Introduction:

In today's world, software systems are becoming more complex, distributed, and dependent on external services. These services could fail or become unresponsive, causing the entire system to fail or slow down, leading to a degraded user experience. To prevent these types of failures, developers can use the circuit breaker pattern.

The circuit breaker pattern is a design pattern used to improve the resiliency and fault tolerance of a software system. It prevents a service from continuously making requests to an unresponsive or failing service, causing the system to degrade or fail. The pattern uses a "circuit breaker" to detect and break the connection to the unresponsive service, allowing the system to continue operating.

In this article, we will discuss how to implement the circuit breaker pattern in .NET Core applications.

Implementing Circuit Breaker Pattern in .NET Core Applications:

Step 1: Install the Polly NuGet Package

To implement the circuit breaker pattern, we will use the Polly NuGet package. Polly is a .NET resilience and transient-fault-handling library that allows us to implement the circuit breaker pattern easily.

To install the Polly NuGet package, open the NuGet package manager console and run the following command:

Install-Package Polly

Step 2: Create a Circuit Breaker Policy

To create a circuit breaker policy, we need to define the conditions under which the circuit breaker should be opened and closed. We can do this using the CircuitBreakerPolicy class provided by the Polly library.

Here is an example of creating a circuit breaker policy:

var circuitBreakerPolicy = Policy
    .Handle<Exception>()
    .CircuitBreaker(
        exceptionsAllowedBeforeBreaking: 3,
        durationOfBreak: TimeSpan.FromSeconds(30)
    );

In this example, the circuit breaker policy is configured to break the circuit after three exceptions occur within a 30-second window.

Step 3: Wrap External Service Calls in the Circuit Breaker Policy

To wrap external service calls in the circuit breaker policy, we need to use the Execute method provided by the Polly library.

Here is an example of wrapping an external service call in the circuit breaker policy:

var result = circuitBreakerPolicy.Execute(() =>
{
    // Make external service call here
});

In this example, the external service call is made inside the Execute method, which is wrapped in the circuit breaker policy. If the circuit breaker is open, the Execute method will throw a BrokenCircuitException.

Step 4: Handle Broken Circuit Exceptions

To handle broken circuit exceptions, we need to catch the BrokenCircuitException thrown by the Execute method.

Here is an example of handling broken circuit exceptions:

try
{
    var result = circuitBreakerPolicy.Execute(() =>
    {
        // Make external service call here
    });
}
catch (BrokenCircuitException ex)
{
    // Handle the exception here
}

In this example, the BrokenCircuitException is caught in a try-catch block, and we can handle the exception in the catch block.

Step 5: Update Circuit Breaker State

When the circuit breaker is open, we need to update its state periodically to check if the external service is available again. We can do this using the Isolate method provided by the Polly Library.

Here is an example of updating the circuit breaker state:

circuitBreakerPolicy.Execute(() =>
{
    // Check if external service is available
}).Isolate();

In this example, we use the Execute method to check if the external service is available, and then call the Isolate method to update the circuit breaker state.

Step 6: Implementing a Fallback Strategy

When the circuit breaker is open, we may want to implement a fallback strategy to provide a degraded user experience instead of failing completely. We can do this using the Fallback method provided by the Polly Library.

Here is an example of implementing a fallback strategy:

var fallbackResult = "Fallback result";

var result = circuitBreakerPolicy.Execute(() =>
{
    // Make external service call here
},
fallbackAction: () =>
{
    return fallbackResult;
});

In this example, we provide a fallback action to the Execute method using the fallbackAction parameter. If the circuit breaker is open, the fallback action will be executed instead of the external service call.

Conclusion:

The circuit breaker pattern is an effective way to improve the resiliency and fault tolerance of software systems. By using the Polly library, we can easily implement the circuit breaker pattern in .NET Core applications. We can define the conditions under which the circuit breaker should be opened and closed, wrap external service calls in the circuit breaker policy, handle broken circuit exceptions, update the circuit breaker state, and implement a fallback strategy.