added background and solution in README.md

Author: jfan9

typescript/api-gateway-async-lambda-invocation/README.md

@@ -5,6 +5,42 @@ Sample architecture to process events asynchronously using API Gateway and Lambd
 ## Architecture
 ![architecture](./images/architecture.png)
 
+## Background:
+
+In Lambda non-proxy (custom) integration, the backend Lambda function is invoked synchronously by default. This is the desired behavior for most REST API operations.  
+Some applications, however, require work to be performed asynchronously (as a batch operation or a long-latency operation), typically by a separate backend component.  
+In this case, the backend Lambda function is invoked asynchronously, and the front-end REST API method doesn't return the result.
+
+## Solution:
+
+### API Gateway:
+
+- `POST` `/job`: Integrates with the Lambda function for job submission.
+- `GET` `/job/{jobId}`: Direct DynamoDB integration to fetch the job status by jobId.
+
+### DynamoDB Integration:
+
+- The stack includes the DynamoDB table for storing job statuses with jobId as the partition key.
+- The Lambda function has permissions to write to the DynamoDB table.
+
+### IAM Role:
+- An IAM role is created for API Gateway with permissions to access the DynamoDB table for the GET /job/{jobId} method
+
+### Example structure:
+```
+/api-gateway-async-lambda-invocation
+  ├── /assets
+  │   └── /lambda-functions
+  │       └── job_handler.js
+  ├── /lib
+  |   |-- app.ts
+  │   └── api-gateway-async-lambda-invocation-stack.ts
+  ├── node_modules
+  ├── package.json
+  ├── cdk.json
+  └── ...
+```
+
 ## Test:
 - `POST` curl command:
 ```shell
@@ -20,14 +56,6 @@ curl -X POST https://<API-ID>.execute-api.<REGION>.amazonaws.com/<stage>/job \
 curl https://<API-ID>.execute-api.<REGION>.amazonaws.com/<stage>/job/<jobId>
 ```
 
-
-```
-In Lambda non-proxy (custom) integration, the backend Lambda function is invoked synchronously by default.  
-This is the desired behavior for most REST API operations.  
-Some applications, however, require work to be performed asynchronously (as a batch operation or a long-latency operation), typically by a separate backend component.  
-In this case, the backend Lambda function is invoked asynchronously, and the front-end REST API method doesn't return the result.
-```
-
-### Reference:
+## Reference:
 [1] Set up asynchronous invocation of the backend Lambda function  
 https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-integration-async.html