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Migrating XML files into Drupal

Today we will learn how to migrate content from a XML file into Drupal using the Migrate Plus module. We will show how to configure the migration to read files from the local file system and remote locations. We will also talk about the difference between two data parsers provided the module. The example includes node, images, and paragraphs migrations. Let's get started.

Note: Migrate Plus has many more features. For example, it contains source plugins to import from JSON files and SOAP endpoints. It provides many useful process plugins for DOM manipulation, string replacement, transliteration, etc. The module also lets you define migration plugins as configurations and create groups to share settings. It offers a custom event to modify the source data before processing begins. In today's blog post, we are focusing on importing XML files. Other features will be covered in future entries.

Getting the code

You can get the full code example at https://github.com/dinarcon/ud_migrations The module to enable is UD XML source migration whose machine name is ud_migrations_xml_source. It comes with four migrations: udm_xml_source_paragraph, udm_xml_source_image, udm_xml_source_node_local, and udm_xml_source_node_remote.

You can get the Migrate Plus module using composer: composer require 'drupal/migrate_plus:^5.0'. This will install the 8.x-5.x branch, where new development will happen. This branch was created to introduce breaking changes in preparation for Drupal 9. As of this writing, the 8.x-4.x branch has feature parity with the newer branch. If your Drupal site is not composer-based, you can download the module manually.

Understanding the example set up

This migration will reuse the same configuration from the introduction to paragraph migrations example. Refer to that article for details on the configuration: the destinations will be the same content type, paragraph type, and fields. The source will be changed in today's example, as we use it to explain XML migrations. The end result will again be nodes containing an image and a paragraph with information about someone's favorite book. The major difference is that we are going to read from XML. In fact, three of the migrations will read from the same file. The following snippet shows a reduced version of the file to get a sense of its structure:

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_people>
    <unique_id>1</unique_id>
    <name>Michele Metts</name>
    <photo_file>P01</photo_file>
    <book_ref>B10</book_ref>
  </udm_people>
  <udm_people>
    ...
  </udm_people>
  <udm_people>
    ...
  </udm_people>
  <udm_book_paragraph>
    <book_id>B10</book_id>
    <book_details>
      <title>The definite guide to Drupal 7</title>
      <author>Benjamin Melançon et al.</author>
    </book_details>
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
  <udm_photos>
    <photo_id>P01</photo_id>
    <photo_url>https://udrupal.com/photos/freescholar.jpg</photo_url>
    <photo_dimensions>
      <width>240</width>
      <height>351</height>
    </photo_dimensions>
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
</data>

Note: You can literally swap migration sources without changing any other part of the migration. This is a powerful feature of ETL frameworks like Drupal's Migrate API. Although possible, the example includes slight changes to demonstrate various plugin configuration options. Also, some machine names had to be changed to avoid conflicts with other examples in the demo repository.

Migrating nodes from a XML file

In any migration project, understanding the source is very important. For XML migrations, there are two major considerations. First, where in the XML tree hierarchy lies the data that you want to import. It can be at the root of the file or several levels deep in the hierarchy. You use an XPath expression to select a set of nodes from the XML document. In this article, the term element when referring to an XML document node to distinguish it from a Drupal node. Second, when you get to the set of elements that you want to import, what child elements are going to be made available to the migration. It is possible that each element contains more data than needed. In XML imports, you have to manually include the child elements that will be required for the migration. The following code snippet shows part of the local XML file relevant to the node migration:

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_people>
    <unique_id>1</unique_id>
    <name>Michele Metts</name>
    <photo_file>P01</photo_file>
    <book_ref>B10</book_ref>
  </udm_people>
  <udm_people>
    ...
  </udm_people>
  <udm_people>
    ...
  </udm_people>
</data>

The set of elements containing node data lies two levels deep in the hierarchy. Starting with data at the root and then descending one level to udm_people. Each element of this array is an object with four properties:

unique_id is the unique identifier for each element returned by the data/udm_people hierarchy.
name is the name of a person. This will be used in the node title.
photo_file is the unique identifier of an image that was created in a separate migration.
book_ref is the unique identifier of a book paragraph that was created in a separate migration.

The following snippet shows the configuration to read a local XML file for the node migration:

source:
  plugin: url
  # This configuration is ignored by the 'xml' data parser plugin.
  # It only has effect when using the 'simple_xml' data parser plugin.
  data_fetcher_plugin: file
  # Set to 'xml' to use XMLReader https://www.php.net/manual/en/book.xmlreader.php
  # Set to 'simple_xml' to use SimpleXML https://www.php.net/manual/en/ref.simplexml.php
  data_parser_plugin: xml
  urls:
    - modules/custom/ud_migrations/ud_migrations_xml_source/sources/udm_data.xml
  # XPath expression. It is common that it starts with a slash (/).
  item_selector: /data/udm_people
  fields:
    - name: src_unique_id
      label: 'Unique ID'
      selector: unique_id
    - name: src_name
      label: 'Name'
      selector: name
    - name: src_photo_file
      label: 'Photo ID'
      selector: photo_file
    - name: src_book_ref
      label: 'Book paragraph ID'
      selector: book_ref
  ids:
    src_unique_id:
      type: integer

The name of the plugin is url. Because we are reading a local file, the data_fetcher_plugin is set to file and the data_parser_plugin to xml. The urls configuration contains an array of file paths relative to the Drupal root. In the example we are reading from one file only, but you can read from multiple files at once. In that case, it is important that they have a homogeneous structure. The settings that follow will apply equally to all the files listed in urls.

