XML Query Routing in P2P Systems | Paper Critique

XML Query Routing in P2P Systems | Paper Critique Critique on XML Query Routing in Structured P2P Systems (Leonidas Fegaras, Weimin He, Gowtham Das, David Levine) Rahul Rayineni The structured peer-to-peer network is a decentralized architecture which consists of large number of nodes that share data and resources with other nodes. They use a distributed hash table to determine the location of data. The nodes in structured P2P systems maintains a list of neighbors so that they from a overlay network in which lookup time for a key take logarithmic number of routing hops between peers. They offer better availability and scalability than unstructured P2P systems but the main difficulty in using these systems lies in data placement and query processing as queries are more complex. If the queries are not properly optimized routing and processing takes lot of time. This paper demonstrates about data placement, querying, and indexing large data repositories distributed over an existing DHT based P2P systems like pastry (Reference 4 and 6). According to Leonidas Fegaras, Weimen He, Gautam das, and David Levine, There are lot of earlier proposals like XPath lookup queries in P2P networks(Reference1) and Locating data sources in large Distributed Systems(reference 3) on indexing and querying XML data distributed data over a P2P network but there is no work reported on complex XML query processing with full text search that uses data synopses to selectively route queries to peers. The framework proposed in this paper is implemented on DHT-based P2P system Pastry (Reference 4 and 6). But this framework can be implemented on any P2P infrastructure. This frameworks works on summarized data namely structural synopsis and data synopsis for mapping XML queries. The high query routing precision, low data placement and low maintenance overheads are achieved through novel data synopses structures. This framework gives more accurate evaluation of textual and containment constraints in a query compared to bloom filters. The publishing process makes the documents available to other peers while unpublishing is removing the document by the owner. If the peer wants to update the document first it will unpublish the document and later publish it again to reflect the document updates. According to Leonidas Fegaras, Weimen He, Gautam Das, and David Levine, The XPath syntax is modified to add the predicate e ~ S, where e is an arbitrary XPath expression that returns true if at least from the sequence returned by e matches the search specification, S. A search specification is an IR-style Boolean keyword query that takes the form term | S1 and S2 | S1 or S2 | (S) where S, S1 and S2 are search specifications. A term is a keyword that must be present in the text of an element returned by the expression e. The label path of an XML document consists of child/attribute steps and can distinguish non-empty set of data nodes in the document. There are two types of data synopses used in this framework in which one is content synopses that contain structural summary nodes associated with bit matrices. The second one is positional filters which are bit vectors consists of internal structure summary nodes and their positions. To achieve good load balance with small number of nodes structural summary information and data synopsis are distributed over the existing P2P network. This framework is capable of finding all possible structural summaries applicable to the query with one DHT lookup. The content synopses consists of index terms along with their positions. Index terms are formed breaking the document in to simple terms. The position of the index term depends on the position of its begin/end tags. The position of the elements tag depends on the number of the begin and end tags preceding the elements. So index terms of a same element consists of same positional range. The positional bit vector consists of all the positions of the document. According to Leonidas Fegaras, Weimen He, Gautam Das, and David Levine, The search specification e ~ t1 and t2 for two terms t1 and t2 becomes true if and only if there is at least one document node returned by e that contains both terms. Using one-dimensional term bitmaps alone, such as bloom filters, and checking whether both the t1 and t2 bits are on, will give us a prohibitive number of false positives. The core operation in this framework is containments filtering, CF(F,V) which test element containment. Here F is positional filter and V is a bit vector. The output of the above function is new positional filter F. In the bit vector V, if there is at least one bit is on with in this range it copies all the bits in F to F. Data placement involves placing of structural summary and data synopsis. The structural summaries are routed to every peer using every different tag name. Thus, with a single DHT lookup we can able to find out all structural summaries matching the structural footprint of the query. The label path of data synopsis is used in placing it. Since Multiple documents consists of label path , the synopsis from these documents is placed at a single peer. Thus with single DHT look up we can locate all the documents. Query routing involves collecting and filtering documents all the way. The triples(peer, document, positional-filter) are used as communication between the peers. They contain id of a matching document and owner of the document along with the document positions. At each peer size of the list is reduced by removing the documents whose positional filter are