# DynamoDB Read/Write Units — Numeric Practice, Set 3 20 questions. Give a numeric answer for each, using `1 = `, `2 = `, and so on. ## Assumptions - Assume provisioned capacity and steady workloads. - All item sizes are exact DynamoDB item sizes, including attribute names and values. - Each read accesses an individual item, not a Query or Scan. Transactional operations count individual items, not transaction API calls. - Normal writes are non-transactional. All writes create new items. - Ignore indexes, replication, retries, failed requests, burst capacity and partition limits. - Questions 1–14 ask for exact throughput requirements: fractional RCU answers are valid; do not round the final result to a whole number. - Questions 15–18 ask for the maximum number of individual item operations per second. - Questions 19–20 ask for whole-number provisioned capacity settings. ## Questions ### 1. You normally write six separate 1.8 KB items every second. How many WCUs are required? Answer: ______ ### 2. You normally write nine separate 0.7 KB items every second. How many WCUs are required? Answer: ______ ### 3. You normally write four separate 3.01 KB items every second. How many WCUs are required? Answer: ______ ### 4. You transactionally write three separate 2.01 KB items every second. How many WCUs are required? Answer: ______ ### 5. You transactionally write six separate 1 KB items every second. How many WCUs are required? Answer: ______ ### 6. You perform strongly consistent reads of five separate 5 KB items every second. How many RCUs of throughput are required? Answer: ______ ### 7. You perform strongly consistent reads of three separate 12 KB items every second. How many RCUs of throughput are required? Answer: ______ ### 8. You perform eventually consistent reads of nine separate 0.8 KB items every second. How many RCUs of throughput are required? Answer: ______ ### 9. You perform eventually consistent reads of five separate 8.01 KB items every second. How many RCUs of throughput are required? Answer: ______ ### 10. You perform eventually consistent reads of seven separate 8 KB items every second. How many RCUs of throughput are required? Answer: ______ ### 11. You transactionally read five separate 4.01 KB items every second. How many RCUs of throughput are required? Answer: ______ ### 12. You transactionally read three separate 11 KB items every second. How many RCUs of throughput are required? Answer: ______ ### 13. Each second, you normally write two 0.6 KB items and three 2.4 KB items to the same table. How many WCUs are required in total? Answer: ______ ### 14. Each second, you perform strongly consistent reads of two 5 KB items and eventually consistent reads of three 1 KB items from the same table. How many RCUs of throughput are required in total? Answer: ______ ### 15. You have 28 WCUs available. What is the maximum number of normal item writes per second you can sustain if each item is 3.2 KB? Answer: ______ ### 16. You have 30 WCUs available. What is the maximum number of transactional item writes per second you can sustain if each item is 2.2 KB? Answer: ______ ### 17. You have 15 RCUs available. What is the maximum number of eventually consistent item reads per second you can sustain if each item is 9 KB? Answer: ______ ### 18. You have 32 RCUs available. What is the maximum number of transactional item reads per second you can sustain if each item is 5 KB? Answer: ______ ### 19. You perform eventually consistent reads of eleven separate 3 KB items every second. What is the minimum WHOLE NUMBER of RCUs you must provision? Answer: ______ ### 20. You perform eventually consistent reads of five separate 12.01 KB items every second. What is the minimum WHOLE NUMBER of RCUs you must provision? Answer: ______ ## Answer key
Show answers and calculations `ceil` means round up to the next whole number. Sizes are divided by the 1 KB write or 4 KB read increment before rounding. 1. **12 WCUs** — `ceil(1.8 / 1) × 6 = 2 × 6`. 2. **9 WCUs** — `ceil(0.7 / 1) × 9 = 1 × 9`. 3. **16 WCUs** — `ceil(3.01 / 1) × 4 = 4 × 4`. 4. **18 WCUs** — `ceil(2.01 / 1) × 3 × 2 = 3 × 3 × 2`. 5. **12 WCUs** — `ceil(1 / 1) × 6 × 2 = 1 × 6 × 2`. 6. **10 RCUs** — `ceil(5 / 4) × 5 = 2 × 5`. 7. **9 RCUs** — `ceil(12 / 4) × 3 = 3 × 3`. 8. **4.5 RCUs** — `ceil(0.8 / 4) × 9 × 0.5 = 1 × 9 × 0.5`. 9. **7.5 RCUs** — `ceil(8.01 / 4) × 5 × 0.5 = 3 × 5 × 0.5`. 10. **7 RCUs** — `ceil(8 / 4) × 7 × 0.5 = 2 × 7 × 0.5`. 11. **20 RCUs** — `ceil(4.01 / 4) × 5 × 2 = 2 × 5 × 2`. 12. **18 RCUs** — `ceil(11 / 4) × 3 × 2 = 3 × 3 × 2`. 13. **11 WCUs** — `(ceil(0.6 / 1) × 2) + (ceil(2.4 / 1) × 3) = 2 + 9`. 14. **5.5 RCUs** — `(ceil(5 / 4) × 2) + (ceil(1 / 4) × 3 × 0.5) = 4 + 1.5`. 15. **7 writes/s** — `28 / ceil(3.2 / 1) = 28 / 4`. 16. **5 transactional item writes/s** — `30 / (ceil(2.2 / 1) × 2) = 30 / 6`. 17. **10 reads/s** — `15 / (ceil(9 / 4) × 0.5) = 15 / 1.5`. 18. **8 transactional item reads/s** — `32 / (ceil(5 / 4) × 2) = 32 / 4`. 19. **6 provisioned RCUs** — `ceil(ceil(3 / 4) × 11 × 0.5) = ceil(5.5)`. 20. **10 provisioned RCUs** — `ceil(ceil(12.01 / 4) × 5 × 0.5) = ceil(10)`. For questions 1–14, RCUs describe exact throughput requirements. Provisioned settings must be whole numbers, as explicitly requested in questions 19–20.