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small spec copy improvements

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Brandon Liu
2022-10-19 23:21:49 +08:00
parent 546cc93b01
commit 4f20a4b933
1 changed files with 9 additions and 9 deletions
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spec/v3/spec.md
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@@ -39,11 +39,11 @@ Entries are stored in memory as integers, but serialized to disk using these com
* The number of entries in the root directory and leaf directories is up to the implementation. * The number of entries in the root directory and leaf directories is up to the implementation.
* However, the compressed size of the header plus root directory is required in v3 to be under **16,384 bytes**. This is to allow latency-optimized clients to prefetch the root directory and guarantee it is complete. A sophisticated writer might need several attempts to optimize this. * However, the compressed size of the header plus root directory is required in v3 to be under **16,384 bytes**. This is to allow latency-optimized clients to prefetch the root directory and guarantee it is complete. A sophisticated writer might need several attempts to optimize this.
* Root size, leaf sizes and depth should be configurable by the user to adjust for optimize for different trade-offs: cost, bandwidth, latency. * Root size, leaf sizes and depth should be configurable by the user to optimize for different trade-offs: cost, bandwidth, latency.
# Header Design # Header Design
*Certain fields belonging to metadata in v2 are promoted to fixed-size header fields. This allows a map container to be initialized to the desired extent or center without blocking on the JSON metadata.* *Certain fields belonging to metadata in v2 are promoted to fixed-size header fields. This allows a map container to be initialized to the desired extent or center without blocking on the JSON metadata, and allows proxies to return well-defined HTTP headers.*
The `Header` is 127 bytes, with little-endian integer values: The `Header` is 127 bytes, with little-endian integer values:
@@ -62,9 +62,9 @@ The `Header` is 127 bytes, with little-endian integer values:
| 72 | # of addressed tiles, 0 if unknown | 8 | | 72 | # of addressed tiles, 0 if unknown | 8 |
| 80 | # of tile entries, 0 if unknown | 8 | | 80 | # of tile entries, 0 if unknown | 8 |
| 88 | # of tile contents, 0 if unknown | 8 | | 88 | # of tile contents, 0 if unknown | 8 |
| 96 | boolean clustered flag, `1` if true | 1 | | 96 | boolean clustered flag, `0x1` if true | 1 |
| 97 | internal compression enum (0 = Unknown, 1 = None, 2 = Gzip, 3 = Brotli, 4 = Zstd) | 1 | | 97 | `InternalCompression` enum (0 = Unknown, 1 = None, 2 = Gzip, 3 = Brotli, 4 = Zstd) | 1 |
| 98 | tile compression enum | 1 | | 98 | `TileCompression` enum | 1 |
| 99 | tile type enum (0 = Unknown/Other, 1 = MVT (PBF Vector Tile), 2 = PNG, 3 = JPEG, 4 = WEBP | 1 | | 99 | tile type enum (0 = Unknown/Other, 1 = MVT (PBF Vector Tile), 2 = PNG, 3 = JPEG, 4 = WEBP | 1 |
| 100 | min zoom | 1 | | 100 | min zoom | 1 |
| 101 | max zoom | 1 | | 101 | max zoom | 1 |
@@ -81,13 +81,13 @@ The `Header` is 127 bytes, with little-endian integer values:
* **# of addressed tiles**: the total number of tiles before run-length encoding, i.e. `Sum(RunLength)` over all entries. * **# of addressed tiles**: the total number of tiles before run-length encoding, i.e. `Sum(RunLength)` over all entries.
* **# of tile entries**: the total number of entries across all directories where `RunLength > 0`. * **# of tile entries**: the total number of entries across all directories where `RunLength > 0`.
* **# # of tile contents**: the number of referenced blobs in the tile section, or the unique # of offsets. If the archive is completely deduplicated, this is equal to the # of unique tile contents. If there is no deduplication, this is equal to the number of tile entries above. * **# # of tile contents**: the number of referenced blobs in the tile section, or the unique # of offsets. If the archive is completely deduplicated, this is equal to the # of unique tile contents. If there is no deduplication, this is equal to the number of tile entries above.
* **boolean clustered flag**: if `True`, blobs in the data section are ordered by Hilbert `TileId`. When writing with deduplication, this means that offsets are either contiguous with the previous offset+length, or refer to a lesser offset. * **boolean clustered flag**: if true, blobs in the data section are ordered by Hilbert `TileId`. When writing with deduplication, this means that offsets are either contiguous with the previous offset+length, or refer to a lesser offset.
* **compression enum**: Mandatory, tells the client how to decompress contents as well as provide correct `Content-Encoding` headers to browsers. * **compression enum**: Mandatory, tells the client how to decompress contents as well as provide correct `Content-Encoding` headers to browsers.
* **tile type**: A hint as to the tile contents. Clients and proxies may use this to: * **tile type**: A hint as to the tile contents. Clients and proxies may use this to:
* Automatically determine a visualization method * Automatically determine a visualization method
* provide a conventional MIME type HTTP `Content-Type` header * provide a conventional MIME type `Content-Type` HTTP header
* Enforce a canonical file path extension e.g. `.mvt`, `png`, `jpeg`, `.webp` * Enforce a canonical extension e.g. `.mvt`, `png`, `jpeg`, `.webp` to prevent duplication in caches
### Organization ### Organization
In most cases, the archive should be in the order `Header`, Root Directory, JSON Metadata, Leaf Directories, Tile Data. It is possible to relocate sections other than `Header` arbitrarily, but no current writers/readers take advantage of this. In most cases, the archive should be in the order `Header`, Root Directory, JSON Metadata, Leaf Directories, Tile Data. It is possible to relocate sections other than `Header` arbitrarily, but no current writers/readers take advantage of this. A future design may allow for reverse-ordered archives to enable single-pass writing.