How do you read and analyse Apex Debug Logs effectively?

Debug Logs record everything Apex does in a transaction: SOQL queries, DML, method calls, governor limit usage, exceptions, callouts, validation rule firings, flow steps, and more. They are essential for production debugging.

Enabling:

Setup -> Debug Logs -> Add User -> set the user, time window, and Trace Flag (log levels per category):

• Apex Code: NONE, ERROR, WARN, INFO, DEBUG, FINE, FINER, FINEST.
• Workflow: similar levels.
• Validation, Database, System, Visualforce, Callout — separate levels for each.

Higher levels capture more detail; trade-off is log size (logs are capped at 5 MB; if you exceed, the log is truncated).

Reading the log:

Each line has: timestamp, line number, event type, content.

Key event types to recognise:

• `EXECUTION_STARTED` / `EXECUTION_FINISHED` — transaction boundary.
• `CODE_UNIT_STARTED [Apex Trigger / Visualforce / API]` — start of a unit of work.
• `SOQL_EXECUTE_BEGIN` / `SOQL_EXECUTE_END` — SOQL execution. END includes row count.
• `DML_BEGIN` / `DML_END` — DML operation; END shows count and any errors.
• `METHOD_ENTRY` / `METHOD_EXIT` — method calls. Indented to show call depth.
• `HEAP_ALLOCATE` — memory allocation (FINEST level only).
• `VARIABLE_ASSIGNMENT` — variable updates.
• `CALLOUT_REQUEST` / `CALLOUT_RESPONSE` — HTTP callouts.
• `USER_DEBUG` — your System.debug() output.
• `LIMIT_USAGE_FOR_NS` — governor limit consumption at end of unit.
• `FATAL_ERROR` — uncaught exception with stack trace.
• `FLOW_START_INTERVIEW` / `FLOW_ELEMENT_BEGIN/END` — Flow execution.
• `VALIDATION_RULE` — validation rule evaluation.

Diagnostic techniques:

1. Search for `FATAL_ERROR` to find exceptions.
2. Look at `LIMIT_USAGE_FOR_NS` to identify governor-limit pressure.
3. Count `SOQL_EXECUTE_BEGIN` to spot SOQL-in-loop anti-patterns.
4. Inspect `DML_BEGIN`/`DML_END` pairs for slow DML.
5. Trace `METHOD_ENTRY`/`METHOD_EXIT` for call hierarchy.
6. Use timestamps to see slow operations — gaps often indicate slow queries or callouts.

Apex Replay Debugger (in VS Code) — replays a debug log step-by-step in the editor, showing variable state at each line. Game-changer for hard bugs.

Production debugging strategy:

• Don't enable FINEST in production by default — log volume explodes.
• Enable for the specific user reporting the issue, for a 30-minute window.
• Reproduce the issue; pull the log; analyse offline.
• Disable the trace flag.

Tools:

• Salesforce Inspector (browser extension) — improved log viewer.
• Apex Log Analyzer for VS Code — UI for parsing logs.
• Event Monitoring (Shield) — gives detailed transaction logs without manual debug-log toggling.

A senior dev reads a debug log fluently, tracing exactly which line triggered which side effect.