zeroes. The network updates like node arrival, departure and failure are handled by novel methods when the node arrives the overlay network, it invokes the Pastry method notifyReady() and start sending and receiving messages. It receives all the information like structural summaries and data synopses from its predecessor. Similarly when a node decides to leave the network, it routes all its structural summaries and data synopses to its successor and leave the network. This can be done using one single message. When peer doesnt find a matching structural summary the predecessor node may be failed. In this case peer chooses another tagname and search request to other peers to find structural summary. The Id of the failed predecessor is used to check for the node failure. If the predecessor is failed it will abort the query and scan the list again for the documents and send a message to publisher to publish the data. The main advantage of this method it will abort only one query at a time and d ata synopsis is restored that is associated with the failed peer The closest work to this is done by L.Galanis, Y.Wang, S. R.Jeffery, and D.J.DeWitt. locating data sources in large distributed systems.(Reference 3). In this framework, the distributed indexing targets location of data sources which is different from the framework in this paper. The structural summaries are similar to the one that are presented in this paper. Here they use the tag name of the element that contain the text as search key which is contrary to the framework in this paper in which text was broken before indexing and label paths are used as keys. They dont address the indexing cost also. According to Leonidas Fegaras, Weimen He, Gautam Das, and David Levine, the framework proposed by Galanis (Reference 3) is more suitable for data-centric XML data rather than to document-centric ones, since the later may include large text portions inside specific tag names, which results to the routing of large parts of document to the same nodes. Another related framework is by A. Boni fati, Xpath Lookup Queries in P2P Networks. WIDM 2004. in which XML data fragments are indexed based on their path. The search key is the hash value of its path. This framework answer simple Xpath queries in one peer hop. The drawback of this framework is it requires additional hops to retrieve complex data fragment. Also this framework doesnt support XPath predicates. There are other distributed summaries for XML data like XSketch(Reference 5) which is used in selectivity estimation than in query routing. In the paper presented by J.M. Bremer and M. Gertz on distributing XML repositories (Reference 2), the structural summary is used as a global scheme to show how XML data are fragmented and distributed over the network. My opinion of this paper, it provides best framework for XML routing in structured P2P networks. The data synopsis and content synopsis used for indexing are better than regular bloom filters. The framework maps a query with full-text search into a distributed program that migrates from peer to peer. The index terms used in this framework are label paths which are better single tag names used in the previous frameworks for routing XML data. The containment filtering of this framework is efficient in addressing the containment relationships between predicates in a query. It can find all structural summaries of a query using one DHT lookup. It can handle complicated XPath queries using structural summaries. The network updates are handled effectively using novel methods which are very crucial in structured P2P networks. The data placement strategy gives load balancing in the system. The framework is easily scalable and it can be implemented on top of any existing P2P infrastructure. References A. Bonifati, et al. XPath Lookup Queries in P2P Networks. WIDM 2004. J.-M. Bremer and M. Gertz. On Distributing XML Repositories. WebDB 2003. L.Galanis, Y.Wang, S.R.Jeery, and D. J. DeWitt. Locating Data Sources in Large Distributed Systems. VLDB 2003. Pastry. http://freepastry .rice.edu/. N. Polyzotis and M. Garofalakis. Structure and Value Synopses for XML Data Graphs. VLDB 2002. A. Rowstron and P . Druschel. Pastry: Scalable, Distributed Object Location and Routing for Large-Scale Peer-to-Peer Systems. International Conference on Distributed Systems Platforms 2001. The Motorcycle Diaries | An Analysis The Motorcycle Diaries | An Analysis During these novels it can be seen how important is the background in there, whether historical background and social one were deeply connected in the way the plot was developed, creating contrast and reflection about that current time in both stories. Besides, even they were written in different years they are a critical informers about the lack of moral in the American society in the 20s and in the other hand, the economical situation in Latin-America -specifically in South America -; the poverty that took part during the 50s and in some way also modified thinking and performance of that society. In a very short review, The Motorcycle Diaries tells the trip of two friends the 29 years old Alberto Granado and the main and later revolutionary character the 23 years old Ernesto Che Guevara, both university students of medicine that travel from Argentina to Venezuela, go passing from Chile, Peru and Colombia. Here the criticism of the social injustices of exploited mine workers; persecuted communists, ostracized lepers, and the tattered descendants of a once-great Incan civilization are the main themes of the novel. Opposite to The Motorcycle Diaries, The