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Monitoring — understand the state of the system, based on gathering predefined sets of metrics or logs. Observability can anticipate the system's health based on the data it generates (logs, metrics, traces) Push vs. Pull vs. Push model, the agents push their data to the monitoring system whereas pull model the system pulls data from the agents. Push-based approach (Nagios, Zabbix) the central monitoring system knows quite a lot about the metrics. Agentless approach (e.g. Splunk forwarder, AppDynamics app agents)