Agent-Skills.md

Agent Skills: dbt Snowflake to BigQuery Translator

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BigDataAndAnalyticsID: aibangjuxin/knowledge/dbt-sf-to-bq-translator

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Skill Metadata

Name
dbt-sf-to-bq-translator
Description
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dbt Snowflake to BigQuery Translator

You are responsible for:

  1. Dialect Translation: Translating Snowflake dbt SQL models to Standardized BigQuery SQL on Google Cloud.
  2. Standardization & Compliance: Enforcing Google-specific standards including copyright headers at the very top of each file, explicit type casting, standardized JSON extraction, and deduplication via QUALIFY with _extracted_at.
  3. Workflow Integration: Preserving dbt Jinja constructs and storing the final BigQuery-compatible models.

Follow the instructions given you under migration_plan/[mig_prefix]/tasks.md. You will add your progress during operation and summary at the end to the tasks file so that human supervisor can track where you are. You recover from errors by checking the tasks file.

Prerequisites & Environment Setup

Before starting the translation, ensure your Google Cloud environment is properly configured:

  1. Google Cloud SDK: Install the Google Cloud SDK if not already installed.
  2. Authentication: Authenticate your CLI session: 
    gcloud auth login
gcloud auth application-default login
  3. Project Configuration: Set your active Google Cloud project: 
    gcloud config set project {project_id}
  4. Billing Account: Verify that an active Google Cloud Billing account is attached to the target project.
  5. Enable Required APIs: Ensure BigQuery, Migration, and Storage services are enabled: 
    gcloud services enable bigquerymigration.googleapis.com storage.googleapis.com bigquery.googleapis.com
  6. Region Selection: Configure your preferred compute/BigQuery region (default recommended: us-central1 or us). See Google Cloud Locations: 
    gcloud config set compute/region us-central1

Steps

  • Initialization & Setup: If you do not have a defined mig_prefix or if the user wants to start a new translation project, you MUST first ask the user for:

    1. Migration Project Name (e.g. my_migration_project).
    2. Input directory containing Snowflake SQL files.
    3. Output directory where BigQuery SQL files should be saved.
    4. Cloud Storage bucket name for staging translation assets.
    5. Google Cloud region (e.g. us or eu).
    6. (Optional) Local path to a directory or .zip file containing source database metadata (such as columns.csv or tables.csv). Once provided, create the tasks checklist file under migration_plan/[mig_prefix]/tasks.md with unchecked tasks representing the migration steps.

  • Automated Execution via Bundled Scripts: Execute the deterministic end-to-end migration using the bundled translation script scripts/bulk_translate_via_gcloud.py:

    python3 scripts/bulk_translate_via_gcloud.py \
  --input {input_dir} \
  --output {output_dir} \
  --bucket {gcs_bucket} \
  --location {region} \
  [--metadata {metadata_path}] \
  [--array-cast-mode {array_cast_mode}] \
  [--struct-columns {struct_columns}]

    | Flag | Default | Description | | :--- | :--- | :--- | | --input | (Required) | Local directory containing source Snowflake dbt .sql and .yml files. | | --output | (Required) | Target directory for translated BigQuery dbt .sql and .yml files. | | --bucket | (Required) | Cloud Storage bucket name used to stage translation inputs and outputs. | | --location | us | Google Cloud region for BigQuery Translation Service execution (e.g., us, eu). | | --metadata | None | Optional path to a directory or .zip archive containing source tables.csv and columns.csv. | | --array-cast-mode | array_string | Target conversion for BQMS ARRAY<JSON> casts. Use array_string (ARRAY<STRING>) by default; use string (STRING), json_extract_string_array, or preserve as escape hatches when required by the source table schema. | | --struct-columns | None | Optional comma-separated list of additional BigQuery STRUCT column names (e.g., struct_col,record_col) to convert from JSON_VALUE to native dot notation (col.field). |

    The migration tools bundled in scripts/ perform the following coordinated actions:

    • scripts/bulk_translate_via_gcloud.py: Orchestrates end-to-end bulk migration, automating pre-processing, Cloud Storage upload, BigQuery Translation Service invocation, download, post-processing, and YAML configuration copying.
    • scripts/dbt_translator.py: Core translation library containing the deterministic AST parser for config(...), Jinja placeholder masking and restoration, JSON/STRUCT/ARRAY extraction sanitization, unimplemented opcode cleanup, macro auditing, casing/join standardization, and copyright header enforcement.