Great Gatsby, written by F. Scott Fitzgerald, tells the story of Jay Gatsby, the Buchanan Family and Nick Carraway, the narrator of the story. The novel shows how the American society of the 20s recreated in the New York city was unwrapped in an environment of luxury, individualism, shallowness and appearances, all reflected in each of the characters in the plot of the book and at the same time are part of what Fitzgerald always idolized and wanted to get for himself. However, it is very important to remember that both stories were written in order to show to the reader deep situations that Fitzgerald like Guevara were involved with, trying to face them and leave a moral maybe. According to Che Guevara this trip was not only another experience gained during youth but also made of him another completely person; in his spirit: The person who recognizes and polishes them, me, is no longer, at least Im not the person once was (à ¢Ã¢â€š ¬Ã‚ ¦). Our America with a capital A, has changed me more than I thought (Page 16). But what are the experiences that made of him a totally different person, as it was mentioned previously, the poverty of this continent, the one of he belonged and the political practices far away of the real needs of people changed his view of the life. According to Guevara the hospitality of Chile To meet in Chile means invite (to a house)à ¢Ã¢â€š ¬Ã‚ ¦ page 101 because in contrast with Argentina, Chile did not suffer the mix-up between Creoles and the Saxons that lived in the south in that country: creoles and Indians lived together in distances of each others. The lack of opportunities that Chilean people had for having a better life style was reflected and also how the community, reflection of society in general, disapproved the other people conditions coexisting in that way a kind of a mutual bitterness between the people who is a little bit better than the other one who lived in bad conditions, and the ones who are under the standard level of quality of life between the other people over their living conditions whose reproach them: and become a purely negative factor in the struggle for life, and consequently, a bitterness for healthy members of the community who resent their illness as if it were a personal insult to those who have to support them. (Page 70) Continuing in Chile and according to different issues that took part in the stay in this country; specifically in Valparaiso, they could find lots of social problems that the established government during that time did not solve, like the overcrowding and the almost inexistence of economical help for the community exposed in the following mention: is time that politicians and put less effort wasting time in the goddesses of their propaganda and more money, ( ), for solving the work in social profits. (Page 72). Moving forward in the story, the corruption, deceive, and the naivety of police and common citizens were involved each other. Moreover, placed in the North of Chile, Chuquicamata, they met a couple in the desert night who were communist, and where the men told them about the time he was in jail due to his political tendency; while the woman, his wife, followed him leaving their child with someone else until they got some money. This situation was particularly hard and sad for both Alberto and Che Guevara; for the last one added: (The couple) was the living image of proletariat in any place around the world (page 77) and according to Guevaras words being communist was considered like a sin while. That spirit produced the long for something better in the future, at least that was what the couple expected to get at the end, but is it worth putting your life in risk, in a work place where you can get a pulmonary disease and even dye, an leaving somebodys children alone just because of dreams? At this moment; at the beginning of the end, Granado and Guevara leave Chile for going on with their objectives at the north of this continent; and something else, even this is the goodbye of this long and narrow country Guevara keeps talking about the social differences evidenced and experienced by them and most of all, how the more powerful people in the history of this country, and in most of the countries for sure, applied his vigour over the rest of the common people immortalized in: the act of Valdivia represents the never deny effort of the men for achieving a place where to exert his irrefutable authority (page 85). Finally, some reflections written tell now the social situation of Chile, like he said in the whole South America and even worse. First of all the lack of awareness of health in everywhere, streets, hospitals and also in Chile existed low opportunities for working and the people; thereby, Chile showed a lower quality of life than Argentina, and no mattering those previous features of this isolated country, the life can be easily carry in the way people work hard and, of course, they do not belong to the communist party. While they were in Peru they could experienced that most of Peruvians believed that Argentina was like the land in which peoples dreams were made come true, and not only Peron was great but also Evita (Perons wife), where everything was good just because the Indians were not punished by the white man. Guevaras life, consciously or unconsciously, went around the pursuit of social justice, corruption and democracy. Since they started their journey and specifically when they arrived in Chile they were focused in different issues that embodied those moral behaviour in people and in the particular idiosyncrasy due to, nevertheless