doc: Improve TxGraph & co docs

Author: danielabrozzoni

crates/chain/src/chain_data.rs

@@ -147,6 +147,8 @@ impl From<(&u32, &BlockHash)> for BlockId {
 
 /// An [`Anchor`] implementation that also records the exact confirmation height of the transaction.
 ///
+/// Note that the confirmation block and the anchor block can be different here.
+///
 /// Refer to [`Anchor`] for more details.
 #[derive(Debug, Default, Clone, PartialEq, Eq, Copy, PartialOrd, Ord, core::hash::Hash)]
 #[cfg_attr(
@@ -186,6 +188,8 @@ impl AnchorFromBlockPosition for ConfirmationHeightAnchor {
 /// An [`Anchor`] implementation that also records the exact confirmation time and height of the
 /// transaction.
 ///
+/// Note that the confirmation block and the anchor block can be different here.
+///
 /// Refer to [`Anchor`] for more details.
 #[derive(Debug, Default, Clone, PartialEq, Eq, Copy, PartialOrd, Ord, core::hash::Hash)]
 #[cfg_attr(

crates/chain/src/chain_oracle.rs

@@ -3,7 +3,7 @@ use crate::BlockId;
 /// Represents a service that tracks the blockchain.
 ///
 /// The main method is [`is_block_in_chain`] which determines whether a given block of [`BlockId`]
-/// is an ancestor of another "static block".
+/// is an ancestor of the `chain_tip`.
 ///
 /// [`is_block_in_chain`]: Self::is_block_in_chain
 pub trait ChainOracle {

crates/chain/src/indexed_tx_graph.rs

@@ -1,7 +1,5 @@
-//! Contains the [`IndexedTxGraph`] structure and associated types.
-//!
-//! This is essentially a [`TxGraph`] combined with an indexer.
-
+//! Contains the [`IndexedTxGraph`] structure and associated types. Refer to the
+//! [`IndexedTxGraph`] documentation for more.
 use alloc::vec::Vec;
 use bitcoin::{Block, OutPoint, Transaction, TxOut, Txid};
 
@@ -160,7 +158,7 @@ where
     /// Batch insert unconfirmed transactions, filtering out those that are irrelevant.
     ///
     /// Relevancy is determined by the internal [`Indexer::is_tx_relevant`] implementation of `I`.
-    /// Irrelevant tansactions in `txs` will be ignored.
+    /// Irrelevant transactions in `txs` will be ignored.
     ///
     /// Items of `txs` are tuples containing the transaction and a *last seen* timestamp. The
     /// *last seen* communicates when the transaction is last seen in the mempool which is used for
@@ -223,7 +221,7 @@ where
     /// [`AnchorFromBlockPosition::from_block_position`].
     ///
     /// Relevancy is determined by the internal [`Indexer::is_tx_relevant`] implementation of `I`.
-    /// Irrelevant tansactions in `txs` will be ignored.
+    /// Irrelevant transactions in `txs` will be ignored.
     pub fn apply_block_relevant(
         &mut self,
         block: Block,

crates/chain/src/lib.rs

@@ -12,9 +12,8 @@
 //!    you do it synchronously or asynchronously. If you know a fact about the blockchain, you can just
 //!    tell `bdk_chain`'s APIs about it, and that information will be integrated, if it can be done
 //!    consistently.
-//! 2. Error-free APIs.
-//! 3. Data persistence agnostic -- `bdk_chain` does not care where you cache on-chain data, what you
-//!    cache or how you fetch it.
+//! 2. Data persistence agnostic -- `bdk_chain` does not care where you cache on-chain data, what you
+//!    cache or how you retrieve it from persistent storage.
 //!
 //! [Bitcoin Dev Kit]: https://bitcoindevkit.org/
 

crates/chain/src/tx_data_traits.rs

@@ -5,21 +5,25 @@ use alloc::vec::Vec;
 
