(For more resources related to this topic, see here.)
What is CMIS?
The goal of CMIS is to provide a standard method for accessing content from different content repositories. Using CMIS service calls, it is possible to navigate through and create content in a repository. CMIS also includes a query language for searching both the metadata and full-text content stored that is stored in a repository.
The CMIS standard defines the protocols and formats for the requests and responses of API service calls made to a repository.
CMIS acts as a standard interface and protocol for accessing content repositories, something similar to how ANSI-SQL acts as a common-denominator language for interacting with different databases.
The use of the CMIS API for accessing repositories brings with it a number of benefits. Perhaps chief among these is the fact that access to CMIS is language neutral. Any language that supports HTTP services can be used to access a CMIS-enabled repository. Client software can be written to use a single API and be deployed to run against multiple CMIS-compliant repositories.
Alfresco and CMIS
The original draft for CMIS 0.5 was written by EMC, IBM and Microsoft. Shortly after that draft, Alfresco and other vendors joined the CMIS standards group. Alfresco was an early CMIS adopter and offered an implementation of CMIS version 0.5 in 2008.
In 2009, Alfresco began hosting an on-line preview of the CMIS standard. The server, accessible via the http://cmis.alfresco.com URL, still exists and implements the latest CMIS standard. As of this writing, that URL hosts a preview of CMIS 1.1 features.
In mid-2010, just after the CMIS 1.0 standard was approved, Alfresco released CMIS in both the Alfresco Community and Enterprise editions.
In 2012, with Alfresco version 4.0, Alfresco moved from a home grown CMIS runtime implementation to one that uses the Apache Chemistry OpenCMIS Server Framework. From that release, developers have been able to customize Alfresco using the OpenCMIS Java API.
Overview of the CMIS Standard
Next we discuss the details of the CMIS specification, particularly the domain model, the different services that it provides, and the supported protocol bindings.
Domain model (Object model)
Every content repository vendor has their own definition of a content or object model. Alfresco, for example, has rich content modeling capabilities, such as types and aspects that can inherit from other types and aspects, and properties that can be assigned attributes like data-type, multi-valued and required.
But there are wide differences in the ways in which different vendors have implemented content modeling. In the Documentum ECM system, for example, the generic content type is called dm_document, while in Alfresco it is called cm:content. Another example is the concept of an aspect as used in Alfresco – many repositories do not support that idea.
The CMIS Domain Model is an attempt by the CMIS Standardization group to define a framework generic enough that can describe content models and map to concepts used by many different repository vendors.
The CMIS Domain Model defines a Repository as a container and an entry point to all content items, from now on called objects. All objects are classified by an Object Type , which describes a common set of Properties (like Type ID, Parent, and Display Name). There are five base types of objects: Document, Folder, Relationship, Policy , Item(available from CMIS 1.1), and these all inherit from Object Type.
In addition to the five base object types there are also a number of property types that can be used when defining new properties for an Object type. These are shown in the figure: String, Boolean, Decimal, Integer, and DateTime. Besides these property types there are also the URI, Id, and HTML property types, not shown in the figure.
Taking a closer look at each one of the base types, we can see that:
- Document almost always corresponds to a file, although it need not have any content (when you upload a file via, for example, the AtomPub binding the metadata is created with the first request and the content for the file is posted with the second request).
- Folder is a container for file-able objects such as folders and documents. Immediately after filing a folder or document into a folder, an implicit parent-child relationship is automatically created. The fileable property of the object type definition specifies whether an object is file-able or not.
- Relationship object defines a relationship between a target and source object. Objects can have multiple relationships with other objects. The support for relationship objects is optional.
- Policy is a way of defining administrative policies to manage objects. An object to which a policy may be applied is called a controllable object (controllablePolicy property has to be set to true). For example, a CMIS policy could be used to define a retention policy. A policy is opaque and has no meaning to the repository. It must be implemented and enforced in a repository-specific way. For example, rules might be used in Alfresco to enforce a policy. The support for policy objects is optional.
- Item (CMIS 1.1) object represents a generic type of a CMIS information asset. For example, this could be a user or group object. Item objects are not versionable and do not have content streams like documents, but they do have properties like all other CMIS objects. The support for item objects is optional.
