Glean migration deep dive

Glean Migration Deep Dive

Overview

Comprehensive guide for migrating enterprise search from Elasticsearch or Algolia to Glean. Covers connector migration (replacing custom crawlers with Glean's push indexing API), permission model changes (mapping ACLs to Glean's datasource-level permissions), and full index rebuilds using bulkindexdocuments. Typical timeline is 2-4 weeks for a mid-size deployment with 100K-1M documents across multiple datasources.

Migration Assessment

// Scan current integration for deprecated patterns and index health
const assessment = {
  source: process.env.SEARCH_PROVIDER ?? 'elasticsearch',
  indices: await sourceClient.cat.indices({ format: 'json' }),
  totalDocs: 0, connectors: [] as string[], permissionModel: '',
};
for (const idx of assessment.indices) {
  assessment.totalDocs += parseInt(idx['docs.count'] ?? '0', 10);
  assessment.connectors.push(idx.index);
}
assessment.permissionModel = assessment.source === 'elasticsearch' ? 'index-level' : 'api-key';
console.log(`Source: ${assessment.source}`);
console.log(`Indices: ${assessment.connectors.length} | Total docs: ${assessment.totalDocs}`);
console.log(`Permission model: ${assessment.permissionModel} → Glean datasource ACLs`);

Step-by-Step Migration

Phase 1: Prepare

Export all documents from the current search provider and map them to Glean's document schema. Each document needs id, title, url, body, and permissions.

interface GleanDocument {
  id: string;
  datasource: string;
  title: string;
  url: string;
  body: { mimeType: 'text/plain' | 'text/html'; content: string };
  permissions: { allowedUsers?: string[]; allowedGroups?: string[] };
  updatedAt: string;
}

async function exportAndTransform(sourceIndex: string): Promise<GleanDocument[]> {
  const docs: GleanDocument[] = [];
  let scrollId: string | undefined;
  do {
    const batch = scrollId
      ? await sourceClient.scroll({ scroll: '2m', scroll_id: scrollId })
      : await sourceClient.search({ index: sourceIndex, scroll: '2m', size: 500 });
    scrollId = batch._scroll_id;
    for (const hit of batch.hits.hits) {
      docs.push({
        id: hit._id, datasource: 'custom_' + sourceIndex,
        title: hit._source.title, url: hit._source.url,
        body: { mimeType: 'text/plain', content: hit._source.body },
        permissions: { allowedGroups: hit._source.acl_groups ?? ['everyone'] },
        updatedAt: hit._source.updated_at ?? new Date().toISOString(),
      });
    }
  } while (scrollId && docs.length < 1_000_000);
  return docs;
}

Phase 2: Migrate

Create the Glean datasource and bulk-index all transformed documents.

// Create datasource
await gleanClient.post('/api/index/v1/adddatasource', {
  name: 'custom_knowledge_base',
  displayName: 'Knowledge Base',
  homeUrl: 'https://kb.example.com',
  objectDefinitions: [{ name: 'Article', propertyDefinitions: [] }],
});

// Bulk index in batches of 200
const BATCH_SIZE = 200;
for (let i = 0; i < docs.length; i += BATCH_SIZE) {
  const batch = docs.slice(i, i + BATCH_SIZE);
  await gleanClient.post('/api/index/v1/bulkindexdocuments', {
    datasource: 'custom_knowledge_base',
    documents: batch.map(d => ({
      id: d.id, title: d.title, url: d.url, body: d.body,
      permissions: d.permissions, updatedAt: d.updatedAt,
    })),
  });
  console.log(`Indexed ${Math.min(i + BATCH_SIZE, docs.length)}/${docs.length}`);
}

Phase 3: Validate

Run test queries against both old and new systems and compare result quality.

const testQueries = ['onboarding process', 'security policy', 'API authentication'];
for (const query of testQueries) {
  const result = await gleanClient.post('/api/client/v1/search', {
    query, pageSize: 5, requestOptions: { datasourceFilter: 'custom_knowledge_base' },
  });
  console.log(`"${query}" → ${result.results.length} results, top: ${result.results[0]?.title}`);
}

Rollback Plan

# Delete the Glean datasource (removes all indexed documents)
curl -X DELETE "https://your-instance.glean.com/api/index/v1/deletedatasource" \
  -H "Authorization: Bearer $GLEAN_API_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"name": "custom_knowledge_base"}'

# Re-enable old search provider
export SEARCH_PROVIDER=elasticsearch
echo "Rollback complete — old search provider re-enabled"

Migration Checklist

• Inventory all source indices and document counts
• Map permission model (index-level ACLs to Glean datasource permissions)
• Export and transform documents to Glean schema
• Create datasource via adddatasource API
• Bulk index all documents in batches of 200
• Run validation queries and compare relevance
• Update search UI to use Glean Client API
• Decommission old search crawlers and indices
• Monitor search quality metrics for 7 days post-migration
• Remove old provider SDK dependencies from codebase

Error Handling

Issue	Cause	Fix
401 Unauthorized	Invalid Glean API token	Regenerate token in admin console
409 Datasource exists	Name collision	Delete or rename existing datasource
413 Payload Too Large	Batch exceeds 10 MB	Reduce batch size from 200
422 Invalid document	Missing required field	Validate id, title, url before indexing
Stale search results	Index not refreshed	Wait 5 min or trigger manual refresh

Resources

• Glean Developer Portal
• Indexing API
• Search API

Next Steps

After migration, configure Glean's built-in connectors for Google Drive, Confluence, and Slack to replace custom crawlers. See glean-core-workflow-a for search integration.