves the task in the database and waits for it to be completed.
When the task is finished, the WCF service updates the status of the task in the database.

The challenge here is that we need to implement this flow without polling the database repeatedly. This is known as an “event-driven” architecture where the WCF service only checks for changes to the task when a new one is received or when the current task is completed.

How it Can be Done Without Polling

There are several ways to achieve this without polling the database:

1. Message Queue

One approach is to use a message queue like MSMQ (Microsoft Message Queue) or Azure Service Bus. These queues provide a way for messages to be stored and retrieved asynchronously.

When Client 1 sends a request, the WCF service can enqueue the task in the message queue with an associated deadline. The WCF service can then poll the message queue periodically to check if any new tasks have been received before checking on the status of its own task.

However, this approach introduces additional complexity and overhead due to the need for message queue management.

2. Event-Driven Architecture (EDA)

Another approach is to use an event-driven architecture where the WCF service publishes events related to task completion.

When a client completes a task, it can publish an “event” that the WCF service can listen to and update accordingly.

This approach eliminates the need for polling but requires additional infrastructure to manage events.

3. Database-Trigger

A simpler approach is to use database triggers.

When the WCF service saves a new task in the database, it can also create a trigger on the table that updates the status of the task when any changes occur.

The WCF service can then listen for these changes and update its own task accordingly.

This approach does not require polling or message queues but may introduce additional complexity due to the need for trigger management.

4. Windows Service

Another option is to use a Windows Service that runs in the background and updates the status of tasks on behalf of the WCF service.

When Client 1 sends a request, the WCF service can enqueue it with an associated deadline. The WCF service can then start this Windows Service which periodically checks for new requests or task completion events.

This approach introduces additional complexity due to the need for a separate process but eliminates the need for polling and message queues.

Implementation Example

To illustrate these concepts, let’s consider an implementation example using the event-driven architecture approach:

{< highlight csharp >}
using System;
using System.Timers;

namespace WcfService
{
    public class TaskService : ITaskService
    {
        private readonly IObservable<Task> _tasks;
        private readonly Timer _timer;

        public TaskService(IObservable<Task> tasks)
        {
            _tasks = tasks;
            _timer = new Timer(1000);
            _timer.Elapsed += (sender, args) => UpdateTasks();
        }

        public void EnqueueTask(Task task)
        {
            // Create an event handler to update the task status
            var handler = new TaskEventHandler(task.Id);
            _tasks.OnNext += handler;
        }

        private void UpdateTasks()
        {
            // Check if any tasks have been completed
            _tasks.OnCompleted?.Invoke();
        }
    }

    public class TaskHandler : IObserver<Task>
    {
        private readonly int _id;

        public TaskHandler(int id)
        {
            _id = id;
        }

        public void OnNext(Task value)
        {
            // Update the task status
            value.Status = TaskStatus.Completed;
        }

        public void OnCompleted()
        {
            Console.WriteLine($"Task {_id} has been completed");
        }
    }
}
{</ highlight >}

In this example, we have a TaskService that uses an observable to listen for new tasks. When a task is received, it creates an event handler (TaskHandler) that updates the task status.

The TaskService also starts a timer that periodically checks if any tasks have been completed.

This implementation demonstrates how the WCF service can wait for changes to the database without polling by using events and async processing.

Last modified on 2024-06-23