Uptime computation

app/models/service.rb

@@ -34,6 +34,34 @@ class Service < ApplicationRecord
     end
   end
 
+  UPTIME_DURATIONS = [1.year, 1.month, 1.week, 1.day].freeze
+
+  def uptime
+    first_check = self.checks.order(created_at: :asc).limit(1).first
+    return nil unless first_check
+
+    now    = Time.now.utc
+    uptime = UPTIME_DURATIONS.map { |d| [d, 0.second] }.to_h
+    ago    = UPTIME_DURATIONS.map { |d| [d, d.ago(now).utc] }.to_h
+    start  = ago.fetch UPTIME_DURATIONS.first
+
+    self.downs.where('"to" >= ?', start).each do |down|
+      UPTIME_DURATIONS.each do |duration|
+        start            = ago[duration]
+        uptime[duration] += down.to.utc - [start, down.from.utc].max if start <= down.to
+      end
+    end
+
+    check_start = first_check.created_at.utc
+    uptime.map do |duration, downtime|
+      start = ago[duration]
+      max   = [check_start, start].max
+      diff  = (now - max).seconds
+      percentage = 1 - downtime / diff
+      [duration, percentage]
+    end.to_h
+  end
+
   def last_check
     self.checks.order(created_at: :desc).limit(1).first
   end

spec/models/service_spec.rb

@@ -13,7 +13,6 @@ describe Service do
                                     .and_return(check)
 
       now = Time.utc 2020, 01, 01, 00, 00, 00
-      from = nil
       Timecop.freeze now do
         allow(check).to receive(:ok?).and_return(true).at_least(:once)
         expect { service.check! }.to not_change { ActionMailer::Base.deliveries.count } \
@@ -54,4 +53,37 @@ describe Service do
       end
     end
   end
+
+  describe :uptime do
+    let!(:service) { Service.create! name: :test, type: :ping, config: { host: 'example.org' } }
+    let!(:check) { service.checks.create! created_at: '2020-01-01', result: true }
+
+    it 'must exclude downtime prior to uptime period' do
+      service.downs.create! from: '2019-01-01', to: '2020-01-01'
+      Timecop.freeze 2021, 01, 01 do
+        expect(service.uptime[1.year]).to eq 1
+      end
+    end
+
+    it 'must only include the last downtime part of overlapping uptime period' do
+      service.downs.create! from: '2019-07-01', to: '2020-07-01'
+      Timecop.freeze 2021, 01, 01 do
+        expect(service.uptime[1.year]).to eq 1-(182.0/366)
+      end
+    end
+
+    it 'must include all the downtime later to uptime period' do
+      service.downs.create! from: '2020-03-01', to: '2020-07-01'
+      Timecop.freeze 2021, 01, 01 do
+        expect(service.uptime[1.year]).to eq 1-(122.0/366)
+      end
+    end
+
+    it 'must restrict uptime period if check created before' do
+      service.downs.create! from: '2020-03-01', to: '2020-07-01'
+      Timecop.freeze 2020, 06, 01 do
+        expect(service.uptime[1.year]).to eq 1-(122.0/152)
+      end
+    end
+  end
 end