algorithm tests

app/models/district.rb

@@ -258,9 +258,12 @@ def total_demanded(resource)
   def instance_count_demanded(resource)
     per_instance = total_registered(resource) / container_instances.count
 
-    # naively determine the number of instances needed for each service
+    # naively determine the number of instances needed for each service.
+    # this algo gives at worst n + 2 servers where n is the number of types
+    # of service memory requirements and at best the exact number of instances.
+    # please see tests for details.
     demand_structure(resource).map do |k, v|
-      (k / 1699.to_f * v).ceil
+      (k / per_instance.to_f * v).ceil + 1
     end.sum
   end
 

spec/models/district_spec.rb

@@ -147,4 +147,52 @@
       district.publish_sns("message")
     end
   end
+
+  describe '#instances_recommended' do
+    it 'gives the maximum from cpu and memory requirements' do
+      allow(district).to receive(:instance_count_demanded).with(:cpu) { 100 }
+      allow(district).to receive(:instance_count_demanded).with(:memory) { 10 }
+
+      expect(district.send(:instances_recommended)).to eq 100
+    end
+  end
+
+  describe '#instance_count_demanded' do
+    before do
+      # set some constants
+      allow(district).to receive(:container_instances) { [1] }
+      allow(district).to receive(:total_registered) { 1000 }
+    end
+
+    it 'gives 1 more server than required if we have only 1 service type with exact occupancy' do
+      allow(district).to receive(:demand_structure) { { 1000 => 3 } }
+
+      expect(district.send(:instance_count_demanded, :something)).to eq 4
+    end
+
+    it 'gives one more server required if we have 1 service type with less than half occupancy' do
+      allow(district).to receive(:demand_structure) { { 400 => 3 } }
+
+      expect(district.send(:instance_count_demanded, :something)).to eq 3
+    end
+
+    it 'gives exactly the number of servers required if we have 1 service type with more than half occupancy' do
+      allow(district).to receive(:demand_structure) { { 600 => 3 } }
+
+      expect(district.send(:instance_count_demanded, :something)).to eq 3
+    end
+
+    it 'gives two more servers than required if we have 1 service type with less than half occupancy and a minor type' do
+      allow(district).to receive(:demand_structure) { { 400 => 3, 100 => 3 } }
+
+      expect(district.send(:instance_count_demanded, :something)).to eq 5
+    end
+
+    it 'gives two more servers than required if we have 1 service type with more than half occupancy and a minor type' do
+      allow(district).to receive(:demand_structure) { { 600 => 3, 100 => 3 } }
+
+      expect(district.send(:instance_count_demanded, :something)).to eq 5
+    end
+
+  end
 end