Introduce LogStore trait

This trait is supposed to serve as the entry point to read and write
commits in the Delta log.

rust/src/lib.rs

@@ -84,6 +84,7 @@ compile_error!(
 
 pub mod data_catalog;
 pub mod errors;
+pub mod logstore;
 pub mod operations;
 pub mod protocol;
 pub mod schema;

rust/src/logstore/mod.rs

@@ -0,0 +1,36 @@
+//! Delta log store.
+use crate::errors::DeltaResult;
+use bytes::Bytes;
+
+use crate::protocol::Action;
+
+/// Trait for critical operations required to read and write commit entries in Delta logs.
+///
+/// The correctness is predicated on the atomicity and durability guarantees of
+/// the implementation of this interface. Specifically,
+///
+/// - Atomic visibility: Any commit created via `write_commit_entry` must become visible atomically.
+/// - Mutual exclusion: Only one writer must be able to create a commit for a specific version.
+/// - Consistent listing: Once a commit entry for version `v` has been written, any future call to
+///   `get_latest_version` must return a version >= `v`, i.e. the underlying file system entry must
+///   become visible immediately.
+#[async_trait::async_trait]
+pub trait LogStore {
+    /// Read data for commit entry with the given version.
+    /// TODO: return the actual commit data, i.e. Vec<Action>, instead?
+    async fn read_commit_entry(&self, version: i64) -> DeltaResult<Bytes>;
+
+    /// Write list of actions as delta commit entry for given version.
+    ///
+    /// This operation can be retried with a higher version in case the write
+    /// fails with `TransactionError::VersionAlreadyExists`.
+    async fn write_commit_entry(
+        &self,
+        version: i64,
+        actions: Vec<Action>,
+        overwrite: bool,
+    ) -> DeltaResult<()>;
+
+    /// Find latest version currently stored in the delta log.
+    async fn get_latest_version(&self) -> DeltaResult<i64>;
+}