Request and Response Failure Handling and Recovery Package

A significant feature of the request and responsecommunications pattern that supports the transfer of information between an agent and a client. interaction modelmodel that defines how information is exchanged across an interface to enable interactions between independent systems. is the ability for either piece of equipment to detect a failure associated with either the requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. or responseresponse interface which responds to a request. actions. When either a failure or unexpected action occurs, the requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. and the responseresponse interface which responds to a request. portion of the interaction modelmodel that defines how information is exchanged across an interface to enable interactions between independent systems. can announce a FAIL state upon detecting a problem. The following are graphical models describing multiple scenarios where either the requesterentity that initiates a request for information in a communications exchange. or responderentity that responds to a request for information in a communications exchange. detects and reacts to a failure. In these examples, either the requesterentity that initiates a request for information in a communications exchange. or responderentity that responds to a request for information in a communications exchange. announces the detection of a failure by setting either the requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. or the responseresponse interface which responds to a request. state to FAIL.

Once a failure is detected, the interaction modelmodel that defines how information is exchanged across an interface to enable interactions between independent systems. provides information from each piece of equipment as they attempt to recover from a failure, reset all of their functions associated with the interfacemeans by which communication is achieved between independent systems. to their original state, and return to normal operation.

The following sections are scenarios that describe how pieces of equipment may react to different types of failures and how they indicate when they are again ready to request a service or respond to a request for service after recovering from those failures:

Responder Fails Immediately

In this scenario, a failure is detected by the responderentity that responds to a request for information in a communications exchange. immediately after a requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. for service has been initiated by the requesterentity that initiates a request for information in a communications exchange..

In this case, the requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. transitions to ACTIVE and the responderentity that responds to a request for information in a communications exchange. immediately detects a failure before it can transition the responseresponse interface which responds to a request. state to ACTIVE. When this occurs, the responderentity that responds to a request for information in a communications exchange. transitions the responseresponse interface which responds to a request. state to FAIL.

After detecting that the responderentity that responds to a request for information in a communications exchange. has transitioned its state to FAIL, the requesterentity that initiates a request for information in a communications exchange. MUST change its state to FAIL.

The requesterentity that initiates a request for information in a communications exchange., as part of clearing a failure, resets any partial actions that were initiated and attempts to return to a condition where it is again ready to request a service. If the recovery is successful, the requesterentity that initiates a request for information in a communications exchange. changes its state from FAIL to READY. If for some reason the requesterentity that initiates a request for information in a communications exchange. cannot return to a condition where it is again ready to request a service, it transitions its state from FAIL to NOT_READY.

The responderentity that responds to a request for information in a communications exchange., as part of clearing a failure, resets any partial actions that were initiated and attempts to return to a condition where it is again ready to perform a service. If the recovery is successful, the responderentity that responds to a request for information in a communications exchange. changes its responseresponse interface which responds to a request. state from FAIL to READY. If for some reason the responderentity that responds to a request for information in a communications exchange. is not again prepared to perform a service, it transitions its state from FAIL to NOT_READY.

Responder Fails While Providing a Service

This is the most common failure scenario. In this case, the responderentity that responds to a request for information in a communications exchange. will begin the actions required to provide a service. During these actions, the responderentity that responds to a request for information in a communications exchange. detects a failure and transitions its responseresponse interface which responds to a request. state to FAIL.

When a requesterentity that initiates a request for information in a communications exchange. detects a failure of a responderentity that responds to a request for information in a communications exchange., it transitions it state from ACTIVE to FAIL.

The requesterentity that initiates a request for information in a communications exchange. resets any partial actions that were initiated and attempts to return to a condition where it is again ready to request a service. If the recovery is successful, the requesterentity that initiates a request for information in a communications exchange. changes its state from FAIL to READY if the failure has been cleared and it is again prepared to request another service. If for some reason the requesterentity that initiates a request for information in a communications exchange. cannot return to a condition where it is again ready to request a service, it transitions its state from FAIL to NOT_READY.

The responderentity that responds to a request for information in a communications exchange., as part of clearing a failure, resets any partial actions that were initiated and attempts to return to a condition where it is again ready to perform a service. If the recovery is successful, the responderentity that responds to a request for information in a communications exchange. changes its responseresponse interface which responds to a request. state from FAIL to READY if it is again prepared to perform a service. If for some reason the responderentity that responds to a request for information in a communications exchange. is not again prepared to perform a service, it transitions its state from FAIL to NOT_READY.

Requester Failure During a Service Request

In this scenario, the responderentity that responds to a request for information in a communications exchange. will begin the actions required to provide a service. During these actions, the requesterentity that initiates a request for information in a communications exchange. detects a failure and transitions its requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. state to FAIL.

When the responderentity that responds to a request for information in a communications exchange. detects that the requesterentity that initiates a request for information in a communications exchange. has transitioned its requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. state to FAIL, the responderentity that responds to a request for information in a communications exchange. also transitions its responseresponse interface which responds to a request. state to FAIL.

The requesterentity that initiates a request for information in a communications exchange., as part of clearing a failure, resets any partial actions that were initiated and attempts to return to a condition where it is again ready to request a service. If the recovery is successful, the requesterentity that initiates a request for information in a communications exchange. changes its state from FAIL to READY. If for some reason the requesterentity that initiates a request for information in a communications exchange. cannot return to a condition where it is again ready to request a service, it transitions its state from FAIL to NOT_READY.

The responderentity that responds to a request for information in a communications exchange., as part of clearing a failure, resets any partial actions that were initiated and attempts to return to a condition where it is again ready to perform a service. If the recovery is successful, the responderentity that responds to a request for information in a communications exchange. changes its responseresponse interface which responds to a request. state from FAIL to READY. If for some reason the responderentity that responds to a request for information in a communications exchange. is not again prepared to perform a service, it transitions its state from FAIL to NOT_READY.

