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# Needle Layout Format |
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This document describes the binary layout of the Needle structure as used in SeaweedFS storage, for all supported versions (v1, v2, v3). |
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A Needle represents a file or data blob stored in a volume file. The layout determines how the Needle is serialized to disk for efficient storage and retrieval. |
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--- |
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## Common Field Sizes |
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| Field | Size (bytes) | |
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|---------------|--------------| |
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| Cookie | 4 | |
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| NeedleId | 8 | |
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| Size | 4 | |
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| DataSize | 4 | |
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| Flags | 1 | |
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| NameSize | 1 | |
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| MimeSize | 1 | |
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| LastModified | 5 | |
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| Ttl | 2 | |
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| PairsSize | 2 | |
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| Checksum | 4 | |
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| Timestamp | 8 | |
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--- |
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## Needle Layouts by Version |
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### Version 1 |
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| Offset | Field | Size (bytes) | Description | |
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|----------|----------|--------------|-----------------------------------------------| |
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| 0 | Cookie | 4 | Random number to mitigate brute force lookups | |
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| 4 | Id | 8 | Needle ID | |
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| 12 | Size | 4 | Length of Data | |
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| 16 | Data | N | File data (N = Size) | |
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| 16+N | Checksum | 4 | CRC32 of Data | |
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| 20+N | Padding | 0-7 | To align to 8 bytes | |
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### Version 2 |
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| Offset | Field | Size (bytes) | Description | |
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|----------|-------------|--------------|-----------------------------------------------| |
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| 0 | Cookie | 4 | Random number | |
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| 4 | Id | 8 | Needle ID | |
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| 12 | Size | 4 | Total size of the following fields | |
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| 16 | DataSize | 4 | Length of Data (N) | |
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| 20 | Data | N | File data | |
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| 20+N | Flags | 1 | Bit flags | |
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| 21+N | NameSize | 1 (opt) | Optional, if present | |
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| 22+N | Name | M (opt) | Optional, if present (M = NameSize) | |
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| ... | MimeSize | 1 (opt) | Optional, if present | |
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| ... | Mime | K (opt) | Optional, if present (K = MimeSize) | |
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| ... | LastModified| 5 (opt) | Optional, if present | |
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| ... | Ttl | 2 (opt) | Optional, if present | |
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| ... | PairsSize | 2 (opt) | Optional, if present | |
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| ... | Pairs | P (opt) | Optional, if present (P = PairsSize) | |
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| ... | Checksum | 4 | CRC32 | |
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| ... | Padding | 0-7 | To align to 8 bytes | |
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### Version 3 |
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| Offset | Field | Size (bytes) | Description | |
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|----------|-------------|--------------|-----------------------------------------------| |
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| 0 | Cookie | 4 | Random number | |
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| 4 | Id | 8 | Needle ID | |
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| 12 | Size | 4 | Total size of the following fields | |
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| 16 | DataSize | 4 | Length of Data (N) | |
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| 20 | Data | N | File data | |
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| 20+N | Flags | 1 | Bit flags | |
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| 21+N | NameSize | 1 (opt) | Optional, if present | |
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| 22+N | Name | M (opt) | Optional, if present (M = NameSize) | |
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| ... | MimeSize | 1 (opt) | Optional, if present | |
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| ... | Mime | K (opt) | Optional, if present (K = MimeSize) | |
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| ... | LastModified| 5 (opt) | Optional, if present | |
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| ... | Ttl | 2 (opt) | Optional, if present | |
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| ... | PairsSize | 2 (opt) | Optional, if present | |
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| ... | Pairs | P (opt) | Optional, if present (P = PairsSize) | |
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| ... | Checksum | 4 | CRC32 | |
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| ... | Timestamp | 8 | Append time in nanoseconds | |
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| ... | Padding | 0-7 | To align to 8 bytes | |
