RFC 38: OGR Faster Open

Author: Even Rouault

Contact: even dot rouault at spatialys.com

Status: Withdrawn.

Covered by RFC 46: GDAL/OGR unification

Summary

It is proposed that the OGR datasource opening mechanism relies on the GDALOpenInfo class, already used by GDAL drivers, to speed-up datasource opening. The speed-up is due to the fact that the file passed to OGROpen() will be opened and stat’ed only once, whereas currently, it is opened and closed as many times as there are OGR drivers. This should be particularly beneficial for network filesystems, or when trying to open a file that is not a OGR datasource at all.

E.g., trying to open a file that is not a OGR datasource currently requires 45 file opening or stat operations :

$ strace ogrinfo -ro NEWS 2>&1 | grep NEWS | wc -l
45

It is expected that if/once all drivers are migrated, it will decrease to 2 operations only.

Implementation

Similarly to GDALDriver, the OGRSFDriver class is extended to have a pfnOpen member, that drivers will set to point to their own Open method.

/* -------------------------------------------------------------------- */
/*      The following are semiprivate, not intended to be accessed      */
/*      by anyone but the formats instantiating and populating the      */
/*      drivers.                                                        */
/* -------------------------------------------------------------------- */
    OGRDataSource       *(*pfnOpen)( GDALOpenInfo * );

The OGRSFDriverRegistrar::Open() method is updated to call pfnOpen when iterating over the drivers. When pfnOpen is not set, it will try to call the Open() method of OGRSFDriver (which enables a progressive migration of drivers).

Mainly for compatibility reasons, the virtual method Open() of OGRSFDriver that is currently pure virtual, will now be a regular virtual method, that will have a default implementation, that will try to call pfnOpen.

The patch with the changes to OGR core is attached to this page.

Backward Compatibility

Proposed additions will not have any impact on C binary compatibility.

C++ binary interface will be broken (due to the addition of a new member in OGRSFDriver class and the Open() method changed from pure virtual to virtual).

Source level compatibility will be preserved for third-party OGR drivers.

Impact on drivers

Existing drivers are not required to migrate to RFC38, but are strongly encouraged to. New drivers should use RFC38 mechanism to preserve the overall faster opening.

An example of the migration for a few drivers is attached to this page.

Timeline

Even Rouault is responsible to implement this proposal. New API will be available in GDAL 2.0. Most in-tree OGR drivers will be migrated to the new mechanism.