Reading a Debug Log Under Pressure
By the end of this lesson, you'll be able to:
- Scan a debug log efficiently for the relevant section rather than reading top to bottom
- Recognize the key log line types: USER_DEBUG, SOQL_EXECUTE_BEGIN, DML_BEGIN, EXCEPTION_THROWN
- Apply this skill to a realistic "something broke in production" scenario
Prerequisites: "Debug Log Levels"
The key line types worth recognizing
13:42:01.1 (1234567)|USER_DEBUG|[42]|DEBUG|Processing 200 accounts
13:42:01.2 (2345678)|SOQL_EXECUTE_BEGIN|[45]|Aggregations:0|SELECT Id, Name FROM Account
13:42:01.3 (3456789)|DML_BEGIN|[52]|Op:Update|Type:Account|Rows:200
13:42:01.5 (5678901)|EXCEPTION_THROWN|[58]|System.NullPointerException: Attempt to de-reference a null objectUSER_DEBUG is exactly your own System.debug(...) output. SOQL_EXECUTE_BEGIN and DML_BEGIN mark exactly where a query or DML statement ran — genuinely useful for confirming a suspected SOQL-in-a-loop problem (Module 25) actually happened. EXCEPTION_THROWN marks precisely where and what broke.
Scanning, not reading
Under real time pressure, searching a log file for EXCEPTION_THROWN first (jumping straight to the failure) is far faster than reading from the top — then working backward from that line to see what led up to it. Most debug logs are read this way: locate the failure, then trace its immediate context, not consumed start to finish like a narrative.
A realistic scenario, traced
13:42:01.4 (4567890)|USER_DEBUG|[55]|DEBUG|About to process account: null
13:42:01.5 (5678901)|EXCEPTION_THROWN|[58]|System.NullPointerException: Attempt to de-reference a null objectScanning for EXCEPTION_THROWN, then looking one or two lines above it, immediately reveals the cause here: a null account reached line 58, and the line just before confirms it — the exact debugging shortcut this lesson exists to teach.
Exercise
As a comment, describe the two-step scanning strategy for finding the cause of a production error in a large debug log.
Show hint
Search for something specific first, then work backward.
Reading a Debug Log Under Pressure Quiz
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Summary
A real debug log can run thousands of lines — this lesson covers scanning it efficiently under time pressure, rather than reading it top to bottom like a book.