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The famous “OAIS Diagram” shows us a stylized high-level view of an archive. [Bearing in mind the fundamental fact that “the map is not the terrain, “] we can differentiate five kinds of features: the blue “functional entity groups”, the “information packages” in the white ovals, three external generalized Actors, some solid and dotted lines suggesting relationships among the blue entity groups and the actors, and some arrows implying a directed data flow. If this diagram has never really made sense to you, you’re not alone. We have to look deeper to understand how this model describes all archives. Over the past few years we have become familiar with the terms "Ingest", "Access", "Archival Storage", "Preservation planning", Administration" and "Data Management" as the components of an OAIS archive. And we've learned to talk about metadata in terms of the OAIS Information Package: the SIP, AIP, and DIP. If understanding these terms were all there is to creating an archive, we would all be practicing good long-term preservation of our digital assets. Of course, there is more to the OAIS reference model than these basic concepts, and from the beginning of the EATMOT project we knew we needed a deeper understanding of them to build a working digital archive.
We understand the OAIS Reference Model to be a collection of all the functions that take place in the ideal, full-service archive. The group that conceived the model identified approximately thirty functions: negotiating the agreement between a depositor and the archive; various kinds of compliance and error checking; generating reports; billing; managing, storing, and retrieving data; and many more. Some of the functions are purely technical, such as replacing storage media, but just as many are knowledge processes: monitoring the needs of the user community, planning preservation strategies, managing the system configuration, establishing archive policies, among others. The OAIS document's "Composite of Functional Entities" diagram lays all of these functions out in a data-flow diagram that can confuse an experienced archivist (see Fig. 1); looking at this diagram we wondered where we going to find a place to start unraveling this complexity to find a place to start building our archive.  We knew we could not “eat the elephant” in a single bite (byte?), but instead we’d have to divide it up in relatively small, easily consumable pieces. 
 Extreme Programming (XP) proponents advocate four values in system development: communication, feedback, simplicity, and courage. They envision the development process as a communication loop that involves the customer, the requirements, the system designer, and the programmer. One of the areas where they look for simplicity is in deciding what tasks need to be done next. The customers, the designers, and the programmers look at the system requirements (as they know them at the moment). The customers decide which small parts of the system are most important to them at the moment and write a description on "story cards". The designers and programmers then estimate how long implementing each "story" will take to complete. Knowing what is needed next and how long each task might take, they choose the stories that the programmers will work on next. When they have completed those parts of the system, they all repeat the story card process. (Courage is an XP value because all the participants have to have the courage to jump right in and make decisions that might later prove to be wrong--the stories may not have been the right ones to work on: requirements may change, designs may not work, programmers may have to rewrite components.)
We adapted this technique to prioritizing our understanding of the complete OAIS model. We considered each of us to be a customer in the XP sense; all of us have been involved in digital libraries for some years and know our institutions' current requirements for a digital archive. The Cornell team, that developed the stories includes an archivist, a systems technology administrator, a metadata librarian, and a system designer. We put each OAIS function on a 3x5 card, along with a brief description. Each member of the team received a deck of cards and pulled out the cards he/she considered to be important.  As a group, we discussed each story and decided on three we wanted to start on.  We then refined those three stories;
I’m going to use this story, the Update Storage story, to illustrate how we’re eating the elephant one bite at a time.
The next slide is a simplification, showing only the four functions we’re focusing on in this story.
The next slide is a simplification, showing only the four functions we’re focussing on in this story.
The arrows in this drawing show the relationships among the functions—which functions talk with each other or pass each other messages of one sort or another.
You’ll notice that one of the arrows is bi-directional;
it indicates that some sort of request and response conversation goes on between the two functions it joins. You can also see that two  functions interact with other functions outside the boundaries of this story.
What is the nature of all these interactions?  They all have this in common: the functions communicate with each other by passing information to each other. The arrows show the direction the information travels.
 Let’s call the individual packets of information “messages”.
I’ve added the names of the messages to the next slide.
These messages are the actual stuff of an archive. That stuff can be the different forms of the OAIS Information Package --you’ll notice that the AIP is being passed from one function to another—but all the other kinds of messages are important, too. The next slide categorizes the messages in a way that I hope makes them more understandable.
BLUE – Metadata—parts or the whole of information packages and descriptive information. On the well-known, high-level OAIS diagram, they appear in little ovals.
 GRAY – Internal computer messages—not meant for human consumption
Next we’ll hear from Markus Enders from Gottingen who will talk about metadata, then I’ll come back and talk about some open questions
 PINK – Human-readable policies and reports.
When I look at this diagram, with the messages highlighted this way, I start to understand that the functions are information processing functions. They modify the messages they receive and send them on—ultimately to storage or to the human actors identified in the high level OIAS diagram: the Management, the Producer, or the Consumer.
Summary—last 5 minutes?
How much interoperation?
Archivists might flag objects to indicate importance or longevity.
We’ll respect other archivists’ decisions
OR We’ll take responsibility after negotiation
OR We’ll preserve regardless
A confirmation message need not be a tagged message. I’m using this XML snippet to show the information that comes back to the Co-ordinate Updates functions. The information will be passed to the Data Management functions to enable  document discovery and access.
Next, let’s look at a diagram that shows all the individual functions grouped under these labels: