Semantic Search

Semantic Search is a GitLab framework that uses vector embeddings to find semantically similar content based on meaning rather than keyword matching. This enables AI features like Duo Chat to retrieve relevant context for user queries.

Overview

Semantic Search converts text into vector embeddings and stores them in a vector store. When a user makes a query, the query is also converted to an embedding and compared against stored vectors to find the most similar results. This approach captures semantic meaning, allowing searches to find relevant content even when exact keywords don’t match.

Semantic Code Search

Semantic Code Search is the first implementation of the Semantic Search framework. It enables Duo Chat and other AI features to find relevant code snippets from a repository. The feature is available as an MCP tool (semantic_code_search) that can be used by GitLab Duo Agent Platform and other AI platforms.

Please refer to the Semantic Code Search Architecture for further details.

Architecture

The Semantic Search framework is powered by the gitlab-active-context gem. This gem provides a translation layer for different vector stores (Elasticsearch, OpenSearch, PostgreSQL with pgvector), allowing the same code to work with any supported vector store without needing vector store-specific implementations.

The framework is extensible and designed to support multiple types of semantic search. Each semantic search type is implemented using:

• Collections (Ai::ActiveContext::Collections::<Type>): Define what content is indexed and how it’s stored
• References (Ai::ActiveContext::References::<Type>): Track and manage embeddings for content updates
• Queries (Ai::ActiveContext::Queries::<Type>): Retrieve similar content from the vector store
• Queues (Ai::ActiveContext::Queues::<Type>): Manage asynchronous processing of embedding generation
• Migrations: Execute schema changes and data transformations on the vector store

New semantic search types can be added by implementing these components for different content types (for example, merge requests or documentation).

Embedding generation

Embeddings are generated asynchronously through a queue system using reference classes like Ai::ActiveContext::References::Code:

1. References are tracked: When content is created or updated, embedding references are tracked in the appropriate reference class
2. Batch processing: References are processed in batches by the Ai::ActiveContext::BulkProcessWorker
3. Vector storage: Generated embeddings are stored in the configured vector store

The BulkProcessWorker is a cron job that runs every minute and processes embedding references from the queue. It fetches references, generates embeddings, and removes them from the queue. If the queue is not empty after processing, the worker re-enqueues itself to continue processing. If embedding generation fails, it gets retried once and is then placed on a dead queue.

Embedding models

Currently, semantic search uses Vertex AI’s text-embedding-005 model for generating embeddings. The model configuration is defined in the collection classes (for example, Ai::ActiveContext::Collections::Code).

Support for selecting different embedding models is planned in epic 20110. Once available, administrators will be able to choose from multiple embedding models based on their needs.

Query execution

When a query is executed:

1. Embedding generation: The user’s query is converted to an embedding using the same model as the indexed content
2. Vector search: The embedding is compared against stored vectors using k-nearest neighbors (KNN) search
3. Filtering: Results are filtered by relevant criteria (for example: project, file path)
4. Authorization: Results are filtered to only include content the user has access to
5. Result limit: By default, the 10 most similar results are returned

Migrations

The Semantic Search framework uses a migration system to manage schema changes and data transformations for the connected vector store. Migrations are tracked in the database and executed asynchronously by a worker process.

Ai::ActiveContext::MigrationWorker runs as a cron job every 5 minutes to execute uncompleted migrations.

Vector stores

An instance can use one of the following vector stores:

• Elasticsearch
• OpenSearch
• PostgreSQL with pgvector

A vector store connection must be created before semantic search can be used. There are two ways to configure the connection:

Option 1: Using the GitLab UI

For Elasticsearch or OpenSearch clusters used by Advanced Search:

1. Navigate to Admin > Settings > Search
2. Click the button to connect to the Advanced Search cluster
3. The connection is automatically created and configured

Option 2: Using Rails console

connection = Ai::ActiveContext::Connection.create!(
  name: "os",
  options: { url: ["http://localhost:9202"] },
  adapter_class: "ActiveContext::Databases::OpenSearch::Adapter"
)
connection.activate!

Supported adapter classes:

• ActiveContext::Databases::Elasticsearch::Adapter
• ActiveContext::Databases::OpenSearch::Adapter
• ActiveContext::Databases::Postgres::Adapter

The options hash should contain the connection details specific to your vector store (URL, credentials, etc.).

