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# <a name="Cancellation_forwarding"> Cancellation forwarding </a>
## <a name="Rationale"> Rationale </a>
When a request to one or more servers is needed from a server to process a request from it's client, and the client cancels it's request, there will be at least temporary resource leakage in the chain of servers.
If the last server in the chain completes processing and gives a response to it's client, there will be unnecessary CPU and memory usage from all the servers. And if one of the servers never completes, there will be permanent memory leakage.
Timeouts are a way to avoid the permament memory leakage, at the cost of rendering the whole communication impossible above some system load. They also only bring the duration of the memory leakage from infinite to some time.
Cancellation forwarding is a mechanism that can be used to propagate, without additional overt communication, the information that initial request has been cancelled.
## <a name="Protocol"> Protocol </a>
* Each client that want to forward cancellation to it's server increment the protected payload of the FCRB for which a sender's capability has been given to the server, thus invalidating the capability.
* Each server that wants to notice cancellation forwarding will set up a watchdog, and ask the kernel to send heartbeats. At each heartbeat, the watchdog check that the reply capability to the FCRB of the client is not invalid, with a Discrim capability.
## <a name="Example"> Example </a>
Communication is described between 3 processes, client C and servers S and T.
Notation:
* FCRB->A means a FCRB whose receiver process is A
### <a name="Successful_operation_"> Successful operation: </a>
* C invokes a capability to S, giving S a capability c1 to a FCRB->C
* S sets up a watchdog that check that discrim.classify(c1) != clVoid
* S invokes a cap to T, giving T a cap c2 to a FCRB->S
* T sets up a watchdog that check that discrim.classify(c2) != clVoid
( T successfully treat the request, now goes completion )
* T invokes c2
* S reads the answer, and increment the PP of the FCRB->S
* S invokes c1
* C reads the answer, and increment the PP of the FCRB->C
### <a name="Uncomplete_operation_"> Uncomplete operation: </a>
* C invokes a cap to S, giving S a cap c1 to a FCRB->C
* S sets up a watchdog that check that discrim.classify(c1) != clVoid
* S invokes a cap to T, giving T a cap c2 to a FCRB->S
* T sets up a watchdog that check that discrim.classify(c2) != clVoid
( for any reason, C decides to stop, now goes cancellation )
* C increments the PP of the FCRB->C
* S watchdog notifies S of cancellation
* S increments the PP of the FCRB->S
* T watchdog notifies T of cancellation
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-- [[Main/NowhereMan]] - 28 Apr 2006
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