Mastering cache key resolution in Apollo iOS involves using the @typePolicy and @fieldPolicy directives to efficiently handle the storage and retrieval of cached data, thereby minimizing network requests and avoiding data duplication. These directives allow developers to define how cache keys are constructed, ensuring that objects with the same identity are stored consistently and can be retrieved intelligently based on query parameters. The @typePolicy directive specifies how cache keys are derived from fields within the fetched objects, promoting consistent storage, while the @fieldPolicy directive focuses on deriving cache keys from field arguments to facilitate effective data retrieval. This approach not only enhances data consistency and reduces memory footprint but also significantly improves application responsiveness by cutting down unnecessary network traffic. For complex scenarios that require more than the declarative power of directives, programmatic configurations offer additional flexibility, allowing developers to implement custom logic for cache key resolution. By mastering these techniques, developers can optimize Apollo iOS applications for efficient caching, ensuring faster and more reliable user experiences.