{"record":{"id":"9401869139031e62","repo":"nautechsystems/nautilus_trader","slug":"chunk-must-have-at-least-one-element-to-encode","errorCode":null,"errorMessage":"Chunk must have at least one element to encode","messagePattern":"Chunk must have at least one element to encode","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"error","filePath":"crates/serialization/src/arrow/delta.rs","lineNumber":132,"sourceCode":"\n    fn metadata(&self) -> HashMap<String, String> {\n        Self::get_metadata(\n            &self.instrument_id,\n            self.order.price.precision,\n            self.order.size.precision,\n        )\n    }\n\n    /// Extracts metadata from the first non-clear delta, falling back to the first clear.\n    ///\n    /// Clear deltas use sentinel values whose precision does not describe the following book data.\n    fn chunk_metadata(chunk: &[Self]) -> HashMap<String, String> {\n        chunk\n            .iter()\n            .find(|delta| delta.action != BookAction::Clear)\n            .or_else(|| chunk.first())\n            .map(EncodeToRecordBatch::metadata)\n            .expect(\"Chunk must have at least one element to encode\")\n    }\n}\n\nimpl DecodeFromRecordBatch for OrderBookDelta {\n    fn decode_batch(\n        metadata: &HashMap<String, String>,\n        record_batch: RecordBatch,\n    ) -> Result<Vec<Self>, EncodingError> {\n        let (instrument_id, price_precision, size_precision) = parse_price_size_metadata(metadata)?;\n        let cols = record_batch.columns();\n\n        let action_values = extract_column::<UInt8Array>(cols, \"action\", 0, DataType::UInt8)?;\n        let side_values = extract_column::<UInt8Array>(cols, \"side\", 1, DataType::UInt8)?;\n        let price_values = extract_column::<FixedSizeBinaryArray>(\n            cols,\n            \"price\",\n            2,\n            DataType::FixedSizeBinary(PRECISION_BYTES),","sourceCodeStart":114,"sourceCodeEnd":150,"githubUrl":"https://github.com/nautechsystems/nautilus_trader/blob/18893faf8b356be3320add8de2f861b0b647cf06/crates/serialization/src/arrow/delta.rs#L114-L150","documentation":"OrderBookDelta's EncodeToRecordBatch::chunk_metadata inspects the chunk to derive Arrow record metadata; with an empty slice there is no delta to inspect, so the expect panics. The or_else fallback to chunk.first() still yields None for an empty chunk.","triggerScenarios":"Encoding an empty Vec<OrderBookDelta> through the Arrow encoder, which calls chunk_metadata(&[]) — find and first both return None, triggering the panic.","commonSituations":"Flushing a writer/buffer when no deltas were captured (quiet market session, filter removed all events, or a batch boundary that produced zero rows).","solutions":["Guard the call site: skip encoding/flush when chunk.is_empty().","Batch data so each write contains at least one delta.","Handle the empty-batch case explicitly in the serialization pipeline before invoking the encoder."],"exampleFix":"// before\nlet batch = encoder.encode_chunk(&chunk); // panics when chunk is empty\n// after\nif chunk.is_empty() {\n    return Ok(());\n}\nlet batch = encoder.encode_chunk(&chunk);","handlingStrategy":"validation","validationCode":"if chunk.is_empty() {\n    return Ok(()); // nothing to encode\n}","typeGuard":"fn non_empty<T>(chunk: &[T]) -> bool { !chunk.is_empty() }","tryCatchPattern":"let result = std::panic::catch_unwind(|| OrderBookDelta::chunk_metadata(&chunk));","preventionTips":["Skip flush/encode calls when buffers are empty","Assert non-empty chunks at pipeline boundaries in debug builds","Handle quiet periods where no deltas accumulate"],"tags":["rust","serialization","arrow","empty-collection"],"backgroundTag":"empty-required-field","analyzedSha":"18893faf8b356be3320add8de2f861b0b647cf06","analyzedAt":"2026-09-08T20:49:34.690Z","contentChangedAt":"2026-09-08T20:49:34.690Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}