In this post, I want to discuss repository architecture philosophies, although I will focus primarily on Fedora and California Digital Library microservices, there are some generalizations one can pull out of this. It would also be interesting to pull in some very different repository models, like iRODS or a triple-store-backed system, but that's outside of my expertise.

The basics

This is not a section I really want to write, but I don't know of a high-level answer to "when we say repository, this is what we mean". I spent a little time looking around for a summary, but more often than not I found more questions (or, perhaps more useful yet inappropriate for my purposes, technology-based answers rather than use-driven), so I've taken a stab at addressing what I believe are some key issues:

Repositories are a collection of services, with well-defined interfaces, for storing and managing data (both content and metadata) in a format-neutral, display-independent manner way. Repositories can be used as preservation repositories, as access repositories, as centralized aggregations of far-flung data, etc and operate on any scale for any audience. Furthermore, there are existing standards and agreements about what it means to be a certain type of repository (TDR, OAIS, etc). All of these repositories, however, share some common services -- whether implemented as software, external processes, or manual processes.

Some essential repository services are:

  • Identifier services, which may include assignment + registration
  • Storage services (although the content stored may be only pointers to the "actual" content)
  • Content identification, matching identifiers to content items
  • Ingest workflows
  • Access mechanisms

Without these services in place, a repository system would face some difficult obstacles in creating and providing value-added services. Repositories may provide multiple flavors of these services, some of which may be defined in generally accepted standards, models, and specifications.

Other basic services which operate on top of the above services are fairly common in most well-developed repository frameworks include:

  • Dissemination services, to transform repository data into other forms + formats
  • Authorization services

More advanced services may include:

  • preservation services, including checksum (generation + verification), file format migration, support for models like LOCKSS
  • relationship services, using an RDF triplestore or similar, offering SPARQL endpoints, interferencing, etc
  • discovery services, using Lucene/Solr/etc, to provide relevancy, optimized user experience, drill-down faceting

These more advanced services are likely separate applications in the repository ecosystem and are generally useful utilities independent of any repository system. Repositories generally integrate with these external applications in a modular, mix-and-match manner using well-defined interfaces.

Fedora

One approach to repository services is the "repository-in-a-box" model, where you can install and configure a base set of services provided by a single application. Within this group of services, Fedora provides a very basic implementation of the core repository services (vs a full-stack application like DSpace, which provides production-ready user interfaces). Fedora bills itself as a Flexible, Extensible Digital Object Repository Architecture.

  • Identifier services, through PIDGen which provides sequential identifiers per-namespace
  • maps http uris to deferenceable uris to files
  • REST + SOAP APIs for Ingest + Delivery
  • Dissemination services using WSDL
  • Authorization using XACML (and authentication using a number of plugins)
  • Integrates with the Mulgara triplestore and a Lucene index (by default)

Fedora provides a many opportunities for customization and enhancements through custom development:

As services go beyond the basic, common applications present in institutional repositories, enhanced repository services require custom development or supplemental services outside of the repository services. For most, this includes integration with a more advanced search provider (like Solr). At some point, additional services can blur the lines between the repository services and front-end user interfaces (which have to respond to local customization to meet user needs).

Repository-independent services, or third-party services, require some wrapper to make them interoperable with the Fedora APIs, which makes integration with existing technology more difficult. Even Duraspace's Duracloud offering is (currently) built as separate services with some possibility of storage-level integration. Preservation support services will bypass the repository APIs and provide those services against the file system instead.

Considering the services Fedora doesn't provide or the obstacles Fedora creates in integration, many ask why they should start using Fedora anyway. The strongest response to this, I believe, is that it provides a common structure to basic repository services, while at the same time not creating major obstacles to future expansion or migration outside Fedora. Out of the box, Fedora provides a set of "training wheels" (ht Mike Giarlo <http://lackoftalent.org/michael/blog/>) for repository services development that can be removed when unnecessary, but in the meantime offers structure for the creation of new repositories and support for repository services as needed.

CDL Microservices

Another approach to repository services are "microservices" like those designed by the California Digital Library (CDL), provide standards and specifications for individual repository services, which form a structure for standardized, mix-and-match repository services that can integrate, interoperate and take advantage of existing technology independent of a repository application like Fedora. This, conceivably, allows all domain developers to take advantage of these common projects without using a specific technology. CDL provides microservices specifications for:

  • identifier assignment + registration, using NOID, which can act as a CLI tool or a CGI service
  • file-system structures, using the Pairtree convention
  • data exchange and verification, using BagIt
  • access standards, using the ARK URL format

The standards are developed inline the "UNIX philosophy":

Write programs that do one thing and do it well. Write programs to work together. -- Doug McIlroy

These basic services can be organized and crafted using the existing capabilities in web servers, file systems, etc. More advanced services can act within this structure, using individual standards when needed. While significant development and customization may be required to get a microservices architecture to a useable state, the end result is more flexible and targeted to an institutions needs.

Flexing Fedora

These two approaches are certainly not incompatible, and Fedora is quite capable of using some of these micro-services standards under the hood (replacing custom developed approaches to these basic services). By taking this approach, Fedora could act as a management application on top of generic repository data, allow both Fedora-based and microservices-based services to operate on the data, and make it easier to reach around Fedora when necessary (or, go so far as to remove it entirely).

What follows is a short summary of on-going work in this area, which mostly focus on removing the Fedora-centric definitions of /how/ or /where/ services act. The majority of these ideas build on new developments and best practices (since Fedora was initially created) in the repository community as a result increased adoption or awareness of issues. Where available, I've included links to projects in-the-works.

Some of this work is quite easy to do:

Other projects that are more involved, and require more work than just creating new modules for Fedora:

More advanced microservices integration is highly involved and would require a major re-work of the application:

  • Two-way messaging queues (or file alteration monitors, or database update hooks) to allow Fedora to receive updates
  • decreased reliance on self-generated registries, I think the situation is getting better, but I'm not sure its fully there..
  • pluggable storage modules with intelligent filtering, routing, multiplexing, and rules mechanisms -- the Akubra project may be doing (part of?) this <http://www.fedora-commons.org/confluence/display/AKUBRA/Akubra+Project>
  • workflow support hooks, to allow integration and automation of workflow tools (possibly a result of Hydra?)