Add client function for fetching topic replica count

lib/kafka/client.rb

@@ -694,6 +694,15 @@ def partitions_for(topic)
       @cluster.partitions_for(topic).count
     end
 
+    # Counts the number of replicas for a topic's partition
+    #
+    # @param topic [String]
+    # @return [Integer] the number of replica nodes for the topic's partition
+
+    def replica_count_for(topic)
+      @cluster.partitions_for(topic).first.replicas.count
+    end
+
     # Retrieve the offset of the last message in a partition. If there are no
     # messages in the partition -1 is returned.
     #

lib/kafka/protocol/metadata_response.rb

@@ -34,7 +34,7 @@ module Protocol
     #
     class MetadataResponse
       class PartitionMetadata
-        attr_reader :partition_id, :leader
+        attr_reader :partition_id, :leader, :replicas
 
         attr_reader :partition_error_code
 

spec/functional/client_spec.rb

@@ -130,6 +130,23 @@
     expect(kafka.partitions_for(topic)).to be > 0
   end
 
+  example "fetching the replica count for a topic" do
+    expect(kafka.replica_count_for(topic)).to eq 1
+  end
+
+  example "fetching the replica count for a topic that doesn't yet exist" do
+    topic = "unknown-topic-#{SecureRandom.uuid}"
+
+    expect { kafka.replica_count_for(topic) }.to raise_exception(Kafka::LeaderNotAvailable)
+
+    # Eventually the call should succeed.
+    expect {
+      10.times { kafka.replica_count_for(topic) rescue nil }
+    }.not_to raise_exception
+
+    expect(kafka.replica_count_for(topic)).to be > 0
+  end
+
   example "delivering a message to a topic" do
     kafka.deliver_message("yolo", topic: topic, key: "xoxo", partition: 0)