they went all over Chile, they could identified the cultural characteristics between southern and the more north people. And being in Peru they could be aware about their same life philosophy and ideals. However, some believes of those friends were not jut part or applicable in hierarchical relationships but also in the more typical important ones; between the community itself. When we kept on the route, the old Indian took off from his clothes a very appetizing corn and he offered it to us. Quickly we realized that he democratically divided into the same share out of the corn. (Page 115). In the same way, the emphasis of people who had been fired because of their political thinking, it is also an issue that must be not only for reader be aware of that but also Ernesto Guevara as itself and most of all to understand what was happening on that time; oppression against who were apposite to the government believes. Those situations were reflected in the couple of Chuquicamata and Puno a Peruvian indigenous who was expelled of the government due to the already mentioned facts. Summary from this fragment can be assumable during that decade South America was overwhelmed by the power of its leaders. Moreover, Punos words reflected about that current situation between the conqueror who have the nation and the forced ones who were trying to do their best in order to obtain what belongs(ed) to them; however, Guevara had interpreted those words like offensive to the Inca people: to themselves, so at that moment more than a potential Marxist el Che seemed an anarchist; against everything and everyone that subjugated to other, the Incas in this opportunity. At this moment, Guevara showed himself like a critical thinking person and at the same time a dreamer with clear aims for his life and for the other too. The Great Gatsby , like The Motorcycle Diaries, presented in its plot a social issue during the 20s, placed in America the story moved through the high society. The lack of values like honesty, friendship, loyalty, love, among many others is present into those peoples hearts. The well known American dream evoked the worst part of people for both the upper class and the recent one. But this is not just a fiction and successful story; whereby, the story itself was yet the reproduction of that society involved in individualism and shallowness. Therefore, that reality was embodied by Fitzgerald in this book. Throughout the story a big number of themes can be deeper developed. Every character is different to the other but all of them share the same behaviour, laying each other, being aware of appearances, and making the other pay for their own mistakes. Finally, whereas Gay Gatsby, the main character of the novel, it is important to focus on his personality and his life was developing since he worked to the rich man and then when he met Daisy and the goals in his life made them strong. After that, during the reunion with Daisy he showed for once one hundred percent honest and that proved the power of his dreams, which they came true partly but consequently, made of him a totally different person, ostentatious, layer and criminal and finally triggered his dead. For concluding, both novels are recognized by the whole globe, like inb Time Magazine on June the 14th of 1928 (page 5) due to his relation with the historical and social context in which both were written. The Motorcycle Diaries novel revealed the poverty in Latin American and in the other hand The Great Gatsby reflect the high American society before the Black Thursday in which the American dream was the symbolic icon of this society. Thereupon, some reflection can be drawing assuming that no matter how much money somebody have do not define what people are and there is plenty of evidence for that when Guevara told about the poor economical development in Latin America, however, he and Alberto could did their trip because the peoples help in most of the places they arrived. Opposite to that reality, Americas financial situation seemed good as well as the comfortable life for Americans, but if Guevara and company had decided do their journey to America they would certainly had to gi ve up to that odyssey, because people were not as generous as the one in the South. Expression totally opposite can be contrasted quickly, in order to understand the huge and deep feelings and thinking that separates our main characters. Expressions like proletariat and bourgeois page 125 can be easily found in the Latin American novel and that represented the social communist tendency of the writer. But in accordance to the social context of Fitzgerald novel, we can find lots of quotations which named luxury, fashionable, exuberant, money, and so on. So it can be easily to reader identify the environment in which this two celebrated plays were written. But as it was mentioned before, is it worth leave important thing away from you just for getting a dream?, or not just things but also people like leave your children them suffering in a place instead leave your dreams that make them suffering in a place where their dreams did not care at all. The best examples for each ones are the life in Valparaiso and Chuquica-mata. Or putted it in another way, is it worth to fight for a dream that maybe never would become real, until putting your life in danger, like Gay Gatsby did it with not worthwhile people like Daisy; and most of all, the characterization in Great Expectations suggests that money causes people unconsciously to isolate themselves from the rest of the world. Here you can reflect by yourself.