BigQuery ETL Core

Composable: Foundation skill that works with model-requirements, query-writer, metadata-manager, sql-test-generator, and bigconfig-generator skills When to use: Understanding project structure, conventions, common patterns, and finding schema descriptions for construction

Project Overview

The bigquery-etl project manages BigQuery table definitions, queries, and associated metadata for Mozilla. Similar to dbt, the repository maintains query definitions with associated metadata and schemas.

Each table/query typically consists of three files:

• query.sql OR query.py - The query definition (SQL or Python)
• metadata.yaml - Metadata about scheduling, ownership, and dependencies (see metadata-manager skill)
• schema.yaml - BigQuery schema definition with field types and descriptions (see metadata-manager skill)

Note: Most tables use query.sql (~95%). Use query.py for API calls, multi-project queries, or complex Python operations. See query-writer skill for details.

🚨 REQUIRED READING - Start Here

When starting work in bigquery-etl, READ these foundational references:

1. Naming Conventions: READ references/naming_conventions.md

   • Table naming patterns
   • Dataset organization
   • Version suffix conventions

2. Dataset Organization: READ references/dataset_naming_conventions.md

   • Common dataset suffixes (_derived, _stable, _live)
   • When to use each dataset type
   • Dataset naming rules

3. Schema Resources: READ references/discovery_resources.md

   • Schema description sources (Glean Dictionary, ProbeInfo API, DataHub)
   • Priority order for schema lookup during construction
   • Common mozfun UDFs

4. Privacy Guidelines: READ references/privacy_guidelines.md

   • Data handling requirements
   • PII considerations
   • Workgroup access patterns

Directory Structure

sql/{project}/{dataset}/{table_name}/
├── query.sql OR query.py
├── metadata.yaml
└── schema.yaml

See assets/directory_structure_example.txt for detailed examples.

Key principles:

• Always flat: sql/{project}/{dataset}/{table_name}/
• Never use subdirectories within table directories
• Table names always include version suffix (_v1, _v2, etc.)

Schema & Description Resources for Construction

Finding Schema Descriptions

Priority order for schema lookup during construction:

1. Local files first: Check sql/*/schema.yaml and metadata.yaml files

   • Most reliable and up-to-date source
   • Contains field descriptions written by table owners

2. Glean Dictionary: For _live and _stable tables

   • URL: https://dictionary.telemetry.mozilla.org/
   • Contains metric descriptions from Glean schema definitions
   • Use WebFetch with targeted prompts to extract specific field descriptions

3. ProbeInfo API: For Glean metric metadata

   • Endpoints: https://probeinfo.telemetry.mozilla.org/glean/{product}/metrics
   • Provides metric definitions and descriptions programmatically
   • Use for validating metric references in queries

4. DataHub MCP: Only as last resort

   • MUST READ references/datahub_best_practices.md BEFORE any DataHub queries
   • Use for schema lookup when not available in local files or Glean Dictionary
   • Extract ONLY necessary fields (column names, types, descriptions)
   • Use for downstream impact analysis when modifying tables

See references/discovery_resources.md for:

• Detailed guidance on each schema source
• ProbeInfo API endpoints and usage patterns
• Glean Dictionary URL patterns for different products
• DataHub MCP best practices for construction
• Common mozfun UDFs
• Key documentation links

Naming Conventions

Table Names:

• Use snake_case with version suffix: clients_daily_event_v1
• Common suffixes: _daily, _hourly, _aggregates, _summary

Field Names:

• Use snake_case: submission_date, client_id, n_total_events
• Prefix counts with n_: n_events, n_sessions
• Standard Mozilla fields: submission_date, client_id, sample_id, normalized_channel, normalized_country_code, app_version

See references/naming_conventions.md for:

• Complete naming patterns and conventions
• Reserved/common patterns to avoid
• BigQuery project naming conventions

Dataset Organization

See references/dataset_naming_conventions.md for:

• Dataset naming patterns by suffix (_derived, _external, etc.)
• Common dataset prefixes by product/source
• Table versioning patterns
• Incremental vs full refresh query patterns

Privacy & Data Handling

Mozilla follows strict data privacy policies:

• No PII in derived tables
• Use client-level identifiers (client_id) not individual identifiers
• Respect data retention policies (~2 years for client-level data)
• Label client-level tables with table_type: client_level in metadata.yaml

See references/privacy_guidelines.md for:

• Key principles from Mozilla's data platform
• Geo IP lookup and user agent parsing policies
• Best practices for data handling
• Deletion request support
• Sample ID usage for sampling

BigQuery & Mozilla Conventions

Partitioning & Clustering

• Most tables use day partitioning on submission_date
• Clustering improves query performance for filtered/joined fields
• See metadata-manager skill for detailed partitioning and clustering configuration

Common UDFs (mozfun)

Browse available functions: https://mozilla.github.io/bigquery-etl/mozfun/

Common functions:

• mozfun.map.get_key() - Extract values from key-value maps
• mozfun.norm.truncate_version() - Normalize version strings
• mozfun.stats.mode_last() - Statistical mode calculation

UDF source code in sql/mozfun/ directory.

