[jira] [Resolved] (KUDU-2065) Support cancellation for outbound client RPC

["Michael Ho (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KUDU-2065?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Michael Ho resolved KUDU-2065.
------------------------------
       Resolution: Fixed
    Fix Version/s: 1.5.0

0ec793e32dd70da501ff6e967ad775ca3244eeae implemented the logic for cancelling outbound calls in {{READY}}, {{ON_OUTBOUND_QUEUE}} and {{SENT}} states. The solution proposed for cancelling calls in {{SENDING}} state seems too complicated for the minute benefit it provides. We expect most calls to be drained quickly and only time a call is stuck in {{SENDING}} state for extended period of time is when the network is really slow or when the remote end is unresponsive (see KUDU-2192). We may consider using socket option such as TCP_USER_TIMEOUT to timeout unresponsive peers. For Impala, we haven't seen slow network being an issue for query cancellation yet. Closing this JIRA for now and file a new one if it becomes necessary to handle cancellation for RPCs in {{SENDING}} state.

> 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
>            Assignee: Michael Ho
>            Priority: Minor
>             Fix For: 1.5.0
>
>
> 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 Proxy::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 assigning 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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