Commits · 26c11fc542310ef6841d8382418a319502de78a3 · parity / Mirrored projects / polkadot-sdk

Oct 15, 2024

fork-aware transaction pool added (#4639) · 26c11fc5

### Fork-Aware Transaction Pool Implementation

This PR introduces a fork-aware transaction pool (fatxpool) enhancing
transaction management by maintaining the valid state of txpool for
different forks.

### High-level overview
The high level overview was added to
[`sc_transaction_pool::fork_aware_txpool`](https://github.com/paritytech/polkadot-sdk/blob/3ad0a1b7/substrate/client/transaction-pool/src/fork_aware_txpool/mod.rs#L21)
module. Use:
```
cargo doc --document-private-items -p sc-transaction-pool --open
```
to build the doc. It should give a good overview and nice entry point
into the new pool's mechanics.

<details>
<summary>Quick overview (documentation excerpt)</summary>

#### View
For every fork, a view is created. The view is a persisted state of the
transaction pool computed and updated at the tip of the fork. The view
is built around the existing `ValidatedPool` structure.

A view is created on every new best block notification. To create a
view, one of the existing views is chosen and cloned.

When the chain progresses, the view is kept in the cache
(`retracted_views`) to allow building blocks upon intermediary blocks in
the fork.

The views are deleted on finalization: views lower than the finalized
block are removed.

The views are updated with the transactions from the mempool—all
transactions are sent to the newly created views.
A maintain process is also executed for the newly created
views—basically resubmitting and pruning transactions from the
appropriate tree route.

##### View store
View store is the helper structure that acts as a container for all the
views. It provides some convenient methods.

##### Submitting transactions
Every transaction is submitted to every view at the tips of the forks.
Retracted views are not updated.
Every transaction also goes into the mempool.

##### Internal mempool
Shortly, the main purpose of an internal mempool is to prevent a
transaction from being lost. That could happen when a transaction is
invalid on one fork and could be valid on another. It also allows the
txpool to accept transactions when no blocks have been reported yet.

The mempool removes its transactions when they get finalized.
Transactions are also periodically verified on every finalized event and
removed from the mempool if no longer valid.

#### Events
Transaction events from multiple views are merged and filtered to avoid
duplicated events.
`Ready` / `Future` / `Inblock` events are originated in the Views and
are de-duplicated and forwarded to external listeners.
`Finalized` events are originated in fork-aware-txpool logic.
`Invalid` events requires special care and can be originated in both
view and fork-aware-txpool logic.

#### Light maintain
Sometime transaction pool does not have enough time to prepare fully
maintained view with all retracted transactions being revalidated. To
avoid providing empty ready transaction set to block builder (what would
result in empty block) the light maintain was implemented. It simply
removes the imported transactions from ready iterator.

#### Revalidation
Revalidation is performed for every view. The revalidation process is
started after a trigger is executed. The revalidation work is terminated
just after a new best block / finalized event is notified to the
transaction pool.
The revalidation result is applied to the newly created view which is
built upon the revalidated view.

Additionally, parts of the mempool are also revalidated to make sure
that no transactions are stuck in the mempool.

#### Logs
The most important log allowing to understand the state of the txpool
is:
```
maintain: txs:(0, 92) views:[2;[(327, 76, 0), (326, 68, 0)]] event:Finalized { hash: 0x8...f, tree_route: [] } took:3.463522ms
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
unwatched txs in mempool ────┘ │ │ │ │ │ │ │ │ │ │
watched txs in mempool ───────┘ │ │ │ │ │ │ │ │ │
views ───────────────┘ │ │ │ │ │ │ │ │
1st view block # ──────────┘ │ │ │ │ │ │ │
number of ready tx ───────┘ │ │ │ │ │ │
numer of future tx ─────┘ │ │ │ │ │
2nd view block # ──────┘ │ │ │ │
number of ready tx ──────────┘ │ │ │
number of future tx ───────┘ │ │
event ────────┘ │
duration ──────────────────────────────────────────────────┘
```
It is logged after the maintenance is done.

The `debug` level enables per-transaction logging, allowing to keep
track of all transaction-related actions that happened in txpool.
</details>

### Integration notes

For teams having a custom node, the new txpool needs to be instantiated,
typically in `service.rs` file, here is an example:

https://github.com/paritytech/polkadot-sdk/blob/9c547ff3

/cumulus/polkadot-omni-node/lib/src/common/spec.rs#L152-L161

To enable new transaction pool the following cli arg shall be specified:
`--pool-type=fork-aware`. If it works, there shall be information
printed in the log:
```
2024-09-20 21:28:17.528 INFO main txpool: [Parachain] creating ForkAware txpool.
````

For debugging the following debugs shall be enabled:
```
"-lbasic-authorship=debug",
"-ltxpool=debug",
```
*note:* trace for txpool enables per-transaction logging.

### Future work
The current implementation seems to be stable, however further
improvements are required.
Here is the umbrella issue for future work:
- https://github.com/paritytech/polkadot-sdk/issues/5472

Partially fixes: #1202

---------

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>
Co-authored-by: Iulian Barbu <14218860+iulianbarbu@users.noreply.github.com>