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fork-aware transaction pool added #4639
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Across the codebase a following pattern is repeated over and over:
e.g. here: polkadot-sdk/substrate/client/service/src/builder.rs Lines 784 to 812 in d31bb8a
and polkadot-sdk/substrate/client/service/src/builder.rs Lines 815 to 841 in d31bb8a
Introducion of dynamic dispatching for transaction pool may require touching of all these statements, depending on choosen approach. I tried three approaches with using transaction pool trait object. (1) Relaxing Sized constraint in generic params.Basically type used to refer to the transaction pool implementation is
The generic parameter in
is by default constrained by
This was done initially and rejected. Please note that this approach is backward compatible and does not require to change type of field referencing transaction pool instance ( (2) Using wrapperThe type is used to reference the tranaction pool instance is:
This approach does not require any changes in generic parameters or constrains used across code base. (3) Using ArcThe type used to reference the tranaction pool instance is:
As we don't want to have
nees to be converted into:
Additionally, as dynamic upcast conversion is not yet suported in stable rust, we need to add methods that allows to do that. We need a method of converting (4) Getting rid of pool generic parameterThere is one more option:
to
but the problem is that TransactionPool related traits are either generic or have associated types. Which means we need to extend those structurs what make this a quite heavy refactor of many module. (3) seems more elegant for me, but requires many changes (also rebasing becomes more annoying as many files are touched). |
IMO we should pick the most pragmatic solution that gets this merged. That seems to be (2) in my book. Wrapper might not be beautiful, but changes are minimal and the PR is quite big already, we should not add another refactoring on top. All the other proposed options are still possible later. |
I really don't really see the need for the generic parameter here. Just makes it more complicated and without the generic parameter the code should be easier. I would favor this. But if you say 2 is faster for now, pick this one and then later do 4. |
Filed here: #5489 |
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
module. Use:Quick overview (documentation excerpt)
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:
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.Integration notes
For teams having a custom node, the new txpool need to be instantiated, typically in
service.rs
file, here is example:polkadot-sdk/cumulus/polkadot-parachain/polkadot-parachain-lib/src/common/spec.rs
Lines 159 to 168 in 8be61ec
To enable new transaction pool the following
--pool-type=fork-aware
. There shall be information printed in the log:For debugging the following debugs shall be enabled:
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:
fatxpool
: Master issue for upcoming work #5472Partially fixes: #1202