  • Detailed Translation Lifecycle (Executed by Scripts):

    1. Compile to Standard SQL using Placeholders (Pre-Translation):
      • Read the original source dbt .sql files. Extract and strip the {{ config(...) }} header block from the top of each file.
      • Replace dbt macro calls with standard-SQL-compliant placeholder identifiers to prevent BigQuery Translation Service from throwing syntax errors:
        • Replace {{ source('src_name', 'table_name') }} with _DBT_SOURCE_src_name_DBTSEP_table_name_
        • Replace {{ ref('model_name') }} with _DBT_REF_model_name_
      • Normalize Snowflake bracket notation (col['prop'] -> col:prop), ::variant:field, and Jinja {% else %} / loop UNION ALL branches, eliminating Jinja curly braces ({{ ... }}) prior to translation so the transpiler processes 100% valid Snowflake dialect SQL.
    2. Isolate the SQL Files:
      • Save these pre-processed, Jinja-free files to a staging input directory ready for Cloud Storage upload.
    3. Pre-Process Metadata & Translate SQL via BigQuery Translation Service:
      • If a metadata path is provided, map table entries matching discovered dbt models/sources to their placeholder names in columns.csv and tables.csv, clear catalog names to prevent namespace resolution errors, package into metadata.zip, and upload to Cloud Storage.
      • Upload staging SQL files to Cloud Storage: gcloud storage cp <staging_input_dir>/*.sql gs://[YOUR_BUCKET]/migration_input/
      • Create migration_config.yaml specifying snowflakeDialect as source and bigqueryDialect as target (with schemaPath pointing to metadata.zip if provided).
      • Trigger translation workflow: gcloud bq migration-workflows create --location=<region> --config-file=migration_config.yaml --no-async
      • Download translated GoogleSQL files from Cloud Storage: gcloud storage cp gs://[YOUR_BUCKET]/migration_output/*.sql <translated_output_dir>/
    4. Restore Placeholders & Re-Embed dbt Logic:
      • Take the translated BigQuery SQL files and perform advanced post-processing:
        • AST-Based Config Transformation: Parse the original {{ config(...) }} block, stripping Snowflake-specific parameters like copy_grants, transient, and secure. Sanitize hooks (pre_hook and post_hook) to remove invalid Snowflake commands like ALTER ICEBERG TABLE ... REFRESH, UNSET SECURE, or COPY INTO, while preserving valid ones.
        • Reference Resolver (Namespace Resolution): Scan the SQL for hardcoded Snowflake database/schema table paths in FROM and JOIN clauses, and map them back to native dbt {{ ref(...) }} or {{ source(...) }} macros by resolving against discovered project models and sources.
        • Macro & Syntax Audit: Scan all {{ ... }} Jinja expressions and log warnings for any custom/non-allowlisted database-specific macros. Also audit these blocks for Snowflake-specific syntax (e.g. ::date, dateadd, to_date) that may have been skipped or masked, listing warning comments directly in the file.
        • Balanced SQL Edge-Cases Sanitization: Convert date cast suffixes (::date -> CAST(... AS DATE)), datetime cast suffixes (::timestamp -> CAST(... AS TIMESTAMP)), float cast suffixes (::float -> CAST(... AS FLOAT64)), nested dateadd(...) calls, and intervals (- interval '5 month') inside and outside control blocks using balanced-parentheses parsers. Fix displaced Jinja tags around CTEs and strip 16MB substr(string(...), 1, 16777216) wrappers.
        • Array Type & STRUCT Access Normalization: Convert BQMS untyped ARRAY<JSON> casts to ARRAY<STRING> (or the configured --array-cast-mode target), convert json_value on STRUCT columns to direct dot notation (col.field) and on ARRAY<STRUCT> columns to col[SAFE_OFFSET(idx)].field, and clean error_unimplemented opcodes.
        • Copyright Header Placement: Prepend the mandatory Google copyright header at the very top of the file above the config block.
    5. Write to the New BigQuery dbt File & Copy YAML Configurations:
      • Save the newly assembled, BigQuery-compatible files preserving directory structure. Also, copy all .yml/.yaml files from the input directory to the output directory.

  • Put a summary to the tasks file at the end.

  • Before handing over, ask for user approval for the outcome. Apply necessary changes from the user.

  • Mark your task is done in the tasks file.

Mandates & Behavioral Rules

1. Mandatory File Header

Every translated file MUST start with the following exact header at the very top:

# Copyright 2026 Google. This software is provided as-is, without warranty or
# representation for any use or purpose. Your use of it is subject to your
# agreement with Google.

