YAML Schema Reference

A project is defined by four YAML files. load_project() deep-merges them in order (bronze → silver → gold into main) and validates the result before the pipeline runs.

---

File Layout

projects/<project>/
├── main.yaml      ← pipeline name, paths, bi_export, includes
├── bronze.yaml    ← source connection, destination, incremental
├── silver.yaml    ← bronze_to_silver transforms + derived UDFs
└── gold.yaml      ← silver_to_gold aggregations + pre-agg UDFs

---

main.yaml

pipeline:
  name: my_project          # (required) pipeline name — used in dlt and gold output dirs

includes:
  bronze: bronze.yaml       # (required) filename of the bronze config in this folder
  silver: silver.yaml       # (required) filename of the silver config
  gold:   gold.yaml         # (required) filename of the gold config

paths:
  bronze: ./data/bronze     # (required) bronze output directory — local or s3://
  silver: ./data/silver     # (required) silver output directory
  gold:   ./data/gold       # (required) gold output directory
  export: ./data/export     # (required) export output directory

bi_export:                  # (optional) BI export config
  enabled: true
  projects:
    - name: default
      tables:
        - summary.parquet

pipeline

Key	Type	Required	Description
name	string	✅	Project/pipeline name. Used as dataset_name in dlt and as the subfolder name under gold/.

includes

Key	Type	Required	Description
bronze	string	✅	Filename of the bronze YAML (relative to the project folder).
silver	string	✅	Filename of the silver YAML.
gold	string	✅	Filename of the gold YAML.

paths

All paths support local directory strings and s3://bucket/prefix URIs.

Key	Type	Required	Description
bronze	string	✅	Where BronzeLoader writes raw Parquet.
silver	string	✅	Where SilverTransformer reads/writes.
gold	string	✅	Root of gold output; actual files go into gold/<project_name>/.
export	string	✅	Where BIExporter copies files for BI consumption.

bi_export

Key	Type	Default	Description
enabled	bool	true	Skip the export step entirely when false.
projects	list	[]	List of BI project specs.
projects[].name	string	—	Subfolder name under gold/ to export from.
projects[].tables	list[string]	—	Filenames to copy + convert to CSV.

Environment variable expansion

Any string value in any YAML file may contain ${VAR} or ${VAR:-default} placeholders. Expansion happens after all four files are merged.

# Raises EnvironmentError if ORACLE_USER is not set
connection_string: "oracle+oracledb://${ORACLE_USER}:${ORACLE_PASSWORD}@${ORACLE_DSN}"

# Falls back to "us-east-1" if AWS_DEFAULT_REGION is not set
bucket_url: "s3://${BUCKET:-my-default-bucket}/data"

---

bronze.yaml

source:
  type: sql_database         # sql_database | rest_api | filesystem
  dialect: oracle            # oracle | postgres | mysql | mssql | sqlite
  connection_string: "oracle+oracledb://${USER}:${PASS}@${DSN}"
  schema: MY_SCHEMA          # (optional) database schema
  tables:
    - name: ORDERS
      incremental:
        mode: append         # replace | append | merge
        cursor_column: UPDATED_AT
        initial_value: "2024-01-01T00:00:00"
    - name: CUSTOMERS
      incremental:
        mode: merge
        primary_key: CUSTOMER_ID

destination:
  type: filesystem           # filesystem | duckdb | bigquery | snowflake
  bucket_url: ./data/bronze  # local path or s3:// URI

source

Key	Type	Required	Description
type	enum	✅	sql_database, rest_api, or filesystem.
dialect	enum	creds file	Required when credentials_file is used. One of oracle, postgres, mysql, mssql, sqlite.
credentials_file	string	—	Path to a credentials YAML (see secrets.yaml.example). Preferred over connection_string. Supports ${VAR} expansion.
connection_string	string	—	Raw SQLAlchemy URL. Supports ${VAR} expansion. Used only when credentials_file is absent.
schema	string	—	Database schema to scope table discovery and SELECT statements.
tables	list	sql/filesystem	Tables to ingest.
base_url	string	rest only	REST API base URL.
resource	string	rest only	Resource/endpoint name.
bucket_url	string	filesystem	Source bucket or directory.
file_glob	string	filesystem	Glob pattern, e.g. **/*.parquet.
format	string	filesystem	parquet (default) or csv.

