Can a blockchain only store transaction proofs or any kind of data in it?

Question

I am currently studying about the blockchain technology. Everywhere they say that transactions on a blockchain cannot be tempered with. I assumed any kind of data can be stored in a blockchain. But in IBM's blockchain for dummies book it said that a blockchain cannot replace a database and can only store transactional proofs. Can someone tell what's accurate?

Quoting the book:

while the blockchain contains transaction data, it’s not a replacement for databases, messaging technology, trans action processing, or business processes. Instead, the blockchain contains verified proof of transactions. However, while block chain essentially serves as a database for recording transactions, its benefits extend far beyond those of a traditional database.

Answer 1

No time to write a very complete answer, but here is the basics:

Storing data in blockchain has the following disadvantages:

• Increase the data that needs to be stored by the chain.

But as everyone must get the exact same information, you need to share this data

• Increase bandwith usage

But for public blockchain, you need to pay to use. The more storage you require in your transaction, the more you pay

• Increase transaction fees.

Note: this is not true for Hyperledger Fabric, as far as I know, because there is no concept of fee

These disadvantages are huge. So, as you talk about "classical database", instead of storing your classical data in the database, you could hash the new data of your database (say you update a line in the database, hash the new line state). To make sure this data has not been modified, create a transaction in your blockchain that says: In database X, line Y, the new hash of the line is Z.

To answer directly to your questions:

I assumed any kind of data can be stored in a blockchain

You can, but it is not cost effective and requires likely too much resources of any kind

blockchain cannot replace a database

This is technicaly "false", but taking into accounts drawbacks, it is true. "cannot" should be replaced with "should not".

can only store transactional proofs

This is false, but also true because it is, if not the best, a very good way to use blockchain. "can only" should be replaced by "should only".