Platform Events Developer Guide

Delivering Custom Notifications with Platform Events
Defining Your Platform Event
Publishing Platform Events
Subscribing to Platform Events
Considerations and Testing
Defining Platform Events
Processes and Flows
Apex and API
Test Your Platform Event Trigger in Apex
What Is the Difference Between the Salesforce Events?
End-to-End Example Using a Process and a Flow
Reference
PDF

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Considerations for Publishing and Subscribing to Platform Events with Apex and API

Before you use Apex or Salesforce APIs to publish and subscribe to platform events, familiarize yourself with these considerations.
Support Only for after insert Triggers
Only after insert triggers are supported for platform events because event notifications can’t be updated. They’re only inserted (published).
Infinite Trigger Loop and Limits
Be careful when publishing events from triggers because you could get into an infinite trigger loop and exceed limits. For example, if you publish an event from a trigger that’s associated with the same event object, the trigger is fired in an infinite loop.
Apex DML Limits for Publishing Events
Each EventBus.publish method call is considered a DML statement, and DML limits apply.
Platform Event Triggers: ownerId Fields of New Records
In platform event triggers, if you create a Salesforce record that contains an ownerId field, set the ownerId field explicitly to the appropriate user. Platform event triggers run under the Automated Process entity. If you don’t set the ownerId field on records that contain this field, the system sets the default value of Automated Process. This example explicitly populates the ownerId field for an opportunity with an ID obtained from another record.
1Opportunity newOpp = new Opportunity(
2    OwnerId = customerOrder.createdById,
3    AccountId = acc.Id, 
4    StageName = 'Qualification', 
5    Name = 'A ' + customerOrder.Product_Name__c + ' opportunity for ' + acc.name, 
6    CloseDate = Date.today().addDays(7));
For cases and leads, you can alternatively use assignment rules for setting the owner. See AssignmentRuleHeader for the SOAP API or Setting DML Options for Apex.
API Request Limits for Publishing Events
Because platform events are published by inserting the event sObjects, API request limits apply. For more information, see API Request Limits and Allocations in the Salesforce Limits Quick Reference Guide.
Replaying Past Events
You can replay platform events that were sent in the past 24 hours. You can replay platform events through the API (CometD) but not Apex. The process of replaying platform events is the same as for other Streaming API events. For more information, see the following resources.

In rare occasions, some maintenance activities on the Salesforce application servers wipe retained events. For example, maintenance activities, such as disaster recovery of servers after unexpected outages, a site switch of a Salesforce instance, or an org migration to a new data center, can wipe retained events. To prevent losing events, receive only the latest events by subscribing to the tip of the queue (-1 replay option).

Note

Filtered Subscriptions
Filtered subscriptions in Streaming API aren’t supported for platform events.
Publishing Events in Read-Only Mode
During read-only mode, publishing platform events results in an exception and the events aren't published. In contrast, you can publish high-volume platform events in read-only mode. Your org is in read-only mode during Salesforce maintenance activities.