Credential file format (credentials_file)

openmedallion reads the YAML file, looks up the dialect key, and builds the SQLAlchemy connection string internally. Connection is tested and available tables are printed before ingestion starts.

# /workspace/secrets.yaml  (keep outside repo — never commit real credentials)

oracle:
  host:     db.corp.com
  port:     1521          # default
  service:  XE
  username: hr
  password: secret

postgres:
  host:     pg.corp.com
  port:     5432          # default
  database: analytics
  username: etl
  password: secret

sqlite:
  path: data/mydb.db      # relative to CWD where medallion runs

Set the path and dialect via env vars (supports ${VAR:-default} expansion):

# bronze.yaml
source:
  type:             sql_database
  dialect:          "${SECRETS_DIALECT:-sqlite}"
  credentials_file: "${SECRETS_PATH:-dev_credentials.yaml}"
  schema:           HR

source.tables[]

Key	Type	Required	Description
name	string	✅	Table name in the source database.
select	list[string]	—	Column names to ingest. Omit to ingest all columns. For SQL sources the projection is pushed to the database; for local_files it is applied after reading. Always include the cursor_column when using append mode.
incremental	object	—	Omit for full-replace each run.
incremental.mode	enum	—	replace (default), append, or merge.
incremental.cursor_column	string	append	Column used to track the high-watermark. Always include this column in select.
incremental.initial_value	string	append	Value to use on the very first run.
incremental.primary_key	string or list	merge	Column(s) that uniquely identify a row in the Parquet file (used to build the file index).
incremental.merge_key	string or list	merge	Column(s) used to match incoming rows against existing rows. Usually identical to primary_key.
explore	list	—	List of report specs to generate after this table is written. See Inline explore.

Incremental modes

Mode	When to use	Required keys
replace	Small lookup/reference tables. Full re-pull and overwrite every run. Safe when source rows can be deleted.	—
append	Immutable event or log tables. Fetches only rows newer than the cursor and appends to the existing Parquet. No deduplication.	cursor_column, initial_value
merge	Tables where rows can be updated after their first write (e.g. a permit whose status changes over time). Upserts by merge_key — updates matching rows, inserts new ones.	primary_key, merge_key

merge and cursor_column can be combined: the cursor limits how many rows are fetched from the source database, while merge_key controls how they land in the Parquet file. This is the right pattern for large tables where only recent rows are added but existing rows may also change.

# Large table — limit source pull by cursor, upsert by primary key
- name: folder
  incremental:
    mode: merge
    primary_key: folderrsn
    merge_key: folderrsn
    cursor_column: indate
    initial_value: "2025-01-01 00:00:00"

# Lookup table — full re-pull every run, upsert to avoid duplicates across runs
- name: validstatus
  incremental:
    mode: merge
    primary_key: statuscode
    merge_key: statuscode

# Composite key — child records keyed on two columns
- name: folderprocess
  incremental:
    mode: merge
    primary_key: [folderrsn, processrsn]
    merge_key: [folderrsn, processrsn]

# Append-only event log
- name: audit_log
  incremental:
    mode: append
    cursor_column: created_at
    initial_value: "2024-01-01T00:00:00"

destination

Key	Type	Required	Description
type	enum	✅	filesystem, duckdb, bigquery, or snowflake.
bucket_url	string	filesystem	Local path or s3:// URI. Should equal paths.bronze.
db_path	string	duckdb	Path to the .duckdb file (default: bronze.duckdb).

