Rent
Lamports that keep an account alive
Rent-exempt is the rule you meet
On Solana, account data is paid for with lamports. The rent-exempt minimum is the balance at which an account of that size is left alone. Create the account below that line and the runtime rejects it. The older idea of rent dripping away on a schedule is not what you will meet when you create accounts now.
The minimum depends on the data length. A larger state struct costs more lamports to open. That is why the circle writes the byte length next to the struct before anyone asks how the account will be funded.
Who may move the lamports
Lamports are not a decorative balance. Moving them is a privileged act. A program can debit accounts it owns, under the runtime’s rules, and the system program is the owner of ordinary wallets. If your instruction expects a wallet to pay, the system program has to be the one asked, and the wallet has to sign.
Mixing those two stories — the program’s own data account, and a user’s wallet — is a common stall on Wednesday afternoons. The packet uses two different verbs on purpose: fund the new account, and later close it.
Closing
Closing an account zeros the data and sends the lamports to a destination account. The destination should be named in the instruction, not implied. After a close, anything still pointing at that address as if the old struct were there is reading an account that no longer holds the record.
In the August escrow, the vault close was a separate instruction from the settle. We keep them apart on the map so a group does not ‘clean up’ lamports inside the settle without saying where they go.
A small check before you create
Before the room agrees a create instruction is ready, we ask three questions. How many bytes? Which account supplies the lamports, and does that account sign? If the record is later closed, which account receives the lamports? If any answer is ‘the client will decide’, the instruction is not specified.