kevo/docs/iterator.md

Iterator Package Documentation

The iterator package provides a unified interface and implementations for traversing key-value data across the Kevo engine. Iterators are a fundamental abstraction used throughout the system for ordered access to data, regardless of where it's stored.

Overview

Iterators in the Kevo engine follow a consistent interface pattern that allows components to access data in a uniform way. This enables combining and composing iterators to provide complex data access patterns while maintaining a simple, consistent API.

Key responsibilities of the iterator package include:

• Defining a standard iterator interface
• Providing adapter patterns for implementing iterators
• Implementing specialized iterators for different use cases
• Supporting bounded, composite, and hierarchical iteration

Iterator Interface

Core Interface

The core Iterator interface defines the contract that all iterators must follow:

type Iterator interface {
    // Positioning methods
    SeekToFirst()                // Position at the first key
    SeekToLast()                 // Position at the last key
    Seek(target []byte) bool     // Position at the first key >= target
    Next() bool                  // Advance to the next key
    
    // Access methods
    Key() []byte                 // Return the current key
    Value() []byte               // Return the current value
    Valid() bool                 // Check if the iterator is valid
    
    // Special methods
    IsTombstone() bool           // Check if current entry is a deletion marker
}

This interface is used across all storage layers (MemTable, SSTables, transactions) to provide consistent access to key-value data.

Iterator Types and Patterns

Adapter Pattern

The package provides adapter patterns to simplify implementing the full interface:

1. Base Iterators:

   • Implement the core interface directly for specific data structures
   • Examples: SkipList iterators, Block iterators

2. Adapter Wrappers:

   • Transform existing iterators to provide additional functionality
   • Examples: Bounded iterators, filtering iterators

Bounded Iterators

Bounded iterators limit the range of keys an iterator will traverse:

1. Key Range Limiting:

   • Apply start and end bounds to constrain iteration
   • Skip keys outside the specified range

2. Implementation Approach:

   • Wrap an existing iterator
   • Filter out keys outside the desired range
   • Maintain the underlying iterator's properties otherwise

Composite Iterators

Composite iterators combine multiple source iterators into a single view:

1. MergingIterator:

   • Merges multiple iterators into a single sorted stream
   • Handles duplicate keys according to specified policy

2. Implementation Details:

   • Maintains a priority queue or similar structure
   • Selects the next appropriate key from all sources
   • Handles edge cases like exhausted sources

Hierarchical Iterators

Hierarchical iterators implement the LSM tree's multi-level view:

1. LSM Hierarchy Semantics:

   • Newer sources (e.g., MemTable) take precedence over older sources (e.g., SSTables)
   • Combines multiple levels into a single, consistent view
   • Respects the "newest version wins" rule for duplicate keys

2. Source Precedence:

   • Iterators are provided in order from newest to oldest
   • When multiple sources contain the same key, the newer source's value is used
   • Tombstones (deletion markers) hide older values

Implementation Details

Hierarchical Iterator

The HierarchicalIterator is a cornerstone of the storage engine:

1. Source Management:

   • Maintains an ordered array of source iterators
   • Sources must be provided in newest-to-oldest order
   • Typically includes MemTable, immutable MemTables, and SSTable iterators

2. Key Selection Algorithm:

   • During Seek, Next, etc., examines all valid sources
   • Tracks seen keys to handle duplicates
   • Selects the smallest key that satisfies the operation's constraints
   • For duplicate keys, uses the value from the newest source

3. Thread Safety:

   • Mutex protection for concurrent access
   • Safe for concurrent reads, though typically used from one thread

4. Memory Efficiency:

   • Lazily fetches values only when needed
   • Doesn't materialize full result set in memory

Key Selection Process

The key selection process is a critical algorithm in hierarchical iterators:

1. For SeekToFirst:

   • Position all source iterators at their first key
   • Select the smallest key across all sources, considering duplicates

2. For Seek(target):

   • Position all source iterators at the smallest key >= target
   • Select the smallest valid key >= target, considering duplicates

3. For Next:

   • Remember the current key
   • Advance source iterators past this key
   • Select the smallest key that is > current key

Tombstone Handling

Tombstones (deletion markers) are handled specially:

