What are the key differences between Amazon Managed Blockchain and Amazon QLDB ?
Under which scenarios is it advisable to use which service for distributed ledgers and when ?
How can the performance vary between the two services in terms of no of transactions per second ?
Put simply, Amazon Managed Blockchain is "traditional" blockchain whereas Amazon Quantum Ledger Database is something totally different.
First, the official definitions:
Amazon Managed Blockchain is a fully managed service for creating and managing blockchain networks using open source frameworks. Currently, the Hyperledger Fabric open source framework is supported. Blockchain allows you to build applications where multiple parties can securely and transparently run transactions and share data without the need for a trusted, central authority.
So, think of it as providing a traditional blockchain infrastructure.
Amazon Quantum Ledger Database (Amazon QLDB) is a fully managed ledger database owned by a central trusted authority that provides a transparent, immutable, and cryptographically verifiable transaction log of all of your application changes. Amazon QLDB tracks each and every application data change and maintains a complete and verifiable history of changes over time.
This one is quite different. The story is the customers were looking at Blockchain as a way of providing verifiable transactions. Then Amazon realized that this goal does not require a full distributed nature of a blockchain. Instead, it just needs a central database with verifiable transactions.
For example, a normal database cannot "prove" that a transaction has not been changed. DB Admins can always go in and modify a transaction or log to hide something. Amazon QLDB, however, uses blockchain-like verification in each block of transactions to "prove" that it hasn't been modified.
Here's a presentation on the topic: AWS re:Invent 2018: [NEW LAUNCH!] Do I need a ledger database? An intro to Amazon QLDB (DAT370) - YouTube