oxirs_core/storage/

tiered.rs

//! Tiered storage engine with intelligent data placement
//!
//! This module implements a multi-tier storage system that automatically
//! moves data between tiers based on access patterns and age.
//!
//! NOTE: This module requires the rocksdb feature to be enabled.

#[cfg(feature = "rocksdb")]
mod tiered_impl {
    #[allow(unused_imports)] // Used in tests and future implementations
    use super::*;
    use crate::model::{Triple, TriplePattern};
    use crate::storage::{
        ArchiveBackend, CompressionType, QueryMetrics, StorageConfig, StorageEngine, StorageStats,
        TierStat, TierStats,
    };
    use crate::OxirsError;
    use dashmap::DashMap;
    use lru::LruCache;
    use parking_lot::Mutex;
    use serde::{Deserialize, Serialize};
    use std::collections::HashMap;
    use std::path::{Path, PathBuf};
    use std::sync::atomic::{AtomicU64, Ordering};
    use std::sync::Arc;
    use std::time::{Duration, SystemTime};
    use tokio::sync::RwLock;

    /// Access tracking information
    #[derive(Debug, Clone, Serialize, Deserialize)]
    struct AccessInfo {
        /// Last access time
        last_access: SystemTime,
        /// Total access count
        access_count: u64,
        /// Current storage tier
        tier: StorageTier,
        /// Size in bytes
        size_bytes: usize,
    }

    /// Storage tiers
    #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
    enum StorageTier {
        Hot,
        Warm,
        Cold,
        Archive,
    }

    /// Triple with metadata
    #[derive(Debug, Clone, Serialize, Deserialize)]
    struct StoredTriple {
        triple: Triple,
        metadata: TripleMetadata,
    }

    /// Triple metadata for tiering decisions
    #[derive(Debug, Clone, Serialize, Deserialize)]
    struct TripleMetadata {
        /// Creation timestamp
        created_at: SystemTime,
        /// Last modified timestamp
        modified_at: SystemTime,
        /// Access information
        access_info: AccessInfo,
        /// Content hash for deduplication
        content_hash: u64,
        /// Compression info
        compression: Option<CompressionInfo>,
    }

    /// Compression information
    #[derive(Debug, Clone, Serialize, Deserialize)]
    struct CompressionInfo {
        /// Original size
        original_size: usize,
        /// Compressed size
        compressed_size: usize,
        /// Compression algorithm
        algorithm: String,
    }

    /// Tiered storage engine
    pub struct TieredStorageEngine {
        /// Configuration
        config: StorageConfig,
        /// Hot tier - in-memory cache
        hot_tier: Arc<Mutex<LruCache<u64, StoredTriple>>>,
        /// Warm tier - SSD storage
        warm_tier: Arc<RwLock<WarmTier>>,
        /// Cold tier - HDD storage
        cold_tier: Arc<RwLock<ColdTier>>,
        /// Archive tier - long-term storage
        archive_tier: Arc<RwLock<ArchiveTier>>,
        /// Triple index for fast lookups
        index: Arc<DashMap<u64, AccessInfo>>,
        /// Statistics
        stats: Arc<Statistics>,
        /// Background task handle
        background_handle: Option<tokio::task::JoinHandle<()>>,
    }

    /// Warm tier implementation
    struct WarmTier {
        _path: PathBuf,
        storage: rocksdb::DB,
        access_tracker: HashMap<u64, u64>,
    }

    /// Cold tier implementation
    struct ColdTier {
        _path: PathBuf,
        storage: rocksdb::DB,
        compression: compression::Compressor,
    }

    /// Archive tier implementation
    struct ArchiveTier {
        _backend: ArchiveBackend,
        _index: HashMap<u64, ArchiveLocation>,
    }

    /// Archive location information
    #[derive(Debug, Clone, Serialize, Deserialize)]
    struct ArchiveLocation {
        /// Archive file path
        file_path: String,
        /// Offset in file
        offset: u64,
        /// Size in bytes
        size: usize,
        /// Checksum
        checksum: u64,
    }

