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Demian Tretyakov
Demian Tretyakov

Distributed Database Systems: Principles and Practice with Solutions to Exercises



Principles of Distributed Database Systems Solution Manual




In this article, we will explore the principles of distributed database systems and how a solution manual can help students and instructors learn and teach this complex topic. We will cover the main aspects of distributed data management, such as design, query processing, transaction management, and advanced topics. We will also provide some examples and exercises from the solution manual to illustrate the concepts and techniques.




Principles Of Distributed Database Systems Solution Manuall



What are distributed database systems?




A distributed database system (DDBS) is a system that manages a collection of logically related data that is physically distributed over a network of computers. A DDBS provides transparent access to the data, regardless of their location, fragmentation, or replication. A DDBS also ensures that the data are consistent, reliable, secure, and scalable.


Distributed database systems have many advantages over centralized database systems, such as higher availability, fault tolerance, performance, scalability, and autonomy. However, they also pose many challenges, such as complexity, heterogeneity, concurrency, communication, synchronization, and security.


Why do we need a solution manual?




A solution manual is a valuable resource for students and instructors who want to learn and teach distributed database systems. A solution manual provides detailed answers and explanations to the exercises and problems in the textbook. A solution manual can help students to test their understanding of the concepts and techniques, to practice their skills in applying them to realistic scenarios, and to prepare for exams. A solution manual can also help instructors to design assignments and exams, to evaluate students' work, and to provide feedback and guidance.


The solution manual we are referring to is for the fourth edition of the textbook "Principles of Distributed Database Systems" by M. Tamer Özsu and Patrick Valduriez. This textbook is one of the most comprehensive and authoritative books on the topic of distributed data management. It covers both traditional material and emerging areas, such as big data platforms, NoSQL systems, web data management, and blockchain. It also includes many examples and exercises that reflect the changing technology and applications.


Distributed Database Design




Design objectives




The design of a distributed database system involves deciding how to distribute the data over the network of computers. The main objectives of distributed database design are:


  • To achieve high performance by minimizing communication costs, balancing the workload among the nodes, and exploiting parallelism.



  • To ensure high availability by replicating the data on multiple nodes and providing mechanisms for failure recovery.



  • To preserve local autonomy by allowing each node to control its own data and operations.



  • To maintain global consistency by enforcing integrity constraints and ensuring transaction atomicity.



The design of a distributed database system is influenced by many factors, such as the network topology, the data characteristics, the user requirements, the application characteristics, and the system constraints.


Design methods




There are two main methods for designing a distributed database system: top-down and bottom-up.


The top-down method starts with a global conceptual schema that represents the logical structure and relationships of the data. The global schema is then partitioned into fragments that are assigned to different nodes. The fragments can be either horizontal, vertical, or mixed, depending on how the data are divided. The fragmentation process aims to minimize data redundancy and communication costs, while maximizing data locality and parallelism. The fragments are then replicated on multiple nodes to increase availability and reliability. The replication process aims to balance the trade-off between performance and consistency.


The bottom-up method starts with a collection of local schemas that represent the data and operations of each node. The local schemas are then integrated into a global schema that captures the commonalities and differences among them. The integration process aims to resolve the conflicts and inconsistencies among the local schemas, such as naming, structure, and semantics. The integration process also involves defining mappings between the global schema and the local schemas, which are used to translate queries and updates across the nodes.


Design examples




Let us consider some examples of distributed database design from the solution manual.


Example 1: Consider a distributed database system for a multinational company that has branches in different countries. Each branch has its own database that stores information about its employees, products, customers, and orders. The company wants to integrate the data from all the branches into a global database that can support queries and updates from any branch.


A possible solution is to use the bottom-up method to design the global database. The local schemas of each branch can be represented as relational tables, such as:


EMPLOYEE(BranchID, EmpID, Name, Salary, DeptID) PRODUCT(BranchID, ProdID, Name, Price, Category) CUSTOMER(BranchID, CustID, Name, Address, Phone) ORDER(BranchID, OrderID, CustID, ProdID, Quantity, Date)


The global schema can be obtained by integrating the local schemas using a common key attribute BranchID. The global schema can also be represented as relational tables, such as:


EMPLOYEE_GLOBAL(BranchID, EmpID, Name, Salary, DeptID) PRODUCT_GLOBAL(BranchID, ProdID, Name, Price, Category) CUSTOMER_GLOBAL(BranchID, CustID, Name, Address, Phone) ORDER_GLOBAL(BranchID, OrderID, CustID, ProdID, Quantity, Date)


The mappings between the global schema and the local schemas can be defined as simple projections on the BranchID attribute. For example:


EMPLOYEE_GLOBAL = EMPLOYEE_USA EMPLOYEE_UK EMPLOYEE_JAPAN ... EMPLOYEE_USA = πBranchID='USA'(EMPLOYEE_GLOBAL) EMPLOYEE_UK = πBranchID='UK'(EMPLOYEE_GLOBAL) EMPLOYEE_JAPAN = πBranchID='JAPAN'(EMPLOYEE_GLOBAL) ...