---

silver.yaml

bronze_to_silver:

  # Optional: register all silver Parquet files as DuckDB views or tables
  # after the silver run completes.
  duckdb:
    enabled: true                        # false or omit to skip (default: false)
    path: data/silver/silver.duckdb      # where to write the .duckdb file
    mode: views                          # views (default) | tables

  tables:
    - source_file: ORDERS.parquet     # filename in bronze directory
      output_file: orders.parquet     # filename to write in silver directory
      transforms:
        - type: rename
          columns:
            ORDER_ID:    order_id
            UPDATED_AT:  updated_at
        - type: cast
          columns:
            order_id: Int64
            amount:   Float64
        - type: drop
          columns: [INTERNAL_COL, DEBUG_COL]
        - type: udf
          file: projects/my_project/udf/silver/base.py
          function: enrich_orders
          args:
            threshold: 500.0

  derived_tables:
    - output_file: order_lines_enriched.parquet
      udf:
        file: projects/my_project/udf/silver/derived.py
        function: build_enriched
      select: [order_id, product_id, line_revenue]   # optional column projection

bronze_to_silver.duckdb

Optional block. When enabled: true, all *.parquet files in paths.silver are registered in a DuckDB file after the silver run completes.

Key	Type	Default	Description
enabled	bool	false	Set to true to activate DuckDB registration.
path	string	—	Path to write the .duckdb file. Created automatically if absent.
mode	enum	views	views — lightweight pointer, data stays in Parquet, not standalone-shareable. tables — data copied into DuckDB, file is self-contained and shareable.

When to use each mode:

Mode	File size	Shareable alone	Best for
views	Tiny	❌ (needs Parquet files)	Local use, cerebrum queries, notebooks
tables	≈ Parquet total	✅	Sharing with colleagues, attaching to reports

bronze_to_silver.tables[]

Key	Type	Required	Description
source_file	string	✅	Parquet filename to read from paths.bronze.
output_file	string	✅	Parquet filename to write to paths.silver.
transforms	list	—	Ordered list of transform steps.
explore	list	—	List of report specs to generate after this table is written. See Inline explore.

transforms[]

Each entry must have a type field. Valid types: rename, cast, drop, udf, fillna, clip, normalize, deduplicate, filter_rows, map_values, allowed_values, coerce_bool.

renamecastdropudffillnaclipnormalizededuplicatefilter_rowsmap_valuesallowed_valuescoerce_bool

- type: rename
  columns:
    OLD_NAME: new_name
    ANOTHER:  another_name

Renames columns using Polars .rename(). Keys are original names, values are new names.

- type: cast
  columns:
    order_id:  Int64
    amount:    Float64
    is_active: Boolean

Casts columns to Polars dtypes. Use Polars dtype names: Int64, Float64, Utf8, Boolean, Date, Datetime, etc.

- type: drop
  columns: [INTERNAL_COL, DEBUG_COL]

Drops named columns. No-op if the list is empty.

- type: udf
  file: projects/my_project/udf/silver/base.py
  function: enrich_orders
  args:
    threshold: 500.0

Calls a Python function. See Silver base-table UDF.

Key	Required	Description
file	✅	Path to the Python file, relative to the project root (where medallion is run).
function	✅	Name of the function to call.
args	—	Dict of keyword arguments passed to the function.

- type: fillna
  columns:
    score:  0
    status: "unknown"

Fills null values per column with a literal value. Non-null values are unchanged.

# Numeric — clamp to [min, max] (either bound is optional)
- type: clip
  columns:
    age:    {min: 0, max: 120}
    score:  {min: 0}

# Date / Datetime — ISO 8601 string bounds
- type: clip
  columns:
    hired_date: {min: "2000-01-01", max: "2024-12-31"}
    created_at: {max: "2025-01-01"}

Clamps numeric, Date, or Datetime columns to a min/max range. Values outside the range are replaced with the boundary value.

- type: normalize
  columns:
    email:    lower        # upper | lower | strip | strip_lower
    category: strip_lower

Standardises string columns. strip_lower strips whitespace then lowercases.