Semantic Code Search Architecture

Indexing workflow

Initial indexing

When the Semantic Code Search tool is invoked for a project that hasn’t been indexed yet:

1. Repository record creation: An Ai::ActiveContext::Code::Repository record is created with pending state
2. Index worker: The Ai::ActiveContext::Code::RepositoryIndexWorker processes the pending repository
3. Initial indexing:
   1. The Ai::ActiveContext::Code::InitialIndexingService calls the Ai::ActiveContext::Code::Indexer
   2. The Indexer runs the gitlab-elasticsearch-indexer to fetch the repository’s files from Gitaly, chunk the code, and index the chunks in the vector store
   3. The InitialIndexingService enqueues the references/IDs of the indexed content for embeddings generation
4. Async processing: Queued content references are picked up for embeddings generation via the asynchronous Ai::ActiveContext::BulkProcessWorker.
5. Tool not available: The user is notified that indexing is in progress and should try again in a few minutes.
6. Ready check: The Ai::ActiveContext::Code::MarkRepositoryAsReadyEventWorker runs on a 10-minute cron schedule (via SchedulingService) and checks if all embeddings have been generated. Once all embeddings are ready, it marks the repository as ready
7. Available for queries: The next time the tool is invoked, the repository is ready and can be used for semantic search queries

Incremental indexing

When code is merged into the default branch:

1. Push event: A push event triggers the incremental indexing process through the BranchPushService
2. Index worker: The Ai::ActiveContext::Code::RepositoryIndexWorker processes the ready ActiveContext repository
3. Incremental Indexing - only the changed files are processed
   1. The Ai::ActiveContext::Code::IncrementalIndexingService calls the Ai::ActiveContext::Code::Indexer
   2. The Indexer runs the gitlab-elasticsearch-indexer to fetch the changed files from Gitaly, chunk the code, and index the chunks in the vector store. It also deletes orphaned data from the vector store.
   3. The IncrementalIndexingService enqueues the references/IDs of the indexed content for embeddings generation
4. Async processing: Queued content references are picked up for embeddings generation via the asynchronous Ai::ActiveContext::BulkProcessWorker.

Deletion workflow

When a namespace is no longer eligible for indexing, Ai::ActiveContext::Code::ProcessInvalidEnabledNamespaceEventWorker picks it up and deletes the EnabledNamespace record.

When a repository is no longer eligible for indexing, Ai::ActiveContext::Code::MarkRepositoryAsPendingDeletionEventWorker marks it as pending_delete. The Ai::ActiveContext::Code::RepositoryIndexWorker then processes the repository and calls the gitlab-elasticsearch-indexer to delete the project’s documents from the vector store and delete the repository record.

gitlab-elasticsearch-indexer

The gitlab-elasticsearch-indexer Go project handles:

• fetching the repository’s files from Gitaly
• chunking the files
• indexing the chunked content on the vector store
• deleting orphaned data
• deleting all data for a project

Chunking

The gitlab-elasticsearch-indexer makes use of the gitlab-code-parser library to split the code into logic chunks.

The chunking process uses a two-stage approach:

1. AST-aware chunking: The code chunker parses each file and identifies logical split points (function definitions, class definitions, etc.)
2. Size-based fallback: If no AST split points are available, the chunker falls back to splitting on line boundaries while respecting a maximum byte size

This approach ensures chunks are semantically meaningful while staying within size limits for embedding generation.

Namespace eligibility

Not all namespaces are eligible for Semantic Code Search. Eligibility is managed through two workers:

Ai::ActiveContext::Code::CreateEnabledNamespaceEventWorker (runs daily via SchedulingService)

• Identifies and enables eligible namespaces
• Creates EnabledNamespace records for qualifying namespaces

On GitLab.com, a namespace is eligible if:

• AI features are enabled in the namespace settings
• The namespace has a supported AI plan (Premium or higher)
• The subscription is not expired

On self-managed instances, all top-level group namespaces are eligible if:

• Instance-level AI beta features are enabled (instance_level_ai_beta_features_enabled)
• AI features are available in the license

Ai::ActiveContext::Code::MarkRepositoryAsPendingDeletionEventWorker marks repositories for deletion when they no longer meet eligibility criteria.

Ai::ActiveContext::Code::ProcessInvalidEnabledNamespaceEventWorker cleans up EnabledNamespace records for namespaces that no longer meet eligibility criteria.

Supported file types

Semantic Code Search indexes all files in a repository. Currently, results are post-filtered to exclude files matching the project’s exclusion rules. Future versions will stop indexing excluded files entirely for improved efficiency.

MCP implementation

For more information about GitLab MCP implementation and available clients, see the GitLab MCP documentation and the runbook.

Currently, the Semantic Code Search tool is available in IDEs when GitLab MCP is configured. With the rollout of the mcp_client feature flag, it will be available on the web.