Technical note: Migrate Plus provides two data parser plugins for XML files. xml uses XMLReader while simple_xml uses SimpleXML. The parser to use is configured in the data_parser_plugin configuration. Also note that when you use the xml parser, the data_fetcher_plugin setting is ignored. More details below.

The item_selector configuration indicates where in the XML file lies the set of elements to be migrated. Its value is an XPath expression used to traverse the file hierarchy. In this case, the value is /data/udm_people. Verify that your expression is valid and select the elements you intend to import. It is common that it starts with a slash (/).

fields has to be set to an array. Each element represents a field that will be made available to the migration. The following options can be set:

name is required. This is how the field is going to be referenced in the migration. The name itself can be arbitrary. If it contained spaces, you need to put double quotation marks (") around it when referring to it in the migration.
label is optional. This is a description used when presenting details about the migration. For example, in the user interface provided by the Migrate Tools module. When defined, you do not use the label to refer to the field. Keep using the name.
selector is required. This is another XPath-like string to find the field to import. The value must be relative to the subtree specified by the item_selector configuration. In the example, the fields are direct children of the elements to migrate. Therefore, the XPath expression only includes the element name (e.g., unique_id). If you had nested elements, you could use a slash (/) character to go deeper in the hierarchy. This will be demonstrated in the image and paragraph migrations.

Finally, you specify an ids array of field names that would uniquely identify each record. As already stated, the unique_id field servers that purpose. The following snippet shows part of the process, destination, and dependencies configuration of the node migration:

process:
  field_ud_image/target_id:
    plugin: migration_lookup
    migration: udm_xml_source_image
    source: src_photo_file
destination:
  plugin: 'entity:node'
  default_bundle: ud_paragraphs
migration_dependencies:
  required:
    - udm_xml_source_image
    - udm_xml_source_paragraph
  optional: []

The source for the setting the image reference is src_photo_file. Again, this is the name of the field, not the label nor selector. The configuration of the migration lookup plugin and dependencies point to two XML migrations that come with this example. One is for migrating images and the other for migrating paragraphs.

Migrating paragraphs from a XML file

Let's consider an example where the elements to migrate have many levels of nesting. The following snippets show part of the local XML file and source plugin configuration for the paragraph migration:

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_book_paragraph>
    <book_id>B10</book_id>
    <book_details>
      <title>The definite guide to Drupal 7</title>
      <author>Benjamin Melançon et al.</author>
    </book_details>
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
</data>

source:
  plugin: url
  # This configuration is ignored by the 'xml' data parser plugin.
  # It only has effect when using the 'simple_xml' data parser plugin.
  data_fetcher_plugin: file
  # Set to 'xml' to use XMLReader https://www.php.net/manual/en/book.xmlreader.php
  # Set to 'simple_xml' to use SimpleXML https://www.php.net/manual/en/ref.simplexml.php
  data_parser_plugin: xml
  urls:
    - modules/custom/ud_migrations/ud_migrations_xml_source/sources/udm_data.xml
  # XPath expression. It is common that it starts with a slash (/).
  item_selector: /data/udm_book_paragraph
  fields:
    - name: src_book_id
      label: 'Book ID'
      selector: book_id
    - name: src_book_title
      label: 'Title'
      selector: book_details/title
    - name: src_book_author
      label: 'Author'
      selector: book_details/author
  ids:
    src_book_id:
      type: string

The plugin, data_fetcher_plugin, data_parser_plugin and urls configurations have the same values as in the node migration. The item_selector and ids configurations are slightly different to represent the path to paragraph elements and the unique identifier field, respectively.

The interesting part is the value of the fields configuration. Taking data/udm_book_paragraph as a starting point, the records with paragraph data have a nested structure. Particularly, the book_details element has two children: title and author. To refer to them, the selectors are book_details/title and book_details/author, respectively. Note that you can go as many level deeps in the hierarchy to find the value that should be assigned to the field. Every level in the hierarchy could be separated by a slash (/).

In this example, the target is a single paragraph type. But a similar technique can be used to migrate multiple types. One way to configure the XML file is having two children. paragraph_id would contain the unique identifier for the record. paragraph_data would contain a child element to specify the paragraph type. It would also have an arbitrary number of extra child elements with the data to be migrated. In the process section, you would iterate over the children to map the paragraph fields.