Glean Overview

Glean is Mozilla's product analytics & telemetry solution, providing consistent measurement across all Mozilla products.

Key concepts:

• Metric types: Counter, boolean, string, event, etc.
• Pings: Collections of metrics (e.g., baseline, events, metrics)
• Applications: Products using Glean (Fenix, Focus, Firefox iOS, etc.)

Common Glean datasets in BigQuery:

• Pattern: {app_id}.{ping_name} (e.g., org_mozilla_fenix.baseline)
• All have auto-generated schemas based on metric definitions

See references/glean_overview.md for:

• What is Glean and how it differs from Firefox Desktop Telemetry
• Glean SDK and metric type details
• Common Glean datasets in BigQuery
• When to use Glean Dictionary

bigquery-etl CLI Commands

See references/bqetl_cli_commands.md for:

• Key bqetl CLI commands for query creation, validation, schema updates
• How to find the right DAG for scheduling
• Backfill creation commands

Best Practices

General principles:

• Always include field descriptions in schema.yaml (see metadata-manager skill)
• Add header comments explaining query purpose (see query-writer skill)
• Reference bug/ticket numbers for context
• Document any data exclusions or filtering logic

See assets/query_structure_example.sql for standard query structure.

Version migration:

• Create new _v2 table when making breaking schema changes
• Keep _v1 running during migration period
• Update views to point to new version
• Coordinate with downstream consumers before deprecating old version

For detailed best practices, see:

• Query writing: query-writer skill
• Metadata configuration: metadata-manager skill
• Performance optimization: https://docs.telemetry.mozilla.org/cookbooks/bigquery/optimization.html
• Recommended practices: https://mozilla.github.io/bigquery-etl/reference/recommended_practices/

Integration with Other Skills

bigquery-etl-core serves as the foundation skill that other skills build upon:

Works with model-requirements

• Provides naming conventions for new tables and datasets
• Supplies common field naming patterns for requirements gathering
• Offers privacy guidelines for data model planning

Works with query-writer

• Provides project structure and naming conventions
• Supplies common patterns and mozfun UDF references
• Offers schema description lookup guidance for construction

Works with metadata-manager

• Provides DAG naming patterns and scheduling conventions
• Supplies partitioning and clustering best practices
• Offers ownership and labeling patterns

Works with sql-test-generator

• Provides test structure and fixture naming conventions
• Supplies common table patterns for test creation
• Offers query parameter conventions

Works with bigconfig-generator

• Provides table naming conventions for Bigeye monitoring configuration
• Supplies dataset organization patterns
• Offers field naming standards for data quality checks

This skill is always available and does not need to be explicitly invoked - it provides foundational knowledge that other skills reference.

Reference Examples

Real query examples in the repository:

• Simple query: sql/moz-fx-data-shared-prod/mozilla_vpn_derived/users_v1/query.sql
• Aggregation with GROUP BY: sql/moz-fx-data-shared-prod/telemetry_derived/clients_daily_event_v1/query.sql
• Complex query with CTEs: sql/moz-fx-data-shared-prod/telemetry_derived/event_events_v1/query.sql
• Python ETL (INFORMATION_SCHEMA): sql/moz-fx-data-shared-prod/monitoring_derived/bigquery_table_storage_v1/query.py
• Python ETL (External API): sql/moz-fx-data-shared-prod/bigeye_derived/user_service_v1/query.py

For more examples, explore the sql/moz-fx-data-shared-prod/ directory.

Bundled Resources

References

• references/discovery_resources.md - Schema description sources (Glean Dictionary, ProbeInfo API, DataHub MCP), priority order for construction, documentation links
• references/naming_conventions.md - Complete naming patterns for tables, fields, and projects
• references/dataset_naming_conventions.md - Dataset organization and versioning patterns
• references/privacy_guidelines.md - Mozilla data privacy policies and best practices
• references/glean_overview.md - Glean SDK concepts and BigQuery dataset structures
• references/bqetl_cli_commands.md - Key CLI commands and DAG discovery

DataHub Usage (CRITICAL for Token Efficiency)

BEFORE using any DataHub MCP tools (mcp__datahub-cloud__*), you MUST:

• READ references/datahub_best_practices.md - Comprehensive token optimization strategies
• Follow priority order: local files → documentation → DataHub (only as last resort)
• Use search-first patterns and extract minimal fields from responses

Assets

• assets/query_structure_example.sql - Standard query.sql structure with common patterns
• assets/directory_structure_example.txt - File organization examples