Merge pull request #26 from lifeomic/dynamo-stream-handler

[Proposal] Introduce DynamoStreamHandler

README.md

@@ -6,3 +6,83 @@ process AWS data streams.
 ```bash
 yarn add @lifeomic/delta
 ```
+
+### `DynamoStreamHandler`
+
+This helper provides an abstraction over a DynamoDB Stream Lambda handler.
+
+```typescript
+const stream = new DynamoStreamHandler({
+  logger,
+  unmarshall: (object) => {
+    /* ... unmarshall from unknown stream format -> your custom type ... */
+    return { id: object.id };
+  },
+  createRunContext: () => {
+    /* ... create the "context", e.g. data sources ... */
+    return { doSomething: () => null };
+  },
+})
+  .onInsert(async (ctx, entity) => {
+    // INSERT actions receive a single strongly typed new entities
+    // (entities are typed based on the `unmarshall` function)
+    entity.id;
+
+    // `ctx` contains the nice result of `createRunContext`
+    await ctx.dataSources.doSomething();
+
+    // `ctx` contains a logger by default, which already includes niceties like
+    // the AWS request id
+    ctx.logger.info('blah blah');
+  })
+  // The API is chainable to help with readability
+  .onModify(async (ctx, oldEntity, newEntity) => {
+    // MODIFY actions receive strongly typed old + new entities
+    oldEntity.id;
+    newEntity.id;
+  })
+  .onRemove(async (ctx, oldEntity) => {
+    // REMOVE actions receive a single strongly typed old entity
+    oldEntity.id;
+
+    ctx.logger.info('first remove action');
+  })
+  // When multiple actions have been added for the same event (e.g. two `onRemove` calls),
+  // they are executed in.order.
+  .onRemove(async (ctx, oldEntity) => {
+    ctx.logger.info('second remove action');
+  });
+
+// Provides a dead-simple API for creating the Lambda.
+export const handler = stream.lambda();
+```
+
+`DynamoStreamHelper` also comes with a nice helper for testing: `harness(...)`
+
+```typescript
+const harness = stream.harness({
+  marshall: () => {
+    /* marshall from your custom type -> stream format */
+  },
+  /* optionally override the logger */
+  logger,
+  createRunContext: () => {
+    /* optionally override the context, to mock e.g. data sources */
+  }
+})
+
+test('something', async () => {
+  // Provides a simple `sendEvent` function
+  await harness.sendEvent({
+    records: [
+      // Simplified, strongly-typed event types for readability
+      { type: 'remove', entity: ... },
+      { type: 'insert', entity: ... },
+      { type: 'modify', oldEntity: ..., newEntity: ... },
+    ]
+  })
+
+  // Also provides access to the underlying run context for assertions + mocking
+  expect(harness.context.dataSources.doSomething).toHaveBeenCalled()
+})
+```

package.json

@@ -26,13 +26,22 @@
   "devDependencies": {
     "@lifeomic/eslint-config-standards": "^2.1.1",
     "@lifeomic/jest-config": "^1.1.2",
+    "@lifeomic/logging": "^4.0.0",
     "@lifeomic/typescript-config": "^1.0.3",
     "@types/jest": "^27.4.1",
+    "@types/uuid": "^8.3.4",
     "eslint": "^8.9.0",
     "eslint-config-prettier": "^8.4.0",
     "jest": "^27.5.1",
     "prettier": "^2.5.1",
     "ts-jest": "^27.1.3",
-    "typescript": "^4.5.5"
+    "typescript": "^4.5.5",
+    "uuid": "^8.3.2"
+  },
+  "dependencies": {
+    "@types/aws-lambda": "^8.10.92"
+  },
+  "peerDependencies": {
+    "@lifeomic/logging": "*"
   }
 }