How do ExExes work?
ExExes are just Futures that run indefinitely alongside Reth – as simple as that.
An ExEx is usually driven by and acts on new notifications about chain commits, reverts, and reorgs, but it can span beyond that.
They are installed into the node by using the node builder. Reth manages the lifecycle of all ExExes, including:
- Polling ExEx futures
- Sending notifications about new chain, reverts, and reorgs from historical and live sync
- Processing events emitted by ExExes
- Pruning (in case of a full or pruned node) only the data that have been processed by all ExExes
- Shutting ExExes down when the node is shut down
Pruning
Pruning deserves a special mention here.
ExExes SHOULD emit an ExExEvent::FinishedHeight
event to signify what blocks have been processed. This event is used by Reth to determine what state can be pruned.
An ExEx will only receive notifications for block numbers greater than the block in the most recently emitted FinishedHeight event.
To clarify: if an ExEx emits ExExEvent::FinishedHeight(0) it will receive notifications for any block_number > 0.