diff --git a/Gemfile b/Gemfile
index 0b8e97a..8ab5894 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,3 +4,4 @@ gem "async"
 gem "falcon", "0.44.0" # use different version on your own risk; 0.44.0 worked
 gem "sinatra"
 gem "faraday"
+gem "rspec"
diff --git a/Gemfile.lock b/Gemfile.lock
index 4090b46..ccd2a4e 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -30,6 +30,7 @@ GEM
       fiber-annotation
       fiber-local (~> 1.1)
       json
+    diff-lcs (1.6.0)
     falcon (0.44.0)
       async
       async-container (~> 0.17)
@@ -79,6 +80,19 @@ GEM
       rack (>= 3.0.0, < 4)
     rack-session (2.0.0)
       rack (>= 3.0.0)
+    rspec (3.13.0)
+      rspec-core (~> 3.13.0)
+      rspec-expectations (~> 3.13.0)
+      rspec-mocks (~> 3.13.0)
+    rspec-core (3.13.3)
+      rspec-support (~> 3.13.0)
+    rspec-expectations (3.13.3)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.13.0)
+    rspec-mocks (3.13.2)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.13.0)
+    rspec-support (3.13.2)
     ruby2_keywords (0.0.5)
     samovar (2.3.0)
       console (~> 1.0)
@@ -102,6 +116,7 @@ DEPENDENCIES
   async
   falcon (= 0.44.0)
   faraday
+  rspec
   sinatra
 
 BUNDLED WITH
diff --git a/case_study.md b/case_study.md
new file mode 100644
index 0000000..5f5dbc5
--- /dev/null
+++ b/case_study.md
@@ -0,0 +1,17 @@
+# Case-study оптимизации
+
+## Актуальная проблема
+Есть программа, корректно выполняющая свою работу, но очень медленно.
+
+## Формирование метрики
+Время выполнения программы должно укладываться в 7 секунд
+
+## Гарантия корректности работы оптимизированной программы
+Был написан тест на корректность работы программы, который проверяет, что программа работает корректно и после оптимизации.
+
+## Вникаем в детали
+В программе происходит много долгих запросов к серверу, которые можно выполнять параллельно.
+Было принято решение использовать socketry/async для решения задачи.
+
+## Результаты
+В результате проделанной оптимизации удалось улучшить метрику системы с ~19 секунд до ~6 и уложиться в заданный бюджет.
diff --git a/client.rb b/client.rb
index e001ba3..49e97f4 100644
--- a/client.rb
+++ b/client.rb
@@ -1,5 +1,7 @@
 require 'openssl'
 require 'faraday'
+require 'async'
+require 'async/semaphore'
 
 OpenSSL::SSL::VERIFY_PEER = OpenSSL::SSL::VERIFY_NONE
 
@@ -13,20 +15,30 @@
 # Тип C:
 #   - работает 1 секунду
 #   - одновременно можно запускать не более одного
-#
+
+A_SEMAPHORE = Async::Semaphore.new(3)
+B_SEMAPHORE = Async::Semaphore.new(2)
+C_SEMAPHORE = Async::Semaphore.new(1)
+
 def a(value)
-  puts "https://localhost:9292/a?value=#{value}"
-  Faraday.get("https://localhost:9292/a?value=#{value}").body
+  A_SEMAPHORE.acquire do
+    puts "https://localhost:9292/a?value=#{value}"
+    Faraday.get("https://localhost:9292/a?value=#{value}").body
+  end
 end
 
 def b(value)
-  puts "https://localhost:9292/b?value=#{value}"
-  Faraday.get("https://localhost:9292/b?value=#{value}").body
+  B_SEMAPHORE.acquire do
+    puts "https://localhost:9292/b?value=#{value}"
+    Faraday.get("https://localhost:9292/b?value=#{value}").body
+  end
 end
 
 def c(value)
-  puts "https://localhost:9292/c?value=#{value}"
-  Faraday.get("https://localhost:9292/c?value=#{value}").body
+  C_SEMAPHORE.acquire do
+    puts "https://localhost:9292/c?value=#{value}"
+    Faraday.get("https://localhost:9292/c?value=#{value}").body
+  end
 end
 
