Add cached impression writer for batching database writes

[ericholscher]

Summary

Implement a caching layer for ad impression tracking to reduce database load by batching writes. Instead of writing to the database on every impression event (offer, view, click), impressions are now accumulated in Django's cache and periodically flushed to the database in batch.

Key Changes

• New CachedImpressionWriter class (impression_cache.py): Buffers impression increments in Django's cache with deterministic cache keys based on ad, publisher, date, and impression type. Provides increment() to add to cached counters and flush() to batch-write to the database.

• Updated Advertisement.incr() method (models.py): Added conditional logic to use the cached writer when ADSERVER_IMPRESSION_CACHE_ENABLED is True, falling back to direct database writes when disabled.

• New periodic task (tasks.py): Added flush_impression_cache() Celery task to periodically flush cached impressions to the database.

• Configuration setting (settings/base.py): Added ADSERVER_IMPRESSION_CACHE_ENABLED environment variable (defaults to False) to control whether impression caching is active.

• Comprehensive test suite (test_cached_impressions.py): 20 tests covering cache key generation, increment accumulation, dirty key tracking, database flushing, handling of existing records, multiple ads/impression types, null advertisements, concurrent safety, and integration with the Advertisement.incr() method.

Implementation Details

• Cache keys use a deterministic format: impression_cache:{ad_id}:{publisher_id}:{date}:{impression_type}
• Dirty keys are tracked in a separate cache entry to know which keys need flushing
• Flush operation uses atomic F() expressions for safe concurrent updates to existing records
• Supports null advertisement IDs for decisions with no associated offer
• Cache timeout is set to 2 hours to provide a generous buffer for flush intervals
• Graceful error handling with logging if individual flush operations fail

https://claude.ai/code/session_01Cpd47jJhFJ9MEkfobuzKSj