diff --git a/Gemfile.lock b/Gemfile.lock
index fd3a3d5..b0346e8 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,6 +1,6 @@
 GIT
   remote: https://github.com/rails/rails.git
-  revision: 2ab0094ab6e94d30a5a424dc93c2aeb1066cc76b
+  revision: 95deab7b439abba23fdc4bd659116dab5dbe2606
   branch: main
   specs:
     actioncable (8.1.0.alpha)
@@ -127,7 +127,7 @@ GEM
     concurrent-ruby (1.3.4)
     connection_pool (2.4.1)
     crass (1.0.6)
-    date (3.4.0)
+    date (3.4.1)
     debug (1.9.2)
       irb (~> 1.10)
       reline (>= 0.3.8)
@@ -144,11 +144,11 @@ GEM
       activesupport (>= 6.1)
     i18n (1.14.6)
       concurrent-ruby (~> 1.0)
-    io-console (0.7.2)
+    io-console (0.8.0)
     irb (1.14.1)
       rdoc (>= 4.0.0)
       reline (>= 0.4.2)
-    json (2.8.2)
+    json (2.9.0)
     kamal (2.3.0)
       activesupport (>= 7.0)
       base64 (~> 0.2)
@@ -162,7 +162,7 @@ GEM
       zeitwerk (>= 2.6.18, < 3.0)
     keccak (1.3.1)
     language_server-protocol (3.17.0.3)
-    logger (1.6.1)
+    logger (1.6.2)
     loofah (2.23.1)
       crass (~> 1.0.2)
       nokogiri (>= 1.12.0)
@@ -175,7 +175,7 @@ GEM
     matrix (0.4.2)
     mini_mime (1.1.5)
     mini_portile2 (2.8.8)
-    minitest (5.25.2)
+    minitest (5.25.4)
     msgpack (1.7.5)
     net-imap (0.5.1)
       date
@@ -192,27 +192,28 @@ GEM
       net-protocol
     net-ssh (7.3.0)
     nio4r (2.7.4)
-    nokogiri (1.16.7-aarch64-linux)
+    nokogiri (1.16.8-aarch64-linux)
       racc (~> 1.4)
-    nokogiri (1.16.7-arm-linux)
+    nokogiri (1.16.8-arm-linux)
       racc (~> 1.4)
-    nokogiri (1.16.7-arm64-darwin)
+    nokogiri (1.16.8-arm64-darwin)
       racc (~> 1.4)
-    nokogiri (1.16.7-x86-linux)
+    nokogiri (1.16.8-x86-linux)
       racc (~> 1.4)
-    nokogiri (1.16.7-x86_64-darwin)
+    nokogiri (1.16.8-x86_64-darwin)
       racc (~> 1.4)
-    nokogiri (1.16.7-x86_64-linux)
+    nokogiri (1.16.8-x86_64-linux)
       racc (~> 1.4)
     ostruct (0.6.1)
-    pagy (9.3.1)
+    pagy (9.3.2)
     parallel (1.26.3)
     parser (3.3.6.0)
       ast (~> 2.4.1)
       racc
     pg (1.5.9)
     pkg-config (1.5.8)
-    psych (5.2.0)
+    psych (5.2.1)
+      date
       stringio
     public_suffix (6.0.1)
     puma (6.5.0)
@@ -230,9 +231,9 @@ GEM
       activesupport (>= 5.0.0)
       minitest
       nokogiri (>= 1.6)
-    rails-html-sanitizer (1.6.0)
+    rails-html-sanitizer (1.6.1)
       loofah (~> 2.21)
-      nokogiri (~> 1.14)
+      nokogiri (>= 1.15.7, != 1.16.7, != 1.16.6, != 1.16.5, != 1.16.4, != 1.16.3, != 1.16.2, != 1.16.1, != 1.16.0.rc1, != 1.16.0)
     rainbow (3.1.1)
     rake (13.2.1)
     rbsecp256k1 (6.0.0)
@@ -241,18 +242,18 @@ GEM
       rubyzip (~> 2.3)
     rdoc (6.8.1)
       psych (>= 4.0.0)
-    regexp_parser (2.9.2)
-    reline (0.5.11)
+    regexp_parser (2.9.3)
+    reline (0.5.12)
       io-console (~> 0.5)
     rexml (3.3.9)
-    rubocop (1.69.0)
+    rubocop (1.69.1)
       json (~> 2.3)
       language_server-protocol (>= 3.17.0)
       parallel (~> 1.10)
       parser (>= 3.3.0.2)
       rainbow (>= 2.2.2, < 4.0)
-      regexp_parser (>= 2.4, < 3.0)
-      rubocop-ast (>= 1.36.1, < 2.0)
+      regexp_parser (>= 2.9.3, < 3.0)
+      rubocop-ast (>= 1.36.2, < 2.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 2.4.0, < 4.0)
     rubocop-ast (1.36.2)
@@ -275,7 +276,7 @@ GEM
       rubocop-rails
     ruby-progressbar (1.13.0)
     rubyzip (2.3.2)
-    securerandom (0.3.2)
+    securerandom (0.4.0)
     selenium-webdriver (4.27.0)
       base64 (~> 0.2)
       logger (~> 1.4)
@@ -318,7 +319,7 @@ GEM
       unicode-emoji (~> 4.0, >= 4.0.4)
     unicode-emoji (4.0.4)
     uri (1.0.2)
-    useragent (0.16.10)
+    useragent (0.16.11)
     web-console (4.2.1)
       actionview (>= 6.0.0)
       activemodel (>= 6.0.0)
diff --git a/app/controllers/api/events_controller.rb b/app/controllers/api/events_controller.rb
index 8870203..bccd84b 100644
--- a/app/controllers/api/events_controller.rb
+++ b/app/controllers/api/events_controller.rb
@@ -44,6 +44,14 @@ def create
           }
         }, status: :unprocessable_content
       end
+    rescue ActiveRecord::StaleObjectError
+      render json: {
+        status: "error",
+        error: {
+          message: "Event not saved",
+          data: "Too fast requests."
+        }
+      }, status: :unprocessable_content
     end
 
