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Metadata Extractors and Embedders Extension Point

Content Services performs metadata extraction on content automatically, however, you may wish to create custom metadata extractors to handle custom file properties and custom content models.

Architecture Information: Platform Architecture

Introduction

Every time a file is uploaded to the repository the file’s MIME type is automatically detected. Based on the MIME type a related Metadata Extractor is invoked on the file. It will extract common properties from the file, such as author, and set the corresponding content model property accordingly. Each Metadata Extractor has a mapping between the properties it can extract and the content model properties.

Metadata extraction is primarily based on the Apache Tika library. This means that whatever file formats Tika can extract metadata from, Content Services can also handle. To give you an idea of what file formats Content Services can extract metadata from, here is a list of the most common formats:

  • PDF
  • MS Office
  • Open Office
  • MP3, MP4, QuickTime
  • JPEG, TIFF, PNG
  • DWG
  • HTML
  • XML
  • Email

The properties that are extracted are limited to the out-of-the-box content model, which is very generic. Here are some example of extracted property name and what content model property it maps to:

  • author -> cm:author
  • title -> cm:title
  • subject -> cm:description
  • created -> cm:created
  • description -> NOT MAPPED - you could map it in a custom configuration
  • comments -> NOT MAPPED - you could map it in a custom configuration
  • If it is an image file:
  • EXIF metadata -> exif:exif (pixel dimensions, manufacturer, model, software, date-time etc.)
  • Geo metadata -> cm:geographic (longitude & latitude)
  • If it is an audio file -> audio:audio (album, artist, composer, engineer, genre etc.)
  • If it is an email file -> cm:emailed (from, to, subject, sent date)

One thing to note though, even if an extractor can extract any of the system controlled properties, such as created date, it will not be used. Created date, creator, modified date, and modifier is always controlled by the Content Services system, unless you are using the Bulk Import tool, in which case last modified date can be preserved.

Metadata extraction and Transform Engines

The extraction of metadata in the repository is performed in T-Engines (transform engines). Prior to Content Services version 7, it was performed inside the repository. T-Engines provide improved scalability, stability, security and flexibility. New extractors may be added without the need for a new Content Services release or applying an AMP on top of the repository (i.e. alfresco.war).

The Content Services version 6 framework for creating metadata extractors that run as part of the repository still exists, so existing AMPs that add extractors will still work as long as there is not an extractor in a T-Engine that claims to do the same task. The framework is deprecated and could well be removed in a future release.

This page describes how metadata extraction and embedding works, so that it is possible to add a custom T-Engine to do other types. It also lists the various extractors that have been moved to T-Engines.

A framework for embedding metadata into a file was provided as part of the repository prior to Content Services version 7. This too still exists, but has been deprecated. Even though the content repository did not provide any out of the box implementations, the embedding framework of metadata via T-Engines exists.

In the case of an extract, the T-Engine returns a JSON file that contains name value pairs. The names are fully qualified QNames of properties on the source node. The values are the metadata values extracted from the content. The transform defines the mapping of metadata values to properties. Once returned to the repository, the properties are automatically set.

In the case of an embed, the T-Engine takes name value pairs from the transform options, maps them to metadata values which are then updated in the supplied content. The content is then returned to the content repository and the node is updated.

Metadata extraction is just another transform

Metadata extractors and embedders are just a specialist form of transform. The targetMediaType in the T-Engine engine-config.json is set to "alfresco-metadata-extract" or "alfresco-metadata-embed" the following is a snippet from the tika_engine_config.json

    {
      "transformerName": "TikaAudioMetadataExtractor",
      "supportedSourceAndTargetList": [
        {"sourceMediaType": "video/x-m4v",     "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "audio/x-oggflac", "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "application/mp4", "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "audio/vorbis",    "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "video/3gpp",      "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "audio/x-flac",    "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "video/3gpp2",     "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "video/quicktime", "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "audio/mp4",       "targetMediaType": "alfresco-metadata-extract"},
        {"sourceMediaType": "video/mp4",       "targetMediaType": "alfresco-metadata-extract"}
      ],
      "transformOptions": [
        "metadataOptions"
      ]
    },

If a T-Engine definition says it supports a metadata extract or embed, it will be used in preference to any extractor or embedder using the deprecated frameworks in the content repository.

