Add a generic threshold check class

This makes it easier to implement a class of check which compares
a value with a threshold.

docs/healthchecks.md

@@ -47,3 +47,44 @@ This will check:
 
 Each check class gets instanced each time the health check end point is called.
 This allows you to cache any complex queries speeding up performance.
+
+## Built-in Checks
+
+### `SidekiqRedis`
+
+This checks that the app has a connection to Redis via Sidekiq.
+
+### `ActiveRecord`
+
+This checks that the app has a connection to the database via ActiveRecord.
+
+### `ThresholdCheck`
+
+This class is the basis for a check which compares a value with a warning or a
+critical threshold. To implement this kind of check in your application, you
+can inherit from the class.
+
+```ruby
+class MyThresholdCheck < GovukHealthcheck::ThresholdCheck
+  def name
+    :my_threshold_check
+  end
+
+  def value
+    # get the value to be checked
+  end
+
+  def total
+    # (optional) get the total value to be included in the details as extra
+    # information
+  end
+
+  def warning_threshold
+    # if the value is above this threshold, its status is warning
+  end
+
+  def critical_threshold
+    # if the value is above this threshold, its status is critical
+  end
+end
+```

lib/govuk_app_config/govuk_healthcheck.rb

@@ -1,6 +1,7 @@
 require "govuk_app_config/govuk_healthcheck/checkup"
 require "govuk_app_config/govuk_healthcheck/active_record"
 require "govuk_app_config/govuk_healthcheck/sidekiq_redis"
+require "govuk_app_config/govuk_healthcheck/threshold_check"
 require "json"
 
 module GovukHealthcheck

lib/govuk_app_config/govuk_healthcheck/threshold_check.rb

@@ -0,0 +1,50 @@
+module GovukHealthcheck
+  class ThresholdCheck
+    def status
+      if value >= critical_threshold
+        :critical
+      elsif value >= warning_threshold
+        :warning
+      else
+        :ok
+      end
+    end
+
+    def message
+      if value >= critical_threshold
+        "#{value} is above the critical threshold (#{critical_threshold})"
+      elsif value >= warning_threshold
+        "#{value} is above the warning threshold (#{warning_threshold})"
+      else
+        "#{value} is below the critical and warning thresholds"
+      end
+    end
+
+    def details
+      {
+        value: value,
+        total: total,
+        thresholds: {
+          critical: critical_threshold,
+          warning: warning_threshold,
+        },
+      }
+    end
+
+    def value
+      raise "This method must be overridden to be the check value."
+    end
+
+    def total
+      nil # This method can be overriden to provide the total for the check.
+    end
+
+    def critical_threshold
+      raise "This method must be overriden to be the critical threshold."
+    end
+
+    def warning_threshold
+      raise "This method must be overriden to be the warning threshold."
+    end
+  end
+end

spec/lib/govuk_healthcheck/threshold_check_spec.rb

@@ -0,0 +1,67 @@
+require "spec_helper"
+require "govuk_app_config/govuk_healthcheck"
+require_relative "shared_interface"
+
+RSpec.describe GovukHealthcheck::ThresholdCheck do
+  context "an ok check" do
+    subject { TestThresholdCheck.new(0, 10, 20) }
+
+    it_behaves_like "a healthcheck"
+
+    its(:status) { is_expected.to eq(:ok) }
+    its(:message) { is_expected.to match(/below the critical and warning thresholds/) }
+    its(:details) do
+      is_expected.to match(
+        hash_including(value: 0, thresholds: { warning: 10, critical: 20 })
+      )
+    end
+  end
+
+  context "a warning check" do
+    subject { TestThresholdCheck.new(11, 10, 20) }
+
+    it_behaves_like "a healthcheck"
+
+    its(:status) { is_expected.to eq(:warning) }
+    its(:message) { is_expected.to match(/above the warning threshold/) }
+    its(:details) do
+      is_expected.to match(
+        hash_including(value: 11, thresholds: { warning: 10, critical: 20 })
+      )
+    end
+  end
+
+  context "a critical check" do
+    subject { TestThresholdCheck.new(21, 10, 20) }
+
+    it_behaves_like "a healthcheck"
+
+    its(:status) { is_expected.to eq(:critical) }
+    its(:message) { is_expected.to match(/above the critical threshold/) }
+    its(:details) do
+      is_expected.to match(
+        hash_including(value: 21, thresholds: { warning: 10, critical: 20 })
+      )
+    end
+  end
+
+  context "with a total" do
+    subject { TestThresholdCheck.new(0, 10, 20, 40) }
+
+    it_behaves_like "a healthcheck"
+
+    its(:details) { is_expected.to match(hash_including(total: 40)) }
+  end
+
+  class TestThresholdCheck < GovukHealthcheck::ThresholdCheck
+    def initialize(value, warning_threshold, critical_threshold, total = nil, name = :test)
+      @value = value
+      @warning_threshold = warning_threshold
+      @critical_threshold = critical_threshold
+      @total = total
+      @name = name
+    end
+
+    attr_reader :value, :warning_threshold, :critical_threshold, :total, :name
+  end
+end