There are several ways to trigger the External Lookup Data Feed:
One off / ad-hoc
For a one-off lookup, to migrate data or perform an occasional synchronisation, the Test tab contains a Submit Request button, which will trigger the lookup. (Despite the tab being called 'Test', the lookup acts for real, affecting tables and and triggering all other configured cascading events)
You can schedule the lookup to be performed on a regular schedule by configuring the schedule in the lookup's Behavior Editor / Settings / Options
An advanced method of triggering the lookup is via a Search View or Query by Example.
In these scenarios the Search View / Query by Example is based on a Query, and the Query is based not on a Table, but on the Lookup component. From the Query's configuration perspective, the Lookup component appears just like the cache Table the Lookup component is mapped to.
When the user specifies their criteria in the search form, the Lookup is triggered, with the criteria they entered in the search form being passed to the lookup to use in the cache table field mappings. The lookup executes, which may result in data being imported to the cache table, and the query then executes against that to return results to the user as normal.
The user will therefore have triggered the lookup, which may have been personalised with their particular search. They will also be returned the results of their search, which will be based not just on the results of that lookup, but of the entire underlying table.
Note: if you only want the user to see recent results, specify in the query embed it should only return records with an updated date that is more recent than a few seconds ago.
The lookup component's Test tab also provides a search form which can simulate the user entering particular search requests.
When a table record is added or updated, the event can trigger action, including the triggering of a lookup. The triggered lookup does not need to be related to the same table (and would normally be related to a different one).
The event triggered lookup has the unque feature that the lookup can specify the URL to be used as the basis for the lookup, overriding any URL specified in the lookup component itself. This URL can either be specified in the trigger dialog, or it can be taken from a field value in the triggering table's record.
This means that one lookup feed can specify the URLs of subsequent lookups, and the system will follow each in turn as a cascade of lookups.
Care should be taken to avoid infinitely recursive lookups.