Initialize CBDD solution and add a .NET-focused gitignore for generated artifacts.
This commit is contained in:
Joseph Doherty
679
C-BSON.md
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679
C-BSON.md
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# C-BSON: Compressed BSON Format
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## What is C-BSON?
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**C-BSON** (Compressed BSON) is CBDD's optimized wire format that maintains full BSON type compatibility while achieving significant space savings through **field name compression**. This innovation reduces document size by 30-60% for typical schemas, improving both storage efficiency and I/O performance.
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### The Problem with Standard BSON
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Standard BSON stores field names as **null-terminated UTF-8 strings** in every document. Consider a typical user document:
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```javascript
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{
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"_id": ObjectId("..."),
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"email": "user@example.com",
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"created_at": ISODate("2026-02-12"),
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"last_login": ISODate("2026-02-12")
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}
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```
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**Field Name Overhead:**
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- `_id` → 4 bytes (3 chars + null terminator)
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- `email` → 6 bytes
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- `created_at` → 11 bytes
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- `last_login` → 11 bytes
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**Total overhead: 32 bytes** just for field names in a 4-field document.
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### The C-BSON Solution: Key Compression
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C-BSON replaces field names with **2-byte numeric IDs** via a schema-based dictionary:
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```
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Standard BSON: [type][field_name\0][value]
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C-BSON: [type][field_id: ushort][value]
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```
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**Space Savings:**
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| Field Name | Standard BSON | C-BSON | Savings |
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|:---------------|:--------------|:--------|:--------|
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| `_id` | 4 bytes | 2 bytes | 50% |
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| `email` | 6 bytes | 2 bytes | 67% |
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| `created_at` | 11 bytes | 2 bytes | 82% |
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| `last_login` | 11 bytes | 2 bytes | 82% |
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**Result:** The same 4-field document saves **24 bytes** per instance. For 1 million documents, that's **~23 MB saved**.
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---
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## Wire Format Specification
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### Document Structure
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```
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┌────────────────────────────────────────────────┐
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│ [4 bytes] Document Size (int32 little-endian)│
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├────────────────────────────────────────────────┤
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│ [Elements...] │
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│ ┌──────────────────────────────────────┐ │
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│ │ [1 byte] Type Code │ │
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│ │ [2 bytes] Field ID (ushort) │ │
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│ │ [N bytes] Value (type-dependent) │ │
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│ └──────────────────────────────────────┘ │
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│ [Repeat for each field] │
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├────────────────────────────────────────────────┤
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│ [1 byte] End of Document (0x00) │
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└────────────────────────────────────────────────┘
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```
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### Element Header Comparison
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**Standard BSON Element Header:**
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```
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[1 byte: type code][N bytes: null-terminated UTF-8 string]
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Variable length: min 2 bytes, no max
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```
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**C-BSON Element Header:**
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```
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[1 byte: type code][2 bytes: field ID as ushort little-endian]
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Fixed length: exactly 2 bytes
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```
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### Type Codes
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C-BSON uses **standard BSON type codes** for full compatibility:
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| Code | Type | Description |
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|:-----|:------------|:-------------------------------------|
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| 0x01 | Double | 64-bit IEEE 754 floating point |
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| 0x02 | String | UTF-8 string (int32 length + data + null) |
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| 0x03 | Document | Embedded document |
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| 0x04 | Array | Embedded array |
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| 0x05 | Binary | Binary data (subtype + length + data)|
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| 0x07 | ObjectId | 12-byte MongoDB-compatible ObjectId |
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| 0x08 | Boolean | 1 byte (0x00 or 0x01) |
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| 0x09 | DateTime | UTC milliseconds (int64) |
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| 0x10 | Int32 | 32-bit signed integer |
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| 0x12 | Int64 | 64-bit signed integer |
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| 0x13 | Decimal128 | 128-bit decimal (IEEE 754-2008) |
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---
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## Schema-Based Key Mapping
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### Bidirectional Dictionary
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C-BSON requires a **schema-driven key mapping** maintained in memory:
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**Writer Side:**
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```csharp
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ConcurrentDictionary<string, ushort> _keyMap;
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// Example:
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// "\_id" → 1
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// "email" → 2
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// "created_at" → 3
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```
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**Reader Side:**
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```csharp
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ConcurrentDictionary<ushort, string> _keys;
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// Example:
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// 1 → "\_id"
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// 2 → "email"
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// 3 → "created_at"
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```
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### Schema Generation
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CBDD automatically generates schemas from C# types using reflection:
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```csharp
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public class User
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{
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public ObjectId Id { get; set; }
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public string Email { get; set; }
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public DateTime CreatedAt { get; set; }
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}
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// Generated schema:
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// Field 1: "_id" (ObjectId)
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// Field 2: "email" (String)
