Like other services in AWS, DynamoDB has some default soft limits set on the service to prevent us from consuming the backend resources unintentionally. If required, the service soft limits can be extended by contacting AWS. The soft limits in DynamoDB are set as follows:

• Depending on the region, each DynamoDB table will be limited either to 10,000/40,000 read and 10,000/40,000 write capacity units per table and double that for all tables in the account in provisioned mode. There is no soft limit on the cumulative RCU/WCU capacity for all on-demand capacity mode tables.
• Up to 256 tables of unlimited size per region per account.
• A maximum of 20 global secondary indexes per table.

There are also some limitations in the service itself due to the service design, and these are as follows:

• One RCU = one strongly consistent read of up to 4 KB or two eventually consistent reads of up to 4 KB
• One WCU = one write of up to 1 KB in size
• Transactional requests require 2 RCUs and 1 WCU to update the item
• A maximum of five local secondary indexes
• Partition key values are limited to between 1 and 2048 bytes in length
• Sort key values are limited to between 1 and 1024 bytes in length
• Strings and binary values are limited to 400 KB in size
• Number values can have up to 38 digits of precision
• Nested attributes in values can be up to 32 levels deep
• Only two processes can read from a DynamoDB stream shard at the same time

There are some additional limitations for queries, transactions, and APIs, and, as we have mentioned before, DynamoDB has some reserved words that cannot be used as attribute names. For a complete list of these, please see the links in the Further reading section of this chapter.