docs(wallet): Improve docs on ChangeSet persistence requirements

crates/wallet/src/wallet/changeset.rs

@@ -2,12 +2,105 @@ use bdk_chain::{
     indexed_tx_graph, keychain_txout, local_chain, tx_graph, ConfirmationBlockTime, Merge,
 };
 use miniscript::{Descriptor, DescriptorPublicKey};
+use serde::{Deserialize, Serialize};
 
 type IndexedTxGraphChangeSet =
     indexed_tx_graph::ChangeSet<ConfirmationBlockTime, keychain_txout::ChangeSet>;
 
-/// A changeset for [`Wallet`](crate::Wallet).
-#[derive(Default, Debug, Clone, PartialEq, serde::Deserialize, serde::Serialize)]
+/// A change set for [`Wallet`]
+///
+/// ## Definition
+///
+/// The change set is responsible for transmiting data between the persistent storage layer and the
+/// core library components. Specifically, it serves two primary functions:
+///
+/// 1) Recording incremental changes to the in-memory representation that need to be persisted
+///     to disk
+/// 2) Applying aggregate changes from the persistence layer to the in-memory representation at
+///     startup
+///
+/// ## Contract
+///
+/// The change set maintains and enforces the following properties:
+///
+/// * Change sets must implement [`Serialize`] and [`Deserialize`] to meet the definition from
+///     above.
+/// * Change sets must implement [`Default`] as a way of instantiating new empty objects.
+/// * Change sets must implement [`Merge`] so that many instances can be aggregated into a single
+///     instance.
+/// * A change set is composed of a number of individual "sub-change sets" that adhere to the same
+///     rules as above. This is for increased modularity and portability. For example the core
+///     modules each have their own change set (`tx_graph`, `local_chain`, etc).
+///
+/// ## Members and required fields
+///
+/// The change set has certain required fields without which a `Wallet` cannot function.
+/// These include the [`descriptor`] and the [`network`] in use. These are required to be non-empty
+/// *in the aggregate*, meaning the field must be present and non-null in the union of all
+/// persisted changes, but may be empty in any one change set, where "empty" is defined by the
+/// [`Merge`](Merge::is_empty) implementation of that change set. This requirement also applies to
+/// the [`local_chain`] field in that the aggregate change set must include a genesis block.
+///
+/// For example, the descriptor and network are present in the first change set after wallet
+/// creation, but are usually omitted in subsequent updates, as they are not permitted to change
+/// at any point thereafter.
+///
+/// Other fields of the change set are not required to be non-empty, that is they may be empty even
+/// in the aggregate. However in practice they should contain the data needed to recover a wallet
+/// state between sessions. These include:
+/// * [`tx_graph`](Self::tx_graph)
+/// * [`indexer`](Self::indexer)
+///
+/// The [`change_descriptor`] is special in that its presence is optional, however the value of the
+/// change descriptor should be defined at wallet creation time and respected for the life of the
+/// wallet, meaning that if a change descriptor is originally defined, it must also be present in
+/// the aggregate change set.
+///
+/// ## Staging
+///
+/// For greater efficiency the [`Wallet`] is able to *stage* the to-be-persisted changes. Many
+/// operations result in staged changes which require persistence on the part of the user. These
+/// include address revelation, applying an [`Update`], and introducing transactions and chain
+/// data to the wallet. To get the staged changes see [`Wallet::staged`] and similar methods. Once
+/// the changes are committed to the persistence layer the contents of the stage should be
+/// discarded.
+///
+/// Users should persist early and often generally speaking, however in principle there is no
+/// limit to the number or type of changes that can be staged prior to persisting or the order in
+/// which they're staged. This is because change sets are designed to be [merged]. The change
+/// that is ultimately persisted will encompass the combined effect of each change individually.
+///
+/// ## Extensibility
+///
+/// Existing fields may be extended in the future with additional sub-fields. New top-level fields
+/// are likely to be added as new features and core components are implemented. Existing fields may
+/// be removed in future versions of the library.
+///
+/// The authors reserve the right to make breaking changes to the [`ChangeSet`] structure in
+/// a major version release. API changes affecting the types of data persisted will display
+/// prominently in the release notes. Users are advised to look for such changes and update their
+/// application accordingly.
+///
+/// The resulting interface is designed to give the user more control of what to persist and when
+/// to persist it. Custom implementations should consider and account for the possibility of
+/// partial or repeat writes, the atomicity of persistence operations, and the order of reads and
+/// writes among the fields of the change set. BDK comes with support for [SQLite] that handles
+/// the details for you and is recommended for many users. If implementing your own persistence,
+/// please refer to the documentation for [`WalletPersister`] and [`PersistedWallet`] for more
+/// information.
+///
+/// [`change_descriptor`]: Self::change_descriptor
+/// [`descriptor`]: Self::descriptor
+/// [`local_chain`]: Self::local_chain
+/// [merged]: bdk_chain::Merge
+/// [`network`]: Self::network
+/// [`PersistedWallet`]: crate::PersistedWallet
+/// [SQLite]: bdk_chain::rusqlite_impl
+/// [`Update`]: crate::Update
+/// [`WalletPersister`]: crate::WalletPersister
+/// [`Wallet::staged`]: crate::Wallet::staged
+/// [`Wallet`]: crate::Wallet
+#[derive(Default, Debug, Clone, PartialEq, Deserialize, Serialize)]
 pub struct ChangeSet {
     /// Descriptor for recipient addresses.
     pub descriptor: Option<Descriptor<DescriptorPublicKey>>,

crates/wallet/src/wallet/mod.rs

@@ -478,7 +478,7 @@ impl Wallet {
 
     /// Load [`Wallet`] from the given previously persisted [`ChangeSet`] and `params`.
     ///
-    /// Refer to [`Wallet::load`] for more.
+    /// Returns `Ok(None)` if the changeset is empty. Refer to [`Wallet::load`] for more.
     pub fn load_with_params(
         changeset: ChangeSet,
         params: LoadParams,