Handle deadlock

app/controllers/clock_ins_controller.rb

@@ -1,14 +1,16 @@
 class ClockInsController < ApplicationController
   include UserFinder
+  include Retryable
 
   def upsert
     current_time = Time.now
 
-    begin
+    with_retry do
       ActiveRecord::Base.transaction do
         last_unused_clock_in = ClockIn.where(
-          user_id: @user.id, 
-          is_clocked_out: false)
+          is_clocked_out: false,
+          user_id: @user.id
+          )
           .order(clocked_in_at: :desc)
           .limit(1)
           .lock(true)
@@ -27,14 +29,12 @@ def upsert
         else
           ClockIn.create!(
             user_id: @user.id,
-            clocked_in_at: Time.now
+            clocked_in_at: current_time
           )
 
           render_success(@user.clock_ins_json, I18n.t('success.messages.clock_in_success'))
         end
       end
-    rescue ActiveRecord::ActiveRecordError => e
-      render_error(:unprocessable_entity, e.message)
     end
   end
 

app/controllers/concerns/retryable.rb

@@ -0,0 +1,28 @@
+module Retryable
+  MAX_RETRIES = 3
+  RETRY_DELAY = 0.5 # seconds
+
+  def with_retry(max_retries: MAX_RETRIES, delay: RETRY_DELAY)
+    attempts = 0
+
+    begin
+      yield
+    rescue ActiveRecord::Deadlocked => e
+      attempts += 1
+      Rails.logger.warn("Retryable Error: #{e.class} - #{e.message}. Attempt #{attempts} of #{max_retries}.")
+      if attempts < max_retries
+        sleep(delay * attempts)
+        retry
+      else
+        Rails.logger.error("Retries exhausted for #{e.class}: #{e.message}")
+        raise e
+      end
+    rescue ActiveRecord::ActiveRecordError => e
+      Rails.logger.error("ActiveRecord Error: #{e.class} - #{e.message}")
+      render_error(:unprocessable_entity, e.message)
+    rescue StandardError => e
+      Rails.logger.error("Unexpected Error: #{e.class} - #{e.message}")
+      render_error(:internal_server_error, e.message)
+    end
+  end
+end

db/migrate/20250102060751_add_index_to_user_follows_on_follower_and_followee.rb

@@ -4,6 +4,6 @@ def up
   end
 
   def down
-    add_index :user_follows, name: 'index_user_follows_on_follower_id_and_followee_id'
+    remove_index :user_follows, name: 'index_user_follows_on_follower_id_and_followee_id'
   end
 end

db/migrate/20250103014113_add_index_clock_ins_on_is_clocked_out_and_user_id.rb

@@ -0,0 +1,11 @@
+class AddIndexClockInsOnIsClockedOutAndUserId < ActiveRecord::Migration[7.0]
+  def up
+    add_index :clock_ins, [:is_clocked_out, :user_id], name: 'index_clock_ins_on_is_clocked_out_and_user_id'
+    remove_index :clock_ins, [:user_id, :clocked_in_at]
+  end
+
+  def down
+    remove_index :user_follows, name: 'index_clock_ins_on_is_clocked_out_and_user_id'
+    add_index :clock_ins, [:user_id, :clocked_in_at]
+  end
+end

db/schema.rb

@@ -10,15 +10,15 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2025_01_02_060751) do
+ActiveRecord::Schema[7.0].define(version: 2025_01_03_014113) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
 
   create_table "clock_ins", force: :cascade do |t|
     t.bigint "user_id", null: false
     t.datetime "clocked_in_at", null: false
     t.boolean "is_clocked_out", default: false, null: false
-    t.index ["user_id", "clocked_in_at"], name: "index_clock_ins_on_user_id_and_clocked_in_at"
+    t.index ["is_clocked_out", "user_id"], name: "index_clock_ins_on_is_clocked_out_and_user_id"
     t.index ["user_id"], name: "index_clock_ins_on_user_id"
   end
 

system-design/README.md

@@ -23,6 +23,7 @@ This document contains the details of ideas and implementation for time tracker
 3. For any clock-ins, every second clock-in will be considered a clock out for the last clock-in for us to calculate sleep record.
 4. Since the feature is quite simple, most part is enough with simple error message, but it is better to implement proper logging.
 5. Need proper error handling because ActiveModel::Error's full_messages return message with key.
+6. Need to implement retry mechanism for concurrent requests to mitigate deadlocks on locking.
 
 ## Non-Functional Requirements
 1. **Scalability**