Chapter 5.

In this category, you can find general-purpose monitoring systems that rely on metrics. These systems typically employ an agent to collect data and send it to a daemon for aggregation and processing.

Consul: Not What You Probably Meant by “Monitoring”

Consul is mostly a service discovery tool, by which I mean it is a service you run that enables entities like web servers to find other entities like backend API endpoints and databases without the need for hardcoded IPs. It does this either via HTTP/GET requests or via its own DNS service. Generally speaking, you configure your thingy to ask for a node address that can service requests of type foo, and Consul responds with the address of the best node to which to send requests of that type.

Consul is, in fact, a very good and robust service discovery tool, by which I mean, it works quite hard, when queried for a service, to respond with the best possible node address in your infrastructure to handle requests of that type, and especially to avoid responding with the address of a node that is unreachable or overburdened. In the pursuit of that highly accurate response, Consul has its own internal monitoring system, which works with its own gossip protocol and enables you to configure ad hoc service checks (conceptually identical to Nagios checks, to include the return codes).

Therefore, although Consul is not a monitoring system, it does monitor the hosts for which it is responsible. ...

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