Transform interface

Code that transforms a specific document type in a T-Engine generally implements the Transformer interface. In addition to the transform method, extractMetadata and embedMetadata methods will be called depending on the target media type. The implementing class is called from the transformImpl
method of the controller class.

default void transform(String transformName, String sourceMimetype, String targetMimetype,
                       Map<String, String> transformOptions,
                       File sourceFile, File targetFile) throws Exception {
}

default void extractMetadata(String transformName, String sourceMimetype, String targetMimetype,
                             Map<String, String> transformOptions,
                             File sourceFile, File targetFile) throws Exception {
}

default void embedMetadata(String transformName, String sourceMimetype, String targetMimetype,
                           Map<String, String> transformOptions,
                           File sourceFile, File targetFile) throws Exception {
}

It is typical for the extractMetadata method to call another extractMetadata method on a sub class of AbstractMetadataExtractor as this class provides the bulk of the functionality needed to configure metadata extraction or embedding.

    public void extractMetadata(String transformName, String sourceMimetype, String targetMimetype,
                                Map<String, String> transformOptions,
                                File sourceFile, File targetFile) throws Exception
    {
        AbstractMetadataExtractor extractor = ...
        extractor.extractMetadata(sourceMimetype, transformOptions, sourceFile, targetFile);
    }

    // Similar code for embedMetadata

AbstractMetadataExtractor base class

The AbstractMetadataExtractor may be extended to perform metadata extract and embed tasks, by overriding two methods in the sub classes:

    public abstract Map<String, Serializable> extractMetadata(String sourceMimetype, Map<String, String> transformOptions,
                                                              File sourceFile) throws Exception;

    public void embedMetadata(String sourceMimetype, String targetMimetype, Map<String, String> transformOptions,
                              File sourceFile, File targetFile) throws Exception
    {
        // Default nothing, as embedding is not supported in most cases
    }

Method parameters:

  • sourceMimetype mimetype of the source
  • transformOptions transform options from the client
  • sourceFile the source as a file

The extractMetadata should extract and return ALL available metadata from the sourceFile. These values are then mapped into content repository property names and values, depending on what is defined in a <classname>_metadata_extract.properties file. Value may be discarded or a single value may even be used for multiple properties. The selected values are sent back to the repository as JSON as a mapping of fully qualified content model property names to values, where the values are applied to the source node.

Metadata extraction configuration

The AbstractMetadataExtractor class reads the <classname>_metadata_extract.properties file, so that it knows how to map metadata returned from the sub class extractMetadata method onto content model properties. The following is an example for an email (file extension .eml):

#
# RFC822MetadataExtractor - default mapping
#

# Namespaces
namespace.prefix.imap=http://www.alfresco.org/model/imap/1.0
namespace.prefix.cm=http://www.alfresco.org/model/content/1.0

# Mappings
messageFrom=imap:messageFrom, cm:originator
messageTo=imap:messageTo, cm:addressee
messageCc=imap:messageCc, cm:addressees
messageSubject=imap:messageSubject, cm:title, cm:description, cm:subjectline
messageSent=imap:dateSent, cm:sentdate
messageReceived=imap:dateReceived
Thread-Index=imap:threadIndex
Message-ID=imap:messageId

As can be seen, the email’s metadata for messageFrom (if available) will be used to set two properties in the content repository (if they exist): imap:messageFrom, cm:originator. The property names use namespace prefixes specified above.

Property overwrite policy

It is possible to specify if properties in the repository will be set if the extracted values are not null or if the properties already have a value. By default, PRAGMATIC is used. Generally you will not need to change this. Other values (CAUTIOUS, EAGER, PRUDENT) are described in OverwritePolicy. To use a different policy add a sys:overwritePolicy value to the Map returned from the extractMetadata method of the class extending AbstractMetadataExtractor (described above).

The following table shows which conditions must be met for overwriting the value:

overwrite-policy

Aspect property policy

When a property is extracted, which is part of an aspect, it is possible to remove all other properties in the same aspect that do not have an extracted value. In this way only extracted values will be set and any previously set aspect properties will be cleared. By default, this does not take place and newly extracted values are just added to the node’s properties. To clear other aspect properties add sys:carryAspectProperties= false to the Map returned from the extractMetadata method.