    /// Statistics tracker
    struct Statistics {
        total_triples: AtomicU64,
        hot_count: AtomicU64,
        warm_count: AtomicU64,
        cold_count: AtomicU64,
        archive_count: AtomicU64,
        total_size: AtomicU64,
        hot_hits: AtomicU64,
        warm_hits: AtomicU64,
        cold_hits: AtomicU64,
        total_queries: AtomicU64,
    }

    impl TieredStorageEngine {
        /// Create a new tiered storage engine
        pub async fn create(config: StorageConfig) -> Result<Arc<dyn StorageEngine>, OxirsError> {
            // Initialize hot tier
            let hot_capacity = config.tiers.hot_tier.max_size_mb * 1024 * 1024 / 1000; // Approximate
            let hot_tier = Arc::new(Mutex::new(LruCache::new(
                std::num::NonZeroUsize::new(hot_capacity)
                    .unwrap_or(std::num::NonZeroUsize::new(10000).expect("constant is non-zero")),
            )));

            // Initialize warm tier
            let warm_path = PathBuf::from(&config.tiers.warm_tier.path);
            std::fs::create_dir_all(&warm_path)?;
            let mut warm_opts = rocksdb::Options::default();
            warm_opts.create_if_missing(true);
            let warm_storage = rocksdb::DB::open(&warm_opts, warm_path.join("data"))?;
            let warm_tier = Arc::new(RwLock::new(WarmTier {
                _path: warm_path,
                storage: warm_storage,
                access_tracker: HashMap::new(),
            }));

            // Initialize cold tier
            let cold_path = PathBuf::from(&config.tiers.cold_tier.path);
            std::fs::create_dir_all(&cold_path)?;
            let mut cold_opts = rocksdb::Options::default();
            cold_opts.create_if_missing(true);
            let cold_storage = rocksdb::DB::open(&cold_opts, cold_path.join("data"))?;
            let cold_tier = Arc::new(RwLock::new(ColdTier {
                _path: cold_path,
                storage: cold_storage,
                compression: compression::Compressor::new(config.compression.clone()),
            }));

            // Initialize archive tier
            let archive_tier = Arc::new(RwLock::new(ArchiveTier {
                _backend: config.tiers.archive_tier.backend.clone(),
                _index: HashMap::new(),
            }));

            // Initialize index and statistics
            let index = Arc::new(DashMap::new());
            let stats = Arc::new(Statistics {
                total_triples: AtomicU64::new(0),
                hot_count: AtomicU64::new(0),
                warm_count: AtomicU64::new(0),
                cold_count: AtomicU64::new(0),
                archive_count: AtomicU64::new(0),
                total_size: AtomicU64::new(0),
                hot_hits: AtomicU64::new(0),
                warm_hits: AtomicU64::new(0),
                cold_hits: AtomicU64::new(0),
                total_queries: AtomicU64::new(0),
            });

            let mut engine = TieredStorageEngine {
                config,
                hot_tier,
                warm_tier,
                cold_tier,
                archive_tier,
                index,
                stats,
                background_handle: None,
            };

            // Start background tier management
            engine.start_background_tasks();

            Ok(Arc::new(engine))
        }

        /// Start background tasks for tier management
        fn start_background_tasks(&mut self) {
            let hot_tier = self.hot_tier.clone();
            let warm_tier = self.warm_tier.clone();
            let cold_tier = self.cold_tier.clone();
            let archive_tier = self.archive_tier.clone();
            let index = self.index.clone();
            let config = self.config.clone();

            let handle = tokio::spawn(async move {
                let mut interval = tokio::time::interval(Duration::from_secs(60));

                loop {
                    interval.tick().await;

                    // Perform tier management
                    if let Err(e) = Self::manage_tiers(
                        &hot_tier,
                        &warm_tier,
                        &cold_tier,
                        &archive_tier,
                        &index,
                        &config,
                    )
                    .await
                    {
                        tracing::error!("Tier management error: {}", e);
                    }
                }
            });

            self.background_handle = Some(handle);
        }

        /// Manage data movement between tiers
        async fn manage_tiers(
            hot_tier: &Arc<Mutex<LruCache<u64, StoredTriple>>>,
            warm_tier: &Arc<RwLock<WarmTier>>,
            cold_tier: &Arc<RwLock<ColdTier>>,
            archive_tier: &Arc<RwLock<ArchiveTier>>,
            index: &Arc<DashMap<u64, AccessInfo>>,
            config: &StorageConfig,
        ) -> Result<(), OxirsError> {
            let now = SystemTime::now();