     def batch_create
@@ -60,6 +68,8 @@ def batch_create
           errored = event_params
           break
         end
+      rescue ActiveRecord::StaleObjectError
+        errored = event_params
       end
 
       render json: {
diff --git a/app/models/conversation.rb b/app/models/conversation.rb
new file mode 100644
index 0000000..a2a129e
--- /dev/null
+++ b/app/models/conversation.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class Conversation < ApplicationRecord
+  self.primary_key = %i[pubkey session]
+  self.lock_optimistically = true
+
+  has_many :events,
+           foreign_key: %i[pubkey session],
+           dependent: :restrict_with_exception
+
+  def latest_event
+    events.order(id: :desc).first
+  end
+end
diff --git a/app/models/event.rb b/app/models/event.rb
index e080e39..43a7ee6 100644
--- a/app/models/event.rb
+++ b/app/models/event.rb
@@ -8,18 +8,17 @@ class Event < ApplicationRecord
   scope :of_topic, ->(topic) { where(topic:) }
   scope :of_recipient, ->(recipient) { where(recipient:) }
 
+  belongs_to :conversation, foreign_key: %i[pubkey session], required: true, autosave: true
   has_one :merkle_node, dependent: :restrict_with_exception
 
   after_create :add_to_merkle_tree
 
   # A publisher must not send events in the same time which makes harder to sort them.
   validates :created_at,
-            uniqueness: {
-              scope: :pubkey
-            },
+            presence: true,
             comparison: {
               greater_than_or_equal_to: ->(current) {
-                current.latest&.created_at || 0
+                current.conversation&.latest_event_created_at || 0
               }
             }
 
@@ -37,14 +36,20 @@ class Event < ApplicationRecord
             format: { with: /\A\h+\z/ },
             allow_nil: true
 
-  before_validation do
+  before_validation on: :create do
     self.session = tags.find { |tag| tag[0] == "s" }&.[](1)
     self.topic = tags.find { |tag| tag[0] == "t" }&.[](1)
     self.recipient = tags.find { |tag| tag[0] == "p" }&.[](1)
+
+    self.conversation ||= Conversation.find_or_create_by(pubkey: pubkey, session: session) do |c|
+      c.latest_event_created_at = created_at
+      c.events_count = 0
+    end
   end
 
-  def readonly?
-    persisted?
+  after_validation on: :create do
+    self.conversation.latest_event_created_at = created_at
+    self.conversation.events_count += 1
   end
 
   def merkle_tree_hash
@@ -63,19 +68,8 @@ def inclusion_proof
     merkle_node&.inclusion_proof
   end
 
-  def latest
-    @latest ||=
-      Event
-        .of_topic(topic)
-        .of_pubkey(pubkey)
-        .of_session(session)
-        .order(id: :desc)
-        .first
-  end
-
-  def reload(options = nil)
-    @latest = nil
-    super
+  def readonly?
+    persisted?
   end
 
   class << self
diff --git a/config/application.rb b/config/application.rb
index 93f6bb5..7c97837 100644
--- a/config/application.rb
+++ b/config/application.rb
@@ -37,5 +37,12 @@ class Application < Rails::Application
     #
     # config.time_zone = "Central Time (US & Canada)"
     # config.eager_load_paths << Rails.root.join("extras")
+
+    config.i18n.load_path += Dir[Rails.root.join("config", "locales", "**", "*.{rb,yml}")]
+
+    config.generators do |g|
+      g.helper false
+      g.assets false
+    end
   end
 end
diff --git a/db/migrate/20240325200100_create_conversations.rb b/db/migrate/20240325200100_create_conversations.rb
new file mode 100644
index 0000000..e20f05f
--- /dev/null
+++ b/db/migrate/20240325200100_create_conversations.rb
@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class CreateConversations < ActiveRecord::Migration[8.1]
+  def change
+    create_table :conversations, primary_key: %i[pubkey session] do |t|
+      t.string :pubkey, null: false
+      t.string :session, null: false
+
+      t.integer :lock_version
+      t.datetime :latest_event_created_at
+      t.integer :events_count
+
+      t.timestamps
+    end
+  end
+end
diff --git a/db/migrate/20240325200101_create_events.rb b/db/migrate/20240325200101_create_events.rb
index 058d02b..41af6d2 100644
--- a/db/migrate/20240325200101_create_events.rb
+++ b/db/migrate/20240325200101_create_events.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-class CreateEvents < ActiveRecord::Migration[8.0]
+class CreateEvents < ActiveRecord::Migration[8.1]
   def change
     create_table :events do |t|
       t.string :eid, null: false, index: { unique: true }
@@ -10,9 +10,7 @@ def change
       t.string :content, null: false
       t.string :sig, null: false
 
