Northern Trail Outfitters needs to send a highly personalized SMS message to customers.

An Apex Class data provider is the appropriate solution when personalization requires custom runtime logic and data that isn't already available in Data 360. Salesforce supports Apex Class as a personalization data source for both email and SMS content.

The Apex data provider enables custom Apex logic to supply simple or complex values to the marketing message at runtime. This is especially relevant when an external system must be queried, proprietary calculations must be executed, or structured data must be assembled immediately before rendering the message.

A Data Graph would require the relevant information to already exist within Data 360 or be represented through its associated DMOs. That contradicts the requirement that the external information isn't stored there. An Event data provider works well when the triggering event itself contains all required values, but the question specifically states that a complex calculation against external system data must occur at send time. That requirement calls for executable server-side logic rather than simple event-payload personalization.

The uploaded bank keys C. Salesforce explicitly documents Apex Class as a supported data provider for personalized Email and SMS and notes that it can supply complex structured data.

Study Guide Reference: Campaign Design, Flow Orchestration and Content → Advanced Personalization → Apex Data Providers.

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