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Nov 20, 2023
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docs: update doc for ALC #1520

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34 changes: 20 additions & 14 deletions docs/core/logging.md
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Expand Up @@ -596,12 +596,18 @@ via `samplingRate` attribute on annotation.
POWERTOOLS_LOGGER_SAMPLE_RATE: 0.5
```

## AWS Lambda Advanced Logging Controls
With AWS [Lambda Advanced Logging Controls (ALC)](https://docs.aws.amazon.com/lambda/latest/dg/monitoring-cloudwatchlogs.html#monitoring-cloudwatchlogs-advanced), you can control the output format of your logs as either `TEXT` or `JSON` and specify the minimum accepted log level for your application.
Regardless of the output format setting in Lambda, Powertools for AWS Lambda will always output JSON formatted logging messages.
## AWS Lambda Advanced Logging Controls (ALC)

When you have this feature enabled, log messages that don’t meet the configured log level are discarded by Lambda.
For example, if you set the minimum log level to `WARN`, you will only receive `WARN` and `ERROR` messages in your AWS CloudWatch Logs, all other log levels will be discarded by Lambda.
!!!question "When is it useful?"
When you want to set a logging policy to drop informational or verbose logs for one or all AWS Lambda functions, regardless of runtime and logger used.

<!-- markdownlint-disable MD013 -->
With [AWS Lambda Advanced Logging Controls (ALC)](https://docs.aws.amazon.com/lambda/latest/dg/monitoring-cloudwatchlogs.html#monitoring-cloudwatchlogs-advanced){target="_blank"}, you can enforce a minimum log level that Lambda will accept from your application code.

When enabled, you should keep `Logger` and ALC log level in sync to avoid data loss.

Here's a sequence diagram to demonstrate how ALC will drop both `INFO` and `DEBUG` logs emitted from `Logger`, when ALC log level is stricter than `Logger`.
<!-- markdownlint-enable MD013 -->

```mermaid
sequenceDiagram
Expand All @@ -610,29 +616,29 @@ sequenceDiagram
participant Application Logger
Note over Lambda service: AWS_LAMBDA_LOG_LEVEL="WARN"
Note over Application Logger: POWERTOOLS_LOG_LEVEL="DEBUG"
Lambda service->>Lambda function: Invoke (event)
Lambda function->>Lambda function: Calls handler
Lambda function->>Application Logger: logger.warn("Something happened")
Lambda function->>Application Logger: logger.error("Something happened")
Lambda function-->>Application Logger: logger.debug("Something happened")
Lambda function-->>Application Logger: logger.info("Something happened")
Lambda service->>Lambda service: DROP INFO and DEBUG logs
Lambda service--xLambda service: DROP INFO and DEBUG logs
Lambda service->>CloudWatch Logs: Ingest error logs
```

Logger will automatically listen for the `AWS_LAMBDA_LOG_FORMAT` and `AWS_LAMBDA_LOG_LEVEL` environment variables, and change behaviour if they’re found to ensure as much compatibility as possible.

### Priority of log level settings in Powertools for AWS Lambda

When the Advanced Logging Controls feature is enabled, we are unable to increase the minimum log level below the `AWS_LAMBDA_LOG_LEVEL` environment variable value, see [AWS Lambda service documentation](https://docs.aws.amazon.com/lambda/latest/dg/monitoring-cloudwatchlogs.html#monitoring-cloudwatchlogs-log-level) for more details.

We prioritise log level settings in this order:

1. `AWS_LAMBDA_LOG_LEVEL` environment variable
2. `POWERTOOLS_LOG_LEVEL` environment variable

In the event you have set `POWERTOOLS_LOG_LEVEL` to a level lower than the ACL setting, Powertools for AWS Lambda will output a warning log message informing you that your messages will be discarded by Lambda.
If you set `Logger` level lower than ALC, we will emit a warning informing you that your messages will be discarded by Lambda.

> **NOTE**
>
> With ALC enabled, we are unable to increase the minimum log level below the `AWS_LAMBDA_LOG_LEVEL` environment variable value, see [AWS Lambda service documentation](https://docs.aws.amazon.com/lambda/latest/dg/monitoring-cloudwatchlogs.html#monitoring-cloudwatchlogs-log-level){target="_blank"} for more details.
### Timestamp format

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