Flow extension
==============

*Flow* in the context of Tapeti is inspired by what is referred to as a Saga or Conversation in messaging. It enables a controller to communicate with other services, temporarily yielding it's execution while waiting for a response. When the response arrives the controller will resume, retaining the original state of it's public fields.

This process is fully asynchronous, the service initiating the flow can be restarted and the flow will continue when the service is back up (assuming the queues are durable and a persistent flow state store is used).


Request - response pattern
--------------------------
Tapeti implements the request - response pattern by allowing a message handler method to simply return the response message. Tapeti Flow extends on this concept by allowing the sender of the request to maintain it's state for when the response arrives.

See :doc:`indepth` on defining request - response messages.

Enabling Tapeti Flow
--------------------
To enable the use of Tapeti Flow, install the Tapeti.Flow NuGet package and call ``WithFlow()`` when setting up your TapetiConfig:

::

  var config = new TapetiConfig(new SimpleInjectorDependencyResolver(container))
      .WithFlow()
      .RegisterAllControllers()
      .Build();

Starting a flow
---------------
To start a new flow you need to obtain an IFlowStarter from your IoC container. It has one method in various overloads: ``Start``.

Flow requires all methods participating in the flow, including the starting method, to be in the same controller. This allows the state to be stored and restored when the flow continues. The ``IFlowStarter.Start`` call does not need to be in the controller class.

The controller type is passed as a generic parameter. The first parameter to the Start method is a method selector. This defines which method in the controller is called as soon as the flow is initialised.

::

  await flowStart.Start<QueryBunniesController>(c => c.StartFlow);

The start method can have any name, but must be annotated with the ``[Start]`` attribute. This ensures it is not recognized as a message handler. The start method and any further continuation methods must return either Task<IYieldPoint> (for asynchronous methods) or simply IYieldPoint (for synchronous methods).

::

  [MessageController]
  [DynamicQueue]
  public class QueryBunniesController
  {
      public DateTime RequestStart { get; set; }

      [Start]
      public IYieldPoint StartFlow()
      {
          RequestStart = DateTime.UtcNow();
      }
  }



Often you'll want to pass some initial information to the flow. The Start method allows one parameter. If you need more information, bundle it in a class or struct.

::

  await flowStart.Start<QueryBunniesController>(c => c.StartFlow, "pink");

  [MessageController]
  [DynamicQueue]
  public class QueryBunniesController
  {
      public DateTime RequestStart { get; set; }

      [Start]
      public IYieldPoint StartFlow(string colorFilter)
      {
          RequestStart = DateTime.UtcNow();
      }
  }


.. note:: Every time a flow is started or continued a new instance of the controller is created. All public fields in the controller are considered part of the state and will be restored when a response arrives, private and protected fields are not. Public fields must be serializable to JSON (using JSON.NET) to retain their value when a flow continues. Try to minimize the amount of state as it is cached in memory until the flow ends.

Continuing a flow
-----------------
When starting a flow you're most likely want to start with a request message. Similarly, when continuing a flow you have the option to follow it up with another request and prolong the flow. This behaviour is controlled by the IYieldPoint that must be returned from the start and continuation handlers. To get an IYieldPoint you need to inject the IFlowProvider into your controller.

IFlowProvider has a method ``YieldWithRequest`` which sends the provided request message and restores the controller when the response arrives, calling the response handler method you pass along to it.

The response handler must be marked with the ``[Continuation]`` attribute. This ensures it is never called for broadcast messages, only when the response for our specific request arrives. It must also return an IYieldPoint or Task<IYieldPoint> itself.

If the response handler is not asynchronous, use ``YieldWithRequestSync`` instead, as used in the example below:

::

  [MessageController]
  [DynamicQueue]
  public class QueryBunniesController
  {
      private IFlowProvider flowProvider;

      public DateTime RequestStart { get; set; }


      public QueryBunniesController(IFlowProvider flowProvider)
      {
          this.flowProvider = flowProvider;
      }

      [Start]
      public IYieldPoint StartFlow(string colorFilter)
      {
          RequestStart = DateTime.UtcNow();

          var request = new BunnyCountRequestMessage
          {
              ColorFilter = colorFilter
          };

          return flowProvider.YieldWithRequestSync<BunnyCountRequestMessage, BunnyCountResponseMessage>
              (request, HandleBunnyCountResponse);
      }


      [Continuation]
      public IYieldPoint HandleBunnyCountResponse(BunnyCountResponseMessage message)
      {
          // Handle the response. The original RequestStart is available here as well.
      }
  }

You can once again return a ``YieldWithRequest``, or end it.

Ending a flow
-------------
To end the flow and dispose of any stored state, return an end yieldpoint:

::

      [Continuation]
      public IYieldPoint HandleBunnyCountResponse(BunnyCountResponseMessage message)
      {
          // Handle the response.

          return flowProvider.End();
      }


Flows started by a (request) message
------------------------------------
Instead of manually starting a flow, you can also start one in response to an incoming message. You do not need access to the IFlowStarter in that case, simply return an IYieldPoint from a regular message handler:

::

  [MessageController]
  [DurableQueue("hutch")]
  public class HutchController
  {
      private IBunnyRepository repository;
      private IFlowProvider flowProvider;

      public string ColorFilter { get; set; }


      public HutchController(IBunnyRepository repository, IFlowProvider flowProvider)
      {
          this.repository = repository;
          this.flowProvider = flowProvider;
      }

      public IYieldPoint HandleCountRequest(BunnyCountRequestMessage message)
      {
          ColorFilter = message.ColorFilter;

          return flowProvider.YieldWithRequestSync<CheckAccessRequestMessage, CheckAccessResponseMessage>
            (
                new CheckAccessRequestMessage
                {
                    Username = "hutch"
                },
                HandleCheckAccessResponseMessage
            );
      }


      [Continuation]
      public IYieldPoint HandleCheckAccessResponseMessage(CheckAccessResponseMessage message)
      {
          // We must provide a response to our original BunnyCountRequestMessage
          return flowProvider.EndWithResponse(new BunnyCountResponseMessage
          {
              Count = message.HasAccess ? await repository.Count(ColorFilter) : 0
          });
  }

.. note:: If the message that started the flow was a request message, you must end the flow with EndWithResponse or you will get an exception. Likewise, if the message was not a request message, you must end the flow with End.


