Naming Convention

dlt creates table and column identifiers from the data. The data source ie. a stream of JSON documents may have identifiers (i.e. key names in a dictionary) with any Unicode characters, of any length and naming style. On the other hand, destinations require that you follow strict rules when you name tables, columns or collections. A good example is Redshift that accepts case-insensitive alphanumeric identifiers with maximum 127 characters.

dlt groups tables from a single source in a schema. Each schema defines naming convention that tells dlt how to translate identifiers to the namespace that the destination understands. Naming conventions are in essence functions that map strings from the source identifier format into destination identifier format. For example our snake_case (default) naming convention will translate DealFlow source identifier into deal_flow destination identifier.

You can pick which naming convention to use. dlt provides a few to choose from. You can easily add your own as well.

tip

Standard behavior of dlt is to use the same naming convention for all destinations so users see always the same table and column names in their databases.

Use default naming convention (snake_case)

snake_case is case insensitive naming convention, converting source identifiers into lower case snake case identifiers with reduced alphabet.

• Spaces around identifier are trimmed
• Keeps ascii alphanumerics and underscores, replaces all other characters with underscores (with the exceptions below)
• Replaces + and * with x, - with _, @ with a and | with l
• Prepends _ if name starts with number.
• Multiples of _ are converted into single _.
• Replaces all trailing _ with x

Uses __ as patent-child separator for tables and flattened column names.

tip

If you do not like snake_case your next safe option is sql_ci which generates SQL-safe, lower-case, case-insensitive identifiers without any other transformations. To permanently change the default naming convention on a given machine:

1. set an environment variable SCHEMA__NAMING to sql_ci_v1 OR
2. add the following line to your global config.toml (the one in your home dir ie. ~/.dlt/config.toml)

[schema]
naming="sql_ci_v1"

Source identifiers vs destination identifiers

Pick the right identifier form when defining resources

dlt keeps source (not normalized) identifiers during data extraction and translates them during normalization. For you it means:

1. If you write a transformer or a mapping/filtering function, you will see the original data, without any normalization. Use the source identifiers to access the dicts!
2. If you define a primary_key or cursor that participate in cursor field incremental loading use the source identifiers (dlt uses them to inspect source data, Incremental class is just a filtering function).
3. When defining any other hints ie. columns or merge_key you can pick source or destination identifiers. dlt normalizes all hints together with your data.
4. Schema object (ie. obtained from the pipeline or from dlt source via discover_schema) always contains destination (normalized) identifiers.

Understand the identifier normalization

Identifiers are translated from source to destination form in normalize step. Here's how dlt picks the naming convention:

• The default naming convention is snake_case.
• Each destination may define a preferred naming convention in destination capabilities. Some destinations (ie. Weaviate) need specialized naming convention and will override the default.
• You can configure a naming convention explicitly. Such configuration overrides the destination settings.
• This naming convention is used when new schemas are created. It happens when pipeline is run for the first time.
• Schemas preserve naming convention when saved. Your running pipelines will maintain existing naming conventions if not requested otherwise.
• dlt applies final naming convention in normalize step. Jobs (files) in load package now have destination identifiers. Pipeline schema is duplicated, locked and saved in the load package and will be used by the destination.

caution

If you change naming convention and dlt detects that a change in the destination identifiers for tables/collection/files that already exist and store data, the normalize process will fail. This prevents an unwanted schema migration. New columns and tables will be created for identifiers that changed.

Case sensitive and insensitive destinations

Naming convention declare if the destination identifiers they produce are case sensitive or insensitive. This helps dlt to generate case sensitive / insensitive identifiers for the destinations that support both. For example: if you pick case insensitive naming like snake_case or sql_ci_v1, with Snowflake, dlt will generate all upper-case identifiers that Snowflake sees as case insensitive. If you pick case sensitive naming like sql_cs_v1, dlt will generate quoted case-sensitive identifiers that preserve identifier capitalization.

Note that many destinations are exclusively case insensitive, of which some preserve casing of identifiers (ie. duckdb) and some will case-fold identifiers when creating tables (ie. Redshift, Athena do lower case on the names). dlt is able to detect resulting identifier collisions and stop the load process before data is mangled.

Identifier shortening

Identifier shortening happens during normalization. dlt takes the maximum length of the identifier from the destination capabilities and will trim the identifiers that are too long. The default shortening behavior generates short deterministic hashes of the source identifiers and places them in the middle of the destination identifier. This (with a high probability) avoids shortened identifier collisions.

🚧 [WIP] Name convention changes are lossy

dlt does not store the source identifiers in the schema so when naming convention changes (or we increase the maximum identifier length), it is not able to generate a fully correct set of new identifiers. Instead it will re-normalize already normalized identifiers. We are currently working to store full identifier lineage - source identifiers will be stored and mapped to the destination in the schema.

Pick your own naming convention

Configure naming convention

You can use config.toml, environment variables or any other configuration provider to set the naming convention name. Configured naming convention overrides all other settings

• changes the naming convention stored in the already created schema
• overrides the destination capabilities preference.

[schema]
naming="sql_ci_v1"

Configuration above will request sql_ci_v1 for all pipelines (schemas). An environment variable SCHEMA__NAMING set to sql_ci_v1 has the same effect.

You have an option to set naming convention per source:

[sources.zendesk]
config="prop"
[sources.zendesk.schema]
naming="sql_cs_v1"
[sources.zendesk.credentials]
password="pass"

Snippet above demonstrates how to apply certain naming for an example zendesk source.

You can use naming conventions that you created yourself or got from other users. In that case you should pass a full Python import path to the module that contain the naming convention:

[schema]
naming="tests.common.cases.normalizers.sql_upper"

dlt will import tests.common.cases.normalizers.sql_upper and use NamingConvention class found in it as the naming convention.

Available naming conventions

You can pick from a few built-in naming conventions.

• snake_case - the default
• duck_case - case sensitive, allows all unicode characters like emoji 💥
• direct - case sensitive, allows all unicode characters, does not contract underscores
• sql_cs_v1 - case sensitive, generates sql-safe identifiers
• sql_ci_v1 - case insensitive, generates sql-safe lower case identifiers

Set and adjust naming convention explicitly

You can modify destination capabilities to

Avoid identifier collisions

dlt detects various types of identifier collisions and ignores the others.

1. dlt detects collisions if case sensitive naming convention is used on case insensitive destination
2. dlt detects collisions if change of naming convention changes the identifiers of tables already created in the destination
3. dlt detects collisions when naming convention is applied to column names of arrow tables

dlt will not detect collision when normalizing source data. If you have a dictionary, keys will be merged if they collide after being normalized. You can create a custom naming convention that does not generate collisions on data, see examples below.

Write your own naming convention

Custom naming conventions are classes that derive from NamingConvention that you can import from dlt.common.normalizers.naming. We recommend the following module layout:

1. Each naming convention resides in a separate Python module (file)
2. The class is always named NamingConvention

In that case you can use a fully qualified module name in schema configuration or pass module explicitly.

We include two examples of naming conventions that you may find useful:

1. A variant of sql_ci that generates identifier collisions with a low (user defined) probability by appending a deterministic tag to each name.
2. A variant of sql_cs that allows for LATIN (ie. umlaut) characters

note

Note that a fully qualified name of your custom naming convention will be stored in the Schema and dlt will attempt to import it when schema is loaded from storage. You should distribute your custom naming conventions with your pipeline code or via a pip package from which it can be imported.