apache/cassandra · info

text

Error message

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What it means

ClientWarn.warn queues a warning message (the `text` parameter) into the per-request warn/warnedTables state that is returned to drivers in the response's Warnings frame. If no warning state is attached to the current execution (state == null), the warning is silently dropped.

Solutions

  1. If the warning is expected client-side, execute it through a normal query path so ClientWarn state exists; nothing to fix in caller code
  2. For internal code, log via the logger instead of ClientWarn.warn when no client context is present
  3. Check driver logs / QueryWarnings (e.g. driver's warning handler) to consume warnings — unhandled client warnings are easy to miss

Example fix

// before
ClientWarn.instance.warn(msg); // dropped off-request
// after
if (ClientWarn.instance.get() != null) ClientWarn.instance.warn(msg);
else logger.warn(msg);
Defensive patterns

Strategy: try-catch

Type guard

// only warn when a client context exists
State state = ClientWarn.instance.get();
if (state != null) ClientWarn.instance.warn(text); else logger.warn(text);

Try / catch

// server-side guard: fall back to logger when no client is attached
try {
    ClientWarn.instance.warn(text);
} catch (Exception e) {
    logger.warn("Could not deliver warning to client: {}", text, e);
}

Prevention

When it happens

Trigger: Server code calls ClientWarn.instance.warn(text) during query execution; the warning only reaches the client when the ExecutorLocals/ClientState state for the current thread carries a warning State (normal query execution), and it is absent when running outside a client request context.

Common situations: Internal/background code paths (compaction, repair, tooling) that reuse query machinery and invoke warn() with no client attached — warnings vanish; users wondering why a warning from a server-side operation never appears in their driver.

Understand the failure class

Background: "invalid response format", "malformed payload", "missing data field": when an API returns 200 but the response shape is wrong — this error's family across 23 libraries.

Related errors

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/b6c816d927c6c0e1. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/service/ClientWarn.java:46

    private static final String TRUNCATED = " [truncated]";
    public static ClientWarn instance = new ClientWarn();

    private ClientWarn()
    {
    }

    public State get()
    {
        return ExecutorLocals.current().clientWarnState;
    }

    public void set(State value)
    {
        ExecutorLocals current = ExecutorLocals.current();
        ExecutorLocals.Impl.set(current.traceState, value, current.eligibleForArtificialLatency);
    }

    public void warn(String text)
    {
        State state = get();
        if (state != null)
            state.add(text);
    }

    public void captureWarnings()
    {
        set(new State());
    }

    /**
     * Provides an additional control on capturing warnings. When executing SchemaTransformations in the
     * metadata log follower or when committing on a CMS member, we don't want these to be triggered.
     * @see org.apache.cassandra.schema.SchemaTransformation#enterExecution()
     **/
    public void pauseCapture()
    {

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