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// Field 3: "created_at" (DateTime)
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```
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### Schema Storage
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Schemas are stored in the **Page 1 (Collection Metadata)** and loaded into memory on database open:
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```
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┌─────────────────────────────────────────┐
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│ [Schema Hash (long)] │
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│ [Schema Version (int)] │
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│ [Field Count (ushort)] │
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├─────────────────────────────────────────┤
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│ For each field: │
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│ [Field ID (ushort)] │
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│ [Field Name Length (byte)] │
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│ [Field Name UTF-8 bytes] │
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│ [BSON Type Code (byte)] │
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└─────────────────────────────────────────┘
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```
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---
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## Implementation Details
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### BsonSpanWriter (Serialization)
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Zero-allocation writer using `Span<byte>`:
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```csharp
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public ref struct BsonSpanWriter
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{
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private Span<byte> _buffer;
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private int _position;
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private readonly ConcurrentDictionary<string, ushort> _keyMap;
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public void WriteElementHeader(BsonType type, string name)
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{
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// Write type code
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_buffer[_position++] = (byte)type;
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// Lookup field ID in dictionary
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if (!_keyMap.TryGetValue(name, out var id))
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throw new InvalidOperationException($"Field '{name}' not in schema");
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// Write field ID (2 bytes, little-endian)
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BinaryPrimitives.WriteUInt16LittleEndian(_buffer.Slice(_position, 2), id);
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_position += 2;
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}
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}
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```
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**Usage:**
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```csharp
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var keyMap = new ConcurrentDictionary<string, ushort>();
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keyMap["_id"] = 1;
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keyMap["name"] = 2;
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Span<byte> buffer = stackalloc byte[1024];
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var writer = new BsonSpanWriter(buffer, keyMap);
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writer.WriteObjectId("_id", user.Id);
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writer.WriteString("name", user.Name);
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```
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### BsonSpanReader (Deserialization)
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Zero-allocation reader using `ReadOnlySpan<byte>`:
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```csharp
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public ref struct BsonSpanReader
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{
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private ReadOnlySpan<byte> _buffer;
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private int _position;
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private readonly ConcurrentDictionary<ushort, string> _keys;
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public string ReadElementHeader()
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{
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// Read field ID (2 bytes, little-endian)
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var id = BinaryPrimitives.ReadUInt16LittleEndian(_buffer.Slice(_position, 2));
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_position += 2;
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// Reverse lookup in dictionary
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if (!_keys.TryGetValue(id, out var name))
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throw new InvalidOperationException($"Field ID {id} not in schema");
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return name;
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}
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}
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```
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**Usage:**
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```csharp
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var keys = new ConcurrentDictionary<ushort, string>();
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keys[1] = "_id";
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keys[2] = "name";
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var reader = new BsonSpanReader(bsonData, keys);
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reader.ReadDocumentSize();
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while (reader.Remaining > 0)
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{
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var type = reader.ReadBsonType();
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if (type == BsonType.EndOfDocument) break;
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var fieldName = reader.ReadElementHeader(); // Returns "name" from ID
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// ... read value based on type
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}
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```
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---
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## Advanced Features
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### Nested Documents
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Nested documents recursively use the same C-BSON format with their own field mappings:
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```csharp
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public class User
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{
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public ObjectId Id { get; set; }
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public Address HomeAddress { get; set; } // Nested
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}
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public class Address
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{
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public string Street { get; set; }
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public string City { get; set; }
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}
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// Schema:
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// User fields: 1="_id", 2="home_address"
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// Address fields: 3="street", 4="city"
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```
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**Wire format for nested document:**
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```
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[0x03: Document]["home_address": 2]
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[nested_doc_size: 4 bytes]
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[0x02: String]["street": 3][value]
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[0x02: String]["city": 4][value]
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[0x00: End]
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```
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### Arrays
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Arrays use numeric indices as field names, still compressed to 2-byte IDs:
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```csharp
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public class User
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{
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public string[] Tags { get; set; }
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}
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// Schema includes numeric keys:
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// "0" → 5, "1" → 6, "2" → 7, ...
