Principle:Apache Paimon Blob Descriptor Deserialization
| Knowledge Sources | |
|---|---|
| Domains | Data_Lake, Blob_Storage |
| Last Updated | 2026-02-07 00:00 GMT |
Overview
Mechanism for reading back stored blob descriptors from Paimon tables and reconstructing them into usable objects.
Description
After blob metadata is written to a Paimon table, it can be read back using the standard Paimon read pipeline. The blob column contains serialized BlobDescriptor bytes. BlobDescriptor.deserialize() reconstructs the descriptor from its binary representation, recovering the URI, offset, and length.
The deserialization process works as follows:
- The standard table read pipeline (new_read_builder -> new_scan -> plan -> splits -> to_arrow) returns a PyArrow table.
- The blob column in the returned table contains binary values (the serialized descriptor bytes).
- Each binary value is passed to BlobDescriptor.deserialize() to reconstruct the descriptor object.
- The reconstructed descriptor provides uri, offset, and length properties for accessing the referenced blob.
During the read path, the FormatBlobReader handles the blob-specific file format internally, including:
- Magic number validation -- ensures the data file is a valid blob format file
- CRC32 checksum verification -- detects data corruption from storage errors or incomplete writes
These integrity checks happen transparently during the read process, before the serialized descriptor bytes are returned to the caller.
Usage
Use when reading blob-enabled tables to reconstruct BlobDescriptor objects from stored metadata. This is typically performed before lazy-loading the actual blob data, as the descriptor provides the URI, offset, and length needed to locate the external binary object.
This is the fourth step in the blob storage pipeline, following schema definition, descriptor construction, and metadata writing.
Theoretical Basis
Deserialization is the inverse of serialization, reconstructing structured objects from byte sequences. The deterministic binary format (version + URI length + URI bytes + offset + length) enables unambiguous parsing without delimiters or escape characters.
Version checking in the binary format enables forward compatibility. When a newer version of the descriptor format is encountered, the deserializer can either handle it (if backward-compatible) or raise an explicit error with a clear message, rather than silently producing corrupted data.
The CRC32 checksum verification during reads implements the fail-fast principle -- corrupted data is detected immediately at read time rather than propagating silently through the pipeline. This is especially critical for blob descriptors, where a corrupted URI or offset could lead to reading entirely wrong data from external storage.
The read pipeline's use of splits enables parallel deserialization of large tables. Each split can be processed independently, allowing the deserialization workload to be distributed across multiple threads or processes.