 /// Trait that "anchors" blockchain data to a specific block of height and hash.
 ///
-/// [`Anchor`] implementations must be [`Ord`] by the anchor block's [`BlockId`] first.
-///
-/// I.e. If transaction A is anchored in block B, then if block B is in the best chain, we can
+/// If transaction A is anchored in block B, and block B is in the best chain, we can
 /// assume that transaction A is also confirmed in the best chain. This does not necessarily mean
 /// that transaction A is confirmed in block B. It could also mean transaction A is confirmed in a
 /// parent block of B.
 ///
+/// Every [`Anchor`] implementation must contain a [`BlockId`] parameter, and must implement
+/// [`Ord`]. When implementing [`Ord`], the anchors' [`BlockId`]s should take precedence
+/// over other elements inside the [`Anchor`]s for comparison purposes, i.e., you should first
+/// compare the anchors' [`BlockId`]s and then care about the rest.
+///
+/// The example shows different types of anchors:
 /// ```
 /// # use bdk_chain::local_chain::LocalChain;
 /// # use bdk_chain::tx_graph::TxGraph;
 /// # use bdk_chain::BlockId;
 /// # use bdk_chain::ConfirmationHeightAnchor;
+/// # use bdk_chain::ConfirmationTimeAnchor;
 /// # use bdk_chain::example_utils::*;
 /// # use bitcoin::hashes::Hash;
-///
 /// // Initialize the local chain with two blocks.
 /// let chain = LocalChain::from_blocks(
 ///     [
@@ -47,6 +51,7 @@ use alloc::vec::Vec;
 /// );
 ///
 /// // Insert `tx` into a `TxGraph` that uses `ConfirmationHeightAnchor` as the anchor type.
+/// // This anchor records the anchor block and the confirmation height of the transaction.
 /// // When a transaction is anchored with `ConfirmationHeightAnchor`, the anchor block and
 /// // confirmation block can be different. However, the confirmation block cannot be higher than
 /// // the anchor block and both blocks must be in the same chain for the anchor to be valid.
@@ -62,6 +67,25 @@ use alloc::vec::Vec;
 ///         confirmation_height: 1,
 ///     },
 /// );
+///
+/// // Insert `tx` into a `TxGraph` that uses `ConfirmationTimeAnchor` as the anchor type.
+/// // This anchor records the anchor block, the confirmation height and time of the transaction.
+/// // When a transaction is anchored with `ConfirmationTimeAnchor`, the anchor block and
+/// // confirmation block can be different. However, the confirmation block cannot be higher than
+/// // the anchor block and both blocks must be in the same chain for the anchor to be valid.
+/// let mut graph_c = TxGraph::<ConfirmationTimeAnchor>::default();
+/// let _ = graph_c.insert_tx(tx.clone());
+/// graph_c.insert_anchor(
+///     tx.txid(),
+///     ConfirmationTimeAnchor {
+///         anchor_block: BlockId {
+///             height: 2,
+///             hash: Hash::hash("third".as_bytes()),
+///         },
+///         confirmation_height: 1,
+///         confirmation_time: 123,
+///     },
+/// );
 /// ```
 pub trait Anchor: core::fmt::Debug + Clone + Eq + PartialOrd + Ord + core::hash::Hash {
     /// Returns the [`BlockId`] that the associated blockchain data is "anchored" in.

crates/chain/src/tx_graph.rs

@@ -1,12 +1,32 @@
 //! Module for structures that store and traverse transactions.
 //!
-//! [`TxGraph`] is a monotone structure that inserts transactions and indexes the spends. The
-//! [`ChangeSet`] structure reports changes of [`TxGraph`] but can also be applied to a
-//! [`TxGraph`] as well. Lastly, [`TxDescendants`] is an [`Iterator`] that traverses descendants of
-//! a given transaction.
+//! [`TxGraph`] contains transactions and indexes them so you can easily traverse the graph of those transactions.
+//! `TxGraph` is *monotone* in that you can always insert a transaction -- it doesn't care whether that
+//! transaction is in the current best chain or whether it conflicts with any of the
+//! existing transactions or what order you insert the transactions. This means that you can always
+//! combine two [`TxGraph`]s together, without resulting in inconsistencies.
+//! Furthermore, there is currently no way to delete a transaction.
+//!
+//! Transactions can be either whole or partial (i.e., transactions for which we only
+//! know some outputs, which we usually call "floating outputs"; these are usually inserted
+//! using the [`insert_txout`] method.).
+//!
+//! The graph contains transactions in the form of [`TxNode`]s. Each node contains the
+//! txid, the transaction (whole or partial), the blocks it's anchored in (see the [`Anchor`]
+//! documentation for more details), and the timestamp of the last time we saw
+//! the transaction as unconfirmed.
 //!
 //! Conflicting transactions are allowed to coexist within a [`TxGraph`]. This is useful for
-//! identifying and traversing conflicts and descendants of a given transaction.
+//! identifying and traversing conflicts and descendants of a given transaction. Some [`TxGraph`]
+//! methods only consider "canonical" (i.e., in the best chain or in mempool) transactions,
+//! we decide which transactions are canonical based on anchors `last_seen_unconfirmed`;
+//! see the [`try_get_chain_position`] documentation for more details.
+//!
+//! The [`ChangeSet`] reports changes made to a [`TxGraph`]; it can be used to either save to
+//! persistent storage, or to be applied to another [`TxGraph`].
+//!
+//! Lastly, you can use [`TxAncestors`]/[`TxDescendants`] to traverse ancestors and descendants of
+//! a given transaction, respectively.
 //!
 //! # Applying changes
 //!
@@ -49,6 +69,8 @@
 //! let changeset = graph.apply_update(update);
 //! assert!(changeset.is_empty());
 //! ```
+//! [`try_get_chain_position`]: TxGraph::try_get_chain_position
+//! [`insert_txout`]: TxGraph::insert_txout
 
 use crate::{
     collections::*, keychain::Balance, local_chain::LocalChain, Anchor, Append, BlockId,
@@ -90,7 +112,7 @@ impl<A> Default for TxGraph<A> {
     }
 }
 