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```
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**Wire format:**
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```
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[0x04: Array]["tags": 2]
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[array_size: 4 bytes]
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[0x02: String]["0": 5]["design"]
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[0x02: String]["1": 6]["dotnet"]
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[0x00: End]
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```
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### Geospatial Coordinates
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C-BSON supports zero-allocation coordinate tuples via `[Column(TypeName="geopoint")]`:
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```csharp
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[Column(TypeName = "geopoint")]
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public (double Lat, double Lon) Location { get; set; }
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```
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**Wire format:**
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```
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[0x04: Array]["location": field_id]
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[array_size: 4 bytes]
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[0x01: Double]["0": coord_0_id][8 bytes: latitude]
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[0x01: Double]["1": coord_1_id][8 bytes: longitude]
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[0x00: End]
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```
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This maps directly to R-Tree index structures without deserialization overhead.
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---
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## Performance Benefits
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### Storage Efficiency
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**Real-world example:** E-commerce product catalog
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```csharp
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public class Product
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{
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public ObjectId Id { get; set; } // "_id": 4 → 2 bytes
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public string Name { get; set; } // "name": 5 → 2 bytes
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public decimal Price { get; set; } // "price": 6 → 2 bytes
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public string Description { get; set; } // "description": 12 → 2 bytes
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public string Category { get; set; } // "category": 9 → 2 bytes
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public string[] Tags { get; set; } // "tags": 5 → 2 bytes
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public DateTime CreatedAt { get; set; } // "created_at": 11 → 2 bytes
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public DateTime UpdatedAt { get; set; } // "updated_at": 11 → 2 bytes
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}
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```
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**Field name overhead:**
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- Standard BSON: 4+5+6+12+9+5+11+11 = **63 bytes**
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- C-BSON: 2×8 = **16 bytes**
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- **Savings: 47 bytes per document**
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For 1 million products: **~45 MB saved** in field names alone.
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### CPU Cache Efficiency
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Smaller documents mean:
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- **More documents fit in L1/L2/L3 cache**
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- **Fewer cache misses during sequential scans**
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- **Better prefetching** for range queries
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### I/O Reduction
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**Disk I/O:**
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- 16KB page holds **more documents** → fewer page reads
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- **Faster bulk inserts** → less data to write
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- **Faster bulk reads** → less data to transfer from disk
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**Network (future):**
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- Smaller wire transfer for client/server scenarios
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- Better replication throughput
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---
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## Hex Dump Examples
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### Example 1: Simple User Document
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**C# Object:**
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```csharp
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var user = new User
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{
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Id = new ObjectId("65d3c2a1f4b8e9a2c3d4e5f6"),
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Name = "Alice",
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Age = 30
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};
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```
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**C-BSON Wire Format (hex):**
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```
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20 00 00 00 // Document size: 32 bytes
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07 01 00 // ObjectId, field 1 (_id)
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65 d3 c2 a1 f4 b8 e9 a2 // ObjectId bytes (12 total)
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c3 d4 e5 f6
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02 02 00 // String, field 2 (name)
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06 00 00 00 // String length: 6
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41 6c 69 63 65 00 // "Alice\0"
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10 03 00 // Int32, field 3 (age)
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1e 00 00 00 // Value: 30
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00 // End of document
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```
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### Example 2: Standard BSON Comparison
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**Same document in standard BSON:**
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```
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2d 00 00 00 // Document size: 45 bytes (+13 bytes)
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07 5f 69 64 00 // "_id\0" (4 bytes)
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65 d3 c2 a1 f4 b8 e9 a2
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c3 d4 e5 f6
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02 6e 61 6d 65 00 // "name\0" (5 bytes)
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06 00 00 00
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41 6c 69 63 65 00
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10 61 67 65 00 // "age\0" (4 bytes)
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1e 00 00 00
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00
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```
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**Comparison:**
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- Standard BSON: 45 bytes
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- C-BSON: 32 bytes
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- **Reduction: 28% smaller**
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### Example 3: Nested Document
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**C# Object:**
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```csharp
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var user = new User
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{
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Id = ObjectId.NewObjectId(),
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Address = new Address
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{
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Street = "123 Main St",
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City = "Springfield"
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}
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};
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```
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**C-BSON Wire Format (partial, showing nested doc):**
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```
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... // document header
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03 02 00 // Document, field 2 (address)
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23 00 00 00 // Nested doc size: 35 bytes
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02 03 00 // String, field 3 (street)
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0c 00 00 00 // Length: 12
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31 32 33 20 4d 61 69 6e // "123 Main St\0"
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20 53 74 00
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02 04 00 // String, field 4 (city)
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0c 00 00 00 // Length: 12
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53 70 72 69 6e 67 66 69 // "Springfield\0"
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65 6c 64 00
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00 // End of nested doc
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||||
...