- type: deduplicate
  subset: [customer_id]   # omit to use all columns
  keep: first             # first (default) | last | none

Removes duplicate rows. keep: none drops all rows that appear more than once.

- type: filter_rows
  expr: "status != 'deleted' AND score >= 0"

Keeps only rows that satisfy a SQL expression (evaluated by DuckDB via pl.sql_expr). Use this to drop sentinel, test, or out-of-range rows.

- type: map_values
  column: status
  mapping:
    A: active
    B: blocked
  default: unknown        # optional — omit to keep unmapped values as-is

Replaces categorical values using a lookup dict. Unmapped values keep their original value unless default is set.

- type: allowed_values
  columns:
    status: [active, pending, closed]
    tier:   [gold, silver, bronze]

Validates string columns against an allowlist. Values not in the list become null (the row is kept). To drop out-of-range rows instead, use filter_rows.

- type: coerce_bool
  columns: [is_active, has_consent, opted_in]

Converts string representations of booleans to Boolean dtype. Recognised values (case-insensitive):

Result	Accepted strings
true	true, yes, 1, on
false	false, no, 0, off
null	anything else

Columns that are already Boolean pass through unchanged.

bronze_to_silver.derived_tables[]

Key	Type	Required	Description
output_file	string	✅	Filename to write to paths.silver.
udf	object	✅	UDF spec: file, function, optional args.
select	list[string]	—	If present, only these columns are kept from the UDF result.

---

gold.yaml

silver_to_gold:

  # Optional: register all gold Parquet files as DuckDB views or tables
  # after the gold run completes. All projects are registered into one file.
  duckdb:
    enabled: true                      # false or omit to skip (default: false)
    path: data/gold/gold.duckdb        # where to write the .duckdb file
    mode: tables                       # tables recommended for gold (shareable)

  projects:
    - name: analytics                     # subfolder under paths.gold
      aggregations:
        - source_file: orders.parquet     # silver filename to read
          pre_agg_udf:                    # (optional) runs before group_by
            file: projects/my_project/udf/gold/transforms.py
            function: prepare_orders
            args:
              include_region: true
          group_by: [customer_id, region] # columns to group on
          metrics:
            - {column: order_id, agg: count,          alias: total_orders}
            - {column: amount,   agg: sum,             alias: total_revenue}
            - {column: amount,   agg: mean,            alias: avg_order_value}
            - {column: amount,   agg: median,          alias: median_order_value}
            - {column: amount,   agg: count_distinct,  alias: unique_amounts}
          having: "total_orders > 1"     # post-agg filter
          sort:
            columns: [total_revenue]
            descending: true
          limit: 100
          output_file: customer_summary.parquet

        - source_file: orders.parquet     # pass-through (no group_by)
          select: [order_id, amount, status]
          output_file: orders_flat.parquet

silver_to_gold.duckdb

Optional block. Same semantics as bronze_to_silver.duckdb. When enabled: true, Parquet files from all gold project subdirectories are registered into a single .duckdb file after the gold run completes.

Key	Type	Default	Description
enabled	bool	false	Set to true to activate DuckDB registration.
path	string	—	Path to write the .duckdb file.
mode	enum	views	views or tables. Recommend tables for gold (shareable).

silver_to_gold.projects[]

Key	Type	Required	Description
name	string	✅	Subfolder name under paths.gold. Also used for BI export.
aggregations	list	✅	List of aggregation blocks.

aggregations[]

Key	Type	Required	Description
source_file	string	✅	Parquet filename to read from paths.silver.
pre_agg_udf	object	—	UDF to run before group_by. See Gold pre-agg UDF.
group_by	list[string]	—	Column names to group by. Omit for grand-total aggregation.
metrics	list	—	Metric specs. Required unless select is used.
select	list[string]	—	Pass-through mode: select columns without aggregating.
having	string	—	SQL expression to filter rows after aggregation (equivalent to SQL HAVING). Applied before sort and limit.
sort	object	—	Sort the result. Keys: columns (list[string], required) and descending (bool, default false).
limit	int	—	Keep only the top N rows. Applied after sort.
output_file	string	✅	Parquet filename to write under paths.gold/<project>/.
explore	list	—	List of report specs to generate after this aggregation is written. See Inline explore.

