DaX

We discussed about DynamoDB in previous post, but let’s dive deeper into the cost factors associated with DynamoDB and its accelerator, DAX (DynamoDB Accelerator).

Recap

• Amazon DynamoDB (DDB) is AWS’s fully managed NoSQL database service, designed for applications that require consistent performance at any scale.

• Amazon DynamoDB Accelerator (DAX) is an in-memory caching service for DynamoDB. Think of it as a turbocharger — it reduces read latency from milliseconds to microseconds by storing frequently accessed data in memory.

Together, DDB and DAX can significantly improve application performance — but they also come with different cost models you’ll want to understand before adopting.

When to Use DAX?

DAX is particularly useful when:

• Your workload has high read traffic with repeated queries for the same items.
• You want microsecond read latency for real-time user experience.
• You aim to offload read traffic from DynamoDB to reduce provisioned read capacity usage.

Example: A database for AI model training, where the same training data is accessed repeatedly.

Skip DAX when:

• Your workload is write-heavy with low read repetition.
• Your queries are strongly consistent (DAX only supports eventually consistent reads).
• Your access patterns are highly dynamic and unpredictable — the cache hit rate might be low.

Understanding DynamoDB Costs

DynamoDB costs come from three main areas:

• Reading Data
  • Imagine a reading allowance — every time you check a page from a book, it uses part of your allowance.
  • You can either pay per read (On-Demand) or buy a monthly “reading subscription” (Provisioned) if you know your usual usage.
• Writing Data
  • Adding or updating books also uses an allowance — think of it as your “writing subscription” or “per-write” payment.
• Storing Data
  • This is your bookshelf space.
  • Regular storage (Standard) is always ready but costs more.
  • Cheaper storage (Standard-IA) is for books you rarely read, but you’ll pay a small fee each time you take one.

Extras You Might Pay For:

• Backups — like taking daily photos of your bookshelf.
• Copies in other regions — like having the same library in multiple cities.

Understanding DAX Costs

5. DAX Costs DAX pricing is per node-hour, depending on node type:

• Smallest node (dax.t3.small) is the cheapest, suitable for dev/test.
• Larger nodes (dax.r5.large, etc.) cost more but handle higher throughput.
• DAX clusters require at least 3 nodes for fault tolerance in production.

Note: DAX charges are separate from DynamoDB — even if your reads come from the cache.

Cost Comparison

Component	Without DAX (Provisioned)	With DAX (Provisioned)
Read Capacity Cost	High (due to all reads hitting DDB)	Lower (fewer RCUs needed)
Write Capacity Cost	Same	Same
Storage Cost	Same	Same
DAX Cost	$0	Node-hour charges

If your cache hit rate is low, DAX might increase costs without much benefit.

Final Thoughts

• Use DAX if you have heavy, repeated reads and need lightning-fast results.
• Use Standard-IA storage for rarely accessed data — but don’t forget the retrieval cost.
• Always measure first: monitor read/write usage and cache hit rates before committing.