# RFC-0190/Mempool

Maintainer(s): Stanley Bondi, SW van Heerden and Yuko Roodt

## The Mempool for Unconfirmed Transactions on the Tari Base Layer

Copyright 2018 The Tari Development Community

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## Language

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 (covering RFC2119 and RFC8174) when, and only when, they appear in all capitals, as shown here.

## Disclaimer

The purpose of this document and its content is for information purposes only and may be subject to change or update without notice.

This document may include preliminary concepts that may or may not be in the process of being developed by the Tari community. The release of this document is intended solely for review and discussion by the community regarding the technological merits of the potential system outlined herein.

## Goals

This document will introduce the Tari base layer Mempool that consists of an Unconfirmed Pool and Reorg Pool. The Mempool is used for storing and managing unconfirmed transactions.

## Description

### Assumptions

• Each base node is connected to a number of peers that maintain their own copies of the Mempool.

### Abstract

The Mempool is responsible for managing, verifying and maintaining all unconfirmed transactions that have not yet been included in a block and added to the Tari blockchain. It is also responsible for propagating valid transactions and sharing the Mempool state with connected peers. An overview of the required functionality for the Mempool and each of its component pools will be provided.

### Overview

Every base node maintains a transaction pool called Mempool that consists of two separate pools: the Unconfirmed Pool and Reorg Pool. These two pools have different tasks and work together to form the Mempool used for maintaining unconfirmed transactions.

This is the role descriptions for each component pool:

• Unconfirmed Pool: contains all unconfirmed transactions that have been verified, have passed all checks, that only spend valid UTXOs and don't have any time-lock restrictions.
• Reorg Pool: stores a backup of all transactions that have recently been included into blocks, in case a blockchain reorganization occurs and these transactions have to be restored back to the Unconfirmed Pool, so that they can be included in future blocks.

### Prioritizing Unconfirmed Transactions

The maximum storage capacity used for storing unconfirmed transactions by the Mempool and each of its component pools can be configured. When a new transaction is received and the storage capacity limit is reached, then transactions are prioritized by ordering their total fee per gram of all UTXOs used and transaction age, in that order. The transactions of the least priority are discarded to make room for higher priority transactions.

The transaction priority metric has the following behavior:

• transactions with higher fee per gram SHOULD be prioritized over lower fee per gram transactions.
• older transactions in the mempool SHOULD be prioritized over newer ones.

### Memory Pool State: Syncing and Updating

On the initial startup, the complete state of the unconfirmed pool is pulled from the connected peers. Typically, Mempool doesn't persist its state, but can be configured to do so. If the state is locally present, then only the missing, unconfirmed transactions are synced from the peers, otherwise, the full state is requested. The validity and priority of transactions are computed as they are being downloaded from the connected peers. If the base node undergoes a re-org, then the missing state is again pulled from the peers.

The functional bahavior required for the Mempool's synchronization are the following:

• All verified transactions MUST be propagated to neighboring peers.
• Unverified or invalid transactions MUST NOT be propagated to peers.
• Verified transactions that were discarded due to low priority levels MUST be propagated to peers.
• Duplicate transactions MUST NOT be propagated to peers.
• Mempool MUST have an interface, allowing peers to query and download its state, partially and in full.
• Mempool MUST accept all transactions received from peers but MAY decide to discard low-priority transactions.
• Mempool MUST allow wallets to track payments by monitoring that a particular transaction has been added to the Mempool.
• Mempool MAY choose:
• to discard its state on restart and then download the full state from its peers or
• to store its state using persistent storage to reduce communication bandwidth required when reinitializing the pool after a restart.

### Unconfirmed Pool

This Mempool component consists of transactions that have been received, verified and have passed all the checks, but not yet included in the blocks. These transactions are ready to be used to construct new blocks for the Tari blockchain.

Functional behavior required of the Unconfirmed Pool:

• It MUST verify that incoming transactions spend only existing UTXOs.
• It MUST ensure that incoming transactions don't have a processing time-lock or has a time-lock that has expired.
• It MUST ensure that all time-locks of the UTXOs that will be spent by the transaction have expired.
• Transactions that have been used to construct new blocks MUST be removed from the Unconfirmed Pool and added to the Reorg Pool.

### Reorg Pool

The Reorg Pool consists of transactions that have recently been added to blocks, resulting in their removal from the Unconfirmed Pool. When a potential blockchain reorganization occurs that invalidates previously assembled blocks, the transactions used to construct these discarded blocks can be moved back into the Unconfirmed Pool. This ensures that high-priority transactions are not lost during reorganization and can be included in future blocks. The Reorg Pool is an internal, isolated Mempool component and cannot be accessed or queried from outside.

Functional behavior required of the Reorg Pool:

• Copies of the transactions used recently in blocks MUST be stored in the Reorg Pool.
• Transactions in the Reorg Pool MAY be discarded after a set expiration threshold has been reached.
• When reorganization is detected, all affected transactions found in the Reorg Pool MUST be moved back to the Unconfirmed Pool and removed from the Reorg Pool.

### Mempool

The Mempool is responsible for the internal transaction management and synchronization with the peers. New transactions must pass all verification steps to make it into the Unconfirmed Pool and further be propagated to peers.

Functional behavior required of the Mempool:

• If multiple transactions contain the same UTXO, then only the highest priority transaction MUST be kept and the rest (having the said UTXO included) MUST be discarded.
• If the storage capacity limit is reached, then new incoming transactions SHOULD be prioritized according to a set number of rules.
• Transactions of the least priority MUST be discarded to make room for higher priority incoming transactions.
• Transactions with its computed priority being lower than the minimum set threshold MUST be discarded.
• The Mempool MUST verify that incoming transactions do not have duplicate outputs.
• The Mempool MUST verify that only matured coinbase outputs can be spent.
• Each component pool MAY have the storage capacity configured and adjusted.
• The memory pool SHOULD have a mechanism to estimate fee categories from the current Mempool state. For example, the transaction fee can be estimated, ensuring that new transactions will be properly prioritized, to be added into new blocks in a timely manner.

Functional behavior required for the allocation of incoming transactions in the component pools:

• Verified transactions that have passed all checks such as spending of valid UTXOs and expired time-locks MUST be placed in the Unconfirmed Pool.
• Incoming transactions with time-locks, prohibiting them from being included in new blocks SHOULD be discarded.
• Newly received, verified transactions attempting to spend a UTXO that does not yet exist MUST be discarded.
• Transactions that have been added to blocks and were removed from the Unconfirmed Pool SHOULD be added to the Reorg Pool.