apache/druid · error · ISE

Object cannot be deserialized to a Moments Sketch:

Error message

Object cannot be deserialized to a Moments Sketch:

What it means

MomentSketchAggregatorFactory.deserialize throws this IllegalStateException when the object passed in is not one of the supported serialized forms of a Moments Sketch: a String (base64), a byte[], or an already-decoded MomentSketchWrapper. The message appends the offending object's class so the developer can see what type actually arrived.

Source

Thrown at extensions-contrib/momentsketch/src/main/java/org/apache/druid/query/aggregation/momentsketch/aggregator/MomentSketchAggregatorFactory.java:186

  }

  private MomentSketchWrapper deserializeFromByteArray(byte[] bytes)
  {
    return MomentSketchWrapper.fromByteArray(bytes);
  }

  @Override
  public Object deserialize(Object serializedSketch)
  {
    if (serializedSketch instanceof String) {
      String str = (String) serializedSketch;
      return deserializeFromByteArray(StringUtils.decodeBase64(StringUtils.toUtf8(str)));
    } else if (serializedSketch instanceof byte[]) {
      return deserializeFromByteArray((byte[]) serializedSketch);
    } else if (serializedSketch instanceof MomentSketchWrapper) {
      return serializedSketch;
    }
    throw new ISE(
        "Object cannot be deserialized to a Moments Sketch: "
        + serializedSketch.getClass()
    );
  }

  @Nullable
  @Override
  public Object finalizeComputation(@Nullable Object object)
  {
    return object;
  }

  @Override
  @JsonProperty
  public String getName()
  {
    return name;
  }

View on GitHub (pinned to 9b90983fd2)

Solutions

Inspect the logged class name in the message to identify the unexpected type and where it originates.
Rewrite or re-ingest the affected segments so the sketches are serialized with the current format.
Align the momentsketch library and Druid versions across ingestion and query clusters.
If the value comes from your own code, convert it to base64 String or byte[] before calling deserialize.

Example fix

// before
Object sketch = columnValue; // could be Integer, Map, etc.
momentSketch = factory.deserialize(sketch);
// after
if (sketch instanceof String || sketch instanceof byte[] || sketch instanceof MomentSketchWrapper) {
  momentSketch = factory.deserialize(sketch);
} else {
  throw new IllegalArgumentException("Unsupported sketch type: " + sketch.getClass());
}

Defensive patterns

Strategy: type-guard

Validate before calling

boolean isSupportedSketchForm(Object v) {
  return v instanceof String || v instanceof byte[] || v instanceof MomentSketchWrapper;
}

Type guard

boolean canDeserializeSketch(Object serializedSketch) {
  return serializedSketch instanceof String
      || serializedSketch instanceof byte[]
      || serializedSketch instanceof MomentSketchWrapper;
}

MomentSketchWrapper safeDeserialize(MomentSketchAggregatorFactory f, Object v) {
  return canDeserializeSketch(v) ? f.deserialize(v) : null;
}

Try / catch

try {
  sketch = factory.deserialize(value);
} catch (IllegalStateException e) {
  log.warn("Unrecognized sketch payload: {}", e.getMessage());
  sketch = null; // or re-ingest / skip row
}

Prevention

Keep Druid and momentsketch versions consistent between ingestion and query clusters.
Guard deserialization with instanceof checks on the three supported types.
Validate segment data after version upgrades or format migrations.
Wrap deserialization of external/corruptible data in try-catch.

When it happens

Trigger: deserialize receives a serializedSketch object whose runtime class is none of String, byte[], or MomentSketchWrapper — typically a corrupted aggregation result, an object written by an incompatible Druid version, or unexpected data read back from a segment/store that changed the representation.

Common situations: Querying segments written with an older or newer Druid serialization format; a custom aggregation pipeline inserting a wrong-typed object; a corrupted column value stored in deep storage; class-shading/version drift of the momentsketch dependency changing the wrapper type.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/24884e9049891f1d. Report an issue: GitHub.