What are Platform Events?

Platform Events are Salesforce’s built-in publish/subscribe messaging system. You define an event type as a custom object (with __e suffix), publish event records from Apex/Flow/REST, and any subscriber — Apex trigger, Flow, Lightning component, external system — receives the message within seconds.

Think Kafka, but inside Salesforce, with no infrastructure to manage.

What an event looks like

In Setup, create a Platform Event called Order_Placed__e with fields:

• Order_Id__c (Text)
• Customer_Email__c (Email)
• Total__c (Currency)

Now you can publish from anywhere:

Order_Placed__e evt = new Order_Placed__e(
    Order_Id__c = order.Id,
    Customer_Email__c = order.Customer__r.Email,
    Total__c = order.Total__c
);
Database.SaveResult sr = EventBus.publish(evt);

And subscribe with a trigger:

trigger OrderPlacedTrigger on Order_Placed__e (after insert) {
    for (Order_Placed__e evt : Trigger.new) {
        // Send confirmation email, sync to ERP, etc.
    }
}

Why use them instead of regular triggers or callouts

Two delivery models

• Publish Immediately — event is sent right when EventBus.publish() runs, before the transaction commits. Use for diagnostics and high-volume publishing. If the transaction rolls back, the event still went out.
• Publish After Commit — event is sent only if the transaction succeeds. Set EventBus.PublishBehavior.PublishAfterCommit on the event definition. This is what you want for business logic — no orphan events from rolled-back transactions.

External systems subscribing

This is the killer feature for integration. Your Node.js / Python / Java service connects to Salesforce’s Pub/Sub API (gRPC, the modern replacement for the older CometD streaming API), subscribes to /event/Order_Placed__e, and receives every order placed in real-time. No polling, no webhooks to maintain on the Salesforce side.

Limits to remember

• Publish: 250,000 events per 24 hours (or per the org’s event allocation)
• Event size: 1 MB
• Standard Volume vs High-Volume: Use High-Volume for anything serious — 24-hour retention, higher delivery limits
• Replay: subscribers can request events by Replay ID up to 72 hours back (High-Volume) or 24 hours (Standard)

Common interview follow-ups

• Platform Events vs Change Data Capture? — CDC is automatic (any record change), schema-defined. Platform Events are custom, with your own shape and timing.
• What’s the order of execution? — Publisher transaction commits, then the event fires. Subscribers run in a separate transaction with their own governor limits.
• Can a Platform Event trigger fail? — Yes. Use EventBus.RetryableException to ask the platform to redeliver, with a max of 9 retries.

Verified against: Platform Events Developer Guide. Last reviewed 2026-05-17 for Spring ‘26 release.