The following snippet shows part of the process configuration of the paragraph migration:

process:
  field_ud_book_paragraph_title: src_book_title
  field_ud_book_paragraph_author: src_book_author

Migrating images from a XML file

Let's consider an example where the elements to migrate have more data than needed. The following snippets show part of the local XML file and source plugin configuration for the image migration:

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_photos>
    <photo_id>P01</photo_id>
    <photo_url>https://udrupal.com/photos/freescholar.jpg</photo_url>
    <photo_dimensions>
      <width>240</width>
      <height>351</height>
    </photo_dimensions>
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
</data>

source:
  plugin: url
  # This configuration is ignored by the 'xml' data parser plugin.
  # It only has effect when using the 'simple_xml' data parser plugin.
  data_fetcher_plugin: file
  # Set to 'xml' to use XMLReader https://www.php.net/manual/en/book.xmlreader.php
  # Set to 'simple_xml' to use SimpleXML https://www.php.net/manual/en/ref.simplexml.php
  data_parser_plugin: xml
  urls:
    - modules/custom/ud_migrations/ud_migrations_xml_source/sources/udm_data.xml
  # XPath expression. It is common that it starts with a slash (/).
  item_selector: /data/udm_photos
  fields:
    - name: src_photo_id
      label: 'Photo ID'
      selector: photo_id
    - name: src_photo_url
      label: 'Photo URL'
      selector: photo_url
  ids:
    src_photo_id:
      type: string

The plugin, data_fetcher_plugin, data_parser_plugin and urls configurations have the same values as in the node migration. The item_selector and ids configurations are slightly different to represent the path to image elements and the unique identifier field, respectively.

The interesting part is the value of the fields configuration. Taking data/udm_photos as a starting point, the elements with image data have extra children that are not used in the migration. Particularly, the photo_dimensions element has two children representing the width and height of the image. To ignore this subtree, you simply omit it from the fields configuration. In case you wanted to use it, the selectors would be photo_dimensions/width and photo_dimensions/height, respectively.

The following snippet shows part of the process configuration of the image migration:

process:
  psf_destination_filename:
    plugin: callback
    callable: basename
    source: src_photo_url

XML file location

Important: What is described in this section only applies when you use either (1) the xml data parser or (2) the simple_xml parser with the file data fetcher.

When using the file data fetcher plugin, you have three options to indicate the location to the XML files in the urls configuration:

Use a relative path from the Drupal root. The path should not start with a slash (/). This is the approach used in this demo. For example, modules/custom/my_module/xml_files/example.xml.
Use an absolute path pointing to the XML location in the file system. The path should start with a slash (/). For example, /var/www/drupal/modules/custom/my_module/xml_files/example.xml.
Use a fully-qualified URL to any built-in wrapper like http, https, ftp, ftps, etc. For example, https://understanddrupal.com/xml-files/example.xml.
Use a custom stream wrapper.

Being able to use stream wrappers gives you many more options. For instance:

Files located in the public, private, and temporary file systems managed by Drupal. This leveragers functionality already available in Drupal core. For example: public://xml_files/example.xml.
Files located in profiles, modules, and themes. You can use the System stream wrapper module or apply this core patch to get this functionality. For example, module://my_module/xml_files/example.xml.
Files located in AWS Amazon S3. You can use the S3 File System module along with the S3FS File Proxy to S3 module to get this functionality.

Migrating remote XML files

Important: What is described in this section only applies when you use the http data fetcher plugin.

Migrare Plus provides another data fetcher plugin named http. Under the hood, it uses the Guzzle HTTP Client library. You can use it to fetch files using any protocol supported by curl like http, https, ftp, ftps, sftp, etc. In a future blog post we will explain this data fetcher in more detail. For now, the udm_xml_source_node_remote migration demonstrates a basic setup for this plugin. Note that only the data_fetcher_plugin, data_parser_plugin, and urls configurations are different from the local file example. The following snippet shows part of the configuration to read a remote XML file for the node migration:

source:
  plugin: url
  data_fetcher_plugin: http
  # 'simple_xml' is configured to be able to use the 'http' fetcher.
  data_parser_plugin: simple_xml
  urls:
    - https://sendeyo.com/up/d/478f835718
  item_selector: /data/udm_people
  fields: ...
  ids: ...

And that is how you can use XML files as the source of your migrations. Many more configurations are possible when you use the simple_xml parser with the http fetcher. For example, you can provide authentication information to get access to protected resources. You can also set custom HTTP headers. Examples will be presented in a future entry.

XMLReader vs SimpleXML in Drupal migrations

As noted in the module's README file, the xml parser plugin uses the XMLReader interface to incrementally parse XML files. The reader acts as a cursor going forward on the document stream and stopping at each node on the way. This should be used for XML sources which are potentially very large. On the other than, the simple_xml parser plugin uses the SimpleXML interface to fully parse XML files. This should be used for XML sources where you need to be able to use complex XPath expressions for your item selectors, or have to access elements outside of the current item element via XPath.

What did you learn in today's blog post? Have you migrated from XML files before? If so, what challenges have you found? Did you know that you can read local and remote files? Did you know that the data_fetcher_plugin configuration is ignored when using the xml data parser? Please share your answers in the comments. Also, I would be grateful if you shared this blog post with others.

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