Subtrees

Each entry can contain multiple subtrees (e.g., "messages", "_settings"). Subtrees maintain independent parent-child relationships within the DAG.

Subtree Root Entries

A subtree root is an entry that starts a named subtree:

• Contains a SubTreeNode for the subtree
• Has empty subtree parents ([])
• Still has normal main tree parents

Entry {
    tree: TreeNode {
        root: "tree_id",
        parents: ["main_parent_id"],  // Normal main tree parents
    },
    subtrees: [
        SubTreeNode {
            name: "messages",
            parents: [],  // Empty = subtree root
            data: "...",
        }
    ],
}

Subsequent entries reference previous subtree entries as parents:

SubTreeNode {
    name: "messages",
    parents: ["previous_messages_entry_id"],
    data: "...",
}

Automatic Parent Discovery

Transactions automatically determine subtree parents:

1. If using current database tips → get current subtree tips
2. If using custom parents → find subtree tips reachable from those parents
3. If first subtree entry → empty tips (creates subtree root)

Always use transactions for entry creation - they handle parent discovery automatically.

Subtree Heights

Each subtree can have its own height value. Heights provide deterministic ordering when merging DAG branches after network splits. Entries are processed in height order, with ties broken by entry hash:

• Height = None: Subtree inherits the tree (main database) height
• Height = Some(h): Subtree has an independent height value

The Entry.subtree_height() accessor handles inheritance transparently:

// If subtree.height is None, returns tree.height (inheritance)
// If subtree.height is Some(h), returns the independent value
let height = entry.subtree_height("messages")?;

Per-subtree height strategies are configured via the _index registry. System subtrees (_settings, _index, _root) always inherit from the tree.

See Height Strategy Design for implementation details.

Implementation

See src/entry/mod.rs and src/transaction/mod.rs for implementation.