James Clampffer created HDFS-11028:

             Summary: libhdfs++: FileHandleImpl::CancelOperations needs to be 
able to cancel pending connections
                 Key: HDFS-11028
                 URL: https://issues.apache.org/jira/browse/HDFS-11028
             Project: Hadoop HDFS
          Issue Type: Sub-task
          Components: hdfs-client
            Reporter: James Clampffer
            Assignee: James Clampffer

Cancel support is now reasonably robust except the case where a FileHandle 
operation ends up causing the RpcEngine to try to create a new RpcConnection.  
In HA configs it's common to have something like 10-20 failovers and a 20 
second failover delay (no exponential backoff just yet). This means that all of 
the functions with synchronous interfaces can still block for many minutes 
after an operation has been canceled, and often the cause of this is something 
trivial like a bad config file.

The current design makes this sort of thing tricky to do because the 
FileHandles need to be individually cancelable via CancelOperations, but they 
share the RpcEngine that does the async magic.

A non-exhaustive list of design assumptions:
1) multiple users will be doing stuff on the same FS in the same process, and 
some users might be a lot more impatient than others.  This means that it's 
possible that progress is slow and they want to give up but it wasn't stalled 
and other users are still able to make progress. Side effects of a 
FileHandle::CancelOperations call should only be visible to the owner of that 
2) In most use cases the library is spending more time in the read path than 
namenode metadata operations.  At any given time it's unlikely that there are a 
crazy amount of pending RPC requests though this certainly can happen (see 
[~anatoli.shein]'s awesome tools).

Some sparse design plans to help out reviewers:
1a) RPC Request objects get something analogous to the ReaderGroup to track all 
pending requests associated with a FileHandle.  As long as there is a 
transitive dependency on the FH from the request a flag can be pushed down.
1b) FileSystem operations also need the same support.  Since they return their 
result directly there isn't an object to call a cancel method on.  One approach 
here would be to pass in an optional flag (CancelHandle object).

2) Based on assumption 2 it's generally not unacceptably expensive to cancel 
and resend async RPC calls.  Since the RpcConnection is shared for all pending 
requests it needs to be wiped out.  This will cause all pending and on-the-fly 
requests to return asio::operation_aborted status.  If the Request object 
doesn't have it's flag set to canceled it gets placed back in line using the 
same mechanism as common RPC errors.  This retry does not count against the 
retry_count or failover_count since it's a side effect of the cancel.  Nor 
should this cause the RpcEngine to attempt to fail over.

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