Resources:
  BoardMessagesTable:
      Type: AWS::DynamoDB::Table
      Properties:
        TableName: board-messages-table
        AttributeDefinitions:
          - AttributeName: partKey
            AttributeType: S
          - AttributeName: createdAt
            AttributeType: N
        KeySchema:
          - AttributeName: partKey
            KeyType: HASH
          - AttributeName: createdAt
            KeyType: RANGE
        ProvisionedThroughput:
          ReadCapacityUnits: 5
          WriteCapacityUnits: 5

BoardHttpApi:
    Type: AWS::Serverless::HttpApi
    Properties:
      StageName: Test
      CorsConfiguration: True

PostMessageFunction:
      Type: AWS::Serverless::Function
      Properties:
        Handler: src/handlers/postMessage.handler
        Runtime: nodejs12.x
        MemorySize: 128
        Timeout: 5
        Events:
          PostMessage:
            Type: HttpApi
            Properties:
              ApiId: !Ref BoardHttpApi
              Method: POST
              Path: /messages
        Policies:
          - AmazonDynamoDBFullAccess
  GetMessagesFunction:
      Type: AWS::Serverless::Function
      Properties:
        Handler: src/handlers/getMessages.handler
        Runtime: nodejs12.x
        MemorySize: 128
        Timeout: 5
        Events:
          GetMessages:
            Type: HttpApi
            Properties:
              ApiId: !Ref BoardHttpApi
              Method: GET
              Path: /messages
        Policies:
          - AmazonDynamoDBFullAccess

const AWS = require('aws-sdk');
const dynamodb = new AWS.DynamoDB();
exports.handler = async (event) => {
  const { body } = event;
  try {
    const { author, text } = JSON.parse(body);
    if (!author || !text) {
      return {
        statusCode: 403,
        body: 'author and text are required!'
      }
    }
    await dynamodb.putItem({
      TableName: 'board-messages-table',
      Item: {
        msgId: { S: 'board' },
        author: { S: author },
        text: { S: text },
        createdAt: { N: String(Date.now()) } // still expects string!
      }
    }).promise();
    return {
       statusCode: 200,
       body: 'Message posted on board!',
    }
  } catch (err) {
    return {
       statusCode: 500,
       body: 'Something went wrong :(',
    }
  }
};

const AWS = require('aws-sdk');
const dynamodb = new AWS.DynamoDB();
exports.handler = async () => {
  try {
    const result = await dynamodb.query({
      TableName: 'board-messages-table',
      KeyConditionExpression: 'partKey = :partKey',
      ScanIndexForward: false,
      Limit: 3,
      ExpressionAttributeValues: {':partKey': { S: 'board'}}
    }).promise();
    return {
      statusCode: 200,
      headers: {
        'Content-Type': 'application/json',
      },
      body: JSON.stringify(result.Items),
    }
  } catch (err) {
    console.log(err);
    return {
      statusCode: 500,
      body: 'Something went wrong :(',
    }
  }
};

$ sam deploy --guided

curl -d '{"author":"name", "text":"Message text"}' -H "Content-Type: application/json" -X POST https://your-api-id.execute-api.your-region.amazonaws.com/test/messages

curl https://your-api-id.execute-api.your-region.amazonaws.com/test/messages