Bankless leader proposal

0005-bankless-leader.md

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+---
+simd: '0005'
+title: Bankless Leader
+authors:
+  - Tao Zhu (Solana Labs)
+category: Standard
+type: Core
+status: Draft
+created: 2022-12-08
+feature: (fill in with feature tracking issues once accepted)
+---
+
+## Summary
+
+A bankless leader does the minimum amount of work to produce a valid block.
+The leader is tasked with ingress transactions, sorting and filtering valid
+transactions, arranging them into entries, shredding the entries and
+broadcasting the shreds. While a validator only needs to reassemble the block
+and replay execution of well formed entries. 
+
+## Motivation
+
+Without having to execute transactions, leader threads will have more cycles
+to pull packets from receiving buffer, and processing more packets per slot.
+
+Feeding more packets to filtering and sorting logic allows higher prioritized
+transactions to have higher chance be added to the top of queue, therefore have
+priority to be considered to block inclusion.
+
+It also ingests more non-conflicting transactions to be packed into block,
+therefore improve block quality.
+
+Combine with scheduler logic, it'd produce entries that are parallelizable,
+potential simplify replay and reduce time.  
+
+## Detailed Design
+
+Normal bank operation for a spend needs to do 2 loads and 2 stores. With this
+design leader just does 1 load. So 4x less account_db work before generating the
+block. The store operations are likely to be more expensive than reads.
+
+When replay stage starts processing the same transactions, it can assume that
+PoH is valid, and that all the entries are safe for parallel execution. The fee
+accounts that have been loaded to produce the block are likely to still be in
+memory, so the additional load should be warm and the cost is likely to be
+amortized.
+
+### Fee Account
+
+The [fee account](../terminology.md#fee_account) pays for the transaction to be
+included in the block. The leader only needs to validate that the fee account
+has the balance to pay for the fee.
+
+### Balance Cache
+
+For the duration of the leaders consecutive blocks, the leader maintains a
+temporary balance cache for all the processed fee accounts. The cache is a map
+of pubkeys to lamports.
+
+At the start of the first block the balance cache is empty. At the end of the
+last block the cache is destroyed.
+
+The balance cache lookups must reference the same base fork for the entire
+duration of the cache. At the block boundary, the cache can be reset along with
+the base fork after replay stage finishes verifying the previous block.
+
+### Balance Check
+
+Prior to the balance check, the leader validates all the signatures in the
+transaction.
+
+1. Verify the accounts are not in use and BlockHash is valid.
+2. Check if the fee account is present in the cache, or load the account from
+   accounts_db and store the lamport balance in the cache.
+3. If the balance is less than the fee, drop the transaction.
+4. Subtract the fee from the balance.
+5. If account is called by system program, or it is passed to a instruction
+   with system program, then reduce its balance to 0 in the cache.
+
+### Leader Replay
+
+Leaders will need to replay their blocks as part of the standard replay stage
+operation.
+
+### Leader Replay With Consecutive Blocks
+
+A leader can be scheduled to produce multiple blocks in a row. In that scenario
+the leader is likely to be producing the next block while the replay stage for
+the first block is playing.
+
+When the leader finishes the replay stage it can reset the balance cache by
+clearing it, and set a new fork as the base for the cache which can become
+active on the next block.
+
+### Resetting the Balance Cache
+
+1. At the start of the block, if the balance cache is uninitialized, set the
+   base fork for the balance cache to be the parent of the block and create an
+empty cache.
+2. if the cache is initialized, check if block's parents has a new frozen bank
+   that is newer than the current base fork for the balance cache.
+3. if a parent newer than the cache's base fork exist, reset the cache to the
+   parent.
+
+### Impact on Clients
+
+The same fee account can be reused many times in the same block until it is
+called by system program, or it is passed to a instruction alongside system
+program.
+
+Clients that transmit a large number of transactions per second should use a
+dedicated fee account.
+
+Once an account fee is used by system program, it will fail the balance check
+until the balance cache is reset. 
+
+
+## Security Considerations
+
+What security implications/considerations come with implementing this feature?
+Are there any implementation-specific guidance or pitfalls?
+
+## Drawbacks
+
+There will be failed transactions in block;
+