rspec/rules/S6419/csharp/rule.adoc

== Why is this an issue?

An Azure Function should be stateless as there's no control over where and when function instances are provisioned and de-provisioned. Managing and storing data/state between requests can lead to inconsistencies.
If, for any reason, you need to have a stateful function, consider using the Durable Functions extension of Azure Functions.

// If you want to factorize the description uncomment the following line and create the file.
//include::../description.adoc[]

=== Noncompliant code example

[source,csharp]
----
    public static class HttpExample
    {
        private static readonly int port = 2000;
        private static int numOfRequests = 1;

        [FunctionName("HttpExample")]
        public static async Task<IActionResult> Run( [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = null)] HttpRequest request, ILogger log)
        {
            numOfRequests += 1; // Noncompliant
            log.LogInformation($"Number of POST requests is {numOfRequests}.");

            string responseMessage = $"HttpRequest was made on port {port}."; // Compliant, state is only read.

            return new OkObjectResult(responseMessage);
        }
    }
----

=== Compliant solution

[source,csharp]
----
    public static class HttpExample
    {
        private static readonly int port = 2000;

        [FunctionName("HttpExample")]
        public static async Task<IActionResult> Run( [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = null)] HttpRequest request, ILogger log)
        {
            // A compliant solution would be to manage the `numOfRequests` with an entity function or would use storage (e.g., Azure Blob storage, Azure Queue Storage)
            // to share the state between functions.

            string responseMessage = $"HttpRequest was made on port {port}.";

            return new OkObjectResult(responseMessage);
        }
    }
----

== Resources

* https://docs.microsoft.com/en-us/azure/azure-functions/performance-reliability#write-functions-to-be-stateless[Improve the performance and reliability of Azure Functions]
* https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-overview?tabs=csharp[Durable Functions Overview]
* https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-entities?tabs=csharp[Durable Functions - Entity Functions]
* https://docs.microsoft.com/en-us/azure/azure-functions/storage-considerations[Storage considerations for Azure Functions]