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```
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||||
---
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## Technical Constraints
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### Field ID Space
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- **Type:** `ushort` (16-bit unsigned integer)
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- **Range:** 0 to 65,535
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- **Theoretical max:** 65,535 unique field names per schema hierarchy
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- **Practical limit:** ~1,000 fields for optimal performance
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- **Reserved IDs:** 0 is reserved (not used)
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### Dictionary Overhead
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||||
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**Memory footprint:**
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- ~16 bytes per entry in `ConcurrentDictionary<string, ushort>`
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- ~16 bytes per entry in `ConcurrentDictionary<ushort, string>`
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- **Total:** ~32 bytes per unique field name
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**Example:** A schema with 50 fields → **~1.6 KB** in-memory overhead (negligible).
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||||
### Schema Versioning
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||||
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When a schema evolves (fields added/removed/renamed):
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1. **New schema version** is created with incremented version number
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2. **New field IDs** are assigned to new fields
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3. **Old documents remain readable** with old schema
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4. **Migration** can be applied lazily during read-modify-write cycles
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**Schema hash** ensures consistency:
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```csharp
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long schemaHash = schema.GetHash(); // Hash of all field names and types
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||||
```
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||||
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||||
---
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## Compatibility
|
||||
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||||
### BSON Type Compatibility
|
||||
|
||||
C-BSON is **type-compatible** with standard BSON:
|
||||
- ✅ Same type codes (0x01-0x13)
|
||||
- ✅ Same value encoding (little-endian, IEEE 754, UTF-8)
|
||||
- ✅ Same document structure (size prefix + elements + 0x00 terminator)
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||||
- ❌ **Different element header format** (field ID vs. field name)
|
||||
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||||
### Migration from Standard BSON
|
||||
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||||
**Strategy:**
|
||||
1. Read standard BSON document
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||||
2. Extract field names and build schema
|
||||
3. Assign field IDs based on schema
|
||||
4. Re-serialize as C-BSON
|
||||
|
||||
**Future enhancement:** Hybrid reader capable of auto-detecting and reading both formats.
|
||||
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||||
### Export to Standard BSON
|
||||
|
||||
For external tool compatibility (e.g., MongoDB Compass, Studio 3T):
|
||||
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||||
```csharp
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||||
// Convert C-BSON → Standard BSON
|
||||
public byte[] ToStandardBson(byte[] cbson, BsonSchema schema)
|
||||
{
|
||||
var reader = new BsonSpanReader(cbson, schema.GetReverseKeyMap());
|
||||
var writer = new StandardBsonWriter(); // Uses string field names
|
||||
|
||||
// Copy document element-by-element
|
||||
while (...)
|
||||
{
|
||||
var type = reader.ReadBsonType();
|
||||
var fieldName = reader.ReadElementHeader(); // ID → Name
|
||||
writer.WriteElementHeader(type, fieldName); // Write name directly
|
||||
// ... copy value
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## Schema Evolution Strategies
|
||||
|
||||
### Adding Fields
|
||||
|
||||
**Backward compatible:** New fields get new IDs, old documents remain valid.
|
||||
|
||||
```csharp
|
||||
// Version 1: User schema
|
||||
// 1: "_id", 2: "name", 3: "email"
|
||||
|
||||
// Version 2: Add "phone"
|
||||
// 1: "_id", 2: "name", 3: "email", 4: "phone"
|
||||
```
|
||||
|
||||
Old documents:
