Add support for custom notifiers

[gabrieltaylor]

Description

Adds support for sending failing checks to a custom notifier.

Happy to adjust the code however you recommend if this change has value for other users. Happy to close the PR if it doesn't :)

Motivation

In our application we find value in triggering alerts from failing Blazer checks. In order to create alerts in a third party system we need to send a specific payload to that service via an HTTP POST request.

To enable this function we've defined a module that implements:

• state_change
• failing_checks

following the pattern defined by the Blazer::EmailNotifier and Blazer::SlackNotifier.

The interface implemented by the module is

  module Interface
    def self.state_change(check:, state:, state_was:, result:, message:, check_type:)
    end

    def self.failing_checks(checks)
    end
  end

The implementation might look something like this

module BlazerService
  module CheckFailureHttpNotifier
    extend T::Sig

    sig { params(check: Blazer::Check, state: String, state_was: String, result: Blazer::Result, message: T.nilable(String), check_type: String).returns(Net::HTTPResponse) }
    def self.state_change(check:, state:, state_was:, result:, message:, check_type:)
      payload = build_payload(check:, state:, state_was:, result:, message:, check_type:)
      make_request(payload)
    end

    sig { params(checks: T::Array[Blazer::Check]).void }
    def self.failing_checks(checks)
      # noop
    end

    sig { params(payload: T::Hash[Symbol, T.untyped]).returns(Net::HTTPResponse) }
    def self.make_request(payload)
      http = Net::HTTP.new(uri.host, uri.port)
      http.use_ssl = (uri.scheme == "https")

      request = Net::HTTP::Post.new(uri.request_uri, {"Content-Type" => "application/json", "Accept" => "application/json"})
      request.body = payload.to_json

      http.request(request)
    end

    sig { params(check: Blazer::Check, state: String, state_was: String, result: Blazer::Result, message: T.nilable(String), check_type: String).returns(T::Hash[Symbol, T.untyped]) }
    def self.build_payload(check:, state:, state_was:, result:, message:, check_type:)
      {
        check_id: check.id,
        query_name: check.query&.name,
        query_description: check.query&.description,
        query_statement: check.query&.statement,
        row_count: result.rows.size,
        state:,
        state_was:,
        message:,
        check_type:
      }
    end

    sig { returns(URI::HTTP) }
    def self.uri
      T.cast(URI.parse(url), URI::HTTP)
    end

    sig { returns(String) }
    def self.url
      ENV.fetch("BLAZER_HTTP_NOTIFICATION_URL")
    end
  end
end

We're adding the custom notifier in the initializer like this.

Rails.application.config.to_prepare do
  notifier = BlazerService::CheckFailureHttpNotifier
  unless Blazer.custom_notifiers.include?(notifier)
    Blazer.custom_notifiers << notifier
  end
end