-/// An outward-facing view of a (transaction) node in the [`TxGraph`].
+/// A transaction node in the [`TxGraph`].
 #[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub struct TxNode<'a, T, A> {
     /// Txid of the transaction.
@@ -127,7 +149,7 @@ impl Default for TxNodeInternal {
     }
 }
 
-/// An outwards-facing view of a transaction that is part of the *best chain*'s history.
+/// A transaction that is included in the chain, or is still in mempool.
 #[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub struct CanonicalTx<'a, T, A> {
     /// How the transaction is observed as (confirmed or unconfirmed).
@@ -454,7 +476,7 @@ impl<A: Clone + Ord> TxGraph<A> {
     /// Batch insert unconfirmed transactions.
     ///
     /// Items of `txs` are tuples containing the transaction and a *last seen* timestamp. The
-    /// *last seen* communicates when the transaction is last seen in the mempool which is used for
+    /// *last seen* communicates when the transaction is last seen in mempool which is used for
     /// conflict-resolution (refer to [`TxGraph::insert_seen_at`] for details).
     pub fn batch_insert_unconfirmed(
         &mut self,
@@ -480,7 +502,7 @@ impl<A: Clone + Ord> TxGraph<A> {
 
     /// Inserts the given `seen_at` for `txid` into [`TxGraph`].
     ///
-    /// Note that [`TxGraph`] only keeps track of the lastest `seen_at`.
+    /// Note that [`TxGraph`] only keeps track of the latest `seen_at`.
     pub fn insert_seen_at(&mut self, txid: Txid, seen_at: u64) -> ChangeSet<A> {
         let mut update = Self::default();
         let (_, _, update_last_seen) = update.txs.entry(txid).or_default();
@@ -687,8 +709,23 @@ impl<A: Anchor> TxGraph<A> {
 
     /// Get the position of the transaction in `chain` with tip `chain_tip`.
     ///
-    /// If the given transaction of `txid` does not exist in the chain of `chain_tip`, `None` is
-    /// returned.
+    /// If the given `txid` does not exist in the chain of `chain_tip`, and
+    /// we believe that the transaction is not in mempool anymore, `None` is returned.
+    ///
+    /// There are two factors that we use to approximate whether an unconfirmed
+    /// transaction currently exists in mempool:
+    /// 1. Conflicts with confirmed txs. If the transaction `tx` or any of its unconfirmed
+    /// ancestors conflict with a confirmed transaction, then `tx` can't be in mempool.
+    /// 2. The `last_seen_unconfirmed` parameter. Given two conflicting transactions, we say
+    /// that the one with a higher `last_seen_unconfirmed` (i.e., we saw it later) has a higher
+    /// chance of still being in mempool. When trying to figure out if `tx`
+    /// is still in mempool, we first of all calculate `tx`'s `max_last_seen_unconfirmed`,
+    /// which is the max `last_seen_unconfirmed` between `tx` and all its descendants.
+    /// We then look at all the conflicts of `tx`, and if for all of them
+    /// `last_seen_unconfirmed` < `max_last_seen_unconfirmed` holds,
+    /// then we consider `tx` to be still in mempool.
+    /// 3. In case two transactions have the same `last_seen_unconfirmed` parameter, we compare
+    /// their txids by lexicographical order, and arbitrarily pick the one with the higher txid.
     ///
     /// # Error
     ///
@@ -714,7 +751,7 @@ impl<A: Anchor> TxGraph<A> {
             }
         }
 
-        // The tx is not anchored to a block which is in the best chain, which means that it
+        // The tx is not anchored to a block in the best chain, which means that it
         // might be in mempool, or it might have been dropped already.
         // Let's check conflicts to find out!
         let tx = match tx_node {
@@ -917,7 +954,8 @@ impl<A: Anchor> TxGraph<A> {
     /// (`OI`) for convenience. If `OI` is not necessary, the caller can use `()`, or
     /// [`Iterator::enumerate`] over a list of [`OutPoint`]s.
     ///
-    /// Floating outputs are ignored.
+    /// Floating outputs (i.e., outputs for which we don't have the full transaction in the graph)
+    /// are ignored.
     ///
     /// # Error
     ///
@@ -1244,7 +1282,7 @@ impl<A> AsRef<TxGraph<A>> for TxGraph<A> {
 ///
 /// The iterator excludes partial transactions.
 ///
-/// This `struct` is created by the [`walk_ancestors`] method of [`TxGraph`].
+/// This struct is created by the [`walk_ancestors`] method of [`TxGraph`].
 ///
 /// [`walk_ancestors`]: TxGraph::walk_ancestors
 pub struct TxAncestors<'g, A, F> {

crates/chain/tests/test_tx_graph_conflicts.rs

@@ -70,6 +70,7 @@ fn test_tx_conflict_handling() {
                     ..Default::default()
                 },
             ],
+            // the txgraph is going to pick tx_conflict_2 because of higher lexicographical txid
             exp_chain_txs: HashSet::from(["tx1", "tx_conflict_2"]),
             exp_chain_txouts: HashSet::from([("tx1", 0), ("tx_conflict_2", 0)]),
             exp_unspents: HashSet::from([("tx_conflict_2", 0)]),