AWS monitoring alternatives to Nagios: what is the closest equivalent?

Amazon CloudWatch is the closest AWS-native equivalent to Nagios: it collects metrics and logs from AWS resources and triggers alarms when a threshold is crossed, similar to how a Nagios plugin reports a service's state. Unlike Nagios, CloudWatch auto-discovers most AWS resource metrics without needing a config file entry for every host.

What does this mean day to day?

Basic EC2 monitoring in CloudWatch reports metrics like CPU and network usage every five minutes at no extra cost, with one-minute detailed monitoring available for a fee, so there's no agent to install just to see whether an instance is healthy. Alarms route through Amazon SNS to email, Slack, or paging tools, which plays the same role as Nagios's own notification commands.

Where does CloudWatch fit best?

It fits teams running entirely on AWS who want monitoring built into the platform rather than a separate server to patch and scale. Serverless workloads benefit most, since Lambda functions have no host to install a monitoring agent on, and CloudWatch collects their metrics and logs automatically. CloudWatch Synthetics can also run scheduled checks against external endpoints, covering the kind of outside-in check Nagios handles with active plugin checks.

What's worth checking before switching?

CloudWatch only sees AWS resources and whatever the CloudWatch agent is installed to report on, so it doesn't natively cover on-prem servers, other clouds, or network gear the way Nagios can through SNMP and custom plugins. Costs also scale with usage, per metric, alarm, and log volume, instead of Nagios's flat cost of running your own server. Teams with infrastructure outside AWS often keep a separate tool for that portion rather than trying to make CloudWatch cover everything.