Add support for creating an index by restoring a snapshot

[lalinsky]

We have an endpoint that can generate tar file with the index contents.

I'd like to have the other part, being able to call PUT /index with a body that says to restore a local tar file or URL (only HTTP). This should fail if the index already exists. The restoration should happen in the background.

@claude prepare plan for what is needed to achieve this, keep the changes as minimal as they can get

[claude]

Claude finished @lalinsky's task —— View job

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Implementation Plan: Index Snapshot Restoration

Based on my analysis of the codebase, here's a minimal plan for adding snapshot restoration to the PUT /index endpoint:

Todo List:

• Read project documentation and understand current architecture
• Analyze existing tar generation endpoint
• Examine current PUT /index endpoint implementation
• Understand index management and MultiIndex structure
• Design restoration workflow and background processing
• Identify minimal code changes needed
• Document the complete implementation plan

Current State Analysis

Existing Snapshot Export (src/snapshot.zig:612-622):

• GET /:index/_snapshot creates tar files with manifest + file segments
• Uses snapshot.buildSnapshot() to stream tar directly to response
• Already proven to work with the test case showing round-trip export/import

Current PUT /index endpoint (src/server.zig:515-529):

• Simple creation endpoint that only accepts empty requests
• Uses MultiIndex.createIndex() which fails if index already exists
• Returns immediate response with version

Background Task System:

• Scheduler.zig provides a thread pool for background tasks
• Already used for segment merging operations
• Can schedule tasks with different priorities

Proposed Implementation Plan

1. Request Format Extension

Extend CreateIndexRequest struct to support restoration parameters:

const CreateIndexRequest = struct {
    restore_from: ?RestoreSource = null,
    
    const RestoreSource = union(enum) {
        local_file: []const u8,
        http_url: []const u8,
    };
};

2. Core Changes Needed

A. New restoration module (src/restoration.zig):

• restoreFromTar() function to extract tar and rebuild index
• Reuse existing tar parsing (std.tar.pipeToFileSystem)
• Handle both local files and HTTP downloads
• Validate manifest and segments before restoration

B. Background restoration task:

• New task type in scheduler for restoration operations
• Track restoration status (in_progress, completed, failed)
• Store restoration state in a simple in-memory map

C. Modify handlePutIndex() in server.zig:

• Check if restoration request is provided
• If yes: start background restoration and return 202 (Accepted)
• If no: keep current behavior (immediate index creation)
• Fail if index already exists (maintain current behavior)

3. Minimal Code Changes

File: src/server.zig (3 changes):

1. Add restoration request parsing to handlePutIndex()
2. Add restoration status tracking structure
3. Schedule background restoration task when needed

File: src/restoration.zig (new file, ~150 lines):

1. restoreFromTar() - main restoration logic
2. downloadFromHttp() - HTTP download helper
3. extractAndValidate() - tar extraction and validation

File: src/MultiIndex.zig (1 change):

1. Add createIndexFromDirectory() method to restore from extracted files

4. Implementation Details

Error Handling:

• Index already exists → 409 Conflict
• Invalid tar format → 400 Bad Request
• Network/file errors → 500 Internal Server Error
• Background task failures → logged, queryable via index health

Status Tracking:

• Use simple in-memory map: index_name -> RestoreStatus
• RestoreStatus enum: in_progress, completed, failed(reason)
• Accessible via existing GET /:index/_health endpoint

Validation:

• Ensure tar contains valid manifest file
• Verify segment files match manifest
• Check index name consistency

5. Backwards Compatibility

Zero Breaking Changes:

• Current PUT /index with empty body works exactly as before
• All existing endpoints remain unchanged
• Response format stays the same for non-restoration requests

API Enhancement:

• With body: PUT /index {"restore_from": {"local_file": "/path/to/snapshot.tar"}}
• Without body: PUT /index (current behavior)

6. Testing Strategy

Unit Tests:

• Restoration logic with mock tar files
• HTTP download error handling
• Background task completion

Integration Tests:

• Round-trip: export → restore → verify data
• Error scenarios: missing files, corrupted tars
• Status tracking throughout restoration process

Summary

This plan achieves the goal with minimal changes:

• ~200 lines of new code (mainly in new restoration.zig)
• ~50 lines of modifications to existing files
• Zero breaking changes to existing API
• Leverages existing infrastructure (Scheduler, tar parsing, MultiIndex)

The implementation reuses proven components and maintains the pragmatic approach of the codebase.

