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Logging
Logging is one of the foundational pieces of telemetry we need to understand issues at runtime. It is therefore deemed one of the core responsibilities of developers to provide meaningful logs.

Logging infrastructure

Developers should use the logging library that is part of the monorepo and is configured for local as well as production environments. Don't use other methods of logging since that could lead to log statements not getting delivered correctly to the central log store.

Logging levels

The logging levels to be used are error, warn,info and debug. Everything from level info and up (that's info, error and warn) are delivered to the central log store. You can use all the levels of course but the logs with logging level lower than info will be discarded when deployed to one of the environments. We can change the logging level for a specific service to a lower level but that is a manual operation, to be used only as a last resort.
Example - logging

Viewing logs

You can search for and view the logs from all environments at https://app.datadoghq.eu/logs.

Best practices

Use error when you are reporting errors or exceptions.
Use warn when things are in 'suspicious' or potentially problematic state.
Use info when reaching important states - server started, user created, registration complete, etc. or anything worth knowing about
Use debug for additional info.
Avoid logging at level info inside unbounded loops.
Don't log the same exception/error multiple times.
Do not log full personal identifiable or other sensitive information. User database IDs are preferred to users' names for example.