            // Check warm tier for promotion/demotion
            {
                let mut warm = warm_tier.write().await;
                let mut to_promote = Vec::new();
                let mut to_demote = Vec::new();

                for (hash, access_count) in &warm.access_tracker {
                    if *access_count >= config.tiers.warm_tier.promotion_threshold as u64 {
                        to_promote.push(*hash);
                    } else if let Some(info) = index.get(hash) {
                        let days_since_access = now
                            .duration_since(info.last_access)
                            .unwrap_or(Duration::ZERO)
                            .as_secs()
                            / 86400;

                        if days_since_access
                            >= config.tiers.warm_tier.demotion_threshold_days as u64
                        {
                            to_demote.push(*hash);
                        }
                    }
                }

                // Promote to hot tier
                for hash in to_promote {
                    if let Ok(Some(data)) = warm.storage.get(hash.to_be_bytes()) {
                        if let Ok((triple, _)) = oxicode::serde::decode_from_slice::<StoredTriple, _>(
                            &data,
                            oxicode::config::standard(),
                        ) {
                            hot_tier.lock().put(hash, triple);
                            warm.storage.delete(hash.to_be_bytes())?;
                            warm.access_tracker.remove(&hash);

                            if let Some(mut info) = index.get_mut(&hash) {
                                info.tier = StorageTier::Hot;
                            }
                        }
                    }
                }

                // Demote to cold tier
                let cold = cold_tier.write().await;
                for hash in to_demote {
                    if let Ok(Some(data)) = warm.storage.get(hash.to_be_bytes()) {
                        // Compress before storing in cold tier
                        let compressed = cold.compression.compress(&data)?;
                        cold.storage.put(hash.to_be_bytes(), compressed)?;
                        warm.storage.delete(hash.to_be_bytes())?;
                        warm.access_tracker.remove(&hash);

                        if let Some(mut info) = index.get_mut(&hash) {
                            info.tier = StorageTier::Cold;
                        }
                    }
                }
            }

            // Check cold tier for archival
            {
                let _cold = cold_tier.read().await;
                let mut to_archive = Vec::new();

                for entry in index.iter() {
                    let (hash, info) = entry.pair();
                    if info.tier == StorageTier::Cold {
                        let days_since_access = now
                            .duration_since(info.last_access)
                            .unwrap_or(Duration::ZERO)
                            .as_secs()
                            / 86400;

                        if days_since_access >= config.tiers.cold_tier.archive_threshold_days as u64
                        {
                            to_archive.push(*hash);
                        }
                    }
                }

                // Move to archive
                if !to_archive.is_empty() {
                    let _archive = archive_tier.write().await;
                    // Archive implementation would batch multiple triples into archive files
                    // For now, we'll skip the actual archival process
                }
            }

            Ok(())
        }

        /// Calculate hash for a triple
        fn hash_triple(triple: &Triple) -> u64 {
            use std::hash::{Hash, Hasher};
            let mut hasher = std::collections::hash_map::DefaultHasher::new();
            triple.hash(&mut hasher);
            hasher.finish()
        }

        /// Get the appropriate tier for a new triple based on hints
        fn determine_initial_tier(&self, _triple: &Triple) -> StorageTier {
            // For now, all new data goes to warm tier
            // In a real implementation, we might analyze the triple's predicate
            // or other characteristics to make smarter placement decisions
            StorageTier::Warm
        }
    }

    #[async_trait::async_trait]
    impl StorageEngine for TieredStorageEngine {
        async fn init(&mut self, config: StorageConfig) -> Result<(), OxirsError> {
            self.config = config;
            Ok(())
        }

        async fn store_triple(&self, triple: &Triple) -> Result<(), OxirsError> {
            let hash = Self::hash_triple(triple);
            let now = SystemTime::now();

            // Check if triple already exists
            if self.index.contains_key(&hash) {
                return Ok(());
            }

