How can separate custom programs interact with each other?

Question

Beginner questions for you. I have a program that can successfully create and later modify a PDA account with some basic data. The Account struct is very basic:

pub struct CustomPdaAccount {
    pub value: u32,
}

I also have a completely separate on-chain program that simply takes in a regular account's data (one with its own keypair, not a PDA, but has the same account structure) and multiplies the current value by some random number. Nothing fancy at all.

That said, how would/should I proceed to make these two programs work together? Is this the territory of CPIs or something else? It would be neat to have my first on-chain program create and initializes a PDA account, and then my second on-chain program could be called to modify the data. However, my understanding is that PDA accounts can only be modified by the program that originally initialized/created the account.

Obviously, I could simply add the multiplication instruction logic inside the first program, rebuild, deploy, and then I'm left with a single program that does it all. Just curious if there is a Solana feature I should explore further.

Answer 1

If I understood you correctly, you want to do something like this:

There is a PDA state Account that is owned by Program A, that stores the state related to this Program. You want some functionality on Program B to modify this state account.

1. User will pass Account to Program A
2. Program A will pass it to Program B
3. Program B will modify the state of the Account

To modify the state account, the Program must be an owner of this account. So, in this case, Program B MUST be an owner of the Account. This is the territory of CPI, and in your case, it is not possible. Simply put, if you are creating a PDA State Account owned by some Program, only this Program will be able to modify it's state