Example 2: Consider a distributed database system for a social media platform that stores information about users, posts, comments, likes, and followers. The platform has millions of users who generate a large amount of data every day. The platform wants to distribute the data over a cluster of computers to improve performance and scalability.


A possible solution is to use the top-down method to design the distributed database. The global schema of the platform can be represented as relational tables or documents (depending on the data model), such as:


USER(UserID, Name, Email, Password) POST(PostID, UserID, Content, Timestamp) COMMENT(CommentID, PostID, UserID, Content, Timestamp) LIKE(LikeID, PostID, UserID, Timestamp) FOLLOW(FollowerID, FolloweeID)


The global schema can be fragmented into smaller units that are assigned to different nodes in the cluster. A possible fragmentation scheme is to use horizontal fragmentation based on hash partitioning on the key attributes. For example:


USER = USER1 USER2 ... USERN


USERi = σh(UserID) = i(USER), where h is a hash function and i is a node identifier POST = POST1 POST2 ... POSTN


POSTi = σh(PostID) = i(POST) COMMENT = COMMENT1 COMMENT2 ... COMMENTN


LIKE = LIKE1 LIKE2 ... LIKEN


LIKEi = σh(LikeID) = i(LIKE) FOLLOW = FOLLOW1 FOLLOW2 ... FOLLOWN


FOLLOWi = σh(FollowerID) = i(FOLLOW)


The global schema can also be replicated on multiple nodes to increase availability and reliability. A possible replication scheme is to use full replication for the USER and FOLLOW tables, and partial replication for the POST, COMMENT, and LIKE tables. For example:


USER = USER1 = USER2 = ... = USERN


FOLLOW = FOLLOW1 = FOLLOW2 = ... = FOLLOWN


POST = POST1 POST2 ... POSTN


POSTi = σh(PostID) = i or h(UserID) = i(POST) COMMENT = COMMENT1 COMMENT2 ... COMMENTN


COMMENTi = σh(CommentID) = i or h(PostID) = i or h(UserID) = i1 LIKE2 ... LIKEN


LIKEi = σh(LikeID) = i or h(PostID) = i or h(UserID) = i


Distributed Query Processing




Query decomposition




The process of distributed query processing involves transforming a high-level query expressed in a global schema into a set of low-level operations executed on the local schemas. The first step of this process is query decomposition, which consists of analyzing the query and breaking it down into subqueries that can be executed on different nodes.


The query decomposition can be done using a syntax-based approach or a semantics-based approach. The syntax-based approach relies on the structure and operators of the query language, such as SQL. The semantics-based approach relies on the meaning and constraints of the data, such as functional dependencies and keys.


The query decomposition aims to minimize the amount of data that needs to be transferred across the network, to exploit parallelism and locality, and to simplify the subqueries as much as possible.


Data localization




The second step of distributed query processing is data localization, which consists of determining the location and availability of the data that are needed to execute the subqueries. The data localization can be done using a directory-based approach or a broadcast-based approach. The directory-based approach relies on a centralized or distributed catalog that stores information about the data distribution and replication. The broadcast-based approach relies on sending requests to all the nodes and waiting for responses.


The data localization aims to find the optimal sources of data that minimize communication costs, balance the workload among the nodes, and avoid conflicts and failures.


Query optimization




The third step of distributed query processing is query optimization, which consists of finding the best execution plan for each subquery and for the whole query. The execution plan specifies the order and method of performing the operations, such as selection, projection, join, aggregation, etc. The query optimization can be done using a heuristic-based approach or a cost-based approach. The heuristic-based approach relies on a set of rules and guidelines that are based on common sense and experience. The cost-based approach relies on a mathematical model that estimates the cost and benefit of each alternative plan.


The query optimization aims to maximize performance by reducing computation time, communication time, memory usage, and disk access.


Distributed Transaction Management




Concurrency control




A transaction is a sequence of operations that performs a logical unit of work on the data. Concurrency control is the process of ensuring that concurrent transactions do not interfere with each other and preserve the consistency and isolation of the data. Concurrency control can be done using a locking-based approach or a timestamp-based approach. The locking-based approach relies on acquiring and releasing locks on the data items that are accessed by the transactions. The timestamp-based approach relies on assigning timestamps to the transactions and the data items and comparing them to determine the order of execution.


