Data Querying and Views

/**
 * A configurable view that allows iterating over values in a KVStore.
 * Methods can be chained to configure iteration behavior.
 */
public abstract class KVStoreView<T> implements Iterable<T> {
    
    /**
     * Reverses the order of iteration. By default, iterates in ascending order.
     * @return This view instance for method chaining
     */
    public KVStoreView<T> reverse();
    
    /**
     * Iterates according to the given index.
     * @param name The name of the index to use for iteration
     * @return This view instance for method chaining
     */  
    public KVStoreView<T> index(String name);
    
    /**
     * Defines the value of the parent index when iterating over a child index.
     * Required for iterating over child indices.
     * @param value The parent index value to match
     * @return This view instance for method chaining
     */
    public KVStoreView<T> parent(Object value);
    
    /**
     * Iterates starting at the given value of the chosen index (inclusive).
     * @param value The starting value for iteration
     * @return This view instance for method chaining
     */
    public KVStoreView<T> first(Object value);
    
    /**
     * Stops iteration at the given value of the chosen index (inclusive).
     * @param value The ending value for iteration
     * @return This view instance for method chaining
     */
    public KVStoreView<T> last(Object value);
    
    /**
     * Limits iteration to a maximum number of elements.
     * @param max Maximum number of elements to retrieve (must be positive)
     * @return This view instance for method chaining
     */
    public KVStoreView<T> max(long max);
    
    /**
     * Skips a number of elements at the start of iteration.
     * Skipped elements are not counted against max().
     * @param n Number of elements to skip
     * @return This view instance for method chaining
     */
    public KVStoreView<T> skip(long n);
    
    /**
     * Returns an iterator for the current configuration.
     * @return A closeable iterator instance
     * @throws Exception if iterator creation fails
     */
    public KVStoreIterator<T> closeableIterator() throws Exception;
}

/**
 * An iterator for KVStore with resource management and batch operations.
 * Should be explicitly closed after use unless all elements are consumed.
 */
public interface KVStoreIterator<T> extends Iterator<T>, Closeable {
    
    /**
     * Retrieve multiple elements from the store in a single operation.
     * @param max Maximum number of elements to retrieve
     * @return List of up to 'max' elements
     */
    List<T> next(int max);
    
    /**
     * Skip elements in the iterator efficiently.
     * @param n Number of elements to skip
     * @return true if there are items left after skipping, false otherwise
     */
    boolean skip(long n);
}

import org.apache.spark.util.kvstore.*;
import java.util.List;

// Assume we have a Task class with indexed fields
public class Task {
    @KVIndex
    public String id;
    
    @KVIndex("priority")
    public int priority;
    
    @KVIndex("status")
    public String status;
    
    @KVIndex("userId")
    public String userId;
    
    public Task(String id, int priority, String status, String userId) {
        this.id = id;
        this.priority = priority;
        this.status = status;
        this.userId = userId;
    }
}

// Basic iteration over all tasks
KVStoreView<Task> allTasks = store.view(Task.class);
for (Task task : allTasks) {
    System.out.println("Task: " + task.id);
}

// Query high priority tasks (priority >= 8) in descending order
KVStoreView<Task> highPriorityTasks = store.view(Task.class)
    .index("priority")
    .first(8)
    .reverse();

for (Task task : highPriorityTasks) {
    System.out.println("High priority task: " + task.id + " (priority: " + task.priority + ")");
}

// Get first 10 running tasks, skipping the first 5
KVStoreView<Task> runningTasks = store.view(Task.class)
    .index("status")
    .first("running")
    .last("running")
    .skip(5)
    .max(10);

// Using closeable iterator for resource management
try (KVStoreIterator<Task> iterator = runningTasks.closeableIterator()) {
    while (iterator.hasNext()) {
        Task task = iterator.next();
        System.out.println("Running task: " + task.id);
    }
}

// Batch operations with iterator
try (KVStoreIterator<Task> iterator = store.view(Task.class).closeableIterator()) {
    List<Task> batch;
    while (!(batch = iterator.next(100)).isEmpty()) {
        processBatch(batch);
    }
}

// Range queries - tasks with priority between 5 and 9
KVStoreView<Task> mediumPriorityTasks = store.view(Task.class)
    .index("priority")
    .first(5)
    .last(9);

// Skip and pagination
KVStoreView<Task> page2Tasks = store.view(Task.class)
    .skip(20)  // Skip first 20 tasks
    .max(10);  // Get next 10 tasks

// Parent-child relationships (if using parent indices)
// Assuming we have a parent index relationship
KVStoreView<Task> userTasks = store.view(Task.class)
    .index("userId")
    .parent("user-123");

// Get all items of a type
KVStoreView<Task> all = store.view(Task.class);

// Get items sorted by an index
KVStoreView<Task> byPriority = store.view(Task.class).index("priority");

// Reverse sort order
KVStoreView<Task> descending = store.view(Task.class).index("priority").reverse();

// Items with index value >= start
KVStoreView<Task> fromStart = store.view(Task.class).index("priority").first(5);

// Items with index value <= end
KVStoreView<Task> toEnd = store.view(Task.class).index("priority").last(9);

// Items with index value between start and end (inclusive)
KVStoreView<Task> range = store.view(Task.class).index("priority").first(5).last(9);

// Exact match (start and end are the same)
KVStoreView<Task> exact = store.view(Task.class).index("status").first("running").last("running");

// First page (items 1-20)
KVStoreView<Task> page1 = store.view(Task.class).max(20);

// Second page (items 21-40)
KVStoreView<Task> page2 = store.view(Task.class).skip(20).max(20);

// Third page (items 41-60)
KVStoreView<Task> page3 = store.view(Task.class).skip(40).max(20);

// Automatic resource cleanup with enhanced for loop
for (Task task : store.view(Task.class)) {
    // Iterator is automatically closed when loop completes
    processTask(task);
}

// Manual resource management for partial iteration
KVStoreIterator<Task> iterator = store.view(Task.class).closeableIterator();
try {
    if (iterator.hasNext()) {
        Task first = iterator.next();
        // Process only the first task, then exit
        return processTask(first);
    }
} finally {
    iterator.close(); // Important: close manually when not exhausting iterator
}

// Try-with-resources for automatic cleanup
try (KVStoreIterator<Task> iterator = store.view(Task.class).closeableIterator()) {
    while (iterator.hasNext()) {
        Task task = iterator.next();
        if (shouldStop(task)) {
            break; // Iterator will be closed automatically
        }
        processTask(task);
    }
}