Designing Persistency

Delos NoE , Preservation ClusterWorkshop: Persistency in Digital Libraries

14. February 2006, Oxford Internet Institute

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Digital Objects

• Digital objects are the core elements a digital library deals with.

• For long-term preservation they have to be kept persistent.

• Therefore we need DL systems that have the ability to keep digital objects persistent.

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Digital Objects: Structure

• Digital objects are composed entities with a logical structure. Example:

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Life-cycle of Entities

• Persistency appears in the life-cycle of any entity within a digital library.

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Life-cycle of Entities

• The nature of instability of entities appears in the life-cycle of any entity within a digital library.

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Requirements for the Design of a Persistent DL System

• The system should be able to react according to the transient and composite nature of digital library‘s entities.

• Entities in digital libraries can be of different nature. E.g., the system should also be able to keep the structure of a collection as well as of a digital object persistent.

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Requirements for the Design of a Persistent DL System

• It also should be able to react as to the common failures which can occur in a complex system.

• It should cope with different preservation strategies.

• Implementation: The design should be expressed in way that makes implementation of the system and maintenance of it as easy as possible.

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• This includes for example:

– We need a flexibel system that is able to deal with digital objects of any structure.

– The overall processing of the changes to the digital object needs to be strictly controlled.

– The individual processing steps need to be bundled in line with their functional similarity.

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– All events that may have an impact on the longevity of digital objects need to be recorded.

– Modifying access on digital objects (external or internal) need to be „announced“ to the preservation system.

– The design of the system need to be expressed in a flexible, extensible, standardised and widely accepted language.

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Prerequisites for this Approach

• We decided to use the UML for modelling persistent digital library systems. Therefore, the systems which adopt the Preservation Module, must be expressed in UML notation.

• The approach suggested here is focused on the implementation of the system, that is a digital library as a software system.

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Preservation Module

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Preservation Module

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Preservation Module

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Preservation Module

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Preservation Module

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Preservation Module

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Registering Interface

• Core tasks: Receiving and forwarding messages that represent a potentially persistency-sensitive scenario.

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Preservation Module

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Persistency Agent

• Core tasks: Interface between the processing units of the Preservation Module and the DL system (‚Gate‘); Delegation of messages; Controlling the message flow to the surrounding system components.

• Additional tasks could be: Producing logfiles on activation of the Preservation Module, statistical issues

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Processing Controller

• Heart of the core unit. Unit at the top level of control.

• Coordination of the particular over-all processing steps.

• Connected to the other functional units of the Preservation Module.

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Persistency Memory

• Unit that encapsulates all preservation-sensitive parameters.

• Keeps and administrates a machine-readable list of preservation-sensitive parameters.

• Could be realised as a database.

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Persistency Guard

• Unit that controls the execution of basic preservation actions (internal ones, ‚routines‘), like checking if a digital object is physically in good order.

• Initiation and controlling of modifications to the Persistency Memory.

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Object Handler

• Interface between the core unit of the Preservation Module and the system unit that is responsible for the storage of the digital objects (repository).

• Controls all processing steps which are related to the repository.

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Core Unitwith exemplary

functionality

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MHU

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Message Handling Unit

• Message Handler:– Pre-processing of the message: Obtaining its semantic content.– Deciding whether or not a message is preservation-sensitive.

• External Message Handler: Exact semantic interpretation of messages which are forwarded from external units.

• Internal Message Handler: Exact semantic interpretation of messages that are forwarded from the inside.

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Message Handling Unit

Message Handling Unit

with exemplary functionality

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PWPU

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Persistency Workflow Processing Unit

• Unit of the Preservation Module that controls all actions which concern the modification of the object (workflow).

• A workflow is composed of a sequence of smaller workflow units that are encapsulated in the Persistency Workflow class.

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Persistency Workflow Processing Unit

• The Persistency Workflow Handler initiates the workflow process. It also composes the over-all workflow that has to be carried out, according to the messages previously interpreted and forwarded to it.

• The Persistency Workflow Controller is responsible for the control of the particular workflow steps.

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Persistency Workflow Processing Unit

PWPU with exemplary

functionality

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Integration of the PM into Existing Systems

• The PM is conceived as a software module that has only two interfaces which have to be connected to the DL system.

• The first interface, linking the PM via PA to the DL system is called Message Gate, the second, linking the PM to a repository is called Object Gate.

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Integration of the PM into Existing Systems

• Step 1: Specification of the interfaces

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Depending on the analysis of the existing model, there are various possibilities :

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Step 2: Adding the core unit

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Step 3: Adding the Message Handling Unit

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Step 4: Modelling the

PWPU

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Integration of the PM into the conceptual models: EF

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Integration of the PM into the conceptual models: 5S