Verifier for Cross-Region Data Lag | Dataford Interview Questions

You are migrating data from a DB to another datastore with a max acceptable lag of k seconds. Events are generated when data leaves the primary and when it reaches the secondary; build the verifier that raises an alert if data has not appeared in the secondary after k seconds.

Asked in the real-world problem round stage. Candidate did not remember wording exactly; related to the data migration team.

Contract

Implement verify_lag(events, k). events is a chronologically sorted list of [timestamp, event_type, record_id], where event_type is "leave" or "arrive". Each record has one leave event and at most one arrive event. Return record IDs whose arrival is later than k seconds after leaving or never occurs, ordered by alert time. Arrival exactly at the deadline is valid.

Constraints

1 <= len(events) <= 1000
0 <= timestamp <= 10^9
1 <= k <= 10^9
Events are sorted by nondecreasing timestamp
Each record has exactly one leave event and at most one arrive event
Arrival exactly k seconds after leaving is valid

Function Signature

def verify_lag(events, k):

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Problem