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