Endpoints to read from parent state

[adlrocha]

Related: consensus-shipyard/lotus#21

Background

An eudico node running in a child subnet will need to read state and listen to events from its parent's subnet for its operation. In order to decouple completely the child subnet from the specific implementation of its parent blockchain, the child subnet node relies on the IPC agent to pull the required information from its parent state. The IPC agent will have to provide a set of endpoints in its jsonrpc server to serve these requests to child subnet nodes.

Some of the operations that require reading state from the parent blockchain are:

• The consensus algorithm, that needs to track the validator set from its parent to determine when a reconfiguration needs to be triggered due to a change of membership.
• The validation of blocks including top-down cross-net messages. Whenever a block including cross-net message is proposed, one of the consensus checks performed by validators to determine if the blocks is valid is to check if the cross-net messages included in the block have been successfully accepted and finalized in the parent chain.

Implementation

Initially, we can implement this jsonrpc methods so that is Eudico the one that polls the IPC agent to pull information from the parent. Alternatively, we could consider adding also some endpoint in Eudico to support pushing information from the IPC agent to Eudico. This would require all subnet SMR systems to support this method in order to be able to run IPC.

[adlrocha]

This has been implemented, we are pending an end-to-end check to double-check that reading the membership information for Mir from the subnet actor in the parent works fine. We will close this issue then.

[jsoares]

I understand this is still pending the end-to-end check?

[adlrocha]

Oh, no. This is finished, and the integration works. It is used by subnet validator to mine new blocks.