= Logging Guidelines

== Library
We use http://www.slf4j.org[SLF4J] as our logging API.

* **Never use System.out/err**: Always use `LOG.info()` or `LOG.error()`.
* **Use Placeholders**: Use parameterized logging instead of string concatenation.
+
[source,java]
----
// GOOD
LOG.error("Could not... details: {}", something, exception);

// BAD
LOG.error("Could not... details: " + something, exception);
----

== Log Levels

* **`LOG.error()`**: Use for unexpected conditions that need operator attention.
    * Includes technical problems (database unreachable) and bugs.
    * Does NOT include validation errors (which should be user responses).
    * _Note_: SLF4J has no `FATAL` level; use `ERROR`.

* **`LOG.warn()`**: Use sparingly. Decide if it's an error or not.

* **`LOG.info()`**: Use for one-time actions (e.g., startup).
    * Do NOT use for regular application usage info (use Metrics instead).
    * Can be used freely in tests.

* **`LOG.debug()`**: Use for investigation data.
    * Not shown in default configuration but can be enabled for troubleshooting.

* **`LOG.trace()`**: Not used in Fineract.