Extractor Ownership and Lifetime#

Each ViewPart (one per add_part() call) holds a reference to an Extractor object. The Extractor is responsible for issuing the FDB sub-request and decoding the returned data into the chunk buffer.

Ownership Model#

Extractors are held via std::shared_ptr<Extractor>. This enables two important properties:

Sharing across parts: Multiple ViewPart objects can share a single Extractor instance — for example, two parts reading from the same FDB store can reuse the same open handle without duplicating it.
Tied lifetime: Each extractor’s lifetime is bound to the ChunkedDataView that owns it. As long as the view is alive, every extractor it references remains alive. Destroying the view closes all extractor handles cleanly.

Why Extractors Are Non-Copyable#

Concrete extractor implementations are stateful and non-copyable. An FDB-backed extractor owns an open FDB connection handle. Copying such a handle would duplicate a live network or file-system connection, which is unsafe — both copies would race on the same underlying state.

std::shared_ptr provides shared ownership without copying: all parts that reference the same extractor share one instance, and the instance is destroyed exactly once when the last shared reference is dropped.

Extractor Interface#

All extractors implement two methods called by the core during a chunk access:

layout(request): Called once per part during build() to probe the field layout — grid size and axis ordering — without reading actual data values.
extract(request, buffer, offset): Called during each chunk access to retrieve the fields matching request from FDB, decode them, and write them into buffer at offset.

The only extractor currently shipped is GribExtractor, which reads GRIB messages from FDB and decodes them to float32 via eccodes.