buoyant_kernel/

lib.rs

//! # Delta Kernel
//!
//! Delta-kernel-rs is an experimental [Delta](https://github.com/delta-io/delta/) implementation
//! focused on interoperability with a wide range of query engines. It supports reads and
//! (experimental) writes (only blind appends in the write path currently). This library defines a
//! number of traits which must be implemented to provide a working delta implementation. They are
//! detailed below. There is a provided "default engine" that implements all these traits and can
//! be used to ease integration work. See [`DefaultEngine`](engine/default/index.html) for more
//! information.
//!
//! A full `rust` example for reading table data using the default engine can be found in the
//! [read-table-single-threaded] example (and for a more complex multi-threaded reader see the
//! [read-table-multi-threaded] example). An example for reading the table changes for a table
//! using the default engine can be found in the [read-table-changes] example. The [write-table]
//! example demonstrates how to write data to a Delta table using the default engine.
//!
//! [read-table-single-threaded]:
//! https://github.com/delta-io/delta-kernel-rs/tree/main/kernel/examples/read-table-single-threaded
//! [read-table-multi-threaded]:
//! https://github.com/delta-io/delta-kernel-rs/tree/main/kernel/examples/read-table-multi-threaded
//! [read-table-changes]:
//! https://github.com/delta-io/delta-kernel-rs/tree/main/kernel/examples/read-table-changes
//! [write-table]:
//! https://github.com/delta-io/delta-kernel-rs/tree/main/kernel/examples/write-table
//!
//! # Engine trait
//!
//! The [`Engine`] trait allows connectors to bring their own implementation of functionality such
//! as reading parquet files, listing files in a file system, parsing a JSON string etc. This
//! trait exposes methods to get sub-engines which expose the core functionalities customizable by
//! connectors.
//!
//! ## Expression handling
//!
//! Expression handling is done via the [`EvaluationHandler`], which in turn allows the creation of
//! [`ExpressionEvaluator`]s. These evaluators are created for a specific predicate [`Expression`]
//! and allow evaluation of that predicate for a specific batch of data.
//!
//! ## File system interactions
//!
//! Delta Kernel needs to perform some basic operations against file systems like listing and
//! reading files. These interactions are encapsulated in the [`StorageHandler`] trait.
//! Implementers must take care that all assumptions on the behavior of the functions - like sorted
//! results - are respected.
//!
//! ## Reading log and data files
//!
//! Delta Kernel requires the capability to read and write json files and read parquet files, which
//! is exposed via the [`JsonHandler`] and [`ParquetHandler`] respectively. When reading files,
//! connectors are asked to provide the context information they require to execute the actual
//! operation. This is done by invoking methods on the [`StorageHandler`] trait.

#![cfg_attr(all(doc, NIGHTLY_CHANNEL), feature(doc_cfg))]
#![warn(
    unreachable_pub,
    trivial_numeric_casts,
    unused_extern_crates,
    rust_2018_idioms,
    rust_2021_compatibility,
    clippy::unwrap_used,
    clippy::expect_used,
    clippy::panic
)]
// we re-allow panics in tests
#![cfg_attr(test, allow(clippy::unwrap_used, clippy::expect_used, clippy::panic))]

/// This `extern crate` declaration allows the macro to reliably refer to
/// `delta_kernel::schema::DataType` no matter which crate invokes it. Without that, `delta_kernel`
/// cannot invoke the macro because `delta_kernel` is an unknown crate identifier (you have to use
/// `crate` instead). We could make the macro use `crate::schema::DataType` instead, but then the
/// macro is useless outside the `delta_kernel` crate.
// TODO: when running `cargo package -p delta_kernel` this gives 'unused' warning - #1095
#[allow(unused_extern_crates)]
extern crate self as delta_kernel;

use std::any::Any;
use std::fs::DirEntry;
use std::sync::Arc;
use std::time::SystemTime;
use std::{cmp::Ordering, ops::Range};

use bytes::Bytes;
use url::Url;

use self::schema::{DataType, SchemaRef};

mod action_reconciliation;
pub mod actions;
pub mod checkpoint;
pub mod committer;
// Public under test-utils so integration tests can inspect CRC state via Snapshot::get_current_crc_if_loaded_for_testing.
#[cfg(feature = "test-utils")]
pub mod crc;
#[cfg(not(feature = "test-utils"))]
pub(crate) mod crc;
pub mod engine_data;
pub mod error;
pub mod expressions;
mod log_compaction;
mod log_path;
mod log_reader;
pub mod metrics;
pub mod partition;
pub mod scan;
pub mod schema;
pub mod snapshot;
pub mod table_changes;
pub mod table_configuration;
pub mod table_features;
pub mod table_properties;
pub mod transaction;
pub mod transforms;

pub use log_path::LogPath;

mod row_tracking;

pub(crate) mod clustering;

mod arrow_compat;
#[cfg(any(feature = "arrow-57", feature = "arrow-58"))]
pub use arrow_compat::*;

