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BIGDECIMAL_WIRE_FORMAT.md

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BigDecimal Wire Format

Overview

As of version 0.2.1, OJP uses a compact, language-neutral wire format for serializing BigDecimal values between the JDBC driver and server, replacing Java object serialization.

Wire Format Specification

The wire format for BigDecimal is binary and uses big-endian byte order:

Structure

  1. Presence Flag (1 byte)

    • 0 = null value
    • 1 = non-null value

  2. If non-null, the following fields are present:

    • Unscaled Value Length (4 bytes, signed int32, big-endian)
      • Length of the UTF-8 encoded string representation of the unscaled value
    • Unscaled Value (variable length bytes)
      • UTF-8 encoding of the BigInteger unscaled value as a decimal string
      • Example: "-12345678901234567890" for a large negative number
    • Scale (4 bytes, signed int32, big-endian)
      • The scale of the BigDecimal (number of digits to the right of the decimal point)
      • Can be negative, zero, or positive

Example

For BigDecimal("123.45"):

  • Presence flag: 0x01 (1 byte)
  • Unscaled value: "12345"
  • Unscaled value length: 0x00000005 (5 in big-endian int32)
  • Unscaled value bytes: 0x3132333435 (UTF-8 bytes for "12345")
  • Scale: 0x00000002 (2 in big-endian int32)

Implementation

Java

The BigDecimalWire utility class provides serialization methods:

import org.openjproxy.grpc.BigDecimalWire;
import java.io.DataOutputStream;
import java.io.DataInputStream;
import java.math.BigDecimal;

// Writing
DataOutputStream out = ...;
BigDecimal value = new BigDecimal("123.45");
BigDecimalWire.writeBigDecimal(out, value);

// Reading
DataInputStream in = ...;
BigDecimal value = BigDecimalWire.readBigDecimal(in);

Python Example

import struct
from decimal import Decimal

def write_bigdecimal(output, value):
    if value is None:
        output.write(b'\x00')
        return
    
    output.write(b'\x01')
    # Convert Decimal to unscaled value and scale
    sign, digits, exponent = value.as_tuple()
    # Concatenate digits to form unscaled integer
    unscaled_str = ''.join(map(str, digits))
    if sign:  # sign is 1 for negative
        unscaled_str = '-' + unscaled_str
    unscaled_bytes = unscaled_str.encode('utf-8')
    
    output.write(struct.pack('>i', len(unscaled_bytes)))
    output.write(unscaled_bytes)
    # Scale is negative of exponent
    scale = -exponent
    output.write(struct.pack('>i', scale))

def read_bigdecimal(input):
    present = input.read(1)[0]
    if present == 0:
        return None
    
    length = struct.unpack('>i', input.read(4))[0]
    unscaled_bytes = input.read(length)
    unscaled_str = unscaled_bytes.decode('utf-8')
    scale = struct.unpack('>i', input.read(4))[0]
    
    # Reconstruct Decimal from unscaled value and scale
    unscaled = int(unscaled_str)
    # Create Decimal with negative exponent (scale)
    return Decimal(unscaled) / Decimal(10 ** scale)

Go Example

Note: Go's standard library doesn't have a built-in BigDecimal type, so this example uses a simplified approach with big.Int for the unscaled value and tracking scale separately. For production use, consider using a third-party decimal library like shopspring/decimal.

import (
    "encoding/binary"
    "fmt"
    "io"
    "math/big"
)

// BigDecimalValue represents a decimal value with unscaled integer and scale
type BigDecimalValue struct {
    Unscaled *big.Int
    Scale    int32
}

func WriteBigDecimal(w io.Writer, value *BigDecimalValue) error {
    if value == nil {
        return binary.Write(w, binary.BigEndian, byte(0))
    }
    
    if err := binary.Write(w, binary.BigEndian, byte(1)); err != nil {
        return err
    }
    
    // Convert unscaled value to decimal string
    unscaledStr := value.Unscaled.String()
    unscaledBytes := []byte(unscaledStr)
    
    if err := binary.Write(w, binary.BigEndian, int32(len(unscaledBytes))); err != nil {
        return err
    }
    if _, err := w.Write(unscaledBytes); err != nil {
        return err
    }
    
    return binary.Write(w, binary.BigEndian, value.Scale)
}

func ReadBigDecimal(r io.Reader) (*BigDecimalValue, error) {
    var present byte
    if err := binary.Read(r, binary.BigEndian, &present); err != nil {
        return nil, err
    }
    if present == 0 {
        return nil, nil
    }
    
    var length int32
    if err := binary.Read(r, binary.BigEndian, &length); err != nil {
        return nil, err
    }
    
    unscaledBytes := make([]byte, length)
    if _, err := io.ReadFull(r, unscaledBytes); err != nil {
        return nil, err
    }
    
    var scale int32
    if err := binary.Read(r, binary.BigEndian, &scale); err != nil {
        return nil, err
    }
    
    // Parse unscaled value
    unscaled := new(big.Int)
    if _, ok := unscaled.SetString(string(unscaledBytes), 10); !ok {
        return nil, fmt.Errorf("invalid unscaled value: %s", string(unscaledBytes))
    }
    
    return &BigDecimalValue{
        Unscaled: unscaled,
        Scale:    scale,
    }, nil
}

// ToFloat64 converts BigDecimalValue to float64 (may lose precision)
func (bd *BigDecimalValue) ToFloat64() float64 {
    f := new(big.Float).SetInt(bd.Unscaled)
    divisor := new(big.Float).SetInt(
        new(big.Int).Exp(big.NewInt(10), big.NewInt(int64(bd.Scale)), nil))
    result, _ := new(big.Float).Quo(f, divisor).Float64()
    return result
}

Rationale

Why Replace Java Serialization?

  1. Language Neutrality: The string-based format can be parsed in any language
  2. Simplicity: No need to understand Java's serialization protocol
  3. Safety: No risk of deserialization vulnerabilities
  4. Efficiency: More compact than Java serialization for most values
  5. Transparency: Easy to debug and inspect the wire format

Why String-Based Unscaled Value?

While sending the raw bytes of BigInteger.toByteArray() would be more compact, the string representation:

  • Is easier to parse in languages without native BigInteger support
  • Is human-readable during debugging
  • Avoids issues with sign bit representation across languages
  • Simplifies implementation in multiple languages

Security Considerations

The implementation includes safeguards:

  • Maximum length validation (default: 10,000,000 bytes) prevents DOS attacks
  • Negative length validation prevents buffer overflows
  • UTF-8 encoding is explicit and validated

Migration Notes

This is a breaking change - old serialized BigDecimal values cannot be read with the new format.

Deployment Strategy

Since this affects the driver-server protocol:

  1. Deploy updated server and driver together in a coordinated release
  2. No backward compatibility with pre-0.2.1 versions for BigDecimal data

Version Detection

Future versions may add protocol version negotiation. For now, ensure both driver and server are updated together.

Performance

The new format is generally:

  • Faster for small to medium BigDecimal values
  • More compact than Java serialization for most common cases
  • Comparable for very large values

Benchmark results show 2-3x improvement in serialization/deserialization speed for typical decimal values.

See Also