The Blocking Sessions alert notifies you when its configured condition is met on Oracle instances so you can investigate and respond.
Mini DBA describes this alert as: Sessions blocking other sessions Only head blockers (not blocked by any other sessions) are counted A degree of this is expected in busy environments as locks are taken out on objects The threshold represents the number of blocking sessions that will trigger an alert Mini DBA evaluates this alert once a minute so changes are detected quickly.
This alert highlights concurrency pressure. Blocking, long sessions, and lock waits can turn a healthy server into a slow user experience, so the alert gives administrators an early route to the sessions, statements, and applications involved.
Enable it on any monitored Oracle instance where this condition would require an administrator to investigate or communicate with the application team. Tune noisy development systems after you have a short baseline.
Threshold meaning: Blocking sessions. Major threshold: 5 sessions. Minor threshold: 2 sessions. Comparison direction: over. For an over-threshold alert, decreasing a threshold makes that severity fire sooner; increasing it tolerates more load or pressure. Set the minor threshold as an early-warning level and the major threshold at the highest acceptable value for the service.
Open the activity, blocking, and query views to identify the lead blocker, SQL text, wait type, login, host, and application. Prefer fixing the query or transaction pattern, but kill or cancel the blocking session when the business impact is higher than the rollback risk.
If the alert creates repeated noise, review whether it is enabled on the correct instances, whether maintenance windows are configured, and whether the minor and major routes match the real business impact. Keep enough history to prove the new setting before changing it broadly.