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 Transactions are also periodically verified on every finalized event and removed from the mempool if no longer valid.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.