Miscellaneous AEM concepts

What is the difference between jcr:primaryType and jcr:mixinTypes?

A Node can have only one jcr:primaryType but it can have several jcr:mixinTypes, so it is correct to see the mixins as "traits" or additional aspects of a node.

Both properties jcr:primaryType and jcr:mixinTypes are defined in the super node nt:base then inherited by all other nodes.

The jcr:primaryType property specifies the base type of a node and will be assigned at node's creation time and you can think of it as the BASE class of an object in OO world.

1. mix:versionable:
   Usage: Enables versioning capabilities for nodes, allowing multiple versions of a node to be stored in the repository.
   This mixin adds properties and methods related to versioning, such as jcr:isCheckedOut and jcr:predecessors.
2. mix:lockable:
   Usage: Allows nodes to be locked, preventing other users from modifying them.
   This mixin adds properties and methods related to locking, such as jcr:lockOwner and jcr:lockIsDeep.
3. mix:referenceable:
   Usage: Assigns a stable identifier (UUID) to nodes, which can be used to reference them reliably across the repository.
   This mixin adds the jcr:uuid property to nodes.
4. mix:created:
   Usage: Adds properties to track the creation time and user of a node.
   This mixin typically includes properties like jcr:createdBy and jcr:created.
5. mix:lastModified:
   Usage: Adds properties to track the last modification time and user of a node.
   This mixin typically includes properties like jcr:lastModifiedBy and jcr:lastModified.
6. mix:etag:
   Usage: Adds an entity tag (ETag) to nodes, which can be used for cache validation and concurrency control.
   This mixin typically includes properties like mix:etag.
7. mix:language:
   Usage: Indicates that a node represents content in a specific language.
   This mixin typically includes properties like jcr:language.
8. mix:versionHistory:
   Usage: Represents the version history of a versionable node.
   This mixin typically includes properties like jcr:versionHistory and jcr:baseVersion.
9. mix:shareable:
   Usage: Allows nodes to be shared across multiple locations in the repository, enabling the creation of content references.
   This mixin ensures that modifications made to a shared node are reflected across all its references.
10. mix:lockable:
    Usage: Adds locking functionality to nodes, enabling users to lock nodes to prevent concurrent modifications.

What is JCR, JSR, Jackrabbit, Oak and CRX?

JCR : Java Content Repository, a type of repository that provides an interface for Java applications to interact with content repositories. JCR is a specification for a Java platform application programming interface (API) to access content repositories in a uniform manner.

• JCR is a Java technology specification (JSR-283) that defines an API for accessing and manipulating structured content.
• It provides a standard way to interact with content repositories, enabling applications to store, query, and retrieve content in a platform-independent manner.
• JCR implementations typically use hierarchical data structures, similar to file systems, for organizing content.

JSR : Java Specification Request, a formal document that describe proposed technologies and specifications for adding to the Java platform. JCR is based on the principles mentioned in JSR.

• JSR is the process by which specifications for new features or enhancements to Java technologies are developed.
• A JSR is a formal document that outlines the proposed changes and requirements for the technology.
• JCR, as mentioned above, is one example of a JSR.

JCR 283 is the standard that specifies a vendor-independent and implementation-independent way to access content bi-directionally on a granular level within a content repository.

Jackrabbit : Apache Jackrabbit is an example of JCR, an open-source implementation of the JCR standard. It's a software program that provides a content repository according to the JCR specifications. So, it acts like a library that developers can use to store and manage content in their Java applications using the JCR standard.

• Jackrabbit is an open-source implementation of the JCR specification.
• It is developed as part of the Apache Software Foundation's Jakarta project.
• Jackrabbit provides a robust and scalable content repository for Java applications, supporting features such as versioning, transactions, and access control.

CRX : Content Repository Xtreme (CRX), which is Adobe's implementation of the JSR-283. CRX provides additional features such as development tools, clustering capabilities, and has its own storage mechanism. 

• CRX is Adobe's proprietary implementation of the JCR specification.
• It is built on top of Apache Jackrabbit and provides additional features and capabilities tailored for Adobe Experience Manager (AEM) deployments.
• CRX includes features such as replication, clustering, and integration with other Adobe marketing technologies.
• While CRX is specific to Adobe's ecosystem, it is based on the JCR standard and shares many concepts and APIs with other JCR implementations.

Oak : Apache Jackrabbit Oak is a newer and more modern implementation of the JCR standard. It's considered a rewrite of Jackrabbit, with a focus on scalability and performance. It builds upon Jackrabbit but offers a more efficient architecture for handling large volumes of content.

• Oak is a modern, scalable, and performant implementation of the JCR specification.
• It is designed to be the next-generation content repository for Apache Jackrabbit, replacing the older MicroKernel-based architecture with a new, more efficient implementation.
• Oak is optimized for cloud-native environments and supports various deployment scenarios, including traditional data centers, cloud hosting, and hybrid setups.

How are Oak and CRX related in AEM's context?

Oak serves as the underlying content repository technology for AEM, while CRX is Adobe's customized implementation of JCR, built on top of Oak to provide additional features and optimizations specific to AEM's requirements. Together, they form the foundation of the content repository in Adobe Experience Manager.

• Oak forms the foundational layer upon which CRX is built.
• CRX utilizes the capabilities provided by Oak for storing and managing content within AEM.
• While Oak handles the core repository functionalities, CRX adds higher-level features and services that are specific to AEM's use cases.
• Essentially, CRX extends and customizes the capabilities of Oak to provide a comprehensive content repository solution tailored for Adobe Experience Manager.