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7.3Common Log Patterns to Search For
| Pattern | What It Means | Grep Command |
|---|---|---|
ERROR / FATAL |
Application errors | grep -E "ERROR|FATAL" app.log |
NullPointerException |
Missing data or object | grep "NullPointerException" app.log |
Connection refused |
Service dependency down | grep "Connection refused" app.log |
timeout |
Slow dependency or network issue | grep -i "timeout" app.log |
401 / 403 |
Authentication/authorization failure | grep -E "401|403" access.log |
500 / 502 / 503 |
Server errors | grep -E "50[023]" access.log |
OutOfMemoryError |
Memory leak or insufficient allocation | grep "OutOfMemory" app.log |
ECONNREFUSED |
Cannot connect to service (Node.js) | grep "ECONNREFUSED" app.log |