State rent

This article discusses state rent, in particular, state rent with asynchronous garbage collection. Compared with traditional state rent with deterministic garbage collection, asynchrony allows more predictable block processing performance estimation, and freer node implementations.

Special thanks

The earliest state rent work was done by Alexey Akhunov, which highly inspired this article.

State rent with asynchronous garbage collection

Consider a simple account-based blockchain like Ethereum or Substrate. The world state consists of accounts. Accounts consist of some basic information such as balance, nonce, code, together with an account storage. Over lifetime of a blockchain, the collection of accounts can become huge, while a lot of them remain unused. The goal of state rent is to recycle storage space occupied by unused accounts.

Some state rent systems, like the one proposed in Ethereum by Alexey Akhunov, uses a deterministic garbage collection routine. Accounts pay a certain amount of rent to exist for a period of time. This causes the account balances to gradually reduce. If the account balance is too low to pay the rent, it is marked to be evicted. An operational transaction is then due to deterministically evict the account out of the world state. Determinstic garbage collection presents some challenges:

  • Accounts constantly need to pay rent. While the number of times dues happen can be optimized (to only pay it when an account is updated), it still causes unnecessary state updates.

  • Eviction is costly. All accounts that need to be evicted must be done so deterministically. While we can limit the number of evictions per block, this can still cause massive merkle tree updates.

State rent with asynchronous garbage collection aims at solving those challenges.

Protocol Rule

Asynchronous garbage collection requires an append-only merkle tree. Existing accounts can be modified. However, new accounts can only be appended at the end of the merkle tree. Any index-based merkle tree is suitable for this purpose, but not key-value-based merkle tree. Below, we use index-based binary merkle tree to demostrate asynchronous garbage collection, but note that the same system can be used for hex merkle tree.

The protocol rules that constraint block processing is as follows:

  • Each account has a keep-alive period. Pre-defined operations on a blockchain can extend an account’s keep-alive period.

  • If an account is accessed after its keep-alive period, the transaction that accessed it must be accompanied with witness proof of the account, with reinitiation fee and operations to reinitiate the corresponding storage values. Otherwise, the whole transaction fails with no state changes, other than consuming all gases.

We don’t explictly define garbage collection within protocol rules, but it enables the rules to be implemented in a node implementation.

Garbage collection

Upon an account reaches its end of the keep-alive period, a node implementation can safely (but optionally) evict it. It does so by removing the corresponding merkle node in the database, but no merkle hash change is required. Consider the following merkle tree, where we have a world state of 4 accounts D, E, F, G:

   /   \
   B   C
  / \ / \
  D E F G

When D is evicted, no garbage collection is possible. Then, when E is evicted, merkle node D and E can be removed from database, leaving only B. No operation is able to modify evicted accounts and the merkle tree is append-only for new accounts, so B is fixed and will never be modified, unless D or E is reinitiated.

If the node has not invoked garbage collection, then the node can use the account information to check keep-alive period. If an access reaches an evicted node, then it knows that it must have passed the keep-alive period, resulting in revertion of the transaction.