apache/druid · error · IllegalArgumentException

Cannot simultaneously replace and append to existing segment

Error message

Cannot simultaneously replace and append to existing segments. Either dropExisting or appendToExisting should be set to false

What it means

Thrown by AbstractTask.computeIngestionMode when both appendToExisting and dropExisting are true, meaning the task asks to both append to and replace (drop) existing segments at once — a logically contradictory ingestion mode. Druid derives IngestionMode from these two flags and refuses the combination.

Source

Thrown at indexing-service/src/main/java/org/apache/druid/indexing/common/task/AbstractTask.java:421

  protected static IngestionMode computeBatchIngestionMode(@Nullable BatchIOConfig ioConfig)
  {
    final boolean isAppendToExisting = ioConfig == null
        ? BatchIOConfig.DEFAULT_APPEND_EXISTING
        : ioConfig.isAppendToExisting();
    final boolean isDropExisting = ioConfig == null ? BatchIOConfig.DEFAULT_DROP_EXISTING : ioConfig.isDropExisting();
    return computeIngestionMode(isAppendToExisting, isDropExisting);
  }

  private static IngestionMode computeIngestionMode(boolean isAppendToExisting, boolean isDropExisting)
  {
    if (!isAppendToExisting && isDropExisting) {
      return IngestionMode.REPLACE;
    } else if (isAppendToExisting && !isDropExisting) {
      return IngestionMode.APPEND;
    } else if (!isAppendToExisting) {
      return IngestionMode.REPLACE_LEGACY;
    }
    throw new IAE("Cannot simultaneously replace and append to existing segments. "
        + "Either dropExisting or appendToExisting should be set to false");
  }

  /**
   * Emits a metric for this task using the {@link #getMetricBuilder() metric builder}.
   */
  public void emitMetric(
      ServiceEmitter emitter,
      String metric,
      Number value
  )
  {
    if (emitter == null || metric == null || value == null) {
      return;
    }
    emitter.emit(getMetricBuilder().setMetric(metric, value));
  }

View on GitHub (pinned to 9b90983fd2)

Solutions

1. Set dropExisting=false to append to existing segments, or set appendToExisting=false to replace them — keep exactly one true.
2. If replacement is intended (compaction-like behavior), remove appendToExisting from the spec so REPLACE mode is selected.
3. If appending is intended, remove dropExisting so APPEND mode is selected.
4. Validate the spec before submission with a JSON-schema or pre-submit check on ioConfig flags.

Example fix

// before
"ioConfig": { "type": "index_parallel", "appendToExisting": true },
"tuningConfig": { "dropExisting": true }
// after (append intended)
"ioConfig": { "type": "index_parallel", "appendToExisting": true },
"tuningConfig": { "dropExisting": false }

final boolean append = ioConfig.isAppendToExisting();
final boolean drop = tuningConfig.isDropExisting();
if (append && drop) throw new IllegalArgumentException("Set at most one of appendToExisting/dropExisting to true");

try { submitTask(spec); } catch (IAE e) { if (e.getMessage().contains("simultaneously replace and append")) { fix flags and resubmit; } else throw e; }

When it happens

Trigger: Submitting an ingestion or compaction task spec (e.g., ParallelIndexTask, BatchTask, CompactionTask) whose tuningConfig/ioConfig sets dropExisting=true while appendToExisting=true; via native JSON specs or SQL-based compaction configs where both flags were flipped on.

Common situations: Copy-pasting a compaction spec with dropExisting=true into an append task without removing appendToExisting; editing task JSON by hand; migration scripts that set both flags to 'true' defensively.

Understand the failure class

Background: "mutually exclusive" flag errors: what "can't supply both nx and xx", "--raw is not compatible with -i" and "cannot be used with" mean, and how to fix them — this error's family across 29 libraries.

Related errors

• Interval[%s] is empty, must specify a nonempty interval
• GranularitySpec's intervals cannot be empty for replace.
• SeekableStreamSupervisorIOConfig does not support both prope
• taskCountMax or taskCountMin can't be null!
• taskCountMax can't lower than taskCountMin!

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/3b53eb9bee64697e. Report an issue: GitHub.