Parallel requests
-----------------
When you want to send out more than one request, you could chain them in the response handler for each message. An easier way is to use ``YieldWithParallelRequest``. It returns a parallel request builder to which you can add one or more requests to be sent out, each with it's own response handler. In the end, the Yield method of the builder can be used to create a YieldPoint. It also specifies the converge method which is called when all responses have been handled.

An example:

::

  public IYieldPoint HandleBirthdayMessage(RabbitBirthdayMessage message)
  {
      var sendCardRequest = new SendCardRequestMessage
      {
          RabbitID = message.RabbitID,
          Age = message.Age,
          Style = CardStyles.Funny
      };

      var doctorAppointmentMessage = new DoctorAppointmentRequestMessage
      {
          RabbitID = message.RabbitID,
          Reason = "Yearly checkup"
      };

      return flowProvider.YieldWithParallelRequest()
          .AddRequestSync<SendCardRequestMessage, SendCardResponseMessage>(
            sendCardRequest, HandleCardResponse)

          .AddRequestSync<DoctorAppointmentRequestMessage, DoctorAppointmentResponseMessage>(
            doctorAppointmentMessage, HandleDoctorAppointmentResponse)

          .YieldSync(ContinueAfterResponses);
  }

  [Continuation]
  public void HandleCardResponse(SendCardResponseMessage message)
  {
      // Handle card response. For example, store the result in a public field
  }

  [Continuation]
  public void HandleDoctorAppointmentResponse(DoctorAppointmentResponseMessage message)
  {
      // Handle appointment response. Note that the order of the responses is not guaranteed,
      // but the handlers will never run at the same time, so it is safe to access
      // and manipulate the public fields of the controller.
  }

  private IYieldPoint ContinueAfterResponses()
  {
      // Perform further operations on the results stored in the public fields

      // This flow did not start with a request message, so end it normally
      return flowProvider.End();
  }


A few things to note:

#) The response handlers do not return an IYieldPoint themselves, but void (for AddRequestSync) or Task (for AddRequest). Therefore they can not influence the flow. Instead the converge method as passed to Yield or YieldSync determines how the flow continues. It is called immediately after the last response handler.
#) The converge method must be private, as it is not a valid message handler in itself.
#) You must add at least one request, or specify the NoRequestsBehaviour parameter for Yield/YieldSync explicitly.

Note that you do not have to perform all the operations in one go. You can store the result of ``YieldWithParallelRequest`` and conditionally call ``AddRequest`` or ``AddRequestSync`` as many times as required.


Adding requests to a parallel flow
----------------------------------
As mentioned above, you can not start a new parallel request in the same flow while the current one has not converged yet. This is enforced by the response handlers not returning an IYieldPoint.

You can however add requests to the current parallel request while handling one of the responses. This is equivalent to adding the request to the parallel flow builder initially, and will delay calling the converge method until a response has been received to this new request as well.

To add an additional request, include a second parameter in the continuation method of type IFlowParallelRequest. The continuation method also needs to be async to be able to await the IFlowParallelRequest.AddRequest[Sync] methods. For example:

::

  [Continuation]
  public async Task HandleDoctorAppointmentResponse(DoctorAppointmentResponseMessage appointment,
      IFlowParallelRequest parallelRequest)
  {
      // Now that we have the appointment details, we can query the patient data
      await parallelRequest.AddRequestSync<PatientRequestMessage, PatientResponseMessage>(
          new PatientRequestMessage
          {
              PatientID = appointment.PatientID
          },
          HandlePatientResponse);
  }



Persistent state
----------------
By default flow state is only preserved while the service is running. To persist the flow state across restarts and reboots, provide an implementation of IFlowRepository to ``WithFlow()``.

::

  var config = new TapetiConfig(new SimpleInjectorDependencyResolver(container))
      .WithFlow(new MyFlowRepository())
      .RegisterAllControllers()
      .Build();


Tapeti.Flow includes an implementation for SQL server you can use as well. First, make sure your database contains a table to store flow state:

::

    create table Flow
    (
        FlowID uniqueidentifier not null,
        CreationTime datetime2(3) not null,
        StateJson nvarchar(max) null,
        constraint PK_Flow primary key clustered(FlowID)
    );

Then install the Tapeti.Flow.SQL NuGet package and register the SqlConnectionFlowRepository by passing it to WithFlow, or by using the ``WithFlowSqlRepository`` extension method before calling ``WithFlow``:

::

  var config = new TapetiConfig(new SimpleInjectorDependencyResolver(container))
      .WithFlowSqlRepository("Server=localhost;Database=TapetiTest;Integrated Security=true")
      .WithFlow()
      .RegisterAllControllers()
      .Build();


.. caution:: The controller and method names for response handlers and converge methods are stored in the flow and must be valid when they are loaded again. Keep that in mind if you want to refactor the code; either keep the original class and method temporarily for backwards compatibility, optionally redirecting them internally to the new code, or make sure there are no persisted flows remaining.