[redland-dev] Integrating a new storage implementation

[Lauri Aalto]

Hi Andrew,

as you probably already have figured out, writing a custom librdf storage
module is really about writing some function callbacks declared in
rdf_storage_module.h.

When I wrote my first custom rdf_storage module, I used
rdf_storage_sqlite.c as a reference. Eventually everything was written from
scratch. You're probably fine using any of the rdf_storage_*.c as reference.

Registering the new storage is straghtforward with

    REDLAND_API
    int librdf_storage_register_factory(librdf_world *world, const char
*name, const char *label, void (*factory) (librdf_storage_factory*));

where factory is your function callback that populates
librdf_storage_factory with supported functions' pointers. After this,
calling librdf_new_storage() with your storage_name calls your init()
callback to create a new storage instance.

(It's also possible to load storage modules dynamically if librdf is built
without --disable-modular. See code flagged with LIBRDF_MODULAR for more
information. The static registration mentioned above is simpler to get
started with.)

For unsupported functions, just leave the corresponding function pointer in
librdf_storage_factory NULL and librdf itself will, in most cases, either
return an appropriate error code or obtain the results with other, possibly
less optimized callbacks, e.g. get_arcs_in() using find_statements().

A read-only query-only storage strictly only needs the lifecycle functions:

    init()
    terminate()
    open()
    close()

and the query function

    find_statements().

But to avoid segfaults for unsupported functions where librdf indirects
using the function pointer without checking for non-NULL, you also need:

    size() - return negative for unsupported
    contains_statement() - can use find_statements() and check for
non-empty stream
    serialise() - can use find_statements() with all statement parts empty


Lauri

On Sun, May 5, 2013 at 10:24 PM, Andrew Reslan <andrew.reslan at mac.com>wrote:

> I want to integrate a new storage implementation.
>
> This will be a readonly store, data will be side loaded (initially),
> access to the data would be via a query API on the storage layer, data
> persistence/read/write details will be handled by the underlying storage
> framework.
>
> Which of the existing storage implementations would provide the simplest
> template as a starting point for integration?
>
> It would also be really useful to know which operations to implement for a
> query only store and the best way to implement "unsupported" operations so
> that clients fail gracefully if they try to write to the store.
>
> It looks like I will need to register the new implementation with the
> framework, what is the mechanism for doing this?
>
> Any pointers appreciated.
> _______________________________________________
> redland-dev mailing list
> redland-dev at lists.librdf.org
> http://lists.librdf.org/mailman/listinfo/redland-dev
>
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