Subscribing to Salesforce Platform Events using External Java Client - CometD

Platform Events follow event based Architecture, where multiple subscribers can subscribe to a published event in real time manner. 

Below diagram shows the basic Architecture of the event-based communication Model:

Event - This is a change in state for a business process

Event Message - Data about the event

Producer - Publisher of the event

Event Bus - A communication and storage service for published events

Event Consumer - A subscriber to the channel

Platform events can be subscribed internally and also externally

Platfrom events  - Internal Subscription

Below are some scenarios in which you can consume platform events internally:

1. When we need to execute functionalities asynchronously to save govornor limits

2. To build a proper exception logging framework - You can find more details here 

How to subscribe Internally?

we can see more details about this in a different blogpost.

Platfrom events  - External Consumption

Many Organizations are moving to event-based integration approach and Salesforce Platform Events redefines and make the process easy,  when we need to communicate with multiple external systems with same content.

Platform events are useful for external communication in the below scenarios:

• We are looking for real time data delivery
• We need to send same data to multiple subscribers
• Only changed data needs to be send and not entire record

Subscription Using CometD Client

CometD is a web messaging framework/library which supports pub-subscribe model. You can see more details here.

We can build our own CometD client or there is an opensource, community supported client - EMPConnector available, which we can use.

Why I tried EMPConnector?

In our current project, we used platform events as a messaging mechanism to communicate to an external system, whenever an order record is changed in Salesforce. We used Dell BOOMI as a middleware to subscribe to this event, but they started to receive/consume duplicate events. To verify the issue is on Salesoforce side or BOOMI side, we wanted to verify the event subscription using some other client and EMP Connector helped to verify this.

Steps involved to set up and execute EMP Connector:

1. Set up Java Developer Kit 8

Make sure you download and install JDK 8 from here.

By default this will be installed under program Files:

Also make sure the path is set correct under enviornment variables as shown below:

You can verify if Java set up is correct or not by running java -version command in the command line:

2. Set up Apache Maven

You can run sample EMP Connector using eclipse or command line. I used command line, and for this we need to install Apache Maven from this link.

We can download the zip version available and extract it and save to local folder.

Once extracted, set PATH for the bin folder:

We can verify set up using the command: mvn -v, and it should look like below:

3. Create a Custom Platform Event

Once the basic set up is done, we can create a sample platfrom event in our org.

I created a sample Platfrom event called Member with below details:

4. Download and Build EMP Connector Project

EMP Connector project is available here.

You can clone this project using gitcommand:

This will copy the project to your local repository:

Now build the jar file using mvn clean package command:

Once it is success, you can see a message like this:

And executable jar files will be created downloading all the dependencies:

5. Subscribe to Platform event channel from command line

Once the set up is completed successfully,  we can subscribe to the platform event using below command:

java -classpath target/emp-connector-0.0.1-SNAPSHOT-phat.jar com.salesforce.emp.connector.example.LoginExample <username> <password+token> /event/Member__e

If the login and subscription is success, we can see a message like below:

6. Publish Event and Verify Subscription

Once the subscription is success, we can publish an event from salesforce, and verify it from command line.

Publish an event using Apex Execute from developer console:

You can see that EMP Connector has received the published event.

We can replay events published in the past 24 hours usng the replay ID .

When we specify a replay ID, this client will replay all events with a replayID greater than the specified ID, as shown in the below screenshot:

How to Connect to Sandbox

To connect to a sandbox, we need to update endpoint URL in LoginHelper.java class as test.salesforce.com instead of login.salesforce.com as shown below:

And then build the package again using mvn clean package command.

Then you can run subscribe to the channel.

References:

https://developer.salesforce.com/docs/atlas.en-us.platform_events.meta/platform_events/platform_events_intro.htm