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:
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:
1 Michele Metts P01 B10 ... ... B10 The definite guide to Drupal 7 Benjamin Melançon et al. ... ... P01 https://agaric.coop/sites/default/files/pictures/picture-15-1421176712.jpg 240 351 ... ...
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:
1 Michele Metts P01 B10 ... ...
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_idis the unique identifier for each element returned by the
nameis the name of a person. This will be used in the node title.
photo_fileis the unique identifier of an image that was created in a separate migration.
book_refis 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
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
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.
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:
nameis 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.
labelis 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.
selectoris 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_selectorconfiguration. 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: 
source for the setting the image reference is
src_photo_file. Again, this is the
name of the field, not the
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:
B10 The Definitive Guide to Drupal 7 Benjamin Melançon et al. ... ...
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
urls configurations have the same values as in the node migration. The
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:
author. To refer to them, the selectors are
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:
P01 https://agaric.coop/sites/default/files/pictures/picture-15-1421176712.jpg 240 351 ... ...
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 following snippet shows part of the process configuration of the image migration:
process: psf_destination_filename: plugin: callback callable: basename source: src_photo_url
urls configurations have the same values as in the node migration. The
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
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
- 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,
- Use an absolute path pointing to the XML location in the file system. The path should start with a slash (/). For example,
- Use a fully-qualified URL to any built-in wrapper like
ftps, etc. For example,
- 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 leverages functionality already available in Drupal core. For example:
- 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,
- 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.
Migrate 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
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
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.
This blog post series is made possible thanks to these generous sponsors. Contact us if your organization would like to support this documentation project, whether it is the migration series or other topics.
This blog post series, cross-posted at UnderstandDrupal.com as well as here on Agaric.coop, is made possible thanks to these generous sponsors: Drupalize.me by Osio Labs has online tutorials about migrations, among other topics, and Agaric provides migration trainings, among other services. Contact Understand Drupal if your organization would like to support this documentation project, whether it is the migration series or other topics.