Enable tagging

When an extracted property is taggable, it is possible to automatically extract tags from the value. By default, this is disabled, but may be enabled by adding sys:enableStringTagging= true to the Map returned from the extractMetadata method.

Assuming enableStringTagging is true, it is also possible to change the default separators of the tags in the value. The default separators are , ; and \|. This is done by adding a sys:stringTaggingSeparators value to the Map returned from the extractMetadata method. Please note that escaping of characters takes place in both Java and json, so json response would look like "sys:stringTaggingSeparators": ";,\",\",\\|" if the code explicitly sets the default separators.

Overriding metadata extraction request in the repository

The request from the repository to extract metadata goes through RenditionService2, so will use the asynchronous Alfresco Transform Service if available and a synchronous Local transform if not.

Normally the only transform options are timeout and sourceEncoding, so the extractor code only has the source mimetype and content itself to work on. Customisation of the property mapping should really be done in the T-Engine as described above.

However, it is currently possible for code running in the repository (i.e. alfresco.war) to override the default mapping of metadata to content model properties, with an extractMapping transform option. This approach is deprecated and may be removed in a future minor Content Services 7.x release.

An AMP should supply a class that implements the MetadataExtractorPropertyMappingOverride interface and add it to the metadataExtractorPropertyMappingOverrides property of the extractor.Asynchronous spring bean.

/**
 * Overrides the default metadata mappings for PDF documents:
 *
 * <pre>
 * author=cm:author
 * title=cm:title
 * subject=cm:description
 * created=cm:created
 * </pre>
 * with:
 * <pre>
 * author=cm:author
 * title=cm:title,cm:description
 * </pre>
 */
public class PdfMetadataExtractorOverride implements MetadataExtractorPropertyMappingOverride {
    @Override
    public boolean match(String sourceMimetype) {
        return MIMETYPE_PDF.equals(sourceMimetype);
    }

    @Override
    public Map<String, Set<String>> getExtractMapping(NodeRef nodeRef) {
        Map<String, Set<String>> mapping = new HashMap<>();
        mapping.put("author", Collections.singleton("{http://www.alfresco.org/model/content/1.0}author"));
        mapping.put("title",  Set.of("{http://www.alfresco.org/model/content/1.0}title",
                                     "{http://www.alfresco.org/model/content/1.0}description"));
        return mapping;
    }
}

Resulting in a request that contains the following transform options:

{"extractMapping":{
   "author":["{http://www.alfresco.org/model/content/1.0}author"],
   "title":["{http://www.alfresco.org/model/content/1.0}title",
            "{http://www.alfresco.org/model/content/1.0}description"]},
 "timeout":20000,
 "sourceEncoding":"UTF-8"}

Metadata extraction response

The transformed content that is returned to the repository is JSON and specifies what properties that should be updated on the source node. For example:

{"{http://www.alfresco.org/model/content/1.0}description":"Making Bread",
 "{http://www.alfresco.org/model/content/1.0}title":"Making Bread",
 "{http://www.alfresco.org/model/content/1.0}author":"Fred"}

Metadata embed request

An embed request simply contains a transform option called metadata that contains a map of property names to values, resulting in transform options like the following:

{"metadata":
 {"{http://www.alfresco.org/model/content/1.0}author":"Fred",
  "{http://www.alfresco.org/model/content/1.0}title":"Making Bread"
  "{http://www.alfresco.org/model/content/1.0}helpers":["Jane","Paul"]},
 "timeout":20000,
 "sourceEncoding":"UTF-8"}

Values are either a String, or a Collection of Strings. The mappings of these content repository properties to metadata properties is normally the reverse of those defined in the <classname>_metadata_extract.properties file in the T-Engine.

Metadata embed response

This is simply the source content with the metadata embedded. The content repository updates the content of the node with what is returned.

Repository information

The repository still contains metadata extraction code.

Framework

The Content Services version 6 framework for running metadata extractors and embedders still exists. An additional AsynchronousExtractor has been added to communicate with the RenditionService2 from Content Services version 7. The AsynchronousExtractor handles the request and response in a generic way allowing all the content type specific code to be moved to a T-Engine.