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- Offsets marked with `...` depend on the presence and size of previous optional fields. |
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- Fields marked `(opt)` are optional and only present if the corresponding size or flag is non-zero. |
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- N = DataSize, M = NameSize, K = MimeSize, P = PairsSize. |
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--- |
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## Field Explanations |
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- **Cookie**: 4 bytes, random value for security. |
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- **Id**: 8 bytes, unique identifier for the Needle. |
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- **Size**: 4 bytes, total size of the Needle data section (not including header, checksum, timestamp, or padding). |
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- **DataSize**: 4 bytes, length of the Data field. |
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- **Data**: File data (variable length). |
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- **Flags**: 1 byte, bit flags for Needle properties. |
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- **NameSize/Name**: 1 byte + variable, optional file name. |
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- **MimeSize/Mime**: 1 byte + variable, optional MIME type. |
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- **LastModified**: 5 bytes, optional last modified timestamp. |
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- **Ttl**: 2 bytes, optional time-to-live. |
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- **PairsSize/Pairs**: 2 bytes + variable, optional key-value pairs. |
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- **Checksum**: 4 bytes, CRC32 checksum of the Needle data. |
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- **Timestamp**: 8 bytes, append time (only in v3). |
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- **Padding**: 0-7 bytes, to align the total Needle size to 8 bytes. |
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--- |
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## Version Comparison Table |
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| Field | v1 | v2 | v3 | |
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|---------------|----|----|----| |
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| Cookie | ✔ | ✔ | ✔ | |
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| Id | ✔ | ✔ | ✔ | |
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| Size | ✔ | ✔ | ✔ | |
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| DataSize | | ✔ | ✔ | |
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| Data | ✔ | ✔ | ✔ | |
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| Flags | | ✔ | ✔ | |
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| NameSize/Name | | ✔ | ✔ | |
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| MimeSize/Mime | | ✔ | ✔ | |
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| LastModified | | ✔ | ✔ | |
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| Ttl | | ✔ | ✔ | |
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| PairsSize/Pairs| | ✔ | ✔ | |
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| Checksum | ✔ | ✔ | ✔ | |
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| Timestamp | | | ✔ | |
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| Padding | ✔ | ✔ | ✔ | |
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--- |
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## Flags Field Details |
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The `Flags` field (present in v2 and v3) is a bitmask that encodes several boolean properties of the Needle. Each bit has a specific meaning: |
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| Bit Value | Name | Meaning | |
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|-----------|-----------------------|--------------------------------------------------------------| |
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| 0x01 | FlagIsCompressed | Data is compressed (isCompressed) | |
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| 0x02 | FlagHasName | Name field is present (NameSize/Name) | |
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| 0x04 | FlagHasMime | Mime field is present (MimeSize/Mime) | |
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| 0x08 | FlagHasLastModifiedDate| LastModified field is present | |
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| 0x10 | FlagHasTtl | Ttl field is present | |
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| 0x20 | FlagHasPairs | Pairs field is present (PairsSize/Pairs) | |
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| 0x80 | FlagIsChunkManifest | Data is a chunk manifest (for large files) | |
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- If a flag is set, the corresponding field(s) will appear in the Needle layout at the appropriate position. |
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- The `Flags` field is always present in v2 and v3, immediately after the Data field. |
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## Optional Fields |
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- Fields marked as optional in the layout tables are only present if the corresponding flag in the `Flags` field is set (except for Name/Mime/Pairs, which also depend on their size fields being non-zero). |
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- The order of optional fields is fixed and matches the order of their flags. |
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## Special Notes |
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- **isCompressed**: If set, the Data field is compressed (typically using gzip). This is indicated by the lowest bit (0x01) in the Flags byte. |
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- **isChunkManifest**: If set, the Data field contains a manifest describing chunks of a large file, not raw file data. |
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- **All multi-byte fields** are stored in big-endian order. |
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- **Padding** is always added at the end to align the total Needle size to 8 bytes. |
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- **N = DataSize, M = NameSize, K = MimeSize, P = PairsSize** in the layout tables above. |
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For more details, see the implementation in the corresponding Go files in this directory. |
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