Index Support - Springtail

Overview

The Indexer class is the core component responsible for building and managing secondary indexes on tables in the Springtail database system. It operates as a multi-threaded service within the springtail::committer namespace, handling index creation, deletion, and abort operations asynchronously via worker threads. Recovery and reconciliation operations are synchronous—the Committer waits for them to complete before proceeding.

Responsibilities

Building secondary indexes by scanning table data
Building look-aside indexes to reduce write amplification (see Look-Aside Index for details)
Handling index drops - both immediate drops and drops while a build is in progress
Recovering incomplete index operations after system crashes or shutdowns
Reconciling indexes with data changes that occurred during the build phase
Coordinating with committer to signal when index operations are ready for commit

Integration with Committer

The Indexer is owned and managed by the Committer class:

Initialization: Committer creates the Indexer in run() with a configurable worker count
Request routing: Committer calls process_requests() after batching index requests across XIDs
Recovery trigger: Committer invokes recover_indexes() when it receives an INDEX_RECOVERY_TRIGGER message
Reconciliation: Committer calls process_index_reconciliation() when it receives a RECONCILE_INDEX message

Key Components

Index Status States

The lifecycle of an index operation is tracked through three states:

enum class IndexStatus {
    BUILDING,   // Default state - index build is in progress
    DELETING,   // Fresh drop request for an index not currently being built
    ABORTING    // Drop requested while build is in progress
};

State Transitions: Notes:

create_index → BUILDING: Default state when an index build is initiated
drop_index on index in _work_set → ABORTING: Build in progress, mark for abort
drop_index on index NOT in _work_set → DELETING: Fresh drop, no build in progress
_reconcile_index() checks the current status and decides:
- DELETING → calls _drop() → DELETED
- ABORTING → calls _commit_build() (truncate) → DELETED
- BUILDING → calls _commit_build() (finalize) → READY

Core Data Structures

IndexParams

Encapsulates all parameters needed for an index operation:

struct IndexParams {
    uint64_t _db_id;                           // Database identifier
    uint64_t _xid;                             // Transaction ID when index was created
    proto::IndexProcessRequest _index_request;  // Protobuf request containing index metadata + operation request
    IndexStatus _status = IndexStatus::BUILDING; // Current operation status
};

IndexState

Captures the state of an index after initial build phase, used during reconciliation:

struct IndexState {
    MutableBTreePtr _root;            // B-tree root for the secondary index
    Key _key;                         // (db_id, index_id) identifier pair
    IndexParams _idx;                 // Original index parameters
    uint64_t _tid;                    // Table ID this index belongs to
    MutableBTreePtr _look_aside_root; // Look-aside index root (may be nullptr)
};

Key Type

Unique identifier for work items:

using Key = std::pair<uint64_t, uint64_t>;  // (db_id, index_id)

Internal Maps and Queues

Member	Type	Purpose
`_work_set`	`unordered_map<Key, IndexParams>`	Active index operations keyed by (db_id, index_id)
`_queue`	`queue<Key>`	FIFO queue of keys for worker threads to process
`_pending_idx_reconciliation_map`	`db_id → xid → list<IndexState>`	Indexes awaiting reconciliation, grouped by database and XID
`_table_idx_map`	`db_id → table_id → list<Key>`	Maps tables to their in-progress index builds (for abort_indexes)
`_xid_ddl_counter_map`	`xid → atomic<int>`	Tracks pending DDL operations per transaction
`_look_aside_build_tracker`	`table_id → bool`	Prevents duplicate look-aside index builds when multiple indexes created concurrently

Synchronization Primitives

Mutex	Protects
`_m`	`_work_set`, `_queue`, condition variable coordination
`_pending_reconciliation_map_mtx`	`_pending_idx_reconciliation_map`
`_table_idx_map_mtx`	`_table_idx_map`
`_xid_ddl_counter_map_mtx`	`_xid_ddl_counter_map`
`_look_aside_mutex`	`_look_aside_build_tracker`

Data Flow

Index Creation Flow

Index Drop Flow

Index Recovery Flow

Abort Indexes Flow (Table Resync)

Implementation Details

Worker Thread Model

The Indexer spawns a configurable number of worker threads at construction:

Indexer::Indexer(uint32_t worker_count, ...)
{
    CHECK_GT(worker_count, 0);
    for (auto i = 0; i != worker_count; ++i) {
        std::string thread_name = fmt::format("IndexWorker_{}", i);
        _workers.emplace_back([this](std::stop_token st) { task(st); });
        pthread_setname_np(_workers.back().native_handle(), thread_name.c_str());
    }
}

Worker Loop (task()):

Wait on condition variable _cv for work in _queue
Pop key from queue
Fetch IndexParams from _work_set
If status is BUILDING: call _build() and add result to pending reconciliation
If status is DELETING/ABORTING: add directly to pending reconciliation (with null root)

Workers use std::jthread with std::stop_token for graceful shutdown coordination.