#[cfg(feature = "internal-api")]
pub mod column_trie;
#[cfg(not(feature = "internal-api"))]
pub(crate) mod column_trie;
pub mod kernel_predicates;
pub(crate) mod utils;

#[cfg(feature = "internal-api")]
pub use utils::try_parse_uri;

// for the below modules, we cannot introduce a macro to clean this up. rustfmt doesn't follow into
// macros, and so will not format the files associated with these modules if we get too clever. see:
// https://github.com/rust-lang/rustfmt/issues/3253

#[cfg(feature = "internal-api")]
pub mod path;
#[cfg(not(feature = "internal-api"))]
pub(crate) mod path;

#[cfg(feature = "internal-api")]
pub mod log_replay;
#[cfg(not(feature = "internal-api"))]
pub(crate) mod log_replay;

#[cfg(feature = "internal-api")]
pub mod log_segment;
#[cfg(not(feature = "internal-api"))]
pub(crate) mod log_segment;

#[cfg(feature = "internal-api")]
pub mod last_checkpoint_hint;
#[cfg(not(feature = "internal-api"))]
pub(crate) mod last_checkpoint_hint;

pub(crate) mod log_segment_files;

#[cfg(feature = "internal-api")]
pub mod history_manager;
#[cfg(not(feature = "internal-api"))]
pub(crate) mod history_manager;

#[cfg(feature = "internal-api")]
pub mod parallel;
#[cfg(not(feature = "internal-api"))]
pub(crate) mod parallel;

pub use action_reconciliation::{ActionReconciliationIterator, ActionReconciliationIteratorState};
pub use delta_kernel_derive;
use delta_kernel_derive::internal_api;
pub use engine_data::{
    EngineData, FilteredEngineData, FilteredRowVisitor, GetData, RowIndexIterator, RowVisitor,
};
pub use error::{DeltaResult, Error};
pub use expressions::{Expression, ExpressionRef, Predicate, PredicateRef};
pub use log_compaction::{should_compact, LogCompactionWriter};
pub use metrics::MetricsReporter;
pub use snapshot::Snapshot;
pub use snapshot::SnapshotRef;

use expressions::literal_expression_transform;
use expressions::Scalar;
use schema::{StructField, StructType};

#[cfg(any(
    feature = "default-engine-native-tls",
    feature = "default-engine-rustls",
    feature = "arrow-conversion"
))]
pub mod engine;

/// Delta table version is 8 byte unsigned int
pub type Version = u64;

/// Sentinel version indicating a pre-commit state (table does not exist yet).
/// Used for create-table transactions before the first commit.
pub const PRE_COMMIT_VERSION: Version = u64::MAX;

pub type FileSize = u64;
pub type FileIndex = u64;

/// A specification for a range of bytes to read from a file location
pub type FileSlice = (Url, Option<Range<FileIndex>>);

/// Data read from a Delta table file and the corresponding scan file information.
pub type FileDataReadResult = (FileMeta, Box<dyn EngineData>);

/// An iterator of data read from specified files
pub type FileDataReadResultIterator =
    Box<dyn Iterator<Item = DeltaResult<Box<dyn EngineData>>> + Send>;

/// The metadata that describes an object.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct FileMeta {
    /// The fully qualified path to the object
    pub location: Url,
    /// The last modified time as milliseconds since unix epoch
    pub last_modified: i64,
    /// The size in bytes of the object
    pub size: FileSize,
}

impl Ord for FileMeta {
    fn cmp(&self, other: &Self) -> Ordering {
        self.location.cmp(&other.location)
    }
}

impl PartialOrd for FileMeta {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl TryFrom<DirEntry> for FileMeta {
    type Error = Error;

    fn try_from(ent: DirEntry) -> DeltaResult<FileMeta> {
        let metadata = ent.metadata()?;
        let last_modified = metadata
            .modified()?
            .duration_since(SystemTime::UNIX_EPOCH)
            .map_err(|_| Error::generic("Failed to convert file timestamp to milliseconds"))?;
        let location = Url::from_file_path(ent.path())
            .map_err(|_| Error::generic(format!("Invalid path: {:?}", ent.path())))?;
        let last_modified = last_modified.as_millis().try_into().map_err(|_| {
            Error::generic(format!(
                "Failed to convert file modification time {:?} into i64",
                last_modified.as_millis()
            ))
        })?;
        Ok(FileMeta {
            location,
            last_modified,
            size: metadata.len(),
        })
    }
}

impl FileMeta {
    /// Create a new instance of `FileMeta`
    pub fn new(location: Url, last_modified: i64, size: u64) -> Self {
        Self {
            location,
            last_modified,
            size,
        }
    }
}

