Tancred Lindholm

A three-way merge for XML documents

Three-way merging is a technique that may be employed for reintegrating changes to a document in cases where multiple independently modified copies have been made. While tools for three-way merge of ASCII text files exist in the form of the ubiquitous diff and patch tools these are of limited applicability to XML documents. We present a method for three-way merging of XML which is targeted at merging XML formats that model human-authored documents as ordered trees (e.g. rich text formats structured text drawings etc.). To this end we investigate a number of use cases on XML merging (collaborative editing propagating changes across document variants) from which we derive a set of high-level merge rules. Our merge is based on these rules. We propose that our merge is easy to both understand and implement yet sufficiently expressive to handle several important cases of merging on document structure that are beyond the capabilities of traditional text-based tools. In order to justify these claims we applied our merging method to the merging tasks contained in the use cases. The overall performance of the merge was found to be satisfactory. The key contributions of this work are: a set of merge rules derived from use cases on XML merging a compact and versatile XML merge in accordance with these rules and a classification of conflicts in the context of that merge.

Fast and simple XML tree differencing by sequence alignment

With the advent of XML we have seen a renewed interest in methods for computing the difference between trees. Methods that include heuristic elements play an important role in practical applications due to the inherent complexity of the problem. We present a method for differencing XML as ordered trees based on mapping the problem to the domain of sequence alignment, applying simple and efficient heuristics in this domain, and transforming back to the tree domain. Our approach provides a method to quickly compute changes that are meaningful transformations on the XML tree level, and includes subtree move as a primitive operation. We evaluate the feasibility of our approach and benchmark it against a selection of existing differencing tools. The results show our approach to be feasible and to have the potential to perform on par with tools of a more complex design in terms of both output size and execution time.

Xebu: a binary format with schema-based optimizations for XML data

XML is currently being used as the message syntax for Web services. To enable small mobile devices to use Web services, this XML use must not be too resource-consuming. Due to several measurements indicating otherwise, alternate serialization formats for XML data have been proposed. We present here a format for XML data designed from the ground up for the mobile environment. The format is simple, yet gives acceptable document sizes and is efficiently processable. An automaton-based approach gives further improvements when full or partial schema information is available. We provide performance measurements verifying these claims and also consider some issues arising from the use of an alternate XML serialization format.

XML three-way merge as a reconciliation engine for mobile data

Optimistic replication approaches are often employed on mobile devices, which raises the need for reconciliation of concurrently modified data. We propose that three-way merging algorithms, in particular those that are able to process tree-structured data in XML format, make good candidates for a generic data reconciliation engine on mobile devices.By exchanging data through XML files we impose minimal constraints on application design and are able to offer reconciliation services to a large number of existing applications. Reconciliation support can be added to an application in several increments, allowing application developers to choose a suitable level of support compared to implementation effort. We give two examples of reconciliation by three-way merging of XML.

Fuego: Experiences with Mobile Data Communication and Synchronization

In this paper, we present a summary of our experiences with mobile middleware research in the four-year Fuego Core project. The presented work focuses on data communication and synchronization. We present three middleware services for data communication and synchronization, namely the messaging, event, and file synchronizer services, and discuss their development and usage. We conclude with an integrated architecture of these services and the lessons we have learned

Syxaw: Data Synchronization Middleware for the Mobile Web

In this paper, we present a middleware for synchronization of opaque and structured data in a mobile and resource-constrained environment. The presented Syxaw (Synchronizer with XML-awareness) system distinguishes itself from related proposals in that it interoperates transparently with resources on the World Wide Web, and by exhibiting a model of synchronization that is both easy to understand and well suited for weak devices in a mobile and ubiquitous environment. We demonstrate the feasibility of the proposed system by considering several usage scenarios, including working on the Web and collaborative XML editing. We consider system performance on a mobile wireless device, and examine the impact of different optimization techniques to the performance. According to our analysis, Web interoperability suggests that the data share model be kept simple and conservative, and that moving functionality onto the client is advantageous. We find that the techniques of operation bundling and concurrent use of network downlink and uplink improve network utilization, but that achieving full bandwidth usage with a weak client is challenging in practice.

XML messaging for mobile devices: From requirements to implementation

In recent years, both the number and capabilities of mobile devices have increased rapidly to the point where the mobile world is becoming a significant part of the Internet. Another recent trend is the increase in XML use for communication between applications. However, the mobile world has been reluctant to adopt XML due to its verbosity and processing needs. We consider here the problem of providing an XML-based messaging system for mobile devices. We analyze the requirements that the environment places on such a system and elaborate on these requirements by concentrating on three components that seem most amenable to improvements, namely XML processing interfaces, XML serialization, and message transfer protocols. In tandem with the analysis we also present the design and implementation of our messaging system that addresses these requirements. Our experimentation of this system is extensive and performed completely on real devices and real wireless networks. Based on our implementation and experimentation we conclude that there is potential for improvement in XML messaging. The largest gains are achieved by using an asynchronous programming style and by using a compact serialization format. The improvements are also individually integratable into existing systems.

XML Security with Binary XML for Mobile Web Services

In the wireless world, there has recently been much interest in alternate serialization formats for XML data, mostly driven by the weak capabilities of both devices and networks. However, it is difficult to make an alternate serialization format compatible with XML security features such as encryption and signing. We consider here ways to integrate an alternate format with security, and present a solution that we see as a viable alternative. In addition to this, we present extensive performance measurements, including ones on a mobile phone on the effect of an alternate format when using XML-based security. These measurements indicate that, in the wireless world, reducing message sizes is the most pressing concern, and that processing efficiency gains of an alternate format are a much smaller concern. We also make specific recommendations on security usage based on our measurements.

A hybrid approach to optimistic file system directory tree synchronization

There are two main approaches to optimistic file system synchronization: distributed file systems and file synchronizers. The former type is characterized by a log-based approach that depends on access to file system internals, the latter by a state-based approach that utilizes the standard file system interface, which limits the efficiency of change detection.We propose a hybrid approach that 1) defines a minor extension to the semantics of the file system interface that enables efficient state-based file system change detection and 2) employs selectively instantiated XML documents to make the use of state-based algorithms for optimistic synchronization feasible on large file systems.The hybrid approach is simple, well-suited for current file system architectures, and allows us to leverage existing state-based reconciliation algorithms. An initial implementation shows our approach to be feasible, lightweight, and interoperable and to have satisfactory performance.

How to edit gigabyte XML files on a mobile phone with XAS, RefTrees, and RAXS

The Open Source mobility middleware developed in the Fuego Core project provides a stack for efficient XML processing on limited devices. Its components are a persistent map API, advanced XML serialization and out-of-order parsing with byte-level access (XAS), data structures and algorithms for lazy manipulation and random access to XML trees (RefTree), and a component for XML document management (RAXS) such as packaging, versioning, and synchronization. The components provide a toolbox of simple and lightweight XML processing techniques rather than a complete XML database. We demonstrate the Fuego XML stack by building a viewer and multiversion editor capable of processing gigabyte-sized Wikipedia XML files on a mobile phone. We present performance measurements obtained on the phone, and a comparison to implementations based on existing technologies. These show that the Fuego XML stack allows going beyond what is commonly considered feasible on limited devices in terms of XML processing, and that it provides advantages in terms of decreased set-up time and storage space requirements compared to existing approaches.