¶ Exponential Backoff & DynamoDB
Definition: Exponential Backoff is a retry strategy where failed requests are retried with exponentially increasing wait times (e.g., 50ms, 100ms, 200ms, etc.). This prevents overwhelming a system with repeated requests.
DynamoDB enforces request limits, and exceeding them results in ProvisionedThroughputExceededException errors. Exponential Backoff helps mitigate throttling by spacing out retries, improving request success rates without overloading the system. AWS SDKs implement this automatically.
¶ A Write Capacity Unit (WCU)
A Write Capacity Unit (WCU) in Amazon DynamoDB defines the write capacity required for inserting or updating data.
1WCU = 1 × 1KB write per second
If your write operations are larger than 1 KB, you will need more WCU accordingly:
- A 2 KB item requires 2 WCU per write operation.
- A 500-byte item requires 1 WCU (as it is rounded up to 1 KB).
Exercise: Calculate Write Throughput (WCU) in DynamoDB
You need to calculate the write throughput (WCU) required for storing 2,145 items per minute, where each item is 0.5 KB in size.
Step 1: Determine Write Capacity Units (WCU) per item
DynamoDB rounds up each write to the nearest 1 KB.
Since each item is 0.5 KB, it is rounded up to 1 KB, requiring 1 WCU per write.Step 2: Calculate Writes per Second
2,145 items per minute = 2,145 / 60 ≈ 35.75 items per secondStep 3: Calculate Total WCU Required>
Each item requires 1 WCU, so the total WCU is:
35.75 × 1 = 35.75Since WCU must be a whole number, you round up to 36 WCU.
Final Answer:
To support writing 2,145 items per minute, where each item is 0.5 KB, you need at least 36 WCU.
¶ A Read Capacity Unit (RCU)
In AWS DynamoDB, a Read Capacity Unit (RCU) defines the amount of read throughput for your table or index. Here's a breakdown:
1 RCU allows you to perform 1 strongly consistent read of up to 4 KB per second or 2 eventually consistent reads of up to 4 KB per second.
For a strongly consistent read, DynamoDB reads the most recent data and uses 1 RCU per read for up to 4 KB.
For an eventually consistent read, DynamoDB can return stale data and uses only 0.5 RCU per read for up to 4 KB.
Example:
If you want to perform strongly consistent reads on 5 KB of data, you would need 2 RCUs because each RCU covers 4 KB.
If you want to perform eventually consistent reads on 5 KB of data, you would need 1 RCU because each RCU can cover two 4 KB reads.
Exercise:
You have an application that needs to read 50 items per second and each item is 6 KB in size. Your application uses strongly consistent reads. What should you set the read throughput to?¶ Step 1: Determine RCU per Item
For strongly consistent reads, each 1 RCU allows reading up to 4 KB per second
RCU per item = Item Size / 4KB in this case -> 6KB/4KB = 1,5 -> round up = 2
¶ Step 2: Calculate Total RCU for 50 Reads per Second
Final Answer:
Total RCU = 50 reads per secind x RCU per item = 100 RCU
Exercise:
You have an application which reads 80 items of data every second. Each item consists of 3KB. Your application uses eventually consistent reads. What should you set the RCU read throughput to?Each Read Capacity Unit represents 2 x Eventually consistent 4KB reads per second. 3KB / 4KB = 0.75 and round up to 1. Multiply that by 80 to give you the figure for Strongly consistent reads. Divide that by 2 to get Eventually consistent reads. Therefore the answer is 40 RCU.
Important: "First, calculate the RCPU and round up if needed, before multiplying by the number of items you want to read."
¶ Overview of Key DynamoDB Commands
1️⃣ DynamoDB AWS SDK (API) Commands
Used for programmatic access via AWS SDKs (Node.js, Python, Java, etc.).
2️⃣ DynamoDB AWS CLI Commands
Used for manual testing and administrative tasks via command line.
