From 2e5ee323643af731fd49d8e075f1ec2a2b02b396 Mon Sep 17 00:00:00 2001
From: Christopher Watford <christopherw@suzy.com>
Date: Tue, 13 Feb 2024 16:23:29 -0500
Subject: [PATCH] Add expected version to EventStore interface #10

---
 .../src/commandHandling/handleCommand.ts      | 23 ++++++++++-----
 .../handleCommandWithDecider.ts               | 28 ++++++++++++++-----
 packages/emmett/src/eventStore/eventStore.ts  | 14 ++++++++--
 3 files changed, 48 insertions(+), 17 deletions(-)

diff --git a/packages/emmett/src/commandHandling/handleCommand.ts b/packages/emmett/src/commandHandling/handleCommand.ts
index 7187005a..e004f5b8 100644
--- a/packages/emmett/src/commandHandling/handleCommand.ts
+++ b/packages/emmett/src/commandHandling/handleCommand.ts
@@ -3,7 +3,7 @@ import type { Event } from '../typing';
 
 // #region command-handler
 export const CommandHandler =
-  <State, StreamEvent extends Event>(
+  <State, StreamEvent extends Event, NextExpectedVersion = bigint>(
     evolve: (state: State, event: StreamEvent) => State,
     getInitialState: () => State,
     mapToStreamId: (id: string) => string,
@@ -15,15 +15,24 @@ export const CommandHandler =
   ) => {
     const streamName = mapToStreamId(id);
 
-    const state = await eventStore.aggregateStream(streamName, {
-      evolve,
-      getInitialState,
-    });
+    const { entity: state, nextExpectedVersion } =
+      await eventStore.aggregateStream<State, StreamEvent, NextExpectedVersion>(
+        streamName,
+        {
+          evolve,
+          getInitialState,
+        },
+      );
 
     const result = handle(state ?? getInitialState());
 
     if (Array.isArray(result))
-      return eventStore.appendToStream(streamName, ...result);
-    else return eventStore.appendToStream(streamName, result);
+      return eventStore.appendToStream(
+        streamName,
+        nextExpectedVersion,
+        ...result,
+      );
+    else
+      return eventStore.appendToStream(streamName, nextExpectedVersion, result);
   };
 // #endregion command-handler
diff --git a/packages/emmett/src/commandHandling/handleCommandWithDecider.ts b/packages/emmett/src/commandHandling/handleCommandWithDecider.ts
index a9cb394b..4d9ea78a 100644
--- a/packages/emmett/src/commandHandling/handleCommandWithDecider.ts
+++ b/packages/emmett/src/commandHandling/handleCommandWithDecider.ts
@@ -4,7 +4,12 @@ import type { Decider } from '../typing/decider';
 
 // #region command-handler
 export const DeciderCommandHandler =
-  <State, CommandType extends Command, StreamEvent extends Event>(
+  <
+    State,
+    CommandType extends Command,
+    StreamEvent extends Event,
+    NextExpectedVersion = bigint,
+  >(
     {
       decide,
       evolve,
@@ -15,15 +20,24 @@ export const DeciderCommandHandler =
   async (eventStore: EventStore, id: string, command: CommandType) => {
     const streamName = mapToStreamId(id);
 
-    const state = await eventStore.aggregateStream(streamName, {
-      evolve,
-      getInitialState,
-    });
+    const { entity: state, nextExpectedVersion } =
+      await eventStore.aggregateStream<State, StreamEvent, NextExpectedVersion>(
+        streamName,
+        {
+          evolve,
+          getInitialState,
+        },
+      );
 
     const result = decide(command, state ?? getInitialState());
 
     if (Array.isArray(result))
-      return eventStore.appendToStream(streamName, ...result);
-    else return eventStore.appendToStream(streamName, result);
+      return eventStore.appendToStream(
+        streamName,
+        nextExpectedVersion,
+        ...result,
+      );
+    else
+      return eventStore.appendToStream(streamName, nextExpectedVersion, result);
   };
 // #endregion command-handler
diff --git a/packages/emmett/src/eventStore/eventStore.ts b/packages/emmett/src/eventStore/eventStore.ts
index 2afab070..779fca03 100644
--- a/packages/emmett/src/eventStore/eventStore.ts
+++ b/packages/emmett/src/eventStore/eventStore.ts
@@ -2,18 +2,26 @@ import type { Event } from '../typing';
 
 // #region event-store
 export interface EventStore {
-  aggregateStream<Entity, E extends Event>(
+  aggregateStream<Entity, E extends Event, NextExpectedVersion = bigint>(
     streamName: string,
     options: {
       evolve: (currentState: Entity, event: E) => Entity;
       getInitialState: () => Entity;
+      startingVersion?: NextExpectedVersion | undefined;
     },
-  ): Promise<Entity | null>;
+  ): Promise<{
+    entity: Entity | null;
+    nextExpectedVersion: NextExpectedVersion;
+  }>;
 
-  readStream<E extends Event>(streamName: string): Promise<E[]>;
+  readStream<E extends Event, NextExpectedVersion = bigint>(
+    streamName: string,
+    startingVersion?: NextExpectedVersion | undefined,
+  ): Promise<E[]>;
 
   appendToStream<E extends Event, NextExpectedVersion = bigint>(
     streamId: string,
+    expectedVersion?: NextExpectedVersion | undefined,
     ...events: E[]
   ): Promise<NextExpectedVersion>;
 }