/// Extension trait that makes it easier to work with traits objects that implement [`Any`],
/// implemented automatically for any type that satisfies `Any`, `Send`, and `Sync`. In particular,
/// given some `trait T: Any + Send + Sync`, it allows upcasting `T` to `dyn Any + Send + Sync`,
/// which in turn allows downcasting the result to a concrete type.
///
/// For example, the following code will compile:
///
/// ```
/// # use buoyant_kernel as delta_kernel;
/// # use delta_kernel::AsAny;
/// # use std::any::Any;
/// # use std::sync::Arc;
/// trait Foo : AsAny {}
/// struct Bar;
/// impl Foo for Bar {}
///
/// let f: Arc<dyn Foo> = Arc::new(Bar);
/// let a: Arc<dyn Any + Send + Sync> = f.as_any();
/// let b: Arc<Bar> = a.downcast().unwrap();
/// ```
///
/// In contrast, very similar code that relies only on `Any` would fail to compile:
///
/// ```fail_compile
/// # use std::any::Any;
/// # use std::sync::Arc;
/// trait Foo: Any + Send + Sync {}
///
/// struct Bar;
/// impl Foo for Bar {}
///
/// let f: Arc<dyn Foo> = Arc::new(Bar);
/// let b: Arc<Bar> = f.downcast().unwrap(); // `Arc::downcast` method not found
/// ```
///
/// As would this:
///
/// ```fail_compile
/// # use std::any::Any;
/// # use std::sync::Arc;
/// trait Foo: Any + Send + Sync {}
///
/// struct Bar;
/// impl Foo for Bar {}
///
/// let f: Arc<dyn Foo> = Arc::new(Bar);
/// let a: Arc<dyn Any + Send + Sync> = f; // trait upcasting coercion is not stable rust
/// let f: Arc<Bar> = a.downcast().unwrap();
/// ```
///
/// NOTE: `AsAny` inherits the `Send + Sync` constraint from [`Arc::downcast`].
pub trait AsAny: Any + Send + Sync {
    /// Obtains a `dyn Any` reference to the object:
    ///
    /// ```
    /// # use buoyant_kernel as delta_kernel;
    /// # use delta_kernel::AsAny;
    /// # use std::any::Any;
    /// # use std::sync::Arc;
    /// trait Foo : AsAny {}
    /// struct Bar;
    /// impl Foo for Bar {}
    ///
    /// let f: &dyn Foo = &Bar;
    /// let a: &dyn Any = f.any_ref();
    /// let b: &Bar = a.downcast_ref().unwrap();
    /// ```
    fn any_ref(&self) -> &(dyn Any + Send + Sync);

    /// Obtains an `Arc<dyn Any>` reference to the object:
    ///
    /// ```
    /// # use buoyant_kernel as delta_kernel;
    /// # use delta_kernel::AsAny;
    /// # use std::any::Any;
    /// # use std::sync::Arc;
    /// trait Foo : AsAny {}
    /// struct Bar;
    /// impl Foo for Bar {}
    ///
    /// let f: Arc<dyn Foo> = Arc::new(Bar);
    /// let a: Arc<dyn Any + Send + Sync> = f.as_any();
    /// let b: Arc<Bar> = a.downcast().unwrap();
    /// ```
    fn as_any(self: Arc<Self>) -> Arc<dyn Any + Send + Sync>;

    /// Converts the object to `Box<dyn Any>`:
    ///
    /// ```
    /// # use buoyant_kernel as delta_kernel;
    /// # use delta_kernel::AsAny;
    /// # use std::any::Any;
    /// # use std::sync::Arc;
    /// trait Foo : AsAny {}
    /// struct Bar;
    /// impl Foo for Bar {}
    ///
    /// let f: Box<dyn Foo> = Box::new(Bar);
    /// let a: Box<dyn Any> = f.into_any();
    /// let b: Box<Bar> = a.downcast().unwrap();
    /// ```
    fn into_any(self: Box<Self>) -> Box<dyn Any + Send + Sync>;

    /// Convenient wrapper for [`std::any::type_name`], since [`Any`] does not provide it and
    /// [`Any::type_id`] is useless as a debugging aid (its `Debug` is just a mess of hex digits).
    fn type_name(&self) -> &'static str;
}

// Blanket implementation for all eligible types
impl<T: Any + Send + Sync> AsAny for T {
    fn any_ref(&self) -> &(dyn Any + Send + Sync) {
        self
    }
    fn as_any(self: Arc<Self>) -> Arc<dyn Any + Send + Sync> {
        self
    }
    fn into_any(self: Box<Self>) -> Box<dyn Any + Send + Sync> {
        self
    }
    fn type_name(&self) -> &'static str {
        std::any::type_name::<Self>()
    }
}

/// Extension trait that facilitates object-safe implementations of `PartialEq`.
pub trait DynPartialEq: AsAny {
    fn dyn_eq(&self, other: &dyn Any) -> bool;
}

// Blanket implementation for all eligible types
impl<T: PartialEq + AsAny> DynPartialEq for T {
    fn dyn_eq(&self, other: &dyn Any) -> bool {
        other.downcast_ref::<T>().is_some_and(|other| self == other)
    }
}