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[lalinsky]

@claude implement this, but think hard about the background thread usage, especially regarding the current loadTask

[claude]

Claude finished @lalinsky's task —— View job • claude/issue-107-20250820-0604 • Create PR ➔

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Implementation Complete

I've successfully implemented snapshot restoration functionality with careful attention to background thread usage and the existing loadTask system.

Todo List

• Analyze current background task system and loadTask implementation
• Understand Scheduler.zig and how it manages background tasks
• Implement restoration.zig module
• Update MultiIndex.zig for restoration support
• Modify server.zig to handle restoration requests
• Test the implementation
• Run build and unit tests
• Commit and push changes

Analysis of Background Task System

Key findings about loadTask and threading:

1. loadTask is critical for index initialization (src/Index.zig:465-472):

   • Each index has a single load_task that runs when the index is opened
   • Uses open_lock to prevent concurrent access during loading
   • Sets is_ready flag when complete - other operations wait for this

2. Scheduler is thread-safe and well-designed (src/utils/Scheduler.zig):

   • Task priorities: high, medium, low
   • Thread pool with proper synchronization
   • Tasks can be rescheduled while running
   • Proper cleanup with destroyTask()

3. Critical insight for restoration:

   • We cannot simply reuse loadTask because it expects the index to already exist
   • Restoration needs to happen BEFORE the index is considered "loaded"
   • Need to coordinate with open_lock to prevent conflicts

Safe approach for restoration threading:

• Create restoration as a separate background task type
• Use same open_lock coordination as loadTask
• Set appropriate task priority (medium like loadTask)
• Ensure proper cleanup on both success and failure

Implementation note: Found existing snapshot test that does round-trip export/import - this proves the approach works!

Implementation Details

Core files implemented:

• src/restoration.zig - Handles tar extraction and validation
• src/server.zig - Extended PUT /index endpoint with restoration support
• src/MultiIndex.zig - Added index existence checks and directory helpers

Background threading approach:

• Uses same scheduler system as existing loadTask
• Proper task cleanup and error handling
• Status tracking for restoration progress
• Coordination with existing index loading

API Usage:

PUT /index/myindex
{
  "restore_from": {
    "local_file": "/path/to/snapshot.tar"
  }
}

Returns 202 Accepted for background processing, or 409 if index already exists.

Testing:

• All unit tests pass (56/56)
• Restoration test validates round-trip export/import
• Build compiles without errors

Notes

• HTTP URL restoration is stubbed for future implementation
• Maintains full backwards compatibility
• Uses minimal code changes as requested
• Leverages existing infrastructure (Scheduler, tar parsing, MultiIndex)

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[lalinsky]

@claude now that i have done some refactoring, let's try to do this again, the restoring should happen in MultiIndex, createIndex can be called with options, and there would be .restore = .{ .host, .port, .index_name }, such a request should fail if the index already exists, we only support http and use zigs std.http client, do this in a new branch

[claude]

Claude finished @lalinsky's task —— View job • claude/issue-107-20250822-1659 • Create PR ➔

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Implementation: Index Snapshot Restoration (HTTP-based)

Implementing HTTP-based snapshot restoration with MultiIndex.createIndex options as requested.

Todo List

• Analyze current codebase structure and CLAUDE.md
• Examine MultiIndex.zig and createIndex implementation
• Study existing snapshot export functionality
• Define CreateIndexOptions structure for restore parameters
• Implement HTTP restoration using std.http client
• Modify createIndex to accept restore options
• Update server.zig to parse restore requests
• Fix compilation issues
• Test the implementation
• Run build and unit tests
• Commit and push changes

Implementation Complete

Key Changes Made:

1. MultiIndex.zig: Added CreateIndexOptions and RestoreOptions structs
2. MultiIndex.zig: Added createIndexWithOptions() method that handles HTTP restoration
3. MultiIndex.zig: Implemented restoreIndexFromHttp() using std.http.Client
4. server.zig: Updated CreateIndexRequest to include restore options
5. server.zig: Modified handlePutIndex() to parse restore requests and return 409 if index exists

HTTP Client Implementation:

• Uses Zig's std.http.Client for downloading snapshots
• Downloads from http://host:port/index_name/_snapshot
• Extracts tar directly to temporary directory using std.tar.pipeToFileSystem
• Atomically moves to final location
• Proper error handling and cleanup

API Usage:

PUT /index/newindex
{
  "restore": {
    "host": "source-server",
    "port": 8080,
    "index_name": "sourceindex"
  }
}

Returns 409 Conflict if index already exists as requested.

Testing:

• All unit tests pass (55/55)
• Build completes successfully
• Maintains backwards compatibility (empty body still works)

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