XML framework

The following XML based extractors have NOT been removed from the content repository as custom extensions may be using them. There are no out-of-the-box extractors that use them as part of the repository. Ideally any custom extensions should be moved to a custom T-Engine using code based on these classes.

Metadata extractors that have be moved to T-Engines

The following extractors, and their configuration (i.e. property mappings), exist now in T-Engines rather than in the repository (i.e. alfresco.war):

The LibreOffice extractor has also been moved to a T-Engine, even though Tika based extractors are now used for all types it supported. This has been the case since ACS 6.0.1. It was moved into a T-Engine to simplify moving any custom code that may have extended it.

The Tika based classes for extractors using configuration files or spring context files have been removed from the repository as the preferred way to create extractors is via a T-Engine and these approaches require in process extensions.

Changing default property mappings for PDF metadata extraction

A common requirement is to be able to change the mapping of out-of-the-box properties, such as having the subject property mapped to cm:title instead of cm:description for a PDF file. This is quite easy to achieve, just override the out-of-the-box JSON configuration and re-configure the mapping. The out-of-the-box definitions for Metadata Extractors can be found in the places described in the above section.

To change the subject property so it is mapped to content model property cm:title for PDF files re-define the PdfBoxMetadataExtractor_metadata_extract.properties configuration as follows:

#
# PdfBoxMetadataExtracter - custom mapping
#

# Namespaces
namespace.prefix.cm=http://www.alfresco.org/model/content/1.0

# Mappings
author=cm:author
title=cm:title
subject=cm:title

Note that all the namespaces that the content model properties belong to have to be specified as in the above example with namespace.prefix.cm. It is also very important to know that the property names are case sensitive.

Metadata extraction debug logging

Sometimes it can be useful to know what metadata extractor that is actually used when you upload a document. Turning on Metadata Extraction logging is a good idea to get on top of what is happening. Set the following property in log4j.properties:

log4j.logger.org.alfresco.repo.content.metadata=DEBUG

What about the properties? It is likely that you will struggle to figure out what properties are extracted and their names. You can have this logged with the following log file configuration:

log4j.logger.org.alfresco.repo.content.metadata.AbstractMappingMetadataExtracter=DEBUG

This log configuration is set to some other log level out-of-the-box so you need to specifically re-configure it to be able to see something. Now when running you will also see the extracted doc properties.

Using custom content models in property mappings for PDF metadata extraction

Next requirement is most likely to map properties to custom content models. There is an ACME content model tutorial where the base document type has an acme:documentId property. You might want to add a document identifier to the PDFs you are uploading and have it automatically set in the ACME content model. Start by updating the the PdfBoxMetadataExtractor_metadata_extract.properties configuration as follows:

#
# PdfBoxMetadataExtracter - custom mapping
#

# Namespaces
namespace.prefix.cm=http://www.alfresco.org/model/content/1.0
namespace.prefix.acme=http://www.acme.org/model/content/1.0

# Mappings
author=cm:author
title=cm:title
DocumentId=acme:documentId

Here the custom document property DocumentId has been added so it is mapped to the ACME content model property acme:documentId. When doing this you also need to define the new custom namespace acme. For this to work you need to have a rule on the folder that applies the acme:document type to any PDF document uploaded to the folder. This type has the acme:docuementId property.

Changing default property mappings for XML metadata extraction

Now, what if you would like to extract metadata from an XML file, how would you go about that? This can be achieved with the XmlMetadataExtracter, which in-turn uses the XPathMetadataExtracter to navigate the XML and extract metadata. These extractors are still in the repository, see this section.

Let’s say we had XML files looking like this:

<?xml version="1.0" encoding="UTF-8"?>
<doc id="doc001">
    <project>
        <number>PX001</number>
    </project>
    <securityClassification>Company Confidential</securityClassification>
    <text>
        Lorem ipsum dolor sit amet, consectetur adipiscing elit. Praesent tincidunt luctus ante, in pulvinar ante rutrum
        quis. Etiam maximus arcu ut metus sollicitudin laoreet. Pellentesque ac purus nec massa euismod iaculis a sed
        sapien. Integer id nisi eu tellus commodo congue. In bibendum dapibus porttitor. Aenean lobortis sodales risus
         ....
    </text>
</doc>