/// Trait for implementing an Expression evaluator.
///
/// It contains one Expression which can be evaluated on multiple ColumnarBatches.
/// Connectors can implement this trait to optimize the evaluation using the
/// connector specific capabilities.
pub trait ExpressionEvaluator: AsAny {
    /// Evaluate the expression on a given EngineData.
    ///
    /// Produces one value for each row of the input.
    /// The data type of the output is same as the type output of the expression this evaluator is using.
    fn evaluate(&self, batch: &dyn EngineData) -> DeltaResult<Box<dyn EngineData>>;
}

/// Trait for implementing a Predicate evaluator.
///
/// It contains one Predicate which can be evaluated on multiple ColumnarBatches.
/// Connectors can implement this trait to optimize the evaluation using the
/// connector specific capabilities.
pub trait PredicateEvaluator: AsAny {
    /// Evaluate the predicate on a given EngineData.
    ///
    /// Produces one boolean value for each row of the input.
    fn evaluate(&self, batch: &dyn EngineData) -> DeltaResult<Box<dyn EngineData>>;
}

/// Provides expression evaluation capability to Delta Kernel.
///
/// Delta Kernel can use this handler to evaluate a predicate on partition filters,
/// fill up partition column values, and any computation on data using Expressions.
pub trait EvaluationHandler: AsAny {
    /// Create an [`ExpressionEvaluator`] that can evaluate the given [`Expression`]
    /// on columnar batches with the given [`Schema`] to produce data of [`DataType`].
    ///
    /// If the provided output type is a struct, its fields describe the columns of output produced
    /// by the evaluator. Otherwise, the output schema is a single column named "output" of the
    /// specified `output_type`. In all cases, the output schema is only used for its names (all
    /// field names will be updated to match) and nullability (non-nullable columns can be converted
    /// to nullable). Any mismatch in types (including number of columns) will produce an error.
    ///
    /// # Parameters
    ///
    /// - `input_schema`: Schema of the input data.
    /// - `expression`: Expression to evaluate.
    /// - `output_type`: Expected result data type.
    ///
    /// [`Schema`]: crate::schema::StructType
    /// [`DataType`]: crate::schema::DataType
    fn new_expression_evaluator(
        &self,
        input_schema: SchemaRef,
        expression: ExpressionRef,
        output_type: DataType,
    ) -> DeltaResult<Arc<dyn ExpressionEvaluator>>;

    /// Create a [`PredicateEvaluator`] that can evaluate the given [`Predicate`] on columnar
    /// batches with the given [`Schema`] to produce a column of boolean results.
    ///
    /// The output schema is a single nullable boolean column named "output".
    ///
    /// # Parameters
    ///
    /// - `input_schema`: Schema of the input data.
    /// - `predicate`: Predicate to evaluate.
    ///
    /// [`Schema`]: crate::schema::StructType
    fn new_predicate_evaluator(
        &self,
        input_schema: SchemaRef,
        predicate: PredicateRef,
    ) -> DeltaResult<Arc<dyn PredicateEvaluator>>;

    /// Create a single-row all-null-value [`EngineData`] with the schema specified by
    /// `output_schema`.
    // NOTE: we should probably allow DataType instead of SchemaRef, but can expand that in the
    // future.
    fn null_row(&self, output_schema: SchemaRef) -> DeltaResult<Box<dyn EngineData>>;

    /// Create a multi-row [`EngineData`] by applying the given schema to multiple rows of values.
    ///
    /// Each element in `rows` represents one row of data, where each row is a slice of structured
    /// scalar values (one scalar per top-level field in the schema).
    ///
    /// # Parameters
    ///
    /// - `schema`: Schema describing the structure of each row.
    /// - `rows`: Slice of rows, where each row contains one structured scalar per top-level schema
    ///   field.
    ///
    /// # Returns
    ///
    /// A multi-row `EngineData` containing all rows.
    ///
    /// # Errors
    ///
    /// Returns an error if any row has a number of scalars that does not match the number of
    /// top-level fields in `schema`, or if any scalar value cannot be appended to its corresponding
    /// field's builder (e.g. due to a type mismatch).
    ///
    /// # Example
    ///
    /// For a schema with fields `[add: Struct, remove: Struct]`, each row should contain exactly 2
    /// scalars: one for the `add` field and one for the `remove` field.
    fn create_many(
        &self,
        schema: SchemaRef,
        rows: &[&[Scalar]],
    ) -> DeltaResult<Box<dyn EngineData>>;
}

