I know there's an API called accountSubscribe that I can use to get notified when an Account is modified when new blocks arrive. But for historical blocks / transactions, how can I tell if an Account was modified by a given transaction?
I define "modified" as data or lamport changes, instead of being just read-only accessed. Just like in WebSocket subscriptions, if such a change happens in transactions, it should be spotted out.
I noticed that in the getTransaction API, the returned structure has transaction.message.accountKeys where each item has a field writable. I wonder if it's safe to say, if meta.err of the transaction is null and the writable field is true, the Account is modified by that transaction?
Just using the normal RPC endpoints, there's no way to be sure.
If a transaction succeeded and the account was writable, the account data could have stayed the same. For example, if I transfer 0 tokens to you, the transaction will succeed, and be shown as "writable", but no modification was done.
If you only care about SOL / token balances, you can use the preTokenBalances vs postTokenBalances or preBalances vs postBalances when fetching past transactions. More info at https://docs.solana.com/api/http#gettransaction
Past that, there's no way to know currently.