Execution order within an aggregation: group_by → having → sort → limit.

metrics[]

Key	Type	Required	Description
column	string	✅ (except count)	Source column name. For count, any column works.
agg	enum	✅	Aggregation function — see table below.
alias	string	✅	Output column name in the result.

Supported agg values:

agg	Description
count	Row count (no column required)
sum	Sum of values
mean	Arithmetic mean
min	Minimum value
max	Maximum value
median	Median value
std	Sample standard deviation
var	Sample variance
first	First value in group (insertion order)
last	Last value in group (insertion order)
count_distinct	Count of unique non-null values

pre_agg_udf

Key	Type	Required	Description
file	string	✅	Path to the Python file, relative to project root.
function	string	✅	Function name.
args	dict	—	Keyword arguments forwarded to the function.

---

Inline Explore

Add an explore: list directly to any table entry in bronze.yaml, silver.yaml, or gold.yaml. Reports are generated immediately after the Parquet file for that table is written — no separate explore.yaml or paths.explore key needed.

# bronze.yaml — on a source table
- name: employees
  explore:
    - report_type: profile
      output_file:  employees_profile.html
      title:        "Employees Bronze Quality"

# silver.yaml — on a transformed table
- source_file: employees.parquet
  explore:
    - report_type: walker
      output_file:  employees_explorer.html
      title:        "Employees Explorer"

# gold.yaml — on an aggregation
aggregations:
  - output_file: headcount.parquet
    explore:
      - report_type: profile
        output_file:  headcount_profile.html
        title:        "Headcount by Department"

explore[]

Key	Type	Required	Description
report_type	enum	✅	profile (ydata-profiling HTML report) or walker (pygwalker interactive explorer).
output_file	string	✅	HTML filename to write.
title	string	—	Optional title shown in the report.

Report output paths — reports are co-located with their layer's data under an add-ons/ subdirectory:

Layer	Output path
Bronze	paths.bronze/add-ons/<output_file>
Silver	paths.silver/add-ons/<output_file>
Gold	paths.gold/add-ons/<project>/<output_file>

Optional dependencies — install the extras for the report type you want:

pip install "openmedallion[profile]"   # ydata-profiling >= 4.0
pip install "openmedallion[explore]"   # pygwalker >= 0.4

If the optional dependency is not installed, the report is silently skipped with a notice. An unknown report_type also prints a warning and skips without aborting the run.

---

Validation Rules

_validate_config() checks the following automatically after loading:

Rule	Error
pipeline.name must be a non-empty string	pipeline.name must be a non-empty string
All four paths.* keys must be present	paths.bronze is required
source.type (if present) must be in {sql_database, rest_api, filesystem}	source.type must be one of ...
bronze_to_silver.tables[i].source_file and .output_file required	bronze_to_silver.tables[0].source_file is required
Each transforms[j].type must be rename/cast/drop/udf	transforms[0].type must be one of ...
UDF transform blocks must have file and function	transforms[0] (udf): 'file' is required
silver_to_gold.projects[i].name required	silver_to_gold.projects[0].name is required
pre_agg_udf blocks must have file and function	pre_agg_udf: 'file' is required
bronze_to_silver.duckdb.enabled must be a bool	bronze_to_silver.duckdb.enabled must be a bool
bronze_to_silver.duckdb.mode must be views or tables	bronze_to_silver.duckdb.mode must be one of ...
silver_to_gold.duckdb.enabled must be a bool	silver_to_gold.duckdb.enabled must be a bool
silver_to_gold.duckdb.mode must be views or tables	silver_to_gold.duckdb.mode must be one of ...