/// Internal trait to allow us to have a private `create_one` API that's implemented for all
/// EvaluationHandlers.
// For some reason rustc doesn't detect it's usage so we allow(dead_code) here...
#[allow(dead_code)]
#[internal_api]
trait EvaluationHandlerExtension: EvaluationHandler {
    /// Create a single-row [`EngineData`] by applying the given schema to the leaf-values given in
    /// `values`.
    // Note: we will stick with a Schema instead of DataType (more constrained can expand in
    // future)
    fn create_one(&self, schema: SchemaRef, values: &[Scalar]) -> DeltaResult<Box<dyn EngineData>> {
        // just get a single int column (arbitrary)
        let null_row_schema = Arc::new(StructType::new_unchecked(vec![StructField::nullable(
            "null_col",
            DataType::INTEGER,
        )]));
        let null_row = self.null_row(null_row_schema.clone())?;

        // Convert schema and leaf values to an expression
        let row_expr = literal_expression_transform(schema.as_ref(), values)?;

        let eval =
            self.new_expression_evaluator(null_row_schema, row_expr.into(), schema.into())?;
        eval.evaluate(null_row.as_ref())
    }
}

// Auto-implement the extension trait for all EvaluationHandlers
impl<T: EvaluationHandler + ?Sized> EvaluationHandlerExtension for T {}

/// A trait that allows converting a type into (single-row) EngineData
///
/// This is typically used with the `#[derive(IntoEngineData)]` macro
/// which leverages the traits `ToDataType` and `Into<Scalar>` for struct fields
/// to convert a struct into EngineData.
///
/// # Example
/// ```ignore
/// # use std::sync::Arc;
/// # use delta_kernel_derive::{Schema, IntoEngineData};
///
/// #[derive(Schema, IntoEngineData)]
/// struct MyStruct {
///    a: i32,
///    b: String,
/// }
///
/// let my_struct = MyStruct { a: 42, b: "Hello".to_string() };
/// // typically used with ToSchema
/// let schema = Arc::new(MyStruct::to_schema());
/// // single-row EngineData
/// let engine = todo!(); // create an engine
/// let engine_data = my_struct.into_engine_data(schema, engine);
/// ```
#[internal_api]
pub(crate) trait IntoEngineData {
    /// Consume this type to produce a single-row EngineData using the provided schema.
    fn into_engine_data(
        self,
        schema: SchemaRef,
        engine: &dyn Engine,
    ) -> DeltaResult<Box<dyn EngineData>>;
}

/// Provides file system related functionalities to Delta Kernel.
///
/// Delta Kernel uses this handler whenever it needs to access the underlying
/// file system where the Delta table is present. Connector implementation of
/// this trait can hide filesystem specific details from Delta Kernel.
pub trait StorageHandler: AsAny {
    /// List the paths in the same directory that are lexicographically greater than
    /// (UTF-8 sorting) the given `path`. The result should also be sorted by the file name.
    ///
    /// If the path is directory-like (ends with '/'), the result should contain
    /// all the files in the directory.
    fn list_from(&self, path: &Url)
        -> DeltaResult<Box<dyn Iterator<Item = DeltaResult<FileMeta>>>>;

    /// Read data specified by the start and end offset from the file.
    fn read_files(
        &self,
        files: Vec<FileSlice>,
    ) -> DeltaResult<Box<dyn Iterator<Item = DeltaResult<Bytes>>>>;

    /// Copy a file atomically from source to destination. If the destination file already exists,
    /// it must return Err(Error::FileAlreadyExists).
    fn copy_atomic(&self, src: &Url, dest: &Url) -> DeltaResult<()>;

    /// Write data to the specified path.
    ///
    /// If `overwrite` is false and the file already exists, this must return
    /// `Err(Error::FileAlreadyExists)`.
    fn put(&self, path: &Url, data: Bytes, overwrite: bool) -> DeltaResult<()>;

    /// Perform a HEAD request for the given file at a Url, returning the file metadata.
    ///
    /// If the file does not exist, this must return an `Err` with [`Error::FileNotFound`].
    fn head(&self, path: &Url) -> DeltaResult<FileMeta>;
}

/// Provides JSON handling functionality to Delta Kernel.
///
/// Delta Kernel can use this handler to parse JSON strings into Row or read content from JSON files.
/// Connectors can leverage this trait to provide their best implementation of the JSON parsing
/// capability to Delta Kernel.
pub trait JsonHandler: AsAny {
    /// Parse the given json strings and return the fields requested by output schema as columns in [`EngineData`].
    /// json_strings MUST be a single column batch of engine data, and the column type must be string
    fn parse_json(
        &self,
        json_strings: Box<dyn EngineData>,
        output_schema: SchemaRef,
    ) -> DeltaResult<Box<dyn EngineData>>;

