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Optimism Deep Dive

The new implementation is event-driven which causes the code not readable. This document is to help understand the codebase and the flow of the code.

[TOC]

Rollup Node

Driver

The Driver is the entry point of derivation. The core function is eventLoop which is a loop to handle channels, the external inputs will pass through the channels. Once any channel is ready, the eventLoop will call the corresponding function to handle the input.

The inputs are actually L1 blocks and L2 unsafe blocks.

  • OnNewL1Head is to subscribing the latest L1 header.
  • OnNewL1Safe is to polling the latest L1 safe block.
  • OnNewL1Finalized is to polling the latest L1 finalized block.
  • OnUnsafeL2Payload is to handle the unsafe L2 block.

Driver use channels to communicate with other components. However, the internal components are communicating with events. There are several internal drivers to handle different functionalities.

Derivation

The derivation is the core driver to handle the L1 blocks to derive the L2 blocks. It is a pipeline which consists of several stages. Each stage is a function to handle the L1 block and derive the L2 block.

How to trigger the pipeline

When PipelineDeriver receives the PipelineStepEvent, it will call the Step function to trigger the pipeline. But how the PipelineStepEvent is emitted? We go back to the eventLoop function of the Driver to find the answer.

  • There is a reqStep function to request a derivation step. It will first emit the StepReqEvent, through the StepSchedulingDeriver to transform the request to the StepAttemptEvent, and eventually to the StepEvent. The StepSchedulingDeriver will schedule the step request based on the backoff strategy.
  • The StepEvent will be handled by the SyncDeriver which responsible for the synchronization of the opNode and execution engine status which are header, safe, finalized block. It will call SyncStep function, first drain the remaining events, then synchronization with execution engine. If execution engine is ready, it will emit the PendingSafeRequestEvent which notify AttributesHandler to handle. Otherwise, it will trigger ResetStepBackoffEvent and some error events will be triggered. SyncDeriver will handle the error events and trigger the StepReqEvent again.
  • The EngDeriver will transform the PendingSafeRequestEvent to the PendingSafeUpdateEvent, AttributesHandler will handle the PendingSafeUpdateEvent and when no attributes are handling, it will emit PipelineStepEvent.

Stages

L1Traversal

TODO:

Recovery

Sequencer Outages

https://docs.optimism.io/stack/protocol/outages

dc discussion

Key point

  • There is no depositTx in batch, op-batcher will filter deposit txs before submitting to L1

  • Both verifier and sequencer get deposit txs from L1 contract.

  • If sequencer downtime is enough long, the verifier will force produce empty batch, and add deposit txs to generate L2 block

  • When sequencer is backup from a long downtime(verifier already create empty batch), it will also catch up verifier first(create empty batch), after that, it can sequence new batch again. If it create new batch before catch up verifier, the verifier will drop these batches.