Together with traces and metrics, logs are an essential component of an observable system (we'll discuss Observability more generally in Chapter 7, Monitoring and Observability). Logs are an ordered, timestamped sequence of events that originated in a particular system.
In a microservice architecture, the increased complexity of having multiple services makes having good logs essential. The exact criteria that makes logs good is subjective, but generally speaking, good logs should help an engineer piece together events that may have led to a specific error state or bug. Logs are usually organized by levels, a configurable toggle that allows a developer to instruct a service to be more or less verbose with the information sent to logs.
While essential for observing the behavior of systems in production, logs can also present privacy and security risks. Having too much information sent from systems to logs can give a would-be attacker information about users of your system, or...