    /// Read and parse the JSON format file at given locations and return the data as EngineData with
    /// the columns requested by physical schema. Note: The [`FileDataReadResultIterator`] must emit
    /// data from files in the order that `files` is given. For example if files ["a", "b"] is provided,
    /// then the engine data iterator must first return all the engine data from file "a", _then_ all
    /// the engine data from file "b". Moreover, for a given file, all of its [`EngineData`] and
    /// constituent rows must be in order that they occur in the file. Consider a file with rows
    /// (1, 2, 3). The following are legal iterator batches:
    ///    iter: [EngineData(1, 2), EngineData(3)]
    ///    iter: [EngineData(1), EngineData(2, 3)]
    ///    iter: [EngineData(1, 2, 3)]
    /// The following are illegal batches:
    ///    iter: [EngineData(3), EngineData(1, 2)]
    ///    iter: [EngineData(1), EngineData(3, 2)]
    ///    iter: [EngineData(2, 1, 3)]
    ///
    /// Additionally, engines may not merge engine data across file boundaries.
    ///
    /// # Parameters
    ///
    /// - `files` - File metadata for files to be read.
    /// - `physical_schema` - Select list of columns to read from the JSON file.
    /// - `predicate` - Optional push-down predicate hint (engine is free to ignore it).
    fn read_json_files(
        &self,
        files: &[FileMeta],
        physical_schema: SchemaRef,
        predicate: Option<PredicateRef>,
    ) -> DeltaResult<FileDataReadResultIterator>;

    /// Atomically (!) write a single JSON file. Each row of the input data should be written as a
    /// new JSON object appended to the file. this write must:
    /// (1) serialize the data to newline-delimited json (each row is a json object literal)
    /// (2) write the data to storage atomically (i.e. if the file already exists, fail unless the
    ///     overwrite flag is set)
    ///
    /// For example, the JSON data should be written as { "column1": "val1", "column2": "val2", .. }
    /// with each row on a new line.
    ///
    /// NOTE: Null columns should not be written to the JSON file. For example, if a row has columns
    /// ["a", "b"] and the value of "b" is null, the JSON object should be written as
    /// { "a": "..." }. Note that including nulls is technically valid JSON, but would bloat the
    /// log, therefore we recommend omitting them.
    ///
    /// # Parameters
    ///
    /// - `path` - URL specifying the location to write the JSON file
    /// - `data` - Iterator of EngineData to write to the JSON file. Each row should be written as
    ///   a new JSON object appended to the file. (that is, the file is newline-delimited JSON, and
    ///   each row is a JSON object on a single line)
    /// - `overwrite` - If true, overwrite the file if it exists. If false, the call must fail if
    ///   the file exists.
    fn write_json_file(
        &self,
        path: &Url,
        data: Box<dyn Iterator<Item = DeltaResult<FilteredEngineData>> + Send + '_>,
        overwrite: bool,
    ) -> DeltaResult<()>;
}

/// Reserved field IDs for metadata columns in Delta tables.
///
/// These field IDs are reserved and should not be used for regular table columns.
/// They are used to provide file-level metadata as virtual columns during reads.
pub mod reserved_field_ids {
    /// Reserved field ID for the file name metadata column (`_file`).
    /// This column provides the name of the Parquet file that contains each row.
    pub const FILE_NAME: i64 = 2147483646;
}

/// Metadata from a Parquet file footer.
///
/// This struct contains metadata extracted from a Parquet file's footer, including the schema.
/// It is designed to be extensible for future additions such as row group statistics.
#[derive(Debug, Clone)]
pub struct ParquetFooter {
    /// The schema of the Parquet file, converted to Delta Kernel's schema format.
    pub schema: SchemaRef,
}

