Alexey Serbin created KUDU-3747:
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Summary: Clarify on the C++ client meta-cache logic for
create-table-and-immediate-write-and-scan use case
Key: KUDU-3747
URL: https://issues.apache.org/jira/browse/KUDU-3747
Project: Kudu
Issue Type: Task
Components: client
Affects Versions: 1.18.1, 1.17.1, 1.18.0, 1.17.0, 1.16.0, 1.15.0, 1.14.0,
1.13.0
Reporter: Alexey Serbin
With KUDU-1802 implemented, the Kudu C++ client is able to obtain information
on tablet replica locations from scan tokens, avoiding extra calls to Kudu
masters when such information in present. It works very well in the majority
of use cases. However, a couple of surprises were discovered in a relatively
rare use case when a Kudu client application creates a new table, immediately
start writing data into the newly created table, and then reads data back from
the table.
The two unexpected items are:
# After populating its metacache with the information on the location of tablet
replicas and using it while writing data into the newly created table, the
client still calls {{KuduClient::Data::GetTabletServer()}} as a part of
{{KuduScanner::Data::OpenTablet()}}.
# Even if ignoring the already existing tablet location information in the
metacache (say, if it's short-lived and already expired), it would be natural
for the client at least to rely on the information in the scan tokens when
opening a table for scanning, given KUDU-1802 provisions are already there.
This became apparent once increasing the setting for the
{{\-\-raft_heartbeat_interval_ms}} setting that Kudu tablet servers run tenfold
(i.e. from default 500 ms to 5000 ms): the client application that tries to
exercise the described use case exhibited high latency when calling
{{KuduScanner::Open()}} right after writing a few rows into the newly created
table.
It's necessary to clarify what's going on.
Attached is a sample Kudu client application in C++. Compile it similarly to
{{$KUDU_HOME/examples/cpp/example.cc}} and run against a Kudu cluster having at
least three tablet servers.
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