The link checker is a component of the the DNZ harvesting stack. It's
primary purpose is to ensure that records's
landing_url are still
available, thus ensuring that the overall quality of records is high.
In regard to the link checker, whenever
collection is used, it refers
Source Checking (On Demand)
source_url (such as
is accessed, the API submits the link to the link checker.
This gets processed according to the Link Checking Report page for that collection. See below.
Once the rule has been run against the record, it's state (
supressed) will be updated if necessary.
Every 2 hours, the 2 oldest and 2 newest records, by syndication date,
from each collection are checked. If all records fail their collection
rules (below), the collection is marked as
suppressed. After 3 further
attempts (so 4 in total) with exponential back-off, the collection is
Manually Suppress Collection
If a harvest operator knows that a specific collection is going to be down or if they need to remove the collection's items from the API they can do so via the Suppress Collections interface of the harvester. An operator can see which collections are suppressed, and add or remove collections.
Note: Marking a collection as suppressed doesn't delete the collections items (and are thus still available if you their id), they are just added to the API Suppressed Sources and don't show up in search results.
The Collection Rule defines what it means for an item to inaccessible. The Collection Rule defines:
- Collection title: the
primary_collectionfor which this rule applies to
- Xpath: If this expression evaluates as true or returns elements
against the returned document, the record is deemed to be inactive. Examples:
- Status codes: If any of these comma-separated, regular expression
status codes match the HTTP status code returned, the record is deemed
to be inactive. 404 is assumed to be an invalid code, so doesn't need to
be added explicitly. Examples:
Throttle: The delay (in seconds) between requests being sent to the content partner. Analogous with
throttlein the parser DSL
Collection link checking active?: This disables link checking activities for that collection - meaning that 404 errors will keep the record active.
Network Health Awareness
The link checker is aware of it's internet connectivity. It attempts to
www.google.com homepage every 5 minutes, and if it isn't
successful a global
active flag is un-set. This means that if there is
an issue with internet connectivity, DNS or other network issues the
link checked doesn't bring down all collections in one fell swoop.
Statistics on the number of links activated, suppressed and deleted on a given day are shown on the Manager Link Checking Report page. This is also emailed to Harvest Operators
Nitty Gritty Technical Details
Relationship between the Manager, the API and the Worker
The Manager houses all the setup of the collection checking and the
rules. The Manager provides an interface to the API Suppressed Sources
to manually suppress and active whole collections. The work to actually
perform the checking of the
landing_urls and to check the health of the
network is all done in the Worker, running as Sidekiq jobs.