Update docs for eager rent collection

docs/src/apps/README.md

@@ -1,6 +1,6 @@
 # Programming Model
 
-An _app_ interacts with a Solana cluster by sending it _transactions_ with one or more _instructions_. The Solana _runtime_ passes those instructions to user-contributed _programs_. An instruction might, for example, tell a program to transfer _lamports_ from one _account_ to another or create an interactive contract that governs how lamports are transfered. Instructions are executed sequentially and atomically. If any instruction is invalid, all account changes are discarded.
+An _app_ interacts with a Solana cluster by sending it _transactions_ with one or more _instructions_. The Solana _runtime_ passes those instructions to _programs_ deployed by app developers beforehand. An instruction might, for example, tell a program to transfer _lamports_ from one _account_ to another or create an interactive contract that governs how lamports are transferred. Instructions are executed sequentially and atomically for each transaction. If any instruction is invalid, all account changes in the transaction are discarded.
 
 ### Accounts and Signatures
 
@@ -20,25 +20,32 @@ Each instruction specifies a single program account \(which must be marked execu
 
 ![SDK tools](../.gitbook/assets/sdk-tools.svg)
 
-As shown in the diagram above a client creates a program and compiles it to an ELF shared object containing BPF bytecode and sends it to the Solana cluster. The cluster stores the program locally and makes it available to clients via a _program ID_. The program ID is a _public key_ generated by the client and is used to reference the program in subsequent transactions.
+As shown in the diagram above, a program author creates a program and compiles it to an ELF shared object containing BPF bytecode and uploads it to the Solana cluster with a special _deploy_ transaction. The cluster makes it available to clients via a _program ID_. The program ID is a _address_ specified when deploying and is used to reference the program in subsequent transactions.
 
 A program may be written in any programming language that can target the Berkley Packet Filter \(BPF\) safe execution environment. The Solana SDK offers the best support for C/C++ and Rust programs, which are compiled to BPF using the [LLVM compiler infrastructure](https://llvm.org).
 
 ## Storing State between Transactions
 
-If the program needs to store state between transactions, it does so using _accounts_. Accounts are similar to files in operating systems such as Linux. Like a file, an account may hold arbitrary data and that data persists beyond the lifetime of a program. Also like a file, an account includes metadata that tells the runtime who is allowed to access the data and how. Unlike a file, the account includes metadata for the lifetime of the file. That lifetime is expressed in "tokens", which is a number of fractional native tokens, called _lamports_. Accounts are held in validator memory and pay "rent" to stay there. Each validator periodically scan all accounts and collects rent. Any account that drops to zero lamports is purged.
+If the program needs to store state between transactions, it does so using _accounts_. Accounts are similar to files in operating systems such as Linux. Like a file, an account may hold arbitrary data and that data persists beyond the lifetime of a program. Also like a file, an account includes metadata that tells the runtime who is allowed to access the data and how.
 
-If an account is marked "executable", it will only be used by a _loader_ to run programs. For example, a BPF-compiled program is marked executable and loaded by the BPF loader. No program is allowed to modify the contents of an executable account.
+Unlike a file, the account includes metadata for the lifetime of the file. That lifetime is expressed in "tokens", which is a number of fractional native tokens, called _lamports_. Accounts are held in validator memory and pay ["rent"](rent.md) to stay there. Each validator periodically scans all accounts and collects rent. Any account that drops to zero lamports is purged.
 
-An account also includes "owner" metadata. The owner is a program ID. The runtime grants the program write access to the account if its ID matches the owner. If an account is not owned by a program, the program is permitted to read its data and credit the account.
+In the same way that a Linux user uses a path to look up a file, a Solana client uses an _address_ to look up an account. The address is usually a 256-bit public key. To create an account with a public-key address, the client generates a _keypair_ and registers its public key using the `CreateAccount` instruction with preallocated fixed storage size in bytes. In fact, the account address can be an arbitrary 32 bytes, and there is a mechanism for advanced users to create derived addresses (`CreateAccountWithSeed`). Addresses are presented in Base58 encoding on user interfaces.
 