 # Референсное решение, приведённое ниже работает правильно, занимает ~19.5 секунд
@@ -36,43 +48,40 @@ def collect_sorted(arr)
   arr.sort.join('-')
 end
 
-start = Time.now
-
-a11 = a(11)
-a12 = a(12)
-a13 = a(13)
-b1 = b(1)
-
-ab1 = "#{collect_sorted([a11, a12, a13])}-#{b1}"
-puts "AB1 = #{ab1}"
-
-c1 = c(ab1)
-puts "C1 = #{c1}"
-
-a21 = a(21)
-a22 = a(22)
-a23 = a(23)
-b2 = b(2)
-
-ab2 = "#{collect_sorted([a21, a22, a23])}-#{b2}"
-puts "AB2 = #{ab2}"
-
-c2 = c(ab2)
-puts "C2 = #{c2}"
-
-a31 = a(31)
-a32 = a(32)
-a33 = a(33)
-b3 = b(3)
+def a_collection(number)
+  Sync do |parent|
+    [1, 2, 3].map do |n|
+      parent.async do
+        a("#{number}#{n}")
+      end
+    end.map(&:wait)
+  end
+end
 
-ab3 = "#{collect_sorted([a31, a32, a33])}-#{b3}"
-puts "AB3 = #{ab3}"
+def part(number)
+  a_collection, b = Sync do |parent|
+    a_collection = parent.async { a_collection(number) }
+    b = parent.async { b(number) }
+    [a_collection, b].map(&:wait)
+  end
 
-c3 = c(ab3)
-puts "C3 = #{c3}"
+  ab = "#{collect_sorted(a_collection)}-#{b}"
+  puts "AB#{number} = #{ab}"
 
-c123 = collect_sorted([c1, c2, c3])
-result = a(c123)
+  c = c(ab)
+  puts "C#{number} = #{c}"
+  c
+end
 
