Specification of light-mode

[HeinrichApfelmus]

Issue number

ADP-1370

Overview

This pull request adds a specification (DRAFT) for the cardano-wallet light-mode.

See also PR #3102 for the prototype that explores the feasibility of this specification and implements some parts of it for demonstration purposes.

[rvl]

It's great so far, thanks @HeinrichApfelmus

[rvl]

Great!

I've suggested a couple diagrams. If you don't have time, I could do them.

Could we move the document into docs/design/specs/light-mode.md? Then it will be published. It doesn't matter if it's still a DRAFT - as long as the status is shown.

[rvl]

And here is another gratuitous picture opportunity.
Could we please have two block diagrams - full-node and light-mode?
Like a cut-down version of the diagram here: https://input-output-hk.github.io/cardano-wallet/developers/Architecture
In the "full-node" version, wallet links to node in 4 ways: chainsync, localblockfetch, localstatequery, localtxsubmission.
In the "light-mode" version, wallet requests a list of transactions by address from the block service.

[HeinrichApfelmus]

Ok!

Note: We do not use BlockFetch, but ChainSync to receive the blocks from the node. This is line with the network specification, which recommends that

The (full)-node to client protocol uses an instance of the chain synchronisation protocol that transfers full blocks, while the node-to-node instance only transfers block headers. In the node-to-node case, the block fetch protocol (Section 3.8) is used to transfer full blocks.