Add design for the forking bank

book/art/forks-pruned.bob

@@ -0,0 +1,5 @@
+    2
+   /|
+  / |
+ 4  |
+    5

book/art/forks.bob

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+      1
+     / \
+    2   \
+   /|   |
+  / |   |
+ 4  |   |
+    5   /\
+       6  \
+           7

book/src/SUMMARY.md

@@ -30,6 +30,7 @@
   - [Blocktree](blocktree.md)
   - [Data Plane Fanout](data-plane-fanout.md)
   - [Reliable Vote Transmission](reliable-vote-transmission.md)
+  - [Bank Forks](bank-forks.md)
   - [Cluster Economics](ed_overview.md)
     - [Validation-client Economics](ed_validation_client_economics.md)
       - [State-validation Protocol-based Rewards](ed_vce_state_validation_protocol_based_rewards.md)

book/src/bank-forks.md

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+# Bank Fork
+
+This design describes a way to checkpoint the bank state such that it can track
+multiple forks without duplicating data.  It addresses the following
+challenges:
+
+* Forks are potentially created at every slot boundary.
+* Forks can be based on any previously produced block.
+* Forks are eventually finalized such that rollback is impossible.
+* Unreachable forks need to be pruned.
+
+## Architecture
+
+The basic design idea is to maintain a DAG of forks. The DAG is initialized with
+a *root*.  Each subsequent fork must descend from the root.
+
+## Active Forks
+
+An *active fork* is a direct list of connected forks that descend from the
+current root to a specific fork without any descendants.
+
+For example:
+
+<img alt="Forks" src="img/forks.svg" class="center"/>
+
+The following *active forks* are in the forks DAG
+
+* 4,2,1
+* 5,2,1
+* 6,1
+* 7,1
+
+## Merging into root
+
+A validator votes for a frozen fork.  The *active fork* connecting the fork
+to the root is merged.  If the *active fork* is longer than
+`Forks::ROLLBACK_DEPTH` the oldest two forks are merged.  The oldest fork in
+the *active fork* is the current root, so the second oldest is a direct
+descendant of the root fork.  Once merged, the current root is updated to the
+root descendant. Any forks that are not descendants from the new root are
+pruned since they are no longer reachable.
+
+For example:
+
+<img alt="Forks" src="img/forks.svg" class="center"/>
+
+* ROLLBACK\_DEPTH=2, vote=5, *active fork*={5,2,1}
+
+<img alt="Forks after pruning" src="img/forks-pruned.svg" class="center"/>
+
+The new root is 2, and any active forks that are not descendants from 2 are
+pruned.
+
+* ROLLBACK\_DEPTH=2, vote=6, *active fork*={6,1}
+
+<img alt="Forks" src="img/forks.svg" class="center"/>
+
+The tree remains with `root=1`, since the *active fork* starting at 6 is only 2
+forks long.