            // Create stored triple with metadata
            // Calculate serialized size by actually serializing
            let temp_serialized =
                oxicode::serde::encode_to_vec(triple, oxicode::config::standard())?;
            let size_bytes = temp_serialized.len();

            let stored = StoredTriple {
                triple: triple.clone(),
                metadata: TripleMetadata {
                    created_at: now,
                    modified_at: now,
                    access_info: AccessInfo {
                        last_access: now,
                        access_count: 0,
                        tier: self.determine_initial_tier(triple),
                        size_bytes,
                    },
                    content_hash: hash,
                    compression: None,
                },
            };

            // Store in appropriate tier
            match stored.metadata.access_info.tier {
                StorageTier::Hot => {
                    self.hot_tier.lock().put(hash, stored.clone());
                    self.stats.hot_count.fetch_add(1, Ordering::Relaxed);
                }
                StorageTier::Warm => {
                    let data = oxicode::serde::encode_to_vec(&stored, oxicode::config::standard())?;
                    self.warm_tier
                        .write()
                        .await
                        .storage
                        .put(hash.to_be_bytes(), data)?;
                    self.stats.warm_count.fetch_add(1, Ordering::Relaxed);
                }
                _ => unreachable!("New triples should not go directly to cold/archive"),
            }

            // Update index
            self.index.insert(hash, stored.metadata.access_info.clone());
            self.stats.total_triples.fetch_add(1, Ordering::Relaxed);
            self.stats.total_size.fetch_add(
                stored.metadata.access_info.size_bytes as u64,
                Ordering::Relaxed,
            );

            Ok(())
        }

        async fn store_triples(&self, triples: &[Triple]) -> Result<(), OxirsError> {
            // Use parallel processing for batch inserts
            #[cfg(feature = "parallel")]
            {
                use rayon::prelude::*;
                triples
                    .par_iter()
                    .try_for_each(|triple| futures::executor::block_on(self.store_triple(triple)))
            }
            #[cfg(not(feature = "parallel"))]
            {
                for triple in triples {
                    self.store_triple(triple).await?;
                }
                Ok(())
            }
        }

        async fn query_triples(&self, pattern: &TriplePattern) -> Result<Vec<Triple>, OxirsError> {
            self.stats.total_queries.fetch_add(1, Ordering::Relaxed);
            let mut results = Vec::new();

            // Search hot tier first
            {
                let hot = self.hot_tier.lock();
                for (_, stored) in hot.iter() {
                    if pattern.matches(&stored.triple) {
                        results.push(stored.triple.clone());
                        self.stats.hot_hits.fetch_add(1, Ordering::Relaxed);
                    }
                }
            }

            // If not enough results, search warm tier
            if results.is_empty() {
                let warm = self.warm_tier.read().await;
                // Iterate through warm tier storage
                let iter = warm.storage.iterator(rocksdb::IteratorMode::Start);
                for (_key, value) in iter.flatten() {
                    if let Ok((stored, _)) = oxicode::serde::decode_from_slice::<StoredTriple, _>(
                        &value,
                        oxicode::config::standard(),
                    ) {
                        if pattern.matches(&stored.triple) {
                            results.push(stored.triple.clone());
                            self.stats.warm_hits.fetch_add(1, Ordering::Relaxed);
                        }
                    }
                }
            }

            // Update access info for queried triples
            let now = SystemTime::now();
            for triple in &results {
                let hash = Self::hash_triple(triple);
                if let Some(mut info) = self.index.get_mut(&hash) {
                    info.last_access = now;
                    info.access_count += 1;

                    // Track access in warm tier
                    if info.tier == StorageTier::Warm {
                        if let Ok(mut warm) = self.warm_tier.try_write() {
                            *warm.access_tracker.entry(hash).or_insert(0) += 1;
                        }
                    }
                }
            }

            Ok(results)
        }

        async fn delete_triples(&self, pattern: &TriplePattern) -> Result<usize, OxirsError> {
            let triples = self.query_triples(pattern).await?;
            let count = triples.len();

            for triple in triples {
                let hash = Self::hash_triple(&triple);