Index Build Process (`_build`)

Phase 1: Setup

Invalidate table cache at the creation XID
Extract index column positions from IndexInfo (sorted by idx_position)
Get mutable table reference
Create empty B-tree root for the index

Phase 2: Look-Aside Index (if needed)

if (!look_aside_index) {
    std::unique_lock g(_look_aside_mutex);
    if (_look_aside_build_tracker.find(tid) == _look_aside_build_tracker.end()) {
        look_aside_index = mutable_table->create_look_aside_root(...);
        look_aside_index->init_empty();
        _look_aside_build_tracker[tid] = true;
        build_look_aside = true;
    }
}

Look-aside maps internal_row_id → (extent_id, row_id_within_extent)
Only the first secondary index on a table builds it
_look_aside_build_tracker prevents race conditions with concurrent index creation

Phase 3: Table Scan

constexpr int DROP_CHECK_PERIOD = 1000;
for (auto row_i = table->begin(); row_i != table->end(); ++row_i) {
    // Check for shutdown
    if (st.stop_requested()) {
        root->truncate();
        return {nullptr, key, idx, tid, look_aside_index};
    }

    // Check for concurrent drop every 1000 rows
    if (row_cnt % DROP_CHECK_PERIOD == 0 && _was_dropped(key)) {
        return {root, key, idx, tid, look_aside_index};
    }

    // Insert (index_columns, internal_row_id) into B-tree
    root->insert(svalue);
}

Index Reconciliation (`_reconcile_index`)

After the initial build, changes made to the table during the build must be applied: Extent Chain Processing:

auto next_eid = table->get_stats().end_offset;
auto next_extent_result = table->read_extent_from_disk(next_eid);
auto next_extent = next_extent_result.first;

while (next_extent) {
    // Invalidate previous extent (remove old entries)
    if (prev_eid exists && not already invalidated) {
        for each row in prev_extent:
            idx_state._root->remove(svalue);
        invalidated_eids.insert(prev_eid);
    }

    // Populate with new extent entries
    for each row in next_extent:
        idx_state._root->insert(svalue);

    // Move to next extent in chain
    next_eid = next_extent_result.second;
}

Decision Logic:

Work Item Status	Action
`DELETING`	Call `_drop()` directly
`ABORTING`	Call `_commit_build()` to truncate and mark deleted
`BUILDING`	Process extent chain, then call `_commit_build()` to finalize

Index Commit (`_commit_build`)

Finalizes an index build or abort:

void _commit_build(MutableBTreePtr root, const Key& key,
                   const IndexParams& idx, uint64_t end_xid,
                   MutableBTreePtr look_aside_root)
{
    // 1. Fetch latest work item state (may have changed to ABORTING)
    work_item = _work_set.at(key);
    _work_set.erase(key);

    // 2. Get current index state from metadata
    index_info = server->get_index_info(db_id, index_id, xid);

    // 3. Handle based on metadata state and work item status
    if (index_info.state == DELETED) {
        // Table resync occurred - just cleanup
        root->truncate(); root->finalize();
    }
    else if (work_item.is_status(BUILDING)) {
        // Success path
        extent_id = root->finalize();
        meta->roots.emplace_back(index_id, extent_id);
        server->update_roots(...);
        server->set_index_state(..., READY);
    }
    else if (work_item.is_status(ABORTING)) {
        // Abort path
        root->truncate(); root->finalize();
        server->set_index_state(..., DELETED);
    }

    // 4. Cleanup tracking structures
    _remove_index_key(db_id, tid, key);
}

DDL Counter Management

The counter ensures the Committer only receives reconciliation notifications when ALL index operations for a transaction are complete:

// Initialization in process_requests()
_xid_ddl_counter_map[xid].store(index_requests.size());

