TantivyEx Distributed Search

This document outlines the OTP-based distributed search implementation for TantivyEx, leveraging Elixir's robust OTP features for fault-tolerant, scalable distributed search coordination.

Architecture Overview

The TantivyEx distributed search system is built using Elixir's OTP (Open Telecom Platform) patterns, providing:

OTP Supervision Tree

TantivyEx.Distributed.Supervisor
├── Registry (Node discovery)
├── Coordinator (GenServer - orchestration)
├── NodeSupervisor (DynamicSupervisor - node management)
│   ├── SearchNode (GenServer per node)
│   └── SearchNode (GenServer per node)
└── TaskSupervisor (Concurrent operations)

Key Benefits

1. Fault Tolerance

• Automatic process restart on failures
• Supervisor strategies for different failure scenarios
• Graceful degradation when nodes fail
• Built-in circuit breaker patterns

2. Scalability

• Dynamic node addition/removal
• Process-per-node isolation
• Horizontal scaling through distributed Erlang
• Load balancing at the process level

3. Monitoring & Observability

• Built-in process monitoring
• Health check integration
• Performance metrics collection
• Real-time cluster status

4. Maintainability

• Pure Elixir implementation
• Standard OTP patterns
• Clear separation of concerns
• Better testing capabilities

Implementation Details

Core Components

1. Supervisor (TantivyEx.Distributed.Supervisor)

• Manages the entire distributed search infrastructure
• Implements fault tolerance strategies
• Handles system initialization and shutdown

2. Coordinator (TantivyEx.Distributed.Coordinator)

• Central orchestration GenServer
• Manages cluster configuration
• Handles search request distribution
• Implements merge strategies

3. SearchNode (TantivyEx.Distributed.SearchNode)

• Individual node GenServer
• Manages local Tantivy searcher
• Handles health monitoring
• Tracks performance metrics

4. Registry

• Service discovery for nodes
• Process tracking and naming
• Dynamic node registration

Search Flow

1. Request Reception: Coordinator receives search request
2. Node Selection: Apply load balancing strategy to select active nodes
3. Concurrent Execution: Task.Supervisor manages parallel searches
4. Result Collection: Gather results with timeout handling
5. Merge Strategy: Apply configured merge algorithm
6. Response Formation: Return unified response with metadata

Load Balancing Strategies

Round Robin

defp select_nodes_round_robin(active_nodes, state) do
  count = length(active_nodes)
  index = rem(state.node_round_robin_counter, count)
  selected = Enum.at(active_nodes, index)
  {[selected], %{state | node_round_robin_counter: index + 1}}
end

Weighted Round Robin

defp select_nodes_weighted(active_nodes, _state) do
  total_weight = Enum.sum(Enum.map(active_nodes, & &1.weight))
  # Implement weighted selection logic
end

Health-Based

defp select_healthy_nodes(active_nodes, _state) do
  Enum.filter(active_nodes, fn node ->
    SearchNode.get_health_status(node.pid) == :healthy
  end)
end

Health Monitoring

Each SearchNode performs periodic health checks:

def handle_info(:health_check, state) do
  health_status = perform_health_check(state.searcher)

  # Auto-deactivate unhealthy nodes
  new_state = case health_status do
    :unhealthy -> %{state | active: false}
    :healthy -> %{state | active: true}
    _ -> state
  end

  {:noreply, new_state}
end

Migration Plan

Phase 1: Parallel Implementation

• [x] Create OTP-based modules alongside existing native implementation
• [x] Implement core functionality (Supervisor, Coordinator, SearchNode)
• [x] Add comprehensive test suite
• [x] Create clean API interface (TantivyEx.Distributed.OTP)

Phase 2: Feature Parity

• [ ] Implement all merge strategies
• [ ] Add advanced load balancing algorithms
• [ ] Create performance benchmarks
• [ ] Add configuration validation
• [ ] Implement distributed Erlang support

Phase 3: Migration & Deprecation

• [ ] Update documentation to recommend OTP implementation
• [ ] Add migration utilities
• [ ] Deprecate native implementation
• [ ] Remove native coordination code

Usage Examples

Basic Setup

# Start the distributed search system
{:ok, _pid} = TantivyEx.Distributed.OTP.start_link()

# Add search nodes
:ok = TantivyEx.Distributed.OTP.add_node("node1", "local://index1", 1.0)
:ok = TantivyEx.Distributed.OTP.add_node("node2", "local://index2", 1.5)

# Configure behavior
:ok = TantivyEx.Distributed.OTP.configure(%{
  timeout_ms: 5000,
  merge_strategy: :score_desc,
  health_check_interval: 30_000
})

Advanced Configuration

# Custom supervision tree
opts = [
  name: MyApp.DistributedSearch,
  coordinator_name: MyApp.SearchCoordinator,
  registry_name: MyApp.SearchRegistry
]

{:ok, _pid} = TantivyEx.Distributed.OTP.start_link(opts)

# Bulk node addition
nodes = [
  {"primary", "local://primary_index", 3.0},
  {"secondary", "local://secondary_index", 2.0},
  {"cache", "local://cache_index", 1.0}
]

:ok = TantivyEx.Distributed.OTP.add_nodes(nodes)

Production Deployment

# Application supervisor integration
children = [
  {TantivyEx.Distributed.OTP,
   name: MyApp.Search,
   coordinator_name: MyApp.SearchCoordinator}
]

Supervisor.start_link(children, strategy: :one_for_one)

Performance Considerations

Memory Usage

• Each SearchNode maintains its own state
• Registry overhead is minimal
• Process memory isolation prevents memory leaks

Latency

• Process message passing adds minimal latency (~1-5µs)
• Concurrent execution reduces overall response time
• Task supervision enables timeout handling

Throughput

• Multiple concurrent searches supported
• Process-per-node enables true parallelism
• No global locks or bottlenecks

Testing Strategy

Unit Tests

• Individual component testing (GenServers, functions)
• Mock implementations for external dependencies
• Property-based testing for merge algorithms

Integration Tests

• End-to-end search flow testing
• Failure scenario testing
• Performance benchmarking

Fault Tolerance Tests

• Process crash simulation
• Network partition testing
• Recovery time measurement

Future Enhancements

Distributed Erlang

• Multi-node cluster support
• Automatic node discovery
• Cross-node failover

Advanced Monitoring

• Telemetry integration
• Metrics export (Prometheus, etc.)
• Real-time dashboards

Smart Load Balancing

• Machine learning-based routing
• Adaptive algorithms
• Geographic distribution

Conclusion

The OTP-based implementation provides a more robust, scalable, and maintainable foundation for distributed search in TantivyEx. By leveraging Elixir's battle-tested concurrency model and fault tolerance mechanisms, we achieve better reliability and performance while maintaining clean, idiomatic Elixir code.

This implementation serves as a foundation for future enhancements and provides a clear migration path from the native coordination approach.