Michael Ho created KUDU-2065:
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Summary: Support cancellation for outbound client RPC
Key: KUDU-2065
URL: https://issues.apache.org/jira/browse/KUDU-2065
Project: Kudu
Issue Type: Improvement
Components: rpc
Reporter: Michael Ho
Priority: Minor
Currently, there is no way to cancel an outbound client RPC call in Kudu. The
following is a proposal to do so. Please feel free to comment on it.
A new interface {{void RpcController::Cancel()}} will be introduced. It
enqueues a reactor task which will eventually call {{void
OutboundCall::Cancel()}}. A new RpcFeature flag {{CANCELLABLE}} will be added
to indicate whether the server can handle in-flight RPC which is cancelled.
More details below. A client can specify whether such functionality
is needed by setting a bool flag in {{RpcController}} passed to the proxy.
Depending on the state of an OutboundCall, cancellation will happen as follows:
* READY
** it hasn't been scheduled yet. Set the cancellation flag in the OutboundCall
object. When it's eventually scheduled by {{Connection::QueueOutboundCall()}},
it will check the cancellation flag before assigining a {{call_id}}. If it's
set, call {{OutboundCall::SetCancelled()}} and return.
* ON_OUTBOUND_QUEUE
** it's on the outbound transfer queue but transfer hasn't started yet.
{{Connection::WriteHandler()}} will check the cancellation flag before
initiating a transfer. If it's set, the transfer will be popped from the queue
and deleted. Call {{OutboundCall::SetCancelled()}} and return.
* SENDING
** some of the payload has already made its way to the other side. To make sure
the outbound call doesn't hold on to {{sidecars_}} till the end of the
transfer, the outbound call needs to clear {{sidecars_}} and sends the
remaining bytes as 0. The entry in CAR map will be removed and
{{OutboundCall::SetCancelled()}} will be invoked. Please see below on how the
server will handle this incomplete RPC message.
* SENT
** The payload has been sent. Waiting for a response. Call
{{OutboundCall::SetCancelled()}}. Incoming response will be dropped on the
floor.
* NEGOTIATION_TIMED_OUT
* TIMED_OUT
* CANCELLED
* FINISHED_NEGOTIATION_ERROR
* FINISHED_ERROR
* FINISHED_SUCCESS
** No-op. Callback has been invoked already.
{{OutboundCall::SetCancelled()}} will mark {{status_}} to {{Cancelled}}. Set
the state to {{CANCELLED}}. In addition, it will clear {{side_cars_}}, delete
{{header_buf_}} and invoke the callback.
When an OutboundCall already in the {{SENDING}} state is cancelled,
{{sidecars_}} will be cleared. This provides guarantee to the RPC client that
the RPC subsystem doesn't have any reference left to payload pointed by
{{sidecars_}} so the RPC client can safely free them if it's passed in as a
SliceSideCar. It's freed immediately if it's passed in as FastStringSideCar. An
in-flight RPC will be cancelled by sending the remainder of the payload
(encoded in total message length) as 0. An optional bool field is added to the
request header. If it's set, it's expected that a single byte at the end which
is 1 if the payload has been cancelled when it was in-flight. Potentially, we
can do the same treatment for client RPC which times out already.
{noformat}
+------------------------------------------------+
| Total message length (4 bytes) |
+------------------------------------------------+
| RPC Header protobuf length (variable encoding) |
+------------------------------------------------+
| RPC Header protobuf |
+------------------------------------------------+
| Main message length (variable encoding) |
+------------------------------------------------+ --- 0
| Main message protobuf |
+------------------------------------------------+ --- sidecar_offsets(0)
| Sidecar 0 |
+------------------------------------------------+ --- sidecar_offsets(1)
| Sidecar 1 |
+------------------------------------------------+ --- sidecar_offsets(2)
| Sidecar 2 |
+------------------------------------------------+ --- ...
| ... |
+------------------------------------------------+
| cancelled | -- true if RPC has been
cancelled mid-flight
+------------------------------------------------+
{noformat}
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