/// Provides Parquet file related functionalities to Delta Kernel.
///
/// Connectors can leverage this trait to provide their own custom
/// implementation of Parquet data file functionalities to Delta Kernel.
pub trait ParquetHandler: AsAny {
    /// Read and parse the Parquet file at given locations and return the data as EngineData with
    /// the columns requested by physical schema. The ParquetHandler _must_ return exactly the
    /// columns specified in `physical_schema`, and they _must_ be in schema order.
    ///
    /// # Resolving Parquet schema to the physical schema
    ///
    /// When reading the Parquet file, the columns are resolved from the Parquet schema to the
    /// kernel's `physical_schema`. To do so, the parquet reader must match each Parquet column
    /// to a [`StructField`] in the `physical_schema`. All columns in the returned `EngineData`
    /// must be in the same order as specified in `physical_schema`.
    ///
    /// Parquet columns are matched to `physical_schema` [`StructField`]s using the following rules:
    /// 1. **Field ID**: If a [`StructField`] in `physical_schema` contains a field ID
    ///    (specified in [`ColumnMetadataKey::ParquetFieldId`] metadata), use the ID to
    ///    match the Parquet column's field id
    /// 2. **Field Name**: If no field ID is present in the `physical_schema`'s [`StructField`] or no matching parquet field ID is found,
    ///    fall back to matching by column name
    ///
    /// # Metadata Columns
    ///
    /// The ParquetHandler must support virtual metadata columns that provide additional information
    /// about each row. These columns are not stored in the Parquet file but are generated at read time.
    ///
    /// ## Row Index Column
    ///
    /// When a column in `physical_schema` is marked as a row index metadata column (via
    /// [`StructField::create_metadata_column`] with [`schema::MetadataColumnSpec::RowIndex`]), the
    /// ParquetHandler must populate it with the 0-based row position within the Parquet file:
    ///
    /// - **Column name**: User-specified (commonly `"row_index"` or `"_metadata.row_index"`)
    /// - **Type**: `LONG` (non-nullable)
    /// - **Values**: Sequential integers starting at 0 for each file
    /// - **Use case**: Track row positions for downstream processing, or internally used to compute Row IDs
    ///
    /// Example: A file with 5 rows would have row_index values `[0, 1, 2, 3, 4]`.
    ///
    /// ## File Name Column (Reserved Field ID)
    ///
    /// When a column in `physical_schema` has the reserved field ID
    /// [`reserved_field_ids::FILE_NAME`] (2147483646), the ParquetHandler must populate it
    /// with the file path/name:
    ///
    /// - **Column name**: `"_file"`
    /// - **Type**: `STRING` (non-nullable)
    /// - **Field ID**: 2147483646 (reserved)
    /// - **Values**: The file path/URL (e.g., `"s3://bucket/path/file.parquet"`)
    /// - **Use case**: Track which file each row came from in multi-file reads
    ///
    /// Example: All rows from the same file would have the same `_file` value.
    ///
    /// ## Metadata Column Examples
    ///
    /// ```rust,ignore
    /// use delta_kernel::schema::{StructType, StructField, DataType, MetadataColumnSpec};
    ///
    /// // Example 1: Schema with row_index metadata column
    /// let schema_with_row_index = StructType::try_new([
    ///     StructField::nullable("id", DataType::INTEGER),
    ///     StructField::create_metadata_column("row_index", MetadataColumnSpec::RowIndex),
    ///     StructField::nullable("value", DataType::STRING),
    /// ])?;
    ///
    /// // Example 2: Schema with _file metadata column (using reserved field ID)
    /// let schema_with_file_path = StructType::try_new([
    ///     StructField::nullable("id", DataType::INTEGER),
    ///     StructField::create_metadata_column("_file", MetadataColumnSpec::FilePath),
    ///     StructField::nullable("value", DataType::STRING),
    /// ])?;
    /// ```
    ///
    /// ---
    ///
    ///  If no matching Parquet column is found, `NULL` values are returned
    ///  for nullable columns in `physical_schema`. For non-nullable columns, an error is returned.
    ///
    ///
    /// ## Column Matching Examples
    ///
    /// Consider a `physical_schema` with the following fields:
    /// - Column 0:  `"i_logical"` (integer, non-null) with field ID 1 (via [`ColumnMetadataKey::ParquetFieldId`])
    /// - Column 1: `"s"` (string, nullable) with no field ID metadata
    /// - Column 2: `"i2"` (integer, nullable) with no field ID metadata
    ///
    /// [`ColumnMetadataKey::ParquetFieldId`]: crate::schema::ColumnMetadataKey::ParquetFieldId
    ///
    /// And a Parquet file containing these columns:
    /// - Column 0: `"i2"` (integer, nullable) with field ID 3
    /// - Column 1: `"i"` (integer, non-null) with field ID 1
    /// - No `"s"` column present
    ///
    /// The column matching would work as follows:
    /// - `"i_logical"` matches `"i"` by field ID (both have ID 1)
    /// - `"i2"` matches `"i2"` by column name (no field ID to match on)
    /// - `"s"` has no matching Parquet column, so NULL values are returned
    ///
    /// The returned data will contain exactly 3 columns in physical schema order:
    /// `{i_logical: parquet[1], s: NULL.., i2: parquet[0]}`
    ///
    /// # Parameters
    ///
    /// - `files` - File metadata for files to be read.
    /// - `physical_schema` - Select list and order of columns to read from the Parquet file.
    /// - `predicate` - Optional push-down predicate hint (engine is free to ignore it).
    ///
    /// # Returns
    /// A [`DeltaResult`] containing a [`FileDataReadResultIterator`].
    /// Each element of the iterator is a [`DeltaResult`] of [`EngineData`]. The [`EngineData`]
    /// has the contents of `files` and must match the provided `physical_schema`.
    ///
    /// Note: The [`FileDataReadResultIterator`] must emit data from files in the order that `files`
    /// is given. For example if files ["a", "b"] is provided, then the engine data iterator must
    /// first return all the engine data from file "a", _then_ all the engine data from file "b".
    /// Moreover, for a given file, all of its [`EngineData`] and constituent rows must be in order
    /// that they occur in the file. Consider a file with rows
    /// (1, 2, 3). The following are legal iterator batches:
    ///    iter: [EngineData(1, 2), EngineData(3)]
    ///    iter: [EngineData(1), EngineData(2, 3)]
    ///    iter: [EngineData(1, 2, 3)]
    /// The following are illegal batches:
    ///    iter: [EngineData(3), EngineData(1, 2)]
    ///    iter: [EngineData(1), EngineData(3, 2)]
    ///    iter: [EngineData(2, 1, 3)]
    ///
    /// Additionally, engines must not merge engine data across file boundaries.
    ///
    /// [`ColumnMetadataKey::ParquetFieldId`]: crate::schema::ColumnMetadataKey
    fn read_parquet_files(
        &self,
        files: &[FileMeta],
        physical_schema: SchemaRef,
        predicate: Option<PredicateRef>,
    ) -> DeltaResult<FileDataReadResultIterator>;

