The Request Pattern

This document extends the general Observer Pattern with the ability to respond.

Copyright and Language can be found in RFC1

Change Process

This document is governed by the Consensus-Oriented Specification System (COSS).


This pattern is a combination of “Observer Pattern” found in e.g. Qt and “Request-reply Pattern” found in e.g. web-server communication and is useful in inter-process communication between decoupled, yet associated components of a software architecture design; particularly GUI programming.

                     |          |      
 ____________        |  signal  |        _________
|            |------>|__________|------>|         |
|  graphics  |        __________        |  logic  |
|____________|<------|          |<------|_________|
                     |  signal  |       

Picture a graphical user interface (graphics) where a button represents performing a command; e.g. running a program.

Traditionally (e.g. in MVP), graphics isn’t actually performing any actions, but merely communicating that this is what the user has requested to have happen.

logic may be subscribed to the events of graphics. When the button within graphics is pressed, an event may be emitted that is received by logic. Once logic is finished performing the requested operation, logic may signal back to graphics that the action has been performed, so as to let graphics illustrate this to the user; e.g. with a notification widget.

In this example, the signal is a one-way stream of information. Data may only be passed from graphics to logic; subject to observer. However as we’ve just seen, there are two (2) signals for any action taking place.

In this scenario, graphics and logic are both observers and they are both subjects; e.g. they both request and they both respond.

# Subject on emitting, observer on receiving.
  subject          |          |        observer
 __________        |          |        _________
|          |------>|__________|------>|         |
|          |        __________        |         |
|__________|<------|          |<------|_________|
                   |          |       
  observer         |__________|        subject

The Request Pattern is a method of distilling this procedure into a single call; the request.

  subject                                observer
 __________         ___________         _________
|          |------>|           |------>|         |
|  button  |       |  request  |       |  logic  |

Here, an observer is connected to subject via a request. The button in this scenario is said to emit a request as opposed to a signal. When button emits a request, the subscribed observer performs the given action and returns a value.

# Trivial example in Python
#  --------------------------------
#  Marcus doesn't know his own age 
#  --------------------------------- 
>>> observer = type('Marcus', (object,), {})()
>>> observer.get_age = Request()
>>> observer.get_age()

# Trivial example in Python
#  --------------------------------
#  Mom to the rescue
#  --------------------------------- 
>>> observer = type('Marcus', (object,), {})()
>>> observer.get_age = Request()

>>> subject = type('Mom', (object,), {})()
>>> subject.give_age = lambda: 27

>>> observer.get_age.connect(subject.give_age)
>>> observer.get_age()


Multiple observers

As a side-effect to the given loose-coupling of this pattern, request objects may cope with multiple observers.

In the majority of cases, a request will have 0-1 observers. 0 meaning no one is there to answer the request - in which case it may simply return a null - 1 meaning there is exactly one component potentially capable of fullfilling the request. I say “potentially” because even though in the majority of cases, the return value will be what is expected, but the pattern allows for non-expected return values as well.

                    ___________        |         |
 __________        |           |------>| primary |  # In the cloud
|          |------>|           |<------|_________|
| graphics |       |  request  |        ___________
|__________|<------|           |------>|           |
                   |___________|<------| secondary |  # Local

Picture another graphical user interface. This one has a button within graphics which requests the factorial of a very high number, requiring phenomenal computational horse-power.

When the button is pressed, graphics sends a request to a worker in the cloud; primary, a remote workstation with plentiful of computational horse-power. When primary has completed performing this task, it returns a value to graphics.

Now, what if the graphics suddenly got cut-off from the internet? primary would no longer be there to fulfill its duties and yet the user is awaiting an answer. In this scenario, another observer may be subscribed to the request, secondary, to fill in for when primary fails. This observer is running the operation locally and thus is a little slower; but, it gets the job done.

This pattern of handling and discarding requests may be called the Strategy Pattern

# Multiple observers example
observer = type('Math', (object,), {})()
observer.get_factorial = pifou.signal.Request()

subject1 = type('CloudService', (object,), {})()
subject2 = type('LocalService', (object,), {})()

def compute_remote():
    # Fails..
    return None

def compute_local():
    return 27

subject1.give_factorial = compute_remotely  # First one to try
subject2.give_factorial = compute_locally