2. Standardized JSON & Array Extraction

Never use Snowflake colon notation (:) in BigQuery SQL because BigQuery Standard SQL does not recognize colon syntax for semi-structured field access and throws a syntax error. Translate semi-structured lookups using BigQuery's standard functions and consult dbt_migration_patterns.md for detailed patterns and error remedies:

  • Scalar & Stringified JSON Extraction: Use JSON_VALUE(json_col, '$.path') for native JSON columns (omit dots before array indexes, e.g., $.items[1]), and CAST(JSON_EXTRACT_SCALAR(SAFE.PARSE_JSON(CAST(source.col AS STRING)), '$.path') AS TYPE) for stringified JSON columns. Wrap typed extractions in SAFE_CAST(... AS INT64|BOOL|TIMESTAMP|NUMERIC) when strict typing is required.
  • Array Extraction, Exploding & Untyped Arrays (::array): Use JSON_EXTRACT_STRING_ARRAY or JSON_EXTRACT_ARRAY and explode with CROSS JOIN UNNEST(...). Map BQMS ARRAY<JSON> casts from Snowflake ::array to ARRAY<STRING> by default (or STRING / JSON_EXTRACT_STRING_ARRAY based on source schema; see Section 3 of dbt_migration_patterns.md).
  • JSON Timestamps (ISO-8601 with 'Z'): Cast JSON timestamp extractions to TIMESTAMP (CAST(JSON_EXTRACT_SCALAR(json_col, '$.updated_at') AS TIMESTAMP)) because direct CAST(... AS DATETIME) fails at runtime on ISO-8601 strings ending in 'Z'. If DATETIME is required downstream, cast via TIMESTAMP first (see Section 5 of dbt_migration_patterns.md).
  • BigQuery STRUCT & ARRAY<STRUCT> Columns: Use direct dot notation (col.field, e.g., source.struct_col.count) for native STRUCT columns and source.col[SAFE_OFFSET(0)].field (or JSON_VALUE(TO_JSON(source.col), '$[0].field')) for ARRAY<STRUCT> columns, because JSON_VALUE() and CAST(... AS JSON) do not accept STRUCT types (see Section 6 of dbt_migration_patterns.md).
  • Snowflake Bracket Notation (col['field']) & error_unimplemented: Normalize col['field'] to col:field during pre-processing and replace any residual BQMS error_unimplemented or 16MB substr(string(...), 1, 16777216) wrappers with col.field (for STRUCT columns) or JSON_VALUE(col, '$.field') (for JSON columns; see Sections 7–8 of dbt_migration_patterns.md).

3. Explicit Type Safety

  • Comparisons: Always use SAFE_CAST on both sides of a join or filter if types are not identical. Use AS STRING for universal comparison safety if necessary.
  • ID Fields: Prefer INT64 for all system IDs (e.g., ticket_id, user_id).
  • Null Handling:
    • For placeholder columns, always use explicit type casting: SAFE_CAST(NULL AS TYPE).

4. Prescriptive String & Date Functions

  • Truncation: Use LEFT(col, length) or SUBSTR(col, 1, length).
  • Search: Use LOWER(col) LIKE '%pattern%' instead of REGEXP_CONTAINS.
  • Date Add: Use DATE_ADD(SAFE_CAST(col AS DATETIME), INTERVAL num HOUR).

5. Mandatory Deduplication & Joins

  • Deduplication: If the source model requires deduplication on a primary key:
    • Rule: Use a row_number() over (partition by [PRIMARY_KEY] order by [TIMESTAMP] desc) as rn column in the base CTE, and apply qualify rn = 1 directly on that CTE.
  • Joins: Use LEFT JOIN when joining to custom field or attribute tables (e.g., exploded_array patterns) to prevent dropping records.

6. Preserve Jinja Constructs

Do not alter {{ config(...) }}, {{ ref(...) }}, or {{ source(...) }}. Keep {% if is_incremental() %} blocks functional. Do not inject historical data unions or other custom macros/tables unless they are present in the source files.

Outputs

You will output BigQuery-compatible dbt SQL models under migration_plan/[mig_prefix]/translated_models/. The translated files must strictly adhere to the structural pattern and dialectic formatting. For translation examples, see: dbt_migration_patterns.md

Constraints

Before handing over to the root agent, first get approval from the user about the translated models. After applying user's requests, then hand over the root agent.