    /// Write data to a Parquet file at the specified URL.
    ///
    /// This method writes the provided `data` to a Parquet file at the given `url`.
    ///
    /// This will overwrite the file if it already exists. For filesystem-backed
    /// implementations, the parent directories must be created if they do not exist.
    ///
    /// # Parameters
    ///
    /// - `url` - The full URL path where the Parquet file should be written
    ///   (e.g., `s3://bucket/path/file.parquet`).
    /// - `data` - An iterator of engine data to be written to the Parquet file.
    ///
    /// # Returns
    ///
    /// A [`DeltaResult`] indicating success or failure.
    fn write_parquet_file(
        &self,
        location: url::Url,
        data: Box<dyn Iterator<Item = DeltaResult<Box<dyn EngineData>>> + Send>,
    ) -> DeltaResult<()>;

    /// Read the footer metadata from a Parquet file without reading the data.
    ///
    /// This method reads only the Parquet file footer (metadata section), which is useful for
    /// schema inspection, compatibility checking, and determining whether parsed statistics
    /// columns are present and compatible with the current table schema.
    ///
    /// # Parameters
    ///
    /// - `file` - File metadata for the Parquet file whose footer should be read. The `size` field
    ///   should contain the actual file size to enable efficient footer reads without additional
    ///   I/O operations.
    ///
    /// # Returns
    ///
    /// A [`DeltaResult`] containing a [`ParquetFooter`] with the Parquet file's metadata, including
    /// the schema converted to Delta Kernel's format.
    ///
    /// # Field IDs
    ///
    /// If the Parquet file contains field IDs (written when column mapping is enabled), they are
    /// preserved in each [`StructField`]'s metadata. Callers can access field IDs via
    /// [`StructField::get_config_value`] with [`ColumnMetadataKey::ParquetFieldId`].
    ///
    /// # Errors
    ///
    /// Returns an error if:
    /// - The file cannot be accessed or does not exist
    /// - The file is not a valid Parquet file
    /// - The footer cannot be read or parsed
    /// - The schema cannot be converted to Delta Kernel's format
    ///
    /// [`StructField`]: crate::schema::StructField
    /// [`StructField::get_config_value`]: crate::schema::StructField::get_config_value
    /// [`ColumnMetadataKey::ParquetFieldId`]: crate::schema::ColumnMetadataKey::ParquetFieldId
    fn read_parquet_footer(&self, file: &FileMeta) -> DeltaResult<ParquetFooter>;
}

/// The `Engine` trait encapsulates all the functionality an engine or connector needs to provide
/// to the Delta Kernel in order to read the Delta table.
///
/// Engines/Connectors are expected to pass an implementation of this trait when reading a Delta
/// table.
pub trait Engine: AsAny {
    /// Get the connector provided [`EvaluationHandler`].
    fn evaluation_handler(&self) -> Arc<dyn EvaluationHandler>;

    /// Get the connector provided [`StorageHandler`]
    fn storage_handler(&self) -> Arc<dyn StorageHandler>;

    /// Get the connector provided [`JsonHandler`].
    fn json_handler(&self) -> Arc<dyn JsonHandler>;

    /// Get the connector provided [`ParquetHandler`].
    fn parquet_handler(&self) -> Arc<dyn ParquetHandler>;

    /// Get the connector provided [`MetricsReporter`] for metrics collection.
    ///
    /// Returns an optional reporter that will receive metric events from Delta operations.
    /// The default implementation returns None (no metrics reporting).
    fn get_metrics_reporter(&self) -> Option<Arc<dyn MetricsReporter>> {
        None
    }
}

// we have an 'internal' feature flag: default-engine-base, which is actually just the shared
// pieces of default-engine-native-tls and default-engine-rustls. the crate can't compile with _only_
// default-engine-base, so we give a friendly error here.
#[cfg(all(
    feature = "default-engine-base",
    not(any(
        feature = "default-engine-native-tls",
        feature = "default-engine-rustls",
    ))
))]
compile_error!(
    "The default-engine-base feature flag is not meant to be used directly. \
    Please use either default-engine-native-tls or default-engine-rustls."
);

// Rustdoc's documentation tests can do some things that regular unit tests can't. Here we are
// using doctests to test macros. Specifically, we are testing for failed macro invocations due
// to invalid input, not the macro output when the macro invocation is successful (which can/should be
// done in unit tests). This module is not exclusively for macro tests only so other doctests can also be added.
// https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html#include-items-only-when-collecting-doctests
#[cfg(doctest)]
mod doctests;