“On-Demand support” refers to a plugin’s ability to support downloading and creating Content but not downloading their associated Artifacts. By convention, users expect the Remote.policy attribute to determine when Artifacts will be downloaded. See the user docs for specifics on the user expectations there.
Adding Support when using DeclarativeVersion¶
Plugins like pulp-file sync content using DeclarativeVersion.
On-demand support can be added by specifying deferred_download=True at instantiation of
Remote.policy can take several values. To easily translate them, consider a snippet like this one taken from pulp-file.:
async def run(self): # Interpret download policy deferred_download = (self.remote.policy != Remote.IMMEDIATE) <...> da = DeclarativeArtifact( artifact=artifact, url=url, relative_path=relative_path, remote=self.remote, deferred_download=deferred_download, ) <...>
The deferred_download flag is used at the artifact level, to support on-demand concepts for plugins that need some artifacts to download immediately in all cases. See also Multiple level discovery.
Adding Support when using a Custom Stages API Pipeline¶
Plugins like pulp-rpm that sync content using a custom pipeline can enable on-demand support by excluding the QueryExistingArtifacts, ArtifactDownloader and ArtifactSaver stages. Without these stages included, no Artifact downloading will occur. Content unit saving will occur, which will correctly create the on-demand content units.
Remote.policy can take several values. To easily maintain the pipeline consider a snippet like this one inspired by pulp-rpm:
download = (remote.policy == Remote.IMMEDIATE) # Interpret policy to download Artifacts or not stages = [first_stage] if download: stages.extend([QueryExistingArtifacts(), ArtifactDownloader(), ArtifactSaver()]) stages.extend(the_rest_of_the_pipeline) # This adds the Content and Association Stages
Skipping of those Stages does not work with Multiple level discovery. If you need some artifacts downloaded anyway, follow the example on :ref:on-demand-support-with-dv` and include the artifact stages in the custom pipeline.
Consider to also exclude the ResolveContentFutures stage.
What if the Custom Pipeline Needs Artifact Downloading?¶
For example, pulp-container uses a custom Stages API Pipeline, and relies on Artifact downloading to download metadata that is saved and stored as a Content unit. This metadata defines more Content units to be created without downloading their corresponding Artifacts. The on-demand support for this type needs to download Artifacts for those content types, but not others.
By specifying deferred_download=False in the DeclarativeArtifact regardless of the overall sync policy, lazy downloading for that specific artifact can be prohibited.
How Does This Work at the Model Layer?¶
The presence of a RemoteArtifact is what allows the Pulp content app to fetch and serve that Artifact on-demand. So a Content unit is on-demand if and only if:
It has a saved Content unit
A ContentArtifact for each Artifact is saved that the Content unit would have referenced. Note: the ContentArtifact is created in both on-demand and not on-demand cases.
Instead of creating and saving an Artifact, a RemoteArtifact is created. This contains any known digest or size information allowing for automatic validation when the Artifact is fetched.
How does the Content App work with this Model Layer?¶
When a request for content arrives, it is matched against a Distribution and eventually against a specific Artifact path, which actually matches against a ContentArtifact not an Artifact. If an Artifact exists, it is served to the client. Otherwise a RemoteArtifact allows the Artifact to be downloaded on-demand and served to the client.
If remote.policy == Remote.ON_DEMAND the Artifact is saved on the first download. This causes future requests to serve the already-downloaded and validated Artifact.
In situations where multiple Remotes synced and provided the same Content unit, only one Content unit is created but many RemoteArtifact objects may be created. The Pulp Content app will try all RemoteArtifact objects that correspond with a ContentArtifact. It’s possible an unexpected Remote could be used when fetching that equivalent Content unit. Similar warnings are in the user documentation on on-demand.