1. Detection:

   • Identified by nil values in most iterators
   • Allows distinguishing between deleted keys and non-existent keys

2. Impact on Iteration:

   • Tombstones are visible during direct iteration
   • During merging, tombstones from newer sources hide older values
   • This mechanism enables proper deletion semantics in the LSM tree

Common Usage Patterns

Basic Iterator Usage

// Use any Iterator implementation
iter := someSource.NewIterator()

// Iterate through all entries
for iter.SeekToFirst(); iter.Valid(); iter.Next() {
    fmt.Printf("Key: %s, Value: %s\n", iter.Key(), iter.Value())
}

// Or seek to a specific key
if iter.Seek([]byte("target")) {
    fmt.Printf("Found: %s\n", iter.Value())
}

Bounded Range Iterator

// Create a bounded iterator
startKey := []byte("user:1000")
endKey := []byte("user:2000")
rangeIter := bounded.NewBoundedIterator(sourceIter, startKey, endKey)

// Iterate through the bounded range
for rangeIter.SeekToFirst(); rangeIter.Valid(); rangeIter.Next() {
    fmt.Printf("Key: %s\n", rangeIter.Key())
}

Hierarchical Multi-Source Iterator

// Create iterators for each source (newest to oldest)
memTableIter := memTable.NewIterator()
sstableIter1 := sstable1.NewIterator()
sstableIter2 := sstable2.NewIterator()

// Combine them into a hierarchical view
sources := []iterator.Iterator{memTableIter, sstableIter1, sstableIter2}
hierarchicalIter := composite.NewHierarchicalIterator(sources)

// Use the combined view
for hierarchicalIter.SeekToFirst(); hierarchicalIter.Valid(); hierarchicalIter.Next() {
    if !hierarchicalIter.IsTombstone() {
        fmt.Printf("%s: %s\n", hierarchicalIter.Key(), hierarchicalIter.Value())
    }
}

Performance Considerations

Time Complexity

Iterator operations have the following complexity characteristics:

1. SeekToFirst/SeekToLast:

   • O(S) where S is the number of sources
   • Each source may have its own seek complexity

2. Seek(target):

   • O(S * log N) where N is the typical size of each source
   • Binary search within each source, then selection across sources

3. Next():

   • Amortized O(S) for typical cases
   • May require advancing multiple sources past duplicates

4. Key()/Value()/Valid():

   • O(1) - constant time for accessing current state

Memory Management

Iterator implementations focus on memory efficiency:

1. Lazy Evaluation:

   • Values are fetched only when needed
   • No materialization of full result sets

2. Buffer Reuse:

   • Key/value buffers are reused where possible
   • Careful copying when needed for correctness

3. Source Independence:

   • Each source manages its own memory
   • Composite iterators add minimal overhead

Optimizations

Several optimizations improve iterator performance:

1. Key Skipping:

   • Skip sources that can't contain the target key
   • Early termination when possible

2. Caching:

   • Cache recently accessed values
   • Avoid redundant lookups

3. Batched Advancement:

   • Advance multiple levels at once when possible
   • Reduces overall iteration cost

Design Principles

Interface Consistency

The iterator design follows several key principles:

1. Uniform Interface:

   • All iterators share the same interface
   • Allows seamless substitution and composition

2. Explicit State:

   • Iterator state is always explicit
   • Valid() must be checked before accessing data

3. Unidirectional Design:

   • Forward-only iteration for simplicity
   • Backward iteration would add complexity with little benefit

Composability

The iterators are designed for composition:

1. Adapter Pattern:

   • Wrap existing iterators to add functionality
   • Build complex behaviors from simple components

2. Delegation:

   • Delegate operations to underlying iterators
   • Apply transformations or filtering as needed

3. Transparency:

   • Composite iterators behave like simple iterators
   • Internal complexity is hidden from users

Integration with Storage Layers

The iterator system integrates with all storage layers:

1. MemTable Integration:

   • SkipList-based iterators for in-memory data
   • Priority for recent changes

2. SSTable Integration:

   • Block-based iterators for persistent data
   • Efficient seeking through index blocks

3. Transaction Integration:

   • Combines buffer and engine state
   • Preserves transaction isolation

4. Engine Integration:

   • Provides unified view across all components
   • Handles version selection and visibility