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https://issues.apache.org/jira/browse/CASSGO-139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18127743#comment-18127743
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Ao Chen commented on CASSGO-139:
--------------------------------
I opened [PR #1972|https://github.com/apache/cassandra-gocql-driver/pull/1972]
to address this. It preserves the in-flight query when a speculative execution
cannot find a usable host or connection, and includes regression tests for
successful completion and caller deadlines.
> Speculative execution fails an in-flight query when no alternate host is
> available
> ----------------------------------------------------------------------------------
>
> Key: CASSGO-139
> URL: https://issues.apache.org/jira/browse/CASSGO-139
> Project: Apache Cassandra Go driver
> Issue Type: Bug
> Reporter: Ao Chen
> Assignee: Ao Chen
> Priority: Normal
> Time Spent: 10m
> Remaining Estimate: 0h
>
> *Description*
> When speculative execution starts while the original query is still running,
> it may find no additional usable host and return {{{}ErrNoConnections{}}}.
> The driver returns that error to the caller and cancels the original query,
> even though the original could have completed successfully.
> This means enabling speculative execution can cause otherwise successful
> queries to fail.
> *Steps to reproduce*
> # Configure a session with one available host.
> # Run an idempotent query that takes approximately 50 ms to complete.
> # Enable one speculative execution with a 1 ms delay.
> # The speculative execution cannot find another host and returns
> {{{}ErrNoConnections{}}}.
> This can also happen in a multi-node cluster when the other eligible hosts
> are unavailable.
> *Expected behavior*
> A speculative execution that cannot start a database attempt should not fail
> the request. The driver should continue waiting for an active execution,
> subject to the normal timeout or cancellation.
> *Actual behavior*
> The request fails with {{ErrNoConnections}} before the original query
> finishes. In our reproduction, the query succeeds in approximately 52 ms
> without speculation but fails in approximately 1.2 ms with it enabled.
> *Proposed fix*
> Track whether an execution started a database attempt. Do not publish a
> speculative execution’s result if it never started an attempt. Preserve
> existing error handling for executions that actually attempted the query.
> Add regression tests covering unavailable alternate hosts and genuine
> execution errors.
> *References*
> * Existing report: [GitHub issue
> #1707|https://github.com/apache/cassandra-gocql-driver/issues/1707]
> * Reproduced on upstream commit:
> [{{94c308f}}|https://github.com/apache/cassandra-gocql-driver/commit/94c308f18566b3409f5b50e772804d6fecf439f4]
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