Requester Changes to an Unexpected State While Responder is Providing a Service

In some cases, a requesterentity that initiates a request for information in a communications exchange. may transition to an unexpected state after it has initiated a requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. for service.

As demonstrated in Figure Requester Makes Unexpected State Change, the requesterentity that initiates a request for information in a communications exchange. has initiated a requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. for service and its requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. state has been changed to ACTIVE. The responderentity that responds to a request for information in a communications exchange. begins the actions required to provide the service. During these actions, the requesterentity that initiates a request for information in a communications exchange. transitions its requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. state back to READY before the responderentity that responds to a request for information in a communications exchange. can complete its actions. This SHOULD be regarded as a failure of the requesterentity that initiates a request for information in a communications exchange..

In this case, the responderentity that responds to a request for information in a communications exchange. reacts to this change of state of the requesterentity that initiates a request for information in a communications exchange. in the same way as though the requesterentity that initiates a request for information in a communications exchange. had transitioned its requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. state to FAIL (i.e., the same as in Scenario 3 above).

At this point, the responderentity that responds to a request for information in a communications exchange. then transitions its responseresponse interface which responds to a request. state to FAIL.

The responderentity that responds to a request for information in a communications exchange. resets any partial actions that were initiated and attempts to return to its original condition where it is again ready to perform a service. If the recovery is successful, the responderentity that responds to a request for information in a communications exchange. changes its responseresponse interface which responds to a request. state from FAIL to READY. If for some reason the responderentity that responds to a request for information in a communications exchange. is not again prepared to perform a service, it transitions its state from FAIL to NOT_READY.

Note: The same scenario exists if the requesterentity that initiates a request for information in a communications exchange. transitions its requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. state to NOT_READY. However, in this case, the requesterentity that initiates a request for information in a communications exchange. then transitions its requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. state to READY after it resets all of its functions back to a condition where it is again prepared to make a requestcommunications method where a client transmits a message to an agent. That message instructs the agent to respond with specific information. for service.

Responder Changes to an Unexpected State While Providing a Service

Similar to Scenario 5, a responderentity that responds to a request for information in a communications exchange. may transition to an unexpected state while providing a service.

As demonstrated in Figure Responder Makes Unexpected State Change, the responderentity that responds to a request for information in a communications exchange. is performing the actions to provide a service and the responseresponse interface which responds to a request. state is ACTIVE. During these actions, the responderentity that responds to a request for information in a communications exchange. transitions its state to NOT_READY before completing its actions. This should be regarded as a failure of the responderentity that responds to a request for information in a communications exchange..

Upon detecting an unexpected state change of the responderentity that responds to a request for information in a communications exchange., the requesterentity that initiates a request for information in a communications exchange. transitions its state to FAIL.

The requesterentity that initiates a request for information in a communications exchange. resets any partial actions that were initiated and attempts to return to a condition where it is again ready to request a service. If the recovery is successful, the requesterentity that initiates a request for information in a communications exchange. changes its state from FAIL to READY. If for some reason the requesterentity that initiates a request for information in a communications exchange. cannot return to a condition where it is again ready to request a service, it transitions its state from FAIL to NOT_READY.

Since the responderentity that responds to a request for information in a communications exchange. has failed to an invalid state, the condition of the responderentity that responds to a request for information in a communications exchange. is unknown. Where possible, the responderentity that responds to a request for information in a communications exchange. should try to reset to an initial state.

The responderentity that responds to a request for information in a communications exchange., as part of clearing the cause for the change to the unexpected state, should attempt to reset any partial actions that were initiated and then return to a condition where it is again ready to perform a service. If the recovery is successful, the responderentity that responds to a request for information in a communications exchange. changes its responseresponse interface which responds to a request. state from the unexpected state to READY. If for some reason the responderentity that responds to a request for information in a communications exchange. is not again prepared to perform a service, it maintains its state as NOT_READY.

Responder or Requester Become UNAVAILABLE or Experience a Loss of Communication

In this scenario, a failure occurs in the communications connection between the responderentity that responds to a request for information in a communications exchange. and requesterentity that initiates a request for information in a communications exchange.. This failure may result from the InterfaceState from either piece of equipment returning a value of UNAVAILABLE or one of the pieces of equipment does not provide a heartbeat within the desired amount of time (See Fundamentals for details on heartbeat).

When one of these situations occurs, each piece of equipment assumes that there has been a failure of the other piece of equipment.

When normal communications are re-established, neither piece of equipment should assume that the request and responsecommunications pattern that supports the transfer of information between an agent and a client. state of the other piece of equipment remains valid. Both pieces of equipment should set their state to FAIL.

The requesterentity that initiates a request for information in a communications exchange., as part of clearing its FAIL state, resets any partial actions that were initiated and attempts to return to a condition where it is again ready to request a service. If the recovery is successful, the requesterentity that initiates a request for information in a communications exchange. changes its state from FAIL to READY. If for some reason the requesterentity that initiates a request for information in a communications exchange. cannot return to a condition where it is again ready to request a service, it transitions its state from FAIL to NOT_READY.

The responderentity that responds to a request for information in a communications exchange., as part of clearing its FAIL state, resets any partial actions that were initiated and attempts to return to a condition where it is again ready to perform a service. If the recovery is successful, the responderentity that responds to a request for information in a communications exchange. changes its responseresponse interface which responds to a request. state from FAIL to READY. If for some reason the responderentity that responds to a request for information in a communications exchange. is not again prepared to perform a service, it transitions its state from FAIL to NOT_READY.