DurableMediator.OutOfProcess 3.0.0-preview

Durable Mediator

Durable Mediator is an extension to the Durable Task library which allows for running MediatR Requests as activities in orchestrations without any complex ceremony.

Getting started

To get started with running orchestrations which call MediatR Requests as activities, which are called "Workflows", follow these steps:

First start by defining a workflow request:

public record WorkflowRequest(Guid SomeId) : IWorkflowRequest
{
    public string WorkflowName => "Human readable workflow name";
    public string InstanceId => SomeId.ToString();
};

Also, create a IRequest that needs to be called from the workflow:

public record MediatorRequest(Guid SomeId) : IRequest;

Create a request handler that handles this MediatR request:

public class MediatorRequestHandler : IRequestHandler<MediatorRequest>
{
    private readonly ILogger<MediatorRequestHandler> _logger;

    public RequestAHandler(ILogger<MediatorRequestHandler> logger)
    {
        _logger = logger;
    }

    public Task<Unit> Handle(MediatorRequest request, CancellationToken cancellationToken)
    {
        _logger.LogInformation("Processing RequestA");

        return Task.FromResult(Unit.Value);
    }
}

Create a workflow that handles the WorkflowRequest:

public class ExampleWorkflow : IWorkflow<WorkflowRequest, Unit>
{
    private readonly ILogger<ABCWorkflow> _logger;

    public ExampleWorkflow(ILogger<ABCWorkflow> logger) 
    {
        _logger = logger;
    }

    public async Task<Unit> OrchestrateAsync(IWorkflowExecution<WorkflowRequest> execution)
    {
        var logger = execution.OrchestrationContext.CreateReplaySafeLogger(_logger);

        logger.LogInformation("Start with workflow");

        await execution.ExecuteAsync(new MediatorRequest(execution.Request.SomeId));

        logger.LogInformation("Workflow done");

        return Unit.Value;
    }
}

Create a Azure Function that triggers this workflow:

public static class WorkflowTrigger
{
    [FunctionName(nameof(WorkflowTrigger))]
    public static async Task<IActionResult> TriggerOrchestratorAsync(
        [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = "workflow")] HttpRequestMessage req,
        [Workflow] IWorkflowStarter starter)
    {
        var startDetails = await starter.StartNewAsync(new WorkflowRequest(Guid.NewGuid()));

        return new AcceptedResult("", startDetails);
    }
}

In your function app startup, add the required services by including builder.AddDurableMediator(typeof(Startup));.

When running your function app you will see that next to the WorkflowTrigger Http Trigger function, an Orchestration Trigger function called "WorkflowRequest" and a "DurableMediatorEntity" Entity Trigger function are added. When http function triggers the start of the workflow, the orchestration function will orchestrate the workflow and invoke ExampleWorkflow. Each call to the execution.ExecuteAsync will trigger the DurableMediatorEntity to execute the MediatR Request in a separate activity after which the orchestration resumes. No more calling context.CallActivity and guessing what parameters to pass in.

The execution exposes the IDurableOrchestrationContext from the Durable Task library, giving full access to creating timers for durable delays, locking entities for critical sections, or wait for external events. The execution also exposes methods from ISubWorkflowOrchestrator, which allows workflows to initiate other workflows, and even await their responses, making it easy to compose workflows.

Examples

See the WorkflowFunctionApp in the example folder for more examples.

Preview: Activity history and durable entities

Use builder.AddDurableMediator(useExperimentalEntityExecution: true, typeof(Startup)); to enable entity execution which allows for:

• Enable capturing request and response data to and from durable mediator invocations. This allows IWorkflowManagement.GetWorkflowAsync to output that data, making it possible to access the exact requests and responses after the workflow has completed.

The downside of this way feature is that every workflow will create a durable entity history record. This record will be stored in the orchestration history storage (under @durablemediator@{instanceId}) and needs to be cleared using IDurableEntityClient.CleanEntityStorageAsync.