Add migration steps for TemplatedConfigLoader to OmegaConfigLoader

docs/source/configuration/config_loader_migration.md

@@ -60,3 +60,178 @@ In this example, `"catalog"` is the key to the default catalog patterns specifie
 For error and exception handling, most errors are the same. Those you need to be aware of that are different between the original `ConfigLoader` and `OmegaConfigLoader` are as follows:
 * `OmegaConfigLoader` throws a `MissingConfigException` when configuration paths don't exist, rather than the `ValueError` used in `ConfigLoader`.
 * In `OmegaConfigLoader`, if there is bad syntax in your configuration files, it will trigger a `ParserError` instead of a `BadConfigException` used in `ConfigLoader`.
+
+## [`TemplatedConfigLoader`](/kedro.config.TemplatedConfigLoader) to [`OmegaConfigLoader`](/kedro.config.OmegaConfigLoader)
+
+### 1. Install the required library
+The [`OmegaConfigLoader`](advanced_configuration.md#omegaconfigloader) was introduced in Kedro `0.18.5` and is based on [OmegaConf](https://omegaconf.readthedocs.io/). Features that replace `TemplatedConfigLoader` functionality have been released in later versions, so we recommend users
+to install at least Kedro version `0.18.X` to properly replace the `TemplatedConfigLoader` with `OmegaConfigLoader`.
+You can install both this Kedro version and `omegaconf` using `pip`:
+
+```bash
+pip install kedro==0.18.X
+```
+This would be the minimum required Kedro version which includes `omegaconf` as a dependency and the necessary functionality to replace `TemplatedConfigLoader`.
+Or you can run:
+```bash
+pip install -U kedro
+```
+
+This command installs the most recent version of Kedro which also includes `omegaconf` as a dependency.
+
+### 2. Use the `OmegaConfigLoader`
+To use `OmegaConfigLoader` in your project, set the `CONFIG_LOADER_CLASS` constant in your [`src/<package_name>/settings.py`](../kedro_project_setup/settings.md):
+
+```diff
++ from kedro.config import OmegaConfigLoader  # new import
+
++ CONFIG_LOADER_CLASS = OmegaConfigLoader
+```
+
+### 3. Import statements
+Replace the import statement for `TemplatedConfigLoader` with the one for `OmegaConfigLoader`:
+
+```diff
+- from kedro.config import TemplatedConfigLoader
+
++ from kedro.config import OmegaConfigLoader
+```
+
+### 4. File format support
+`OmegaConfigLoader` supports only `yaml` and `json` file formats. Make sure that all your configuration files are in one of these formats. If you were using other formats with `TemplatedConfigLoader`, convert them to `yaml` or `json`.
+
+### 5. Load configuration
+The method to load the configuration using `OmegaConfigLoader` differs slightly from that used by `TemplatedConfigLoader`, which allowed users to access configuration through the `.get()` method and required patterns as argument.
+When you migrate to use `OmegaConfigLoader` it  requires you to fetch configuration through a configuration key that points to [configuration patterns specified in the loader class](configuration_basics.md#configuration-patterns) or [provided in the `CONFIG_LOADER_ARGS`](advanced_configuration.md#how-to-change-which-configuration-files-are-loaded) in `settings.py`.
+
+```diff
+- conf_path = str(project_path / settings.CONF_SOURCE)
+- conf_loader = TemplatedConfigLoader(conf_source=conf_path, env="local")
+- catalog = conf_loader.get("catalog*")
+
++ conf_path = str(project_path / settings.CONF_SOURCE)
++ config_loader = OmegaConfigLoader(conf_source=conf_path, env="local")
++ catalog = config_loader["catalog"] # note the key accessor syntax
+```
+
+In this example, the `"catalog"` key points to the default catalog patterns specified in the `OmegaConfigLoader` class.
+
+### 6. Templating of values
+Templating of values is done through native [variable interpolation in `OmegaConfigLoader`](advanced_configuration.md#how-to-do-templating-with-the-omegaconfigloader). Where in `TemplatedConfigLoader` it was necessary to
+provide the template values in a `globals` file or dictionary, in `OmegaConfigLoader` you can provide these values within the same file that has the placeholders or a file that has a name that follows [the same config pattern specified](configuration_basics.md#configuration-patterns).
+The variable interpolation is scoped to a specific configuration type and environment. If you want to share templated values across configuration types and environments, [you will need to use globals](#7-globals).
+
+Suppose you are migrating a templated **catalog** file from using `TemplatedConfigLoader` to `OmegaConfigLoader` you would do the following:
+1. Rename `conf/base/globals.yml` to match the patterns specified for catalog (`["catalog*", "catalog*/**", "**/catalog*"]`), for example `conf/base/catalog_globals.yml`
+2. Add an underscore `_` to any catalog template values. This is needed because of how catalog entries are validated.
