On-Demand Download and Sync¶
Overview¶
Pulp can sync content in a few modes: immediate, on_demand, and streamed. Each provides a
different behavior on how and when Pulp acquires content. These are set as the policy attribute
of the Remote performing the sync. Policy is an optional parameter and defaults to
immediate.
Sync Modes¶
immediate¶
When performing the sync, download all Artifacts now. Also download all metadata
now to create the content units in Pulp, associated with the
repository version created by the sync. immediate is the default, and
any plugin providing a sync is expected to implement the immediate mode.
on_demand¶
When performing the sync, do not download any Artifacts now. Download all
metadata now to create the content units in Pulp, associated with the
repository version created by the sync. Clients requesting content
trigger the downloading of Artifacts, which are saved into Pulp to be served to
future clients.
This mode is ideal for saving disk space because Pulp never downloads and stores
Artifacts that clients don't need. Units created from this mode are
on-demand content units<on-demand content>.
streamed¶
When performing the sync, do not download any Artifacts now. Download all
metadata now to create the content units in Pulp, associated with the
repository version created by the sync. Clients requesting content
trigger the downloading of Artifacts, which are not saved into Pulp. This
content will be re-downloaded with each client request.
This mode is ideal for content that you especially don't want Pulp to store over time. For
instance, syncing from a nightly repo would cause Pulp to store every nightly ever produced which
is likely not valuable. Units created from this mode are
on-demand content units<on-demand content>.
Plugin support for on-demand/streamed¶
Unless a plugin has enabled either the 'on_demand' or 'streamed' values for the policy attribute
you will receive an error. Check that plugin's documentation also.
Example of the "Create Remote" endpoints for some plugins that supports these features:
Note
Want to add on-demand support to your plugin? See the On-Demand Support documentation for more details on how to add on-demand support to a plugin.
On-Demand Content and Repository Versions¶
An on-demand content unit can be associated and unassociated from a repository version just like a normal unit. Note that the original Remote will be used to download content should a client request it, even as that content is
made available in multiple places.
Deleting a Remote
Learn about the dangers of deleting a Remote in the context of on-demand content.
On-Demand and Streamed limitations¶
On-demand and streamed content can be useful, but they come with some problems.
External dependency and error handling¶
There are two different types of errors that can occur with on-demand streaming:
- Pre-response: For some reason, Pulp can't get any data from the server (e.g, connectivity errors). A response is never started.
- Post-response: Pulp can get data from the remote and start streaming the response, but in the end the data doesn't match the expected digest.
In the first case, Pulp will try all the available remote sources for the requested content and will return a 404 if all of them fail with this same type of error.
In the second case, Pulp already sent the corrupted data to the client and can't recover from it, so it will close the connection to prevent the client from consolidating the file. When this happens, the content-app will ignore that remote source for a certain amount of time, which will enable future requests to select a different remote source. If all remote sources are ignored due to prior failure, then a 404 will be returned for all requests of that content until the cooldown period for one of those sources has expired. Pulp doesn't permanently invalidate the remote because it can't know if the error is transient or not.
The second case is complex and can be confusing to the user. The core reason for this complexity lies in the very nature of on-demand serving, which imposes that Pulp must fetch and stream the content on request time, and has no way to know anything about the remote before that. This constraint great limits the range of actions Pulp can do to properly satisfy the request.
If this behavior is prohibitive, consider using the immediate sync policy.
Context: https://github.com/pulp/pulpcore/issues/5012.
Remote deletion and content sharing¶
Deleting a Remote that was used in a sync with either the on_demand or streamed
options can break published data.
Specifically, clients who want to fetch content that a Remote was providing access to would begin to 404.
Recreating a Remote and re-triggering a sync will cause these broken units to recover again.
In the worst case, the Content is shared across multiple Repositories, and the Remote's removal can invalidate all those repositories at once.
In either case, proceed with the deletion of a remote with great care.
Context: https://github.com/pulp/pulpcore/issues/1975.
Implicit credential sharing within a domain¶
In the same domain, a request for on-demand content may use any available Remotes associated with that content, regardless of which user created it.
An example:
- Given User A and User B both synced the same on-demand content from their separate remotes (there are two different sources for the same content).
- When User B requests the content
- Then the credentials used for the download could potentially be User A's.
If a user doesn't want their registered Remotes to be indirectly used by other users, they should use a separate domain.