warn on low replication

CHANGELOG.md

@@ -50,6 +50,7 @@
 - [Improvement] Introduce config validation.
 - [Improvement] Provide flash messages support.
 - [Improvement] Use replication factor of two by default (if not overridden) for Web UI topics when there is more than one broker.
+- [Improvement] Show a warning when replication factor of 1 is used for Web UI topics in production.
 - [Fix] Return 402 status instead of 500 on Pro features that are not available in OSS.
 - [Fix] Fix a case where errors would not be visible without Rails due to the `String#first` usage.
 - [Fix] Fix a case where live-poll would be disabled but would still update data.

lib/karafka/web/ui/models/status.rb

@@ -113,9 +113,30 @@ def partitions
             )
           end
 
+          # @return [Status::Step] do we have correct replication for given env
+          def replication
+            if partitions.success?
+              status = :success
+              # low replication is not an error but just a warning and a potential problem
+              # in case of a crash, this is why we do not fail but warn only
+              status = :warning if topics_details.values.any? { |det| det[:replication] < 2 }
+              # Allow for non-production setups to use replication 1 as it is not that relevant
+              status = :success unless Karafka.env.production?
+              details = topics_details
+            else
+              status = :halted
+              details = {}
+            end
+
+            Step.new(
+              status,
+              details
+            )
+          end
+
           # @return [Status::Step] Is the initial consumers state present in Kafka
           def initial_consumers_state
-            if partitions.success?
+            if replication.success?
               @current_state ||= Models::ConsumersState.current
               status = @current_state ? :success : :failure
             else
@@ -234,20 +255,25 @@ def topics_errors
 
           # @return [Hash] hash with topics with which we work details (even if don't exist)
           def topics_details
+            base = { present: false, partitions: 0, replication: 1 }
+
             topics = {
-              topics_consumers_states => { present: false, partitions: 0 },
-              topics_consumers_reports => { present: false, partitions: 0 },
-              topics_consumers_metrics => { present: false, partitions: 0 },
-              topics_errors => { present: false, partitions: 0 }
+              topics_consumers_states => base.dup,
+              topics_consumers_reports => base.dup,
+              topics_consumers_metrics => base.dup,
+              topics_errors => base.dup
             }
 
             @cluster_info.topics.each do |topic|
               name = topic[:topic_name]
 
               next unless topics.key?(name)
 
-              topics[name][:present] = true
-              topics[name][:partitions] = topic[:partition_count]
+              topics[name].merge!(
+                present: true,
+                partitions: topic[:partition_count],
+                replication: topic[:partitions].map { |part| part[:replica_count] }.max
+              )
             end
 
             topics

lib/karafka/web/ui/views/status/show.erb

@@ -63,6 +63,21 @@
         )
       %>
 
+      <%==
+        partial(
+          "status/#{@status.replication.to_s}",
+          locals: {
+            title: 'Replication factors',
+            description: partial(
+              'status/warnings/replication',
+              locals: {
+                details: @status.replication.details
+              }
+            )
+          }
+        )
+      %>
+
       <%==
         partial(
           "status/#{@status.initial_consumers_state.to_s}",

lib/karafka/web/ui/views/status/warnings/_replication.erb

@@ -0,0 +1,19 @@
+<p>
+  It is recommended to have a replication factor greater than <code>1</code> for all the Karafka Web UI topics in a production environment.
+</p>
+
+<p>
+  Current replication factors for Karafka Web UI topics:
+</p>
+
+<ul>
+  <% details.each do |name, details| %>
+    <li>
+      <code><%= name %></code>: <code><%= details[:replication] %></code>
+    </li>
+  <% end %>
+</ul>
+
+<p>
+  Please ensure all those topics have a replication factor of at least <code>2</code>.
+</p>

spec/lib/karafka/web/ui/controllers/status_spec.rb

@@ -30,5 +30,20 @@
         expect(body).to include(breadcrumbs)
       end
     end
+
+    context 'when replication factor is less than 2 in production' do
+      before do
+        allow(Karafka.env).to receive(:production?).and_return(true)
+        get 'status'
+      end
+
+      it do
+        expect(response).to be_ok
+        expect(body).to include(support_message)
+        expect(body).to include(breadcrumbs)
+        expect(body).to include('Please ensure all those topics have a replication')
+        expect(body).to include('Warning')
+      end
+    end
   end
 end

spec/lib/karafka/web/ui/models/status_spec.rb

@@ -256,6 +256,22 @@
         expect(result.partial_namespace).to eq('successes')
       end
     end
+
+    context 'when state is present but replication factor is 1 in prod' do
+      before do
+        all_topics
+        produce(states_topic, state)
+        # This will force a warning because in prod replication is expected to be > 1
+        allow(Karafka.env).to receive(:production).and_return(true)
+      end
+
+      it 'expect all to be ok because replication is a warning' do
+        expect(result.success?).to eq(true)
+        expect(result.to_s).to eq('success')
+        expect(result.details).to eq(nil)
+        expect(result.partial_namespace).to eq('successes')
+      end
+    end
   end
 
   describe '#initial_consumers_metrics' do