Introduction

This repository contains the specification for the SOZip (Seek-Optimized Zip) profile to the ZIP file format.

What is SOZip ?

A Seek-Optimized ZIP file (SOZip) is a ZIP file that contains one or several Deflate-compressed files that are organized and annotated such that a SOZip-aware reader can perform very fast random access (seek) within a compressed file.

SOZip makes it possible to access large compressed files directly from a .zip file without prior decompression. It is not a new file format, but a profile of the existing ZIP format, done in a fully backward compatible way. ZIP readers that are non-SOZip aware can read a SOZip-enabled file normally and ignore the extended features that support efficient seek capability.

Software implementations

• : C/C++ open source geospatial library. Try out its sozip branch, pending future inclusion in its master branch.
• Python sozipfile module: drop-in replacement for standard zipfile module, creating and reading SOZip-enabled files.

See Annex A: Software implementations for more details.

Examples of SOZip files

Examples of SOZip-enabled files can be found in the sozip-examples repository.

Other ZIP related specification

This GitHub organization also hosts the KeyValuePairs extra-field specification, to be able to encode arbitrary key-value pairs of metadata associated with a file within a ZIP. For example to store the Content-Type of a file.

Benchmarking

Done with GDAL sozip branch, on a laptop running a Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz (6 cores / 12 virtual CPUs).

• ZIP generation:

Timing	Action
6.1 s	Multithreaded (12 vCPUs) generation of 489 MB SOZip-enabled file from a 1.6 GB uncompresssed GeoPackage file with sozip nz-building-outlines.gpkg.zip nz-building-outlines.gpkg
36 s	Single threaded compression of same file to 480 MB with regular zip utility with zip nz-building-outlines-regular.gpkg.zip nz-building-outlines.gpkg

• Bulk reading: Multithreaded extraction (4 vCPUs) of 3.2 million features with Arrow Array interface

Timing	Action
1.2 s	from SOZip-compressed GeoPackage file with bench_ogr_batch nz-building-outlines.gpkg.zip
0.7 s	from uncompressed GeoPackage file with bench_ogr_batch nz-building-outlines.gpkg

• Subsetting: Extraction of 66,377 features with a spatial filter

Timing	Action
1.2 s	from SOZip-compressed GeoPackage file with ogr2ogr out.gpkg nz-building-outlines.gpkg.zip -spat 1740000 5910000 1750000 5920000
1.1 s	from uncompressed GeoPackage file with ogr2ogr out.gpkg nz-building-outlines.gpkg -spat 1740000 5910000 1750000 5920000

• Extraction of one feature from its identifier:

Timing	Action
45 ms	from SOZip-compressed GeoPackage file with ogr2ogr out.gpkg nz-building-outlines.gpkg.zip -fid 1000000
44 ms	from uncompressed GeoPackage file with ogr2ogr out.gpkg nz-building-outlines.gpkg -fid 1000000

How to contribute ?

We welcome contributions to this specification as issues, pull requests or discussions.

If you use SOZip or plan to use it for your data delivery, or consider doing a SOZip implementation, etc., let us know!

Datasets available as SOZip

• TUDelft3D delivers the 3DBAG data, an open 3D building dataset, for the whole of the Netherlands, 108 GB uncompressed, as a 18.8 GB large GeoPackage: https://3dbag.nl/en/download
• Raw Data API, from Humanitarian OpenStreetMap Team (HOTOSM), now uses SOZip when you download openstreetmap data .zip files through it.

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Credits

The SOZip specification and its GDAL implementation have been developed by Spatialys, with support from Safe Software