+
+```diff
+- bucket_name: "my_s3_bucket"
++ _bucket_name: "my_s3_bucket" # kedro requires `_` to mark templatable keys
+- key_prefix: "my/key/prefix/"
++ _key_prefix: "my/key/prefix/"
+
+- datasets:
++ _datasets:
+    csv: "pandas.CSVDataSet"
+    spark: "spark.SparkDataSet"
+
+```
+
+3. Update `catalog.yml` with the underscores `_` at the beginning of the templated value names.
+```diff
+raw_boat_data:
+-   type: "${datasets.spark}"
++   type: "${_datasets.spark}"
+-   filepath: "s3a://${bucket_name}/${key_prefix}/raw/boats.csv"
++   filepath: "s3a://${_bucket_name}/${_key_prefix}/raw/boats.csv"
+    file_format: parquet
+
+raw_car_data:
+-    type: "${datasets.csv}"
++    type: "${_datasets.csv}"
+-    filepath: "s3://${bucket_name}/data/${key_prefix}/raw/cars.csv"
++    filepath: "s3://${_bucket_name}/data/${_key_prefix}/raw/cars.csv"
+```
+
+#### Providing default values for templates via `oc.select`
+To provide a default for any template values you have to use [the omegaconf `oc.select` resolver](https://omegaconf.readthedocs.io/en/latest/custom_resolvers.html#oc-select).
+
+```diff
+boats:
+  users:
+    - fred
+-    - "${write_only_user|ron}"
++    - "${oc.select:write_only_user,ron}"
+```
+
+### 7. Globals
+If you want to share variables across configuration types, for example parameters and catalog, and environments you need to use [the custom globals resolver with the `OmegaConfigLoader`](advanced_configuration.md#how-to-use-global-variables-with-the-omegaconfigloader).
+The `OmegaConfigLoader` requires global values to be provided in a `globals.yml` file. Note that using a `globals_dict` to provide globals is not supported with `OmegaConfigLoader`. The following section explains the differences between using globals with `TemplatedConfigLoader` and the `OmegaConfigLoader`.
+
+Let's assume your project contains a `conf/base/globals.yml` file with the following contents:
+
+```yaml
+bucket_name: "my_s3_bucket"
+key_prefix: "my/key/prefix/"
+
+datasets:
+    csv: "pandas.CSVDataSet"
+    spark: "spark.SparkDataSet"
+
+folders:
+    raw: "01_raw"
+    int: "02_intermediate"
+    pri: "03_primary"
+    fea: "04_feature"
+```
+
+You no longer need to set `CONFIG_LOADER_ARGS` variable in [`src/<package_name>/settings.py`](../kedro_project_setup/settings.md) to find this `globals.yml` file, because the
+`OmegaConfigLoader` is configured to pick up files named `globals.yml` by default.
+
+```diff
+- CONFIG_LOADER_ARGS = {"globals_pattern": "*globals.yml"}
+```
+
+The globals templating in your catalog configuration will need to be updated to use the globals resolver as follows:
+
+```diff
+raw_boat_data:
+-   type: "${datasets.spark}"
++   type: "${globals:datasets.spark}"  # nested paths into global dict are allowed
+-   filepath: "s3a://${bucket_name}/${key_prefix}/${folders.raw}/boats.csv"
++   filepath: "s3a://${globals:bucket_name}/${globals:key_prefix}/${globals:folders.raw}/boats.csv"
+    file_format: parquet
+
+raw_car_data:
+-   type: "${datasets.csv}"
++   type: "${globals:datasets.csv}"
+-   filepath: "s3://${bucket_name}/data/${key_prefix}/${folders.raw}/${filename|cars.csv}"  # default to 'cars.csv' if the 'filename' key is not found in the global dict
++   filepath: "s3://${globals:bucket_name}/data/${globals:key_prefix}/${globals:folders.raw}/${globals:filename,'cars.csv'}"  # default to 'cars.csv' if the 'filename' key is not found in the global dict
+```
+
+### 8. Deprecation of Jinja2
+`OmegaConfigLoader` does not support Jinja2 syntax in configuration. However, users can achieve similar functionality with the `OmegaConfigLoader` in combination with [dataset factories](../data/kedro_dataset_factories.md).
+If you take the example from [the `TemplatedConfigLoader` with Jinja2 documentation](advanced_configuration.md#how-to-use-jinja2-syntax-in-configuration) you can rewrite your configuration as follows to work with `OmegaConfigLoader`:
+
+```diff
+# catalog.yml
+- {% for speed in ['fast', 'slow'] %}
+- {{ speed }}-trains:
++ "{speed}-trains":
+    type: MemoryDataSet
+
+- {{ speed }}-cars:
++ "{speed}-cars":
+    type: pandas.CSVDataSet
+-    filepath: s3://${bucket_name}/{{ speed }}-cars.csv
++    filepath: s3://${bucket_name}/{speed}-cars.csv
+    save_args:
+        index: true
+
+- {% endfor %}
+```
+
+### 9. Exception handling
+For error and exception handling, most errors are the same. Those you need to be aware of that are different between the original `TemplatedConfigLoader` and `OmegaConfigLoader` are as follows:
+* For missing template values `OmegaConfigLoader` throws `omegaconf.errors.InterpolationKeyError`.