// Decrement on each operation completion
if (--_xid_ddl_counter_map[xid] == 0) {
    _xid_ddl_counter_map.erase(xid);

    // Notify Committer via queue
    _index_reconciliation_queue_mgr->push(db_id,
        std::make_shared<IndexReconcileRequest>(db_id, xid));
}

Counter Decrement Points:

build() - when index already READY (skip)
drop() - when marking existing work item as ABORTING
abort_indexes() - after marking all table indexes as ABORTING
_add_to_pending_reconciliation() - after build/worker processing completes

Index Drop (`_drop`)

Handles direct index deletion:

void _drop(const Key& key, const IndexParams& idx, uint64_t end_xid)
{
    // 1. Verify state and remove from work_set
    CHECK(idx._status == IndexStatus::DELETING);
    _work_set.erase(key);

    // 2. Validate index exists and is in BEING_DELETED state
    info = server->get_index_info(db_id, index_id, xid);
    if (info.id() == 0) return;  // Invalid index

    // 3. Handle table-dropped case
    if (!table_exists) {
        server->set_index_state(..., DELETED);
        return;
    }

    // 4. Truncate the B-tree
    root->init(extent_id);
    root->truncate();
    root->finalize();

    // 5. Handle look-aside if this is the last secondary index
    if (is_last_index) {
        look_aside_root->truncate();
        look_aside_root->finalize();
        meta->roots.emplace_back(INDEX_LOOK_ASIDE, UNKNOWN_EXTENT);
        server->update_roots(...);
    }

    // 6. Mark as deleted and cleanup
    server->set_index_state(..., DELETED);
    _remove_index_key(...);
}

Look-Aside Index

Structure:

Key:   internal_row_id (stable, never changes for a row)
Value: (extent_id, row_id_within_extent)

How it helps:

Secondary indexes store internal_row_id instead of direct (extent_id, row_offset) pointers
When a data extent is rewritten, only the look-aside index needs to be updated with the new physical location
Secondary indexes remain unchanged, eliminating cascading updates

Lifecycle:

Created: With the first secondary index on a table
Updated: During index reconciliation when extents change
Dropped: When the last secondary index on a table is dropped

Coordination for Concurrent Creation:

std::unique_lock g(_look_aside_mutex);
if (_look_aside_build_tracker.find(tid) == _look_aside_build_tracker.end()) {
    // First thread to reach here builds the look-aside index
    _look_aside_build_tracker[tid] = true;
    build_look_aside = true;
}
// Other threads skip look-aside building

Thread Safety

Lock Ordering (to prevent deadlocks):

_m (work_set, queue)
_table_idx_map_mtx
_pending_reconciliation_map_mtx
_xid_ddl_counter_map_mtx
_look_aside_mutex

Patterns Used:

std::scoped_lock for acquiring multiple locks atomically
std::atomic<int> for DDL counters to minimize contention
Separate mutexes for independent data structures to allow parallelism

Error Handling and Edge Cases

Scenario	Handling
Index already READY at build time	Skip processing, decrement DDL counter
Stop requested during build	Truncate partial B-tree, return null root for recovery
Drop while building	Mark ABORTING, worker detects via periodic check
Table dropped before index drop	Mark index DELETED without truncating
Queue unavailable during reconciliation	Abort reconciliation (will retry on recovery)
Table resync during build	Index marked DELETED via abort_indexes

Public API Summary

Method	Description
`process_requests(db_id, xid, requests)`	Entry point for index create/drop/abort operations
`recover_indexes(db_id)`	Recovers incomplete operations after restart
`process_index_reconciliation(db_id, reconcile_xid, end_xid)`	Finalizes pending index operations
`abort_indexes(db_id, table_id, xid)`	Aborts all index builds for a table (used during resync)

​Overview

​Responsibilities

​Integration with Committer

​Key Components

​Index Status States

​Core Data Structures

​IndexParams

​IndexState

​Key Type

​Internal Maps and Queues

​Synchronization Primitives

​Data Flow

​Index Creation Flow

​Index Drop Flow

​Index Recovery Flow

​Abort Indexes Flow (Table Resync)

​Implementation Details

​Worker Thread Model

​Index Build Process (_build)

​Index Reconciliation (_reconcile_index)

​Index Commit (_commit_build)

​DDL Counter Management

​Index Drop (_drop)

​Look-Aside Index

​Thread Safety

​Error Handling and Edge Cases

​Public API Summary