diff --git a/README.md b/README.md
index 17f0939..bda9732 100644
--- a/README.md
+++ b/README.md
@@ -27,3 +27,17 @@ end
 ## Getting started
 
 Check the [Getting started](https://hexdocs.pm/kafkaesque/getting-started.html) guide in Hexdocs.
+
+## Updating versions
+To go from 1.0.0-rc.1 to 1.0.0-rc.2, an additional migration is needed:
+
+```elixir
+defmodule MyApp.Migrations.BumpKafkaesque do
+  use Ecto.Migration
+
+  def up, do: Kafkaesque.Migrations.up(:v1, :v2)
+  def down, do: Kafkaesque.Migrations.down(:v2, :v1)
+end
+```
+
+No extra steps are required if 1.0.0-rc.2 or a newer version was installed.
diff --git a/lib/kafkaesque.ex b/lib/kafkaesque.ex
index fd8c1e2..abd1d06 100644
--- a/lib/kafkaesque.ex
+++ b/lib/kafkaesque.ex
@@ -50,10 +50,10 @@ defmodule Kafkaesque do
   use the library. See the documentation of the `Kafkaesque` module for more
   information.
   """
-  @spec publish(Ecto.Repo.t(), String.t(), term(), String.t()) ::
-          {:ok, Message.t()} | {:error, atom()}
-  def publish(repo, topic, partition, payload) do
-    message = Message.new(topic, partition, payload)
+  @spec publish(Ecto.Repo.t(), String.t(), term(), String.t(), String.t()) ::
+          {:ok, Message.t()} | {:error, Ecto.Changeset.t()}
+  def publish(repo, topic, partition, key, payload) do
+    message = Message.new(topic, partition, key, payload)
     repo.insert(message)
   end
 
@@ -108,11 +108,14 @@ defmodule Kafkaesque do
     {repo, _opts} = Keyword.pop!(opts, :repo)
 
     quote do
-      @spec publish(String.t(), term()) :: {:ok, Kafkaesque.Message.t()} | {:error, atom()}
-      def publish(topic, body) do
+      @on_definition {Kafkaesque.CompileHooks, :on_def}
+
+      @spec publish(String.t(), String.t(), term()) ::
+              {:ok, Kafkaesque.Message.t()} | {:error, Ecto.Changeset.t()}
+      def publish(topic, key \\ "", body) do
         payload = encode(body)
-        partition = partition(topic, body)
-        Kafkaesque.publish(unquote(repo), topic, partition, payload)
+        partition = partition(topic, key, body)
+        Kafkaesque.publish(unquote(repo), topic, partition, key, payload)
       end
 
       @spec encode(term()) :: String.t()
@@ -120,8 +123,8 @@ defmodule Kafkaesque do
         body
       end
 
-      @spec partition(String.t(), term()) :: integer()
-      def partition(_topic, _body) do
+      @spec partition(String.t(), String.t(), term()) :: integer()
+      def partition(_topic, _key, _body) do
         0
       end
 
@@ -137,7 +140,7 @@ defmodule Kafkaesque do
         Kafkaesque.child_spec(opts)
       end
 
-      defoverridable encode: 1, partition: 2
+      defoverridable encode: 1, partition: 3
     end
   end
 end
diff --git a/lib/kafkaesque/compile_hooks.ex b/lib/kafkaesque/compile_hooks.ex
new file mode 100644
index 0000000..dde05a0
--- /dev/null
+++ b/lib/kafkaesque/compile_hooks.ex
@@ -0,0 +1,20 @@
+defmodule Kafkaesque.CompileHooks do
+  @moduledoc false
+
+  # We need to raise a compile error if partition/2 is defined due to a breaking
+  # change: outbox modules are now expected to implement partition/3, not partition/2
+  #
+  # This way we turn the breaking change into a compile error instead of runtime
+  # misbehaviour
+  def on_def(_env, :def, :partition, args, _guards, _body) do
+    if Enum.count(args) == 3 do
+      :ok
+    else
+      raise "partition function is expected to have 3 arguments"
+    end
+  end
+
+  def on_def(_env, _kind, _name, _args, _guards, _body) do
+    :ok
+  end
+end
diff --git a/lib/kafkaesque/message.ex b/lib/kafkaesque/message.ex
index fe0af1b..1f7df60 100644
--- a/lib/kafkaesque/message.ex
+++ b/lib/kafkaesque/message.ex
@@ -34,6 +34,7 @@ defmodule Kafkaesque.Message do
     )
 
     field(:partition, :integer)
+    field(:key, :string, default: "")
     field(:body, :string)
     field(:attempt, :integer, default: 0)
     field(:attempted_by, :string)
@@ -44,12 +45,13 @@ defmodule Kafkaesque.Message do
     timestamps()
   end
 
