How to do it...

1. Create two projects from the following template:

$ sls create --template-url https://github.com/danteinc/js-cloud-native-cookbook/tree/master/ch2/bi-directional-sync --path cncb-1-bi-directional-sync

$ sls create --template-url https://github.com/danteinc/js-cloud-native-cookbook/tree/master/ch2/bi-directional-sync --path cncb-2-bi-directional-sync

2. Review the file named serverless.yml with the following content in each project:

service: cncb-1-bi-directional-sync

provider:
  name: aws
  runtime: nodejs8.10
  iamRoleStatements:
    ...
  environment:
    SERVERLESS_PROJECT: ${self:service}
    ...

functions:
  command:
    handler: handler.command
  trigger:
    handler: handler.trigger
    events:
      - stream:
          type: dynamodb
          ...
  listener:
    handler: handler.listener
    events:
      - stream:
          type: kinesis
          ...
  query:
    handler: handler.query

resources:
  Resources:
    Table:
      ...

3. Review the file named handler.js with the following content:

module.exports.command = (request, context, callback) => {
  const thing = {
    id: uuid.v4(),
    latch: 'open',
    ...request,
  };

  ...
  db.put(params, callback);
};

module.exports.trigger = (event, context, cb) => {
  _(event.Records)
    .filter(forLatchOpen)
    .map(toEvent)
    .flatMap(publish)
    .collect()
    .toCallback(cb);
};

const forLatchOpen = e => e.dynamodb.NewImage.latch.S === 'open';

const toEvent = record => ({
  id: record.eventID,
  ...
  tags: {
    region: record.awsRegion,
    source: process.env.SERVERLESS_PROJECT
  },
  thing: ...,
});

...

module.exports.listener = (event, context, cb) => {
  _(event.Records)
    .map(recordToEvent)
    .filter(forSourceNotSelf)
    .filter(forThingCrud)
    .map(toThing)
    .flatMap(put)
    .collect()
    .toCallback(cb);
};

...
const forSourceNotSelf = e => e.tags.source != process.env.SERVERLESS_PROJECT;
...

const toThing = event => ({
  id: event.thing.new.id,
  ...
  latch: 'closed',
});

...

4. Navigate to the cncb-1-bi-directional-sync directory withcd cncb-1-bi-directional-sync.
5. Install the dependencies with npm install.
6. Run the tests with npm test -- -s $MY_STAGE.
   
7. Review the contents generated in the .serverless directory.
8. Deploy the stack:

$ npm run dp:lcl -- -s $MY_STAGE

> cncb-1-bi-directional-sync@1.0.0 dp:lcl <path-to-your-workspace>/cncb-1-bi-directional-sync
> sls deploy -r us-east-1 "-s" "john"

Serverless: Packaging service...
...
Serverless: Stack update finished...
...
functions:
  command: cncb-1-bi-directional-sync-john-command
  trigger: cncb-1-bi-directional-sync-john-trigger
  listener: cncb-1-bi-directional-sync-john-listener
  query: cncb-1-bi-directional-sync-john-query

9. Review the stack, functions, and table in the AWS Console.
10. Navigate to the cncb-2-bi-directional-sync directory with cd cncb-2-bi-directional-sync.
11. Repeat steps 5-9 for the cncb-2-bi-directional-sync project.
12. Navigate back to the cncb-1-bi-directional-sync directory with cd cncb-1-bi-directional-sync.
13. Invoke the command function to save data to the first service:

$ sls invoke -r us-east-1 -f command -s $MY_STAGE -d '{"id":"77777777-7777-7777-7777-777777777777","name":"thing seven"}'

14. Take a look at the logs for the command and trigger functions: 

$ sls logs -f command -r us-east-1 -s $MY_STAGE

START ...
2018-04-24 02:02:11 ... event: {"id":"77777777-7777-7777-7777-777777777777","name":"thing seven"}
2018-04-24 02:02:11 ... params: {"TableName":"john-cncb-1-bi-directional-sync-things","Item":{"id":"77777777-7777-7777-7777-777777777777","latch":"open","name":"thing seven"}}
END ...
REPORT ... Duration: 146.90 ms    Billed Duration: 200 ms ... Max Memory Used: 40 MB    

$ sls logs -f trigger -r us-east-1 -s $MY_STAGE
START ...
2018-04-24 02:02:13 ... event: {"Records":[{"eventID":"494ec22686941c0d5ff56dee86df47dd","eventName":"INSERT",...,"Keys":{"id":{"S":"77777777-7777-7777-7777-777777777777"}},"NewImage":{"name":{"S":"thing seven"},"id":{"S":"77777777-7777-7777-7777-777777777777"},"latch":{"S":"open"}},...},...}]}
2018-04-24 02:02:13 ... {"id":"494ec22686941c0d5ff56dee86df47dd","type":"thing-created",...,"tags":{"region":"us-east-1","source":"cncb-1-bi-directional-sync"},"thing":{"new":{"name":"thing seven","id":"77777777-7777-7777-7777-777777777777","latch":"open"}}}
...
END ...
REPORT ... Duration: 140.20 ms    Billed Duration: 200 ms ... Max Memory Used: 35 MB    

15. Navigate to the cncb-2-bi-directional-sync directory with cd cncb-2-bi-directional-sync
16. Take a look at the logs for the listener and trigger functions: 

$ sls logs -f listener -r us-east-1 -s $MY_STAGE

$ sls logs -f trigger -r us-east-1 -s $MY_STAGE

17. Invoke the query function to retrieve the synchronized data to the second service:

$ sls invoke -r us-east-1 -f query -s $MY_STAGE -d 77777777-7777-7777-7777-777777777777

18. Remove both stacks once you have finished with npm run rm:lcl -- -s $MY_STAGE.