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# A monitoring system
Nagios is much-maligned, but pretty much has the most solid design of a monitoring system I have seen:
* Nagios is stable because it is old and focused.
* Implementing most functionality by launching customizable processes with an argument API makes Nagios very flexible.
* Configuration as a text file is the best approach.
However, Nagios has some problems:
* The user interface is bad, although there are nicer alternative user interfaces.
* Metrics collection is an afterthought, and the default implementation is not good.
* Although Nagios configuration has macros and other features to make the configuration somewhat declarative, it is insufficient and in most complex configurations, using a program to generate the configuration is necessary. Command definition is clunky.
* The host/service distinction is likely unnecessary, only dependencies are needed.
* The API for commands is a bit clunky and error-prone. (Generating perfdata and status lines is not straightforward. Return code as command status is error-prone to implement in most programming languages.)
I think this can be improved by:
* Instead of providing a user interface, provide an API as a first-class citizen.
* Do not implement metrics. Metrics collection is better implemented separately.
* Make the configuration a JSON-like format that can be generated easily.
* Only implement checks, dependencies and alerting rules.
* Use JSON output as the main API for commands.
## Check API
Check plugins should output a JSON blob through stdout.
The output looks like:
```
{
"status": "OK"|"WARNING"|"CRITICAL"|"UNKNOWN",
"description": "free text here",
"extra": {any JSON value here}
}
```
If the process return code is not 0, then the status is equivalent to:
```
{
"status": "UNKNOWN",
"description": "command {command} exited with return code {return code}\n\nstdout:\n\n{stdout truncated to a reasonable size}\n\nstderr:\n\n{stderr truncated to a reasonable size}",
"extra": {
"return-code": "{return-code}",
"stdout": "{stdout truncated to a reasonable size}",
"stderr": "{stderr truncated to a reasonable size}"
}
}
```
An adapter executable can be provided to adapt Nagios plugins to the new API.
## TODOs
* Are passive checks with liveness useful? Perhaps they overlap a bit with metrics-based alerting, which I would implement as a separate system.
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