1 files changed, 69 insertions, 0 deletions
diff --git a/spec/lib/gitlab/instrumentation/connection_pool_spec.rb b/spec/lib/gitlab/instrumentation/connection_pool_spec.rb
new file mode 100644
index 00000000000..b7cab2e9900
--- /dev/null
+++ b/spec/lib/gitlab/instrumentation/connection_pool_spec.rb
@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'support/helpers/rails_helpers'
+
+RSpec.describe Gitlab::Instrumentation::ConnectionPool, feature_category: :redis do
+  let(:option) { { name: 'test', size: 5 } }
+  let(:pool) { ConnectionPool.new(option) { 'nothing' } }
+
+  let_it_be(:size_gauge_args) { [:gitlab_connection_pool_size, 'Size of connection pool', {}, :all] }
+  let_it_be(:available_gauge_args) do
+    [:gitlab_connection_pool_available_count,
+      'Number of available connections in the pool', {}, :all]
+  end
+
+  subject(:checkout_pool) { pool.checkout }
+
+  describe '.checkout' do
+    let(:size_gauge_double) { instance_double(::Prometheus::Client::Gauge) }
+
+    context 'when tracking for the first time' do
+      it 'initialises gauges' do
+        expect(::Gitlab::Metrics).to receive(:gauge).with(*size_gauge_args).and_call_original
+        expect(::Gitlab::Metrics).to receive(:gauge).with(*available_gauge_args).and_call_original
+
+        checkout_pool
+      end
+    end
+
+    it 'sets the size gauge only once' do
+      expect(::Gitlab::Metrics.gauge(*size_gauge_args)).to receive(:set).with(
+        { pool_name: 'test', pool_key: anything, connection_class: "String" }, 5).once
+
+      checkout_pool
+      checkout_pool
+    end
+
+    context 'when tracking on subsequent calls' do
+      before do
+        pool.checkout # initialise instance variables
+      end
+
+      it 'uses memoized gauges' do
+        expect(::Gitlab::Metrics).not_to receive(:gauge).with(*size_gauge_args)
+        expect(::Gitlab::Metrics).not_to receive(:gauge).with(*available_gauge_args)
+
+        expect(pool.instance_variable_get(:@size_gauge)).not_to receive(:set)
+          .with({ pool_name: 'test', pool_key: anything, connection_class: "String" }, 5)
+        expect(pool.instance_variable_get(:@available_gauge)).to receive(:set)
+          .with({ pool_name: 'test', pool_key: anything, connection_class: "String" }, 4)
+
+        checkout_pool
+      end
+
+      context 'when pool name is omitted' do
+        let(:option) { {} }
+
+        it 'uses unknown name' do
+          expect(pool.instance_variable_get(:@size_gauge)).not_to receive(:set)
+            .with({ pool_name: 'unknown', pool_key: anything, connection_class: "String" }, 5)
+          expect(pool.instance_variable_get(:@available_gauge)).to receive(:set)
+            .with({ pool_name: 'unknown', pool_key: anything, connection_class: "String" }, 4)
+
+          checkout_pool
+        end
+      end
+    end
+  end
+end