Additional object types can be defined in a repository as custom subtypes of the base types. The Legal Case type shown in the figure above is an example. CMIS services are provided for the discovery of object types that are defined in a repository. However, object type management services, such as the creation, modification, and deletion of an object type, are not covered by the CMIS standard.
An object has one primary base object type, such as Document or Folder, which cannot be changed. An object can also have secondary object types applied to it (CMIS 1.1). A secondary type is a named class that may add extra properties to an object in addition to the properties defined by the object's primary base object-type (This is similar to the concept of aspects in Alfresco).
Every CMIS object has an opaque and immutable Object Identity (ID), which is assigned by the repository when the object is created. In the case of Alfresco, a Node Reference is created which becomes the Object ID. The ID uniquely identifies an object within a repository regardless of the type of the object.
All CMIS objects have a set of named, but not explicitly ordered, properties. Within an object, each property is uniquely identified by its Property ID. In addition, a document object can have a Content Stream, which is then used to hold the actual byte content from a file. A document can also have one or more Renditions, like a thumbnail, a different sized image, or an alternate representation of the content stream.
Document or folder objects can have one Access Control List (ACL), which controls access to the document or folder. An ACL is made up of a list of Access Control Entries (ACEs). An ACE in turn represents one or more permissions being granted to a principal, such as a user, group, role, or something similar.
All objects and properties are defined in the cmis name-space. From now on we will refer to the different objects and properties via their fully qualified name, for example cmis:document or cmis:name.
The following CMIS services can access and manage CMIS objects in the repository:
- Repository Services: These are used to discover information about the repository, including repository ids (more than one repository could be managed by the endpoint). Since many features are optional, this provides a way to find out which are supported. CMIS 1.1 compliant repositories also support the creation of new types dynamically. Methods: getRepositories, getRepositoryInfo , getTypeChildren, getTypeDescendants , getTypeDefinition , createType (CMIS 1.1), updateType (CMIS 1.1), deleteType (CMIS 1.1)
- Navigation Services: These are used to navigate the folder hierarchy. Methods: getChildren, getDescendants, getFolderTree, getFolderParent, getObjectParents, getCheckedOutDocs.
- Object Services: These services provide ID-based CRUD (Create, Read, Update, Delete) operations. Methods: createDocument, createDocumentFromSource, createFolder, createRelationship, createPolicy, createItem (CMIS 1.1), getAllowableActions, getObject, getProperties, getObjectByPath, getContentStream, getRenditions, updateProperties, bulkUpdateProperties (CMIS 1.1), moveObject, deleteObject, deleteTree, setContentStream, appendContentStream (CMIS 1.1), deleteContentStream.
- Multi-filing Services: These services (optional) makes it possible to put an object to several folders (multi-filing) or outside the folder hierarchy (un-filing). This service is not used to create or delete objects. Methods: addObjectToFolder, removeObjectFromFolder.
- Discovery Services: These are used to search for query-able objects within the Repository (objects with property queryable set to true ). Methods: query , getContentChanges.
- Versioning Services: These are used to manage versioning of document objects, other objects are not versionable. Whether or not a document can be versioned is controlled by the versionable property in the Object type. Methods: checkOut, cancelCheckOut, checkIn, getObjectOfLatestVersion, getPropertiesOfLatestVersion, getAllVersions.
- Relationship Services: These (optional) are used to retrieve the relationships in which an object is participating. Methods: getObjectRelationships.
- Policy Services: These (optional) are used to apply or remove a policy object to an object which has the property controllablePolicy set to true. Methods: applyPolicy, removePolicy, getAppliedPolicies.
- ACL Services: This service is used to discover and manage the Access Control List (ACL) for an object, if the object has one. Methods: applyACL, and getACL
In this article, we introduced the CMIS standard and how it came about. Then we covered the CMIS domain model with its five base object types: document, folder, relationship, policy, and item (CMIS 1.1.). We also learned that the CMIS standard defines a number of services, such as navigation and discovery, which makes it possible to manipulate objects in a content management system repository.
Resources for Article:
- Content Delivery in Alfresco 3 [Article]
- Getting Started with the Alfresco Records Management Module [Article]
- Managing Content in Alfresco [Article]