And whenever we upload one we want to have the /doc/@id attribute set as acme:documentId, /doc/project/number set as acme:projectNumber, and /doc/securityClassification set as acme:securityClassification. This will require configuration like this, note these are new bean definitions, no overrides:

<bean id="org.alfresco.tutorial.metadataextracter.xml.AcmeDocXPathMetadataExtracter"
    class="org.alfresco.repo.content.metadata.xml.XPathMetadataExtracter"
    parent="baseMetadataExtracter"
    init-method="init">
  <property name="mappingProperties">
      <bean class="org.springframework.beans.factory.config.PropertiesFactoryBean">
          <property name="location">
              <value>
                  classpath:alfresco/module/${project.artifactId}/metadataextraction/acme-content-model-mappings.properties
              </value>
          </property>
      </bean>
  </property>
  <property name="xpathMappingProperties">
      <bean class="org.springframework.beans.factory.config.PropertiesFactoryBean">
          <property name="location">
              <value>
                  classpath:alfresco/module/${project.artifactId}/metadataextraction/acme-xml-doc-xpath-mappings.properties
              </value>
          </property>
      </bean>
  </property>
</bean>

<bean id="org.alfresco.tutorial.metadataextracter.xml.selector.AcmeDocXPathSelector"
    class="org.alfresco.repo.content.selector.XPathContentWorkerSelector"
    init-method="init">
  <property name="workers">
      <map>
          <entry key="/*">
              <ref bean="org.alfresco.tutorial.metadataextracter.xml.AcmeDocXPathMetadataExtracter"/>
          </entry>
      </map>
  </property>
</bean>

<bean id="org.alfresco.tutorial.metadataextracter.xml.AcmeDocXMLMetadataExtracter"
      class="org.alfresco.repo.content.metadata.xml.XmlMetadataExtracter"
      parent="baseMetadataExtracter">
  <property name="overwritePolicy">
      <value>EAGER</value> <!-- Put the extracted metadata into the content model property as long as it is not null -->
  </property>
  <property name="selectors">
      <list>
          <ref bean="org.alfresco.tutorial.metadataextracter.xml.selector.AcmeDocXPathSelector"/>
      </list>
  </property>
</bean>

The acme-content-model-mappings.properties file contains mappings from the extracted XML doc properties to the content model properties:

# Namespaces
namespace.prefix.acme=http://www.acme.org/model/content/1.0

# Mappings - metadata property -> content model property
documentId=acme:documentId
securityClassification=acme:securityClassification
projectNumber=acme:projectNumber

The property mapping can always be done in .properties files if we like, and we could have used a .properties file for the PDFBoxMetadataExtracter too. The other properties file called acme-xml-doc-xpath-mappings.properties contains the XPath expression configuration for where to find the metadata in the XML file:

# XPath Mappings - metadata property -> XML Document XPATH
documentId=/doc/@id
securityClassification=/doc/securityClassification
projectNumber=/doc/project/number

Metadata extractor limits

Metadata extraction limits allows configurations on AbstractMappingMetadataExtracter for:

  • control of the maximum time allowed for an extraction
  • control of the maximum size (MB) of any single document that the extractor will handle
  • control of the maximum number of all the documents being processed at any point in time

The default values for each of these properties are MAX value specified in the java code. These limits are configured per extractor and mimetype.

The limits configured for Content Services are:

Time out configured for all extractor and all mimetypes
content.metadataExtracter.default.timeoutMs=20000

Maximum size of a document to process - configured for PdfBoxMetadataExtracter , pdf files
content.metadataExtracter.pdf.maxDocumentSizeMB=10

Maximum number of concurrent extractions - configured for PdfBoxMetadataExtracter , pdf files
content.metadataExtracter.pdf.maxConcurrentExtractionsCount=5

Deployment

For XML metadata extraction you will still use the SDK and a JAR project applied to the Repository (i.e. alfresco.war).

To change the configuration for the majority of the metadata extractors you will have to generate a new Transform Core AIO Docker image with the new configuration. Another option would be to create a new separate T-Engine that has a higher priority (lower number) for this metadata extraction. That way you can still use the standard T-Engine and the new one from for this one special case.

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