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Update docs vis-a-vis prohibition of RentPaying accounts (#22438)

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* Rent-exempt docs for exchange integrations

* Remove discussion of rent-paying accounts from developing docs

* Improve verbiage
This commit is contained in:
Tyera Eulberg
2022-01-12 12:32:19 -07:00
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parent 4854fadc44
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2 changed files with 22 additions and 68 deletions
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docs/src/developing/programming-model/accounts.md
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@@ -135,72 +135,13 @@ blockchain cluster must actively maintain the data to process any future transac
This is different from Bitcoin and Ethereum, where storing accounts doesn't
incur any costs.
The rent is debited from an account's balance by the runtime upon the first
access (including the initial account creation) in the current epoch by
transactions or once per an epoch if there are no transactions. The fee is
currently a fixed rate, measured in bytes-times-epochs. The fee may change in
the future.
For the sake of simple rent calculation, rent is always collected for a single,
full epoch. Rent is not pro-rated, meaning there are neither fees nor refunds
for partial epochs. This means that, on account creation, the first rent
collected isn't for the current partial epoch, but collected up front for the
next full epoch. Subsequent rent collections are for further future epochs. On
the other end, if the balance of an already-rent-collected account drops below
another rent fee mid-epoch, the account will continue to exist through the
current epoch and be purged immediately at the start of the upcoming epoch.
Accounts can be exempt from paying rent if they maintain a minimum balance. This
rent-exemption is described below.
### Calculation of rent
Note: The rent rate can change in the future.
As of writing, the fixed rent fee is 19.055441478439427 lamports per byte-epoch
on the testnet and mainnet-beta clusters. An [epoch](terminology.md#epoch) is
targeted to be 2 days (For devnet, the rent fee is 0.3608183131797095 lamports
per byte-epoch with its 54m36s-long epoch).
This value is calculated to target 0.01 SOL per mebibyte-day (exactly matching
to 3.56 SOL per mebibyte-year):
```text
Rent fee: 19.055441478439427 = 10_000_000 (0.01 SOL) * 365(approx. day in a year) / (1024 * 1024)(1 MiB) / (365.25/2)(epochs in 1 year)
```
And rent calculation is done with the `f64` precision and the final result is
truncated to `u64` in lamports.
The rent calculation includes account metadata (address, owner, lamports, etc)
in the size of an account. Therefore the smallest an account can be for rent
calculations is 128 bytes.
For example, an account is created with the initial transfer of 10,000 lamports
and no additional data. Rent is immediately debited from it on creation,
resulting in a balance of 7,561 lamports:
```text
Rent: 2,439 = 19.055441478439427 (rent rate) * 128 bytes (minimum account size) * 1 (epoch)
Account Balance: 7,561 = 10,000 (transfered lamports) - 2,439 (this account's rent fee for an epoch)
```
The account balance will be reduced to 5,122 lamports at the next epoch even if
there is no activity:
```text
Account Balance: 5,122 = 7,561 (current balance) - 2,439 (this account's rent fee for an epoch)
```
Accordingly, a minimum-size account will be immediately removed after creation
if the transferred lamports are less than or equal to 2,439.
Currently, all new accounts are required to be rent-exempt.
### Rent exemption
Alternatively, an account can be made entirely exempt from rent collection by
depositing at least 2 years worth of rent. This is checked every time an
account's balance is reduced, and rent is immediately debited once the balance
goes below the minimum amount.
An account is considered rent-exempt if it holds at least 2 years worth of rent.
This is checked every time an account's balance is reduced, and transactions
that would reduce the balance to below the minimum amount will fail.
Program executable accounts are required by the runtime to be rent-exempt to
avoid being purged.