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Question

In provisioned capacity mode, does writing a single row per second with less than 1KB of data use an entire WCU?    
More concretely, if I write 10 rows, each with 50 bytes, per second have I used an entire WCU even though only 500 bytes have been written?  Or am I actually using 10 WCUs since I wrote 10 rows in a single second?  
  
Similarly with RCUs, if I issue a query once per second that returns multiple rows, but with an aggregate size of less than 4KB is the entire RCU used?  
Concretely, I issue a SELECT and get back 10 rows, each with 100 bytes of data I'm below the 4KB limit per RCU.  If I then issue another SELECT within the same second, and get back another 1000 bytes of data have I used 2 RCUs, or 1 RCU since I've only read 2000 bytes in this second?

Answer

If you write 10 rows each with 50 bytes you will consume 10 WCU though you wrote only 500 bytes which is less the 1 KB WCU size. Even if you write these 10 rows in a batch you will consume 10 WCU.  
  
On read side if aggregate size of your single query is less than 4 KB you will consume 1 RCU.  
But your next query also will cost 1 RCU (your 2 queries cost 2 RCU) though aggregate size of two query result is still below 4 KB. Each transaction is charged individually.

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