-In the same way that a Linux user uses a path to look up a file, a Solana client uses public keys to look up accounts. To create an account, the client generates a _keypair_ and registers its public key using the `CreateAccount` instruction. The account created by `CreateAccount` is called a _system account_ and is owned by a built-in program called the System program. The System program allows clients to transfer lamports and assign account ownership.
+## Ownership of Accounts and Assignment to Programs
 
-The runtime only permits the owner to debit the account or modify its data. The program then defines additional rules for whether the client can modify accounts it owns. In the case of the System program, it allows users to transfer lamports by recognizing transaction signatures. If it sees the client signed the transaction using the keypair's _private key_, it knows the client authorized the token transfer.
+The created account is initialized to be _owned_ by a built-in program called the System program and is called a _system account_ aptly. An account includes "owner" metadata. The owner is a program ID. The runtime grants the program write access to the account if its ID matches the owner. For the case of the System program, the runtime allows clients to transfer lamports and importantly _assign_ account ownership, meaning changing owner to different program ID. If an account is not owned by a program, the program is only permitted to read its data and credit the account.
+
+Also, if an account is marked "executable" in metadata, it will only be used by a _loader_ to run programs. For example, a BPF-compiled program is marked executable and loaded by the BPF loader when executing its transactions. No program is allowed to modify the contents of an executable account once deployed.
+
+## Runtime Capability of Programs on Accounts
+
+The runtime only permits the owner program to debit the account or modify its data. The program then defines additional rules for whether the client can modify accounts it owns. In the case of the System program, it allows users to transfer lamports by recognizing transaction signatures. If it sees the client signed the transaction using the keypair's _private key_, it knows the client authorized the token transfer.
+
+In other words, the entire set of accounts owned by a given program can be regarded as a key-value store where a key is the account address and value is program-specific arbitrary binary data. A program author can decide how to manage the program's whole state as possibly many accounts.
 
 After the runtime executes each of the transaction's instructions, it uses the account metadata to verify that none of the access rules were violated. If a program violates an access rule, the runtime discards all account changes made by all instructions and marks the transaction as failed.
 
 ## Smart Contracts
 
 Programs don't always require transaction signatures, as the System program does. Instead, the program may manage _smart contracts_. A smart contract is a set of constraints that once satisfied, signal to a program that a token transfer or account update is permitted. For example, one could use the Budget program to create a smart contract that authorizes a token transfer only after some date. Once evidence that the date has past, the contract progresses, and token transfer completes.
-

docs/src/apps/rent.md

@@ -1 +1,55 @@
-# Accounts and Rent
+# Storage Rent for Accounts
+
+Keeping accounts alive on Solana incurs a storage cost called _rent_ because the cluster must actively maintain the data to process any future transactions on it. 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):
+
+```
+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:
+
+
+```
+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:
+
+```
+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.
+
+## 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.
+
+Program executable accounts are required by the runtime to be rent-exempt to avoid being purged.
+
+Note: Use the [`getMinimumBalanceForRentExemption` RPC endpoint](jsonrpc-api.md#getminimumbalanceforrentexemption) to calculate the minimum balance for a particular account size. The following calculation is illustrative only.
+
+For example, a program executable with the size of 15,000 bytes requires a balance of 105,290,880 lamports (=~ 0.105 SOL) to be rent-exempt:
+
+```
+105,290,880 = 19.055441478439427 (fee rate) * (128 + 15_000)(account size including metadata) * ((365.25/2) * 2)(epochs in 2 years)
+```

docs/src/implemented-proposals/ed_overview/ed_storage_rent_economics.md

@@ -10,7 +10,7 @@ With this approach, accounts with two-years worth of rent deposits secured are e
 
 Method 2: Pay per byte
 
-If an account has less than two-years worth of deposited rent the network charges rent on a per-epoch basis, in credit for the next epoch (but in arrears when necessary). This rent is deducted at a rate specified in genesis, in lamports per kilobyte-year.
+If an account has less than two-years worth of deposited rent the network charges rent on a per-epoch basis, in credit for the next epoch. This rent is deducted at a rate specified in genesis, in lamports per kilobyte-year.
 
