The Storage Layer

The Storage layer is responsible of storing the blocks on behalf of the Consensus layer. It is also involved in serving the data for Chain diffusion.

Some of this data is volatile and relates to the candidate chains and other data is immutable and relates to the historical chain, following the principle described in the k security parameter section.

flowchart TD
    A("Storage") -- Sequential Access --> B("Immutable Chain")
    A("Storage") -- Random Access --> C("Volatile Chain")
    A("Storage") -- Random Access --> D("Recent Leder States")

    subgraph noteA ["Hot - Efficient Rollback"]
        C
        D
    end
    subgraph noteB ["Cold - Efficient Storage"]
        B
    end

Any storage system designed for Cardano must meet certain requirements for the miniprotocols and the Consensus layer to function properly:

• Fast sequential access to immutable blocks: syncing peers request historical chain blocks sequentially,
• Fast sequential access to current chain-selection blocks in the volatile part of the chain: peers request this information during syncing and when caught-up,
• Fast switch to an alternative chain in the volatile part of the chain,
• Fast identification of chains of blocks in the volatile part of the chain: even if blocks arrive in arbitrary order, the Consensus Chain Selection should be invoked to select a better candidate chain once assembled in the volatile part of the chain,
• Fast node restart after shutdown without full chain replay, while supporting k-deep forks.

It is interesting to note that the storage layer does not need to provide the Durability in the ACID acronym: upstream peers will always be available to replace any blocks a node loses.

Chain diffusion

The mathematical model of the Ouroboros Consensus Protocols assumes instantaneous transmission (and validation) of chains, becoming instantly available even for newly joined peers. This is, even if just because of physical real-world limitations, infeasible in practice. For this reason, transmission of chains is done in a block-by-block basis instead. This allows chains to be incrementally sent to peers but comes with its own risks for newly joined peers (see Ouroboros Genesis).

Furthermore, blocks (which in the end is the data transmitted over the network) are subdivided in block headers and block bodies, as described in the Header|body split section.

The diffusion of data in the Cardano network is in fact a responsibility of the Networking layer. The diffusion of chains involves accessing the Storage layer and serving its contents to peers, however it is ultimately the Consensus layer the one that decides how the data in the Storage layer is mutated, as an outcome of Chain Selection.

Chain diffusion is a joint effort of the Consensus, Network and Storage layers.

Diffusion of chains is achieved by means of ChainSync and BlockFetch mini-protocols.

In a sense, the Storage layer has the data to provide the meaning of the messages in the mini-protocol whereas the networking layer describes the kinds of messages a protocol is composed by. It is possible to run an interaction of the protocol exchanging data that does not follow the intended semantics (for example an evil node sending all the chains it knows about instead of only the best selection).