                // Remove from index
                if let Some((_, info)) = self.index.remove(&hash) {
                    // Remove from appropriate tier
                    match info.tier {
                        StorageTier::Hot => {
                            self.hot_tier.lock().pop(&hash);
                            self.stats.hot_count.fetch_sub(1, Ordering::Relaxed);
                        }
                        StorageTier::Warm => {
                            self.warm_tier
                                .write()
                                .await
                                .storage
                                .delete(hash.to_be_bytes())?;
                            self.stats.warm_count.fetch_sub(1, Ordering::Relaxed);
                        }
                        StorageTier::Cold => {
                            self.cold_tier
                                .write()
                                .await
                                .storage
                                .delete(hash.to_be_bytes())?;
                            self.stats.cold_count.fetch_sub(1, Ordering::Relaxed);
                        }
                        StorageTier::Archive => {
                            // Archive deletion is more complex and might not be allowed
                            if !self.config.tiers.archive_tier.immutable {
                                // Implement archive deletion
                            }
                        }
                    }

                    self.stats.total_triples.fetch_sub(1, Ordering::Relaxed);
                    self.stats
                        .total_size
                        .fetch_sub(info.size_bytes as u64, Ordering::Relaxed);
                }
            }

            Ok(count)
        }

        async fn stats(&self) -> Result<StorageStats, OxirsError> {
            let total_queries = self.stats.total_queries.load(Ordering::Relaxed);
            let hot_hits = self.stats.hot_hits.load(Ordering::Relaxed);
            let warm_hits = self.stats.warm_hits.load(Ordering::Relaxed);
            let cold_hits = self.stats.cold_hits.load(Ordering::Relaxed);
            let total_hits = hot_hits + warm_hits + cold_hits;

            Ok(StorageStats {
                total_triples: self.stats.total_triples.load(Ordering::Relaxed),
                total_size_bytes: self.stats.total_size.load(Ordering::Relaxed),
                tier_stats: TierStats {
                    hot: TierStat {
                        triple_count: self.stats.hot_count.load(Ordering::Relaxed),
                        size_bytes: 0, // Calculate from hot tier
                        hit_rate: if total_queries > 0 {
                            (hot_hits as f64 / total_queries as f64) * 100.0
                        } else {
                            0.0
                        },
                        avg_access_time_us: 1, // Sub-microsecond for memory
                    },
                    warm: TierStat {
                        triple_count: self.stats.warm_count.load(Ordering::Relaxed),
                        size_bytes: 0, // Calculate from warm tier
                        hit_rate: if total_queries > 0 {
                            (warm_hits as f64 / total_queries as f64) * 100.0
                        } else {
                            0.0
                        },
                        avg_access_time_us: 100, // ~100μs for SSD
                    },
                    cold: TierStat {
                        triple_count: self.stats.cold_count.load(Ordering::Relaxed),
                        size_bytes: 0, // Calculate from cold tier
                        hit_rate: if total_queries > 0 {
                            (cold_hits as f64 / total_queries as f64) * 100.0
                        } else {
                            0.0
                        },
                        avg_access_time_us: 10000, // ~10ms for HDD
                    },
                    archive: TierStat {
                        triple_count: self.stats.archive_count.load(Ordering::Relaxed),
                        size_bytes: 0,               // Calculate from archive
                        hit_rate: 0.0,               // Archive is rarely accessed
                        avg_access_time_us: 1000000, // ~1s for archive retrieval
                    },
                },
                compression_ratio: 1.5, // Placeholder
                query_metrics: QueryMetrics {
                    avg_query_time_ms: 0.1,                            // Placeholder
                    p99_query_time_ms: 1.0,                            // Placeholder
                    qps: if total_queries > 0 { 1000.0 } else { 0.0 }, // Placeholder
                    cache_hit_rate: if total_queries > 0 {
                        (total_hits as f64 / total_queries as f64) * 100.0
                    } else {
                        0.0
                    },
                },
            })
        }

        async fn optimize(&self) -> Result<(), OxirsError> {
            // Trigger compaction on RocksDB instances
            self.warm_tier
                .read()
                .await
                .storage
                .compact_range(None::<&[u8]>, None::<&[u8]>);
            self.cold_tier
                .read()
                .await
                .storage
                .compact_range(None::<&[u8]>, None::<&[u8]>);

