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2. Standalone NRPE Custom Check module

Date: 2018-12-19

Status

Accepted

Context

We need to add new functionality to the project - the ability to create custom NRPE checks. This means having templated configuration for adding service-specific plugins for the purposes of monitoring.

The base module already contains NRPE specific code. It installs NRPE and ensures it is running. It also configures a set of default monitoring plugins.

Decision

We considered three possible approaches to adding the new NRPE functionality:

  • Add it to the base::nrpe
  • Create a separate nrpe_custom_checks module and keep base the way it is
  • Create a separate nrpe_custom_checks module and refactor base to use it for check creation

Rather than extending the base NRPE class, we chose to create a standalone NRPE custom checks module and keep base independent.

Consequences

This decision has some positive consequences:

  • By not adding the new nrpe_custom_checks functionality to base, we can keep the scope small and tightly focused on being a standalone foundation for CentOS 7 images
  • This guarantees faster builds because there are no dependencies and the contents are deliberately kept minimal
  • The nrpe_custom_checks module can evolve its own design decisions without being constrained by the design restrictions base has

There are some potential negative consequences or arguments against this decision:

  • There is merit in providing the ability to create a custom check as a part of base
  • There is logic duplication for plugin configuration between base::nrpe and nrpe_custom_checks module
  • Testing the nrpe_custom_checks module becomes a bit tricky
    • Some resources in the nrpe_custom_checks module are parameterised to use an external NRPE installation and configuration: base::nrpe
    • base::nrpe depends on hiera data - we must mock this in our tests
    • If the base::nrpe implementation changes it could break the nrpe_custom_checks module tests

This decision is based on the current context and can change in the future if there's new information.