LedgerDB

Internal newtype wrapper around a ledger state l so that we can define a non-blanket Anchorable instance.

Internal state of the ledger DB

The ledger DB looks like

anchor |> snapshots <| current

where anchor records the oldest known snapshot and current the most recent. The anchor is the oldest point we can roll back to.

We take a snapshot after each block is applied and keep in memory a window of the last k snapshots. We have verified empirically (#1936) that the overhead of keeping k snapshots in memory is small, i.e., about 5% compared to keeping a snapshot every 100 blocks. This is thanks to sharing between consecutive snapshots.

As an example, suppose we have k = 6. The ledger DB grows as illustrated below, where we indicate the anchor number of blocks, the stored snapshots, and the current ledger.

anchor |> #   [ snapshots ]                   <| tip
---------------------------------------------------------------------------
G      |> (0) [ ]                             <| G
G      |> (1) [ L1]                           <| L1
G      |> (2) [ L1,  L2]                      <| L2
G      |> (3) [ L1,  L2,  L3]                 <| L3
G      |> (4) [ L1,  L2,  L3,  L4]            <| L4
G      |> (5) [ L1,  L2,  L3,  L4,  L5]       <| L5
G      |> (6) [ L1,  L2,  L3,  L4,  L5,  L6]  <| L6
L1     |> (6) [ L2,  L3,  L4,  L5,  L6,  L7]  <| L7
L2     |> (6) [ L3,  L4,  L5,  L6,  L7,  L8]  <| L8
L3     |> (6) [ L4,  L5,  L6,  L7,  L8,  L9]  <| L9   (*)
L4     |> (6) [ L5,  L6,  L7,  L8,  L9,  L10] <| L10
L5     |> (6) [*L6,  L7,  L8,  L9,  L10, L11] <| L11
L6     |> (6) [ L7,  L8,  L9,  L10, L11, L12] <| L12
L7     |> (6) [ L8,  L9,  L10, L12, L12, L13] <| L13
L8     |> (6) [ L9,  L10, L12, L12, L13, L14] <| L14

The ledger DB must guarantee that at all times we are able to roll back k blocks. For example, if we are on line (*), and roll back 6 blocks, we get

L3 |> []