            // Force tier rebalancing
            Self::manage_tiers(
                &self.hot_tier,
                &self.warm_tier,
                &self.cold_tier,
                &self.archive_tier,
                &self.index,
                &self.config,
            )
            .await?;

            Ok(())
        }

        async fn backup(&self, path: &Path) -> Result<(), OxirsError> {
            // Create backup directory
            std::fs::create_dir_all(path)?;

            // Backup metadata
            let metadata = BackupMetadata {
                version: 1,
                created_at: SystemTime::now(),
                total_triples: self.stats.total_triples.load(Ordering::Relaxed),
                config: self.config.clone(),
            };

            let metadata_path = path.join("metadata.json");
            let metadata_json = serde_json::to_string_pretty(&metadata)?;
            std::fs::write(metadata_path, metadata_json)?;

            // Backup each tier
            // Hot tier
            let hot_backup = path.join("hot.bin");
            let hot_data: Vec<_> = self
                .hot_tier
                .lock()
                .iter()
                .map(|(k, v)| (*k, v.clone()))
                .collect();
            let hot_bytes = oxicode::serde::encode_to_vec(&hot_data, oxicode::config::standard())?;
            std::fs::write(hot_backup, hot_bytes)?;

            // Warm and cold tiers - backup by iterating and saving
            let warm_backup = path.join("warm.bin");
            let warm_data: Vec<(Vec<u8>, Vec<u8>)> = {
                let warm = self.warm_tier.read().await;
                let mut data = Vec::new();
                let iter = warm.storage.iterator(rocksdb::IteratorMode::Start);
                for (key, value) in iter.flatten() {
                    data.push((key.to_vec(), value.to_vec()));
                }
                data
            };
            let warm_bytes =
                oxicode::serde::encode_to_vec(&warm_data, oxicode::config::standard())?;
            std::fs::write(warm_backup, warm_bytes)?;

            let cold_backup = path.join("cold.bin");
            let cold_data: Vec<(Vec<u8>, Vec<u8>)> = {
                let cold = self.cold_tier.read().await;
                let mut data = Vec::new();
                let iter = cold.storage.iterator(rocksdb::IteratorMode::Start);
                for (key, value) in iter.flatten() {
                    data.push((key.to_vec(), value.to_vec()));
                }
                data
            };
            let cold_bytes =
                oxicode::serde::encode_to_vec(&cold_data, oxicode::config::standard())?;
            std::fs::write(cold_backup, cold_bytes)?;

            // Index backup
            let index_backup = path.join("index.bin");
            let index_data: HashMap<_, _> = self
                .index
                .iter()
                .map(|entry| (*entry.key(), entry.value().clone()))
                .collect();
            let index_bytes =
                oxicode::serde::encode_to_vec(&index_data, oxicode::config::standard())?;
            std::fs::write(index_backup, index_bytes)?;

            Ok(())
        }

        async fn restore(&self, path: &Path) -> Result<(), OxirsError> {
            // Read metadata
            let metadata_path = path.join("metadata.json");
            let metadata_json = std::fs::read_to_string(metadata_path)?;
            let metadata: BackupMetadata = serde_json::from_str(&metadata_json)?;

            // Restore hot tier
            let hot_backup = path.join("hot.bin");
            if hot_backup.exists() {
                let hot_bytes = std::fs::read(hot_backup)?;
                let hot_data: Vec<(u64, StoredTriple)> =
                    oxicode::serde::decode_from_slice(&hot_bytes, oxicode::config::standard())
                        .map(|(v, _)| v)?;

                let mut hot = self.hot_tier.lock();
                hot.clear();
                for (k, v) in hot_data {
                    hot.put(k, v);
                }
            }

            // Restore warm tier
            let warm_backup = path.join("warm.bin");
            if warm_backup.exists() {
                let warm_bytes = std::fs::read(warm_backup)?;
                let warm_data: Vec<(Vec<u8>, Vec<u8>)> =
                    oxicode::serde::decode_from_slice(&warm_bytes, oxicode::config::standard())
                        .map(|(v, _)| v)?;

                let warm = self.warm_tier.write().await;
                for (key, value) in warm_data {
                    warm.storage.put(&key, &value)?;
                }
            }

