In this lab, rather than alerting on simple counts, learn to create an alert that will notify you when your 404s increase at a higher rate than your overall traffic (measured by 200s). See this blog post for full explanation.
Search only for messages with status code 200 or 404
Count 200 messages as Successes and 404 messages as Fails
Sum Successes and Fails to get a count
Create a ratio of fails to successes
Use outlier operator to identify anomalies in the ratio
_sourceCategory=Labs/Apache/Access (status_code=200 or status_code=404)
| timeslice 1m
| if (status_code = "200", 1, 0) as successes
| if (status_code = "404", 1, 0) as fails
| sum(successes) as success_cnt, sum(fails) as fail_cnt by _timeslice
| (fail_cnt/(success_cnt+fail_cnt)) * 100 as failure_rate_pct
| outlier failure_rate_pct window=5, threshold=3, consecutive=1, direction=+
Adding this line allows you to filter out only outliers (where ration increase is higher than normal) . You can now create a Scheduled Search to Alert when this query has results.
| where failure_rate_pct_violation > 0