Declare one part of a resource

A part names bytes that are a piece of an artefact rather than the artefact: a chunk of a file split for a host with an upload limit, or a base release and a delta issued against it. Parts are retrieved and verified individually and then combined, in declaration order, into the file resource() names.

Usage

part(urls, sha256, size = NA_real_)

Arguments

urls

Character vector of download locations for this part, tried in order. All must be https://.

sha256

Lowercase hex SHA-256 of this part as served.

size

Expected size in bytes, or NA. Used to detect a truncated transfer before hashing.

Value

An object of class getaca_part.

Details

Each part carries its own checksum and is stored under it, so a base shared by every version of a resource is transferred once and kept once however many versions declare it. Publishing a new version then costs its consumers the delta rather than the whole file.

Parts describe how the bytes arrive. A declaration may re-split an artefact, add mirrors for a piece or drop one, the same way it may repair a mirror list, because what a version means is fixed by the resource's own sha256 and checked against it after composition.

See also

resource(), and combiner() for parts that are not simply concatenated.

Examples

part("https://example.org/backbone-base.bin", sha256 = strrep("c", 64),
     size = 1048576)