Extending Semantic Search

For detailed information on extending the Semantic Search framework, see the gitlab-active-context gem documentation.

To add a new semantic search type (for example, merge requests or documentation), implement the following components:

• Collection (Ai::ActiveContext::Collections::<Type>): Define the collection name, queue, reference class, and how to handle authorization
• Reference (Ai::ActiveContext::References::<Type>): Extend ActiveContext::Reference to track embeddings and define preprocessors for content and embedding generation
• Query (Ai::ActiveContext::Queries::<Type>): Implement query logic to search the vector store
• Queue (Ai::ActiveContext::Queues::<Type>): Define the queue for managing asynchronous processing
• Workers: Create workers for indexing, processing, and managing the lifecycle of the new semantic search type

See the Semantic Code Search implementation for a complete example of how these components work together.

Enabling Semantic Code Search

Prerequisites

• Vector store connection configured (Elasticsearch, OpenSearch, or PostgreSQL with pgvector)
• Vertex AI credentials configured for embedding generation
• Beta experiment features setting enabled for the instance

Verifying prerequisites

Vector store connection

Test that the vector store connection is working:

ActiveContext::adapter.search(
  user: current_user,
  collection: ::Ai::ActiveContext::Collections::Code,
  query: ActiveContext::Query.all
)

This should return results without errors.

Vertex AI credentials

Test that embedding generation is configured:

::ActiveContext::Embeddings.generate_embeddings(
  "test",
  version: ::Ai::ActiveContext::Collections::Code::MODELS[1]
)

This should return a vector.

Beta experiment features

Verify that beta experiment features are enabled for the namespace:

namespace.experiment_features_enabled?

This should return true.

Disabling Semantic Code Search

WARNING: Disabling semantic code search can cause long database locks if there are many repository records to delete. Use with caution on production environments. Upcoming work will allow disabling safely. See issue 582787.

Delete the index and collection record:

ActiveContext.adapter.executor.drop_collection(:code)

Delete the connection and associated records:

::Ai::ActiveContext::Connection.active.destroy!

Developer Guide

Setting up MCP locally

To set up the MCP server locally for development and testing, see the MCP server development guide.

Tip: specifically ask for the semantic_code_search tool in your prompt to ensure the tool is used.

Using Semantic Code Search

To invoke semantic search from your console, use the Ai::ActiveContext::Queries::Code class:

# Check if semantic code search is available
Ai::ActiveContext::Queries::Code.available?

# Perform a semantic search
result = Ai::ActiveContext::Queries::Code.new(
  search_term: "user authentication logic",
  user: current_user
).filter(
  project_id: project.id,
  path: "app/controllers/",  # Optional: filter by directory
  knn_count: 10,             # Number of vectors to compare
  limit: 10                  # Number of results to return
)

Managing queued items

View all queued items waiting to be processed:

ActiveContext::Queues.all_queued_items

Immediately process all queued items without waiting for cron workers:

ActiveContext.execute_all_queues!

Searching the vector store

Find all items in the vector store:

ActiveContext::adapter.search(
  user: current_user,
  collection: ::Ai::ActiveContext::Collections::Code,
  query: ActiveContext::Query.all
)

Resetting the connection

To start fresh with a new connection, destroy all existing data and recreate:

active_connection = ::Ai::ActiveContext::Connection.active
active_connection.migrations.destroy_all
active_connection.repositories.destroy_all
active_connection.enabled_namespaces.destroy_all
active_connection.collections.destroy_all
active_connection.destroy

Then create and activate a new connection. When creating a migration in Rails console, remember to run:

connection.activate!

Troubleshooting

Semantic search returns no results

Possible causes:

1. Repository is not indexed yet (state is embedding_indexing_in_progress)
   • Check: Ai::ActiveContext::Code::Repository.find_by(project_id: project.id).state
   • Solution: Wait for indexing to complete or manually trigger processing by running ActiveContext.execute_all_queues!
2. Namespace is not eligible
   • Check: Ai::ActiveContext::Code::EnabledNamespace.exists?(namespace_id: project.root_namespace.id)
   • Solution: Verify namespace meets eligibility criteria
3. Vector store connection is not configured
   • Check: Ai::ActiveContext::Connection.active.present?
   • Solution: Configure vector store connection

Clearing dead queue items

If embedding generation fails repeatedly, items may be placed on a dead queue. Clear them using:

# Clear all dead queue items
ActiveContext::DeadQueue.clear_tracking!

# Or clear a specific queue
Ai::ActiveContext::Queues::Code.clear_tracking!