 For information on the technical implementation details of this design, see the [Rent](../rent.md) section.
 

docs/src/implemented-proposals/rent.md

@@ -4,11 +4,17 @@ Accounts on Solana may have owner-controlled state \(`Account::data`\) that's se
 
 ## Two-tiered rent regime
 
-Accounts which maintain a minimum balance equivalent to 2 years of rent payments are exempt. Accounts whose balance falls below this threshold are charged rent at a rate specified in genesis, in lamports per kilobyte-year. The network charges rent on a per-epoch basis, in credit for the next epoch \(but in arrears when necessary\), and `Account::rent_epoch` keeps track of the next time rent should be collected from the account.
+Accounts which maintain a minimum balance equivalent to 2 years of rent payments are exempt. The _2 years_ is drawn from the fact hardware cost drops by 50% in price every 2 years and the resulting convergence due to being a geometric series. Accounts whose balance falls below this threshold are charged rent at a rate specified in genesis, in lamports per byte-year. The network charges rent on a per-epoch basis, in credit for the next epoch, and `Account::rent_epoch` keeps track of the next time rent should be collected from the account.
 
-## Collecting rent
+Currently, the rent cost is fixed at the genesis. However, it's anticipated to be dynamic, reflecting the underlying hardware storage cost at the time. So the price is generally expected to decrease as the hardware cost declines as the technology advances.
 
-Rent is due at account creation time for one epoch's worth of time, and the new account has `Account::rent_epoch` of `current_epoch + 1`. After that, the bank deducts rent from accounts during normal transaction processing as part of the load phase.
+## Timings of collecting rent
+
+There are two timings of collecting rent from accounts: \(1\) when referenced by a transaction, \(2\) periodically once an epoch. \(1\) includes the transaction to create the new account itself, and it happens during the normal transaction processing by the bank as part of the load phase. \(2\) exists to ensure to collect rents from stale accounts, which aren't referenced in recent epochs at all. \(2\) requires the whole scan of accounts and is spread over an epoch based on account address prefix to avoid load spikes due to this rent collection.
+
+## Actual processing of collecting rent
+
+Rent is due for one epoch's worth of time, and accounts always have `Account::rent_epoch` of `current_epoch + 1`.
 
 If the account is in the exempt regime, `Account::rent_epoch` is simply pushed to `current_epoch + 1`.
 
@@ -18,13 +24,19 @@ Accounts whose balance is insufficient to satisfy the rent that would be due sim
 
 A percentage of the rent collected is destroyed. The rest is distributed to validator accounts by stake weight, a la transaction fees, at the end of every slot.
 
-## Read-only accounts
+Finally, rent collection happens according to the protocol-level account updates like the rent distribution to validators, meaning there is no corresponding transaction for rent deductions. So, rent collection is rather invisible, only implicitly observable by a recent transaction or predetermined timing given its account address prefix.
+
+## Design considerations
+
+### Current design rationale
 
-Read-only accounts are not being charged rent in current implementation.
+Under the preceding design, it is NOT possible to have accounts that linger, never get touched, and never have to pay rent. Accounts are always pay rent exactly once for each epoch, except rent-exempt, sysvar and executable accounts.
 
-## Design considerations, others considered
+This is intended design choice. Otherwise, it would be possible to trigger unauthorized rent collection with `Noop` instruction by anyone who may unfairly profit the rent (a leader at the moment) or save the rent given anticipated fluctuating rent cost.
 
-Under this design, it is possible to have accounts that linger, never get touched, and never have to pay rent. `Noop` instructions that name these accounts can be used to "garbage collect", but it'd also be possible for accounts that never get touched to migrate out of a validator's working set, thereby reducing memory consumption and obviating the need to charge rent.
+As another side-effect of this choice, also note that this periodic rent collection effectively forces validator not to store stale accounts into a cold storage optimistically and save the storage cost, which is unfavorable for account owners and may cause transactions on them to stall longer than the others. On the flip side, this prevents malicious users from piling significant amount of garbage accounts, burdening validators.
+
+As the overall consequence of this design, all of accounts is stored equally as validator's working set with same performance characteristics, straightly reflecting the uniform rent pricing structure.
 
 ### Ad-hoc collection
 
@@ -45,8 +57,3 @@ Collecting rent via a system instruction was considered, as it would naturally h
 * it would have adversely affected network throughput
 * it would require special-casing by the runtime, as accounts with non-SystemProgram owners may be debited by this instruction
 * someone would have to issue the transactions
-
-### Account scans on every epoch
-
-Scanning the entire Bank for accounts that owe rent at the beginning of each epoch was considered. This would have been an expensive operation, and would require that the entire current state of the network be present on every validator at the beginning of each epoch.
-