-  @spec new(String.t(), String.t(), String.t()) :: Ecto.Changeset.t()
-  def new(topic, partition, body) do
+  @spec new(String.t(), String.t(), String.t(), String.t()) :: Ecto.Changeset.t()
+  def new(topic, partition, key, body) do
     %__MODULE__{}
-    |> cast(%{topic: topic, partition: partition, body: body}, [
+    |> cast(%{topic: topic, partition: partition, body: body, key: key}, [
       :topic,
       :partition,
+      :key,
       :body
     ])
     |> validate_required([:topic, :partition, :body])
diff --git a/lib/kafkaesque/migrations.ex b/lib/kafkaesque/migrations.ex
index a377f6d..abede5f 100644
--- a/lib/kafkaesque/migrations.ex
+++ b/lib/kafkaesque/migrations.ex
@@ -1,12 +1,14 @@
 defmodule Kafkaesque.Migrations do
   use Ecto.Migration
 
-  def up do
+  # Functions independent from version
+  def up() do
     create table(:kafkaesque_messages, primary_key: false) do
       add(:id, :bigserial, primary_key: true)
       add(:state, :string, null: false, default: "pending")
       add(:topic, :string, null: false)
       add(:partition, :integer, null: false)
+      add(:key, :binary, default: "")
       add(:body, :binary)
       add(:attempt, :integer, null: false, default: 0)
       add(:attempted_by, :string)
@@ -17,7 +19,23 @@ defmodule Kafkaesque.Migrations do
     end
   end
 
-  def down do
+  def down() do
     drop(table(:kafkaesque_messages))
   end
+
+  def up(current, next)
+
+  def up(:v1, :v2) do
+    alter table(:kafkaesque_messages) do
+      add(:key, :binary, default: "", null: false)
+    end
+  end
+
+  def down(current, previous)
+
+  def down(:v2, :v1) do
+    alter table(:kafkaesque_messages) do
+      drop(:key)
+    end
+  end
 end
diff --git a/test/kafkaesque/message_test.exs b/test/kafkaesque/message_test.exs
index 031b1da..7b71fae 100644
--- a/test/kafkaesque/message_test.exs
+++ b/test/kafkaesque/message_test.exs
@@ -8,20 +8,22 @@ defmodule Kafkaesque.MessageTest do
       topic = "sample"
       body = "body"
       partition = 0
+      key = "some"
 
       assert %Ecto.Changeset{
                errors: [],
-               changes: %{topic: ^topic, body: ^body, partition: ^partition}
-             } = Message.new(topic, partition, body)
+               changes: %{topic: ^topic, body: ^body, partition: ^partition, key: ^key}
+             } = Message.new(topic, partition, key, body)
     end
 
     test "returns invalid changeset for invalid input" do
       topic = 1
       body = {1, 2}
+      key = 2
       partition = "notanumber"
 
-      assert %Ecto.Changeset{errors: [topic: _, partition: _, body: _]} =
-               Message.new(topic, partition, body)
+      assert %Ecto.Changeset{errors: [topic: _, partition: _, key: _, body: _]} =
+               Message.new(topic, partition, key, body)
     end
   end
 end
diff --git a/test/kafkaesque_test.exs b/test/kafkaesque_test.exs
index e70f589..7c30d78 100644
--- a/test/kafkaesque_test.exs
+++ b/test/kafkaesque_test.exs
@@ -5,16 +5,23 @@ defmodule KafkaesqueTest do
 
   describe "publish/4" do
     test "inserts valid messages" do
-      {:ok, %Kafkaesque.Message{}} = Kafkaesque.publish(Repo, "test_topic", 0, "content")
+      {:ok, %Kafkaesque.Message{}} = Kafkaesque.publish(Repo, "test_topic", 0, "", "content")
     end
 
     test "errors for invalid messages" do
       invalid_topic = 1
       invalid_body = {1, 2}
+      invalid_key = 2
       invalid_partition = "notanumber"
 
-      assert {:error, %Ecto.Changeset{}} =
-               Kafkaesque.publish(Repo, invalid_topic, invalid_partition, invalid_body)
+      assert {:error, %Ecto.Changeset{errors: [topic: _, partition: _, key: _, body: _]}} =
+               Kafkaesque.publish(
+                 Repo,
+                 invalid_topic,
+                 invalid_partition,
+                 invalid_key,
+                 invalid_body
+               )
     end
   end
 end