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.