            // Restore cold tier
            let cold_backup = path.join("cold.bin");
            if cold_backup.exists() {
                let cold_bytes = std::fs::read(cold_backup)?;
                let cold_data: Vec<(Vec<u8>, Vec<u8>)> =
                    oxicode::serde::decode_from_slice(&cold_bytes, oxicode::config::standard())
                        .map(|(v, _)| v)?;

                let cold = self.cold_tier.write().await;
                for (key, value) in cold_data {
                    cold.storage.put(&key, &value)?;
                }
            }

            // Restore index
            let index_backup = path.join("index.bin");
            if index_backup.exists() {
                let index_bytes = std::fs::read(index_backup)?;
                let index_data: HashMap<u64, AccessInfo> =
                    oxicode::serde::decode_from_slice(&index_bytes, oxicode::config::standard())
                        .map(|(v, _)| v)?;

                self.index.clear();
                for (k, v) in index_data {
                    self.index.insert(k, v);
                }
            }

            // Update statistics
            self.stats
                .total_triples
                .store(metadata.total_triples, Ordering::Relaxed);

            Ok(())
        }
    }

    /// Backup metadata
    #[derive(Debug, Serialize, Deserialize)]
    struct BackupMetadata {
        version: u32,
        created_at: SystemTime,
        total_triples: u64,
        config: StorageConfig,
    }

    // Placeholder compression module
    mod compression {
        use super::*;

        pub struct Compressor {
            _compression_type: CompressionType,
        }

        impl Compressor {
            pub fn new(compression_type: CompressionType) -> Self {
                Compressor {
                    _compression_type: compression_type,
                }
            }

            pub fn compress(&self, data: &[u8]) -> Result<Vec<u8>, OxirsError> {
                // Placeholder - would use actual compression libraries
                Ok(data.to_vec())
            }

            #[allow(dead_code)]
            pub fn decompress(&self, data: &[u8]) -> Result<Vec<u8>, OxirsError> {
                // Placeholder - would use actual compression libraries
                Ok(data.to_vec())
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use crate::model::{Literal, NamedNode};
        use crate::storage::ArchiveBackend;

        #[tokio::test]
        async fn test_tiered_storage() {
            let test_dir = format!(
                "/tmp/oxirs_tiered_test_{}",
                std::time::SystemTime::now()
                    .duration_since(std::time::UNIX_EPOCH)
                    .expect("operation should succeed")
                    .as_millis()
            );

            let mut config = StorageConfig::default();
            config.tiers.warm_tier.path = format!("{}/warm", test_dir);
            config.tiers.cold_tier.path = format!("{}/cold", test_dir);
            config.tiers.archive_tier.backend =
                ArchiveBackend::Local(format!("{}/archive", test_dir));

            let engine = TieredStorageEngine::create(config)
                .await
                .expect("async operation should succeed");

            // Create test triple
            let subject = NamedNode::new("http://example.org/subject").expect("valid IRI");
            let predicate = NamedNode::new("http://example.org/predicate").expect("valid IRI");
            let object = crate::model::Object::Literal(Literal::new("test"));
            let triple = Triple::new(subject, predicate, object);

            // Store triple
            engine
                .store_triple(&triple)
                .await
                .expect("async operation should succeed");

            // Query triple
            let pattern = TriplePattern::new(None, None, None);
            let results = engine
                .query_triples(&pattern)
                .await
                .expect("async operation should succeed");
            assert_eq!(results.len(), 1);
            assert_eq!(results[0], triple);

            // Check stats
            let stats = engine
                .stats()
                .await
                .expect("async operation should succeed");
            assert_eq!(stats.total_triples, 1);
        }
    }
} // end of tiered_impl module

// Re-export types when rocksdb feature is enabled
#[cfg(feature = "rocksdb")]
pub use tiered_impl::*;

// Stub implementations when rocksdb feature is disabled
#[cfg(not(feature = "rocksdb"))]
pub struct TieredStorageEngine;

#[cfg(not(feature = "rocksdb"))]
impl TieredStorageEngine {
    pub async fn new(_config: crate::storage::StorageConfig) -> Result<Self, crate::OxirsError> {
        Err(crate::OxirsError::NotSupported(
            "TieredStorageEngine requires rocksdb feature".to_string(),
        ))
    }
}