-puts "FINISHED in #{Time.now - start}s."
-puts "RESULT = #{result}" # 0bbe9ecf251ef4131dd43e1600742cfb
+def run
+  c_collection = Sync do |parent|
+    [1, 2, 3].map do |n|
+      parent.async do
+        part(n)
+      end
+    end.map(&:wait)
+  end
+
+  c123 = collect_sorted(c_collection)
+  a(c123)
+end
diff --git a/log.txt b/log.txt
new file mode 100644
index 0000000..b332835
--- /dev/null
+++ b/log.txt
@@ -0,0 +1,24 @@
+https://localhost:9292/a?value=11
+https://localhost:9292/a?value=12
+https://localhost:9292/a?value=13
+https://localhost:9292/b?value=1
+https://localhost:9292/b?value=2
+https://localhost:9292/a?value=21
+https://localhost:9292/a?value=22
+https://localhost:9292/a?value=23
+https://localhost:9292/b?value=3
+AB1 = 6512bd43d9caa6e02c990b0a82652dca-c20ad4d76fe97759aa27a0c99bff6710-c51ce410c124a10e0db5e4b97fc2af39-6b86b273ff34fce19d6b804eff5a3f5747ada4eaa22f1d49c01e52ddb7875b4b
+https://localhost:9292/c?value=6512bd43d9caa6e02c990b0a82652dca-c20ad4d76fe97759aa27a0c99bff6710-c51ce410c124a10e0db5e4b97fc2af39-6b86b273ff34fce19d6b804eff5a3f5747ada4eaa22f1d49c01e52ddb7875b4b
+https://localhost:9292/a?value=31
+https://localhost:9292/a?value=32
+https://localhost:9292/a?value=33
+AB2 = 37693cfc748049e45d87b8c7d8b9aacd-3c59dc048e8850243be8079a5c74d079-b6d767d2f8ed5d21a44b0e5886680cb9-d4735e3a265e16eee03f59718b9b5d03019c07d8b6c51f90da3a666eec13ab35
+https://localhost:9292/c?value=37693cfc748049e45d87b8c7d8b9aacd-3c59dc048e8850243be8079a5c74d079-b6d767d2f8ed5d21a44b0e5886680cb9-d4735e3a265e16eee03f59718b9b5d03019c07d8b6c51f90da3a666eec13ab35
+C1 = ee54918569d9620a4947a184c097ab1a90b33db6b8a009e01cbbe7238f800ecf846068a36ac7ae289a0796face8b471dcfd7e7d55f9081bccd8f2f50c1d4f888
+AB3 = 182be0c5cdcd5072bb1864cdee4d3d6e-6364d3f0f495b6ab9dcf8d3b5c6e0b01-c16a5320fa475530d9583c34fd356ef5-4e07408562bedb8b60ce05c1decfe3ad16b72230967de01f640b7e4729b49fce
+https://localhost:9292/c?value=182be0c5cdcd5072bb1864cdee4d3d6e-6364d3f0f495b6ab9dcf8d3b5c6e0b01-c16a5320fa475530d9583c34fd356ef5-4e07408562bedb8b60ce05c1decfe3ad16b72230967de01f640b7e4729b49fce
+C2 = 4bc5a33ec7c74626144b5277e887501074b07819ecd9cc506b08e765be1f26a3d7c9fbb59bde46379ca7f0ebf2ef066f38c0a4652d4017a635fc8bdddc070557
+C3 = 35bf8dfb6c327259fd05a56a6d47ecfc21965b0f4ff8926b164c32e31cc91d825da1a73bc145cdec04dfbb0862fb14b687f15a8233be15092c4c85da4b7bf94a
+https://localhost:9292/a?value=35bf8dfb6c327259fd05a56a6d47ecfc21965b0f4ff8926b164c32e31cc91d825da1a73bc145cdec04dfbb0862fb14b687f15a8233be15092c4c85da4b7bf94a-4bc5a33ec7c74626144b5277e887501074b07819ecd9cc506b08e765be1f26a3d7c9fbb59bde46379ca7f0ebf2ef066f38c0a4652d4017a635fc8bdddc070557-ee54918569d9620a4947a184c097ab1a90b33db6b8a009e01cbbe7238f800ecf846068a36ac7ae289a0796face8b471dcfd7e7d55f9081bccd8f2f50c1d4f888
+FINISHED in 6.05257s.
+RESULT = 0bbe9ecf251ef4131dd43e1600742cfb
diff --git a/rspec/client_spec.rb b/rspec/client_spec.rb
new file mode 100644
index 0000000..c1b4c97
--- /dev/null
+++ b/rspec/client_spec.rb
@@ -0,0 +1,29 @@
+require 'rspec'
+require 'sinatra/base'
+require_relative '../client'
+
+RSpec.describe 'Client' do
+  before do
+    allow(Faraday).to receive(:get).with(any_args) do |*args, **_kwargs|
+      uri = URI.parse(args[0])
+      path = uri.path
+      value = CGI.parse(uri.query)['value'].first
+
+      result = case path
+               when '/a'
+                 Digest::MD5.hexdigest(value)
+               when '/b'
+                 Digest::SHA256.hexdigest(value)
+               when '/c'
+                 Digest::SHA512.hexdigest(value)
+               end
+
+      instance_double(Faraday::Response,
+                      status: 200,
+                      body: result,
+                      headers: {'Content-Type' => 'application/json'})
+    end
+  end
+
+  it { expect(run).to eq '0bbe9ecf251ef4131dd43e1600742cfb' }
+end
diff --git a/runner.rb b/runner.rb
new file mode 100644
index 0000000..f35f941
--- /dev/null
+++ b/runner.rb
@@ -0,0 +1,10 @@
+require_relative 'client'
+
+$stdout = File.open('log.txt', 'w')
+
+start = Time.now
+
+result = run
+
+puts "FINISHED in #{Time.now - start}s."
+puts "RESULT = #{result}" # 0bbe9ecf251ef4131dd43e1600742cfb