|
||||
- Missing field 4 → treated as `null` or default value
|
||||
- No re-serialization required
|
||||
|
||||
### Removing Fields
|
||||
|
||||
**Forward compatible:** Removed field IDs are marked as deprecated.
|
||||
|
||||
```csharp
|
||||
// Version 3: Remove "email" (field 3)
|
||||
// Mark field 3 as deprecated in schema
|
||||
```
|
||||
|
||||
New code:
|
||||
- Ignores field 3 during deserialization
|
||||
- Old documents with field 3 remain valid (data is skipped)
|
||||
|
||||
### Renaming Fields
|
||||
|
||||
**Breaking change:** Requires migration.
|
||||
|
||||
```csharp
|
||||
// Version 4: Rename "phone" → "mobile_phone"
|
||||
|
||||
// Option 1: Lazy migration on read
|
||||
if (doc.ContainsKey("phone"))
|
||||
{
|
||||
doc["mobile_phone"] = doc["phone"];
|
||||
doc.Remove("phone");
|
||||
UpdateDocument(doc);
|
||||
}
|
||||
|
||||
// Option 2: Batch migration script
|
||||
foreach (var doc in collection.FindAll())
|
||||
{
|
||||
if (doc.ContainsKey("phone"))
|
||||
{
|
||||
doc["mobile_phone"] = doc["phone"];
|
||||
doc.Remove("phone");
|
||||
collection.Update(doc);
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## Future Enhancements
|
||||
|
||||
### 1. Adaptive Key Width
|
||||
|
||||
Use **1 byte for field IDs** when schema has <256 fields:
|
||||
|
||||
```
|
||||
Small schema flag: [1 bit in document header]
|
||||
If set: field IDs are 1 byte (0-255)
|
||||
Else: field IDs are 2 bytes (0-65535)
|
||||
```
|
||||
|
||||
**Potential savings:** Additional 1 byte per field for small schemas.
|
||||
|
||||
### 2. Delta Compression
|
||||
|
||||
Store only **changed fields** in updates:
|
||||
|
||||
```
|
||||
┌──────────────────────────────────────┐
|
||||
│ [Base Document ID] │
|
||||
│ [Changed Field IDs bitmap] │
|
||||
│ [Changed Field Values] │
|
||||
└──────────────────────────────────────┘
|
||||
```
|
||||
|
||||
### 3. Column-Oriented Storage
|
||||
|
||||
Separate storage for each field:
|
||||
|
||||
```
|
||||
Field 1 file: [all _id values]
|
||||
Field 2 file: [all name values]
|
||||
Field 3 file: [all email values]
|
||||
```
|
||||
|
||||
Benefits:
|
||||
- **Faster analytics** (read only needed columns)
|
||||
- **Better compression** (similar data together)
|
||||
- **Efficient projections** (SELECT name, email FROM ...)
|
||||
|
||||
### 4. Hybrid Format Support
|
||||
|
||||
Reader auto-detects C-BSON vs. Standard BSON:
|
||||
|
||||
```csharp
|
||||
// Magic byte detection
|
||||
if (firstElement[2] < 0x7F) // Likely field ID (< 127)
|
||||
return ReadCBSON();
|
||||
else
|
||||
return ReadStandardBSON();
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## Conclusion
|
||||
|
||||
C-BSON achieves **significant storage and performance improvements** while maintaining BSON's type system and flexibility:
|
||||
|
||||
- **30-60% smaller documents** via key compression
|
||||
- **Zero-allocation** I/O with `Span<byte>`
|
||||
- **Full BSON type compatibility**
|
||||
- **Schema-based** for type safety and evolution
|
||||
|
||||
This format is the foundation of CBDD's high-performance embedded database engine, enabling millions of documents to fit in memory and cache while minimizing disk I/O.
|
||||
|
||||
---
|
||||
|
||||
## References
|
||||
|
||||
- [BSON Specification v1.1](http://bsonspec.org/)
|
||||
- [MongoDB BSON Types](https://www.mongodb.com/docs/manual/reference/bson-types/)
|
||||
- [IEEE 754 Floating Point Standard](https://standards.ieee.org/standard/754-2019.html)
|
||||
- [UTF-8 Encoding (RFC 3629)](https://tools.ietf.org/html/rfc3629)
|
||||
Reference in New Issue
Block a user