-      t.datetime :created_at, null: false
-
-      t.index %i[pubkey created_at], unique: true
+      t.datetime :created_at, null: false, index: true
     end
   end
 end
diff --git a/db/migrate/20240325200102_add_extended_fields_to_events.rb b/db/migrate/20240325200102_add_extended_fields_to_events.rb
index 2d3a080..fe4c5b6 100644
--- a/db/migrate/20240325200102_add_extended_fields_to_events.rb
+++ b/db/migrate/20240325200102_add_extended_fields_to_events.rb
@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
-class AddExtendedFieldsToEvents < ActiveRecord::Migration[8.0]
+class AddExtendedFieldsToEvents < ActiveRecord::Migration[8.1]
   def change
-    change_table :events, id: :string do |t|
+    change_table :events do |t|
       t.string :session, null: false
 
       t.string :topic, null: true, index: true
diff --git a/db/migrate/20240325200103_create_merkle_nodes.rb b/db/migrate/20240325200103_create_merkle_nodes.rb
index 16729cb..0cbf426 100644
--- a/db/migrate/20240325200103_create_merkle_nodes.rb
+++ b/db/migrate/20240325200103_create_merkle_nodes.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-class CreateMerkleNodes < ActiveRecord::Migration[8.0]
+class CreateMerkleNodes < ActiveRecord::Migration[8.1]
   def change
     # 创建顺序：parent to children
     create_table :merkle_nodes do |t|
diff --git a/db/schema.rb b/db/schema.rb
index c75821a..9201790 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -14,6 +14,16 @@
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
 
+  create_table "conversations", primary_key: ["pubkey", "session"], force: :cascade do |t|
+    t.string "pubkey", null: false
+    t.string "session", null: false
+    t.integer "lock_version"
+    t.datetime "latest_event_created_at"
+    t.integer "events_count"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+  end
+
   create_table "events", force: :cascade do |t|
     t.string "eid", null: false
     t.string "pubkey", null: false
@@ -25,8 +35,8 @@
     t.string "session", null: false
     t.string "topic"
     t.string "recipient"
+    t.index ["created_at"], name: "index_events_on_created_at"
     t.index ["eid"], name: "index_events_on_eid", unique: true
-    t.index ["pubkey", "created_at"], name: "index_events_on_pubkey_and_created_at", unique: true
     t.index ["pubkey", "session"], name: "index_events_on_pubkey_and_session"
     t.index ["pubkey"], name: "index_events_on_pubkey"
     t.index ["recipient"], name: "index_events_on_recipient"
diff --git a/rbs_collection.lock.yaml b/rbs_collection.lock.yaml
index 0ad23ce..5aa22e0 100644
--- a/rbs_collection.lock.yaml
+++ b/rbs_collection.lock.yaml
@@ -6,7 +6,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: actionmailer
@@ -14,7 +14,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: actionpack
@@ -22,7 +22,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: actiontext
@@ -30,7 +30,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: actionview
@@ -38,7 +38,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: activejob
@@ -46,7 +46,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: activemodel
@@ -54,7 +54,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: activerecord
@@ -62,7 +62,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: activestorage
@@ -70,7 +70,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: activesupport
@@ -78,7 +78,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: addressable
@@ -86,7 +86,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: base64
@@ -110,7 +110,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: connection_pool
@@ -118,7 +118,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: date
@@ -146,7 +146,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: i18n
@@ -154,7 +154,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: io-console
@@ -170,7 +170,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: marcel
@@ -178,7 +178,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: mini_mime
@@ -186,7 +186,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: minitest
@@ -214,7 +214,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: openssl
@@ -238,7 +238,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: rails-dom-testing
@@ -246,7 +246,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: rails-html-sanitizer
@@ -254,7 +254,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: railties
@@ -262,7 +262,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: rake
@@ -270,7 +270,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: rdoc
@@ -282,7 +282,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: rubyzip
@@ -290,7 +290,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: securerandom
@@ -314,7 +314,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: time
@@ -334,7 +334,7 @@ gems:
   source:
     type: git
     name: ruby/gem_rbs_collection
-    revision: b438fcecafb7fe0faad88c2bf3b8eec17d83e4d9
+    revision: 0a6ea105a0afc7eaee4494585a7775f47eea6145
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
 - name: uri