The concurrency control aims to prevent conflicts and anomalies, such as lost updates, dirty reads, unrepeatable reads, and phantom reads.


Reliability and recovery




Reliability is the ability of a distributed database system to function correctly and continuously despite failures and errors. Recovery is the process of restoring the system to a consistent and correct state after a failure or error. Recovery can be done using a logging-based approach or a shadowing-based approach. The logging-based approach relies on recording the changes made by the transactions in a log file and using it to undo or redo the operations in case of a failure. The shadowing-based approach relies on maintaining a copy of the data before and after each transaction and switching between them in case of a failure.


The recovery aims to ensure atomicity and durability of the transactions, which means that either all or none of the operations are executed and that the effects are permanent.


Replication and consistency




Replication is the process of storing multiple copies of the same data on different nodes to increase availability and reliability. Consistency is the property that ensures that all the copies of the data are identical and reflect the latest updates. Replication and consistency can be achieved using a synchronous approach or an asynchronous approach. The synchronous approach relies on updating all the copies of the data at the same time and ensuring that they are always consistent. The asynchronous approach relies on updating one copy of the data at a time and propagating the changes to the other copies later.


The replication and consistency aim to balance the trade-off between performance and correctness, which means that faster updates may lead to temporary inconsistencies and slower updates may lead to higher consistency.


Advanced Topics in Distributed Data Management




Parallel database systems




A parallel database system is a system that uses multiple processors and disks to perform operations on the data in parallel. A parallel database system can be classified into three types: shared-memory, shared-disk, and shared-nothing. A shared-memory system has multiple processors that share a common memory and disk. A shared-disk system has multiple processors that have their own memory but share a common disk. A shared-nothing system has multiple processors that have their own memory and disk.


Parallel database systems have many advantages over sequential database systems, such as higher performance, scalability, fault tolerance, and load balancing. However, they also pose many challenges, such as synchronization, communication, scheduling, partitioning, allocation, and load balancing.


Distributed object databases




A distributed object database is a system that manages data as objects that have both state (attributes) and behavior (methods). A distributed object database supports object-oriented features, such as encapsulation, inheritance, polymorphism, and persistence. A distributed object database can be implemented using an object-relational mapping (ORM) or an object-oriented database management system (OODBMS). An ORM is a technique that maps objects to relational tables and vice versa. An OODBMS is a system that stores objects directly in an object-oriented data model.


Distributed object databases have many advantages over relational databases, such as higher expressiveness, flexibility, reusability, extensibility, and compatibility. However, they also pose many challenges, such as complexity, heterogeneity, interoperability, query processing, transaction management, and schema evolution.


Peer-to-peer data management




A peer-to-peer data management system is a system that manages data in a decentralized network of autonomous nodes (peers) that cooperate with each other without any central authority or server. A peer-to-peer data management system can be classified into three types: structured, unstructured, and hybrid. A structured system has a predefined network topology and a deterministic routing mechanism for locating data. An unstructured system has a random network topology and a probabilistic routing mechanism for locating data. A hybrid system combines both structured and unstructured features.


Peer-to-peer data management systems have many advantages over client-server systems, such as scalability, robustness, self-organization, anonymity, and resource sharing. However, they also pose many challenges, such as heterogeneity, inconsistency, security, privacy, incentive, and trust.


Web data management




can be classified into three types: web search engines, web databases, and web services. A web search engine is a system that crawls, indexes, and ranks web pages based on keywords and links. A web database is a system that extracts, integrates, and queries structured or semi-structured data from web sources. A web service is a system that provides a standardized interface for accessing and exchanging data and functionality over the web.


Web data management systems have many advantages over traditional data management systems, such as universality, accessibility, interactivity, and diversity. However, they also pose many challenges, such as scalability, heterogeneity, volatility, quality, and security.


Big data platforms and NoSQL systems




A big data platform is a system that manages large-scale and complex data that exceed the capabilities of conventional data management systems. A big data platform typically consists of three layers: storage, processing, and analysis. A storage layer provides distributed and fault-tolerant storage systems for storing and retrieving data. A processing layer provides parallel and distributed processing frameworks for transforming and manipulating data. An analysis layer provides tools and techniques for extracting insights and value from data.


A NoSQL system is a type of big data platform that provides non-relational and schema-less data models for storing and querying data. A NoSQL system can be classified into four types: key-value, document, column-family, and graph. A key-value system stores data as pairs of keys and values. A document system stores data as collections of documents. A column-family system stores data as tables of columns. A graph system stores data as networks of nodes and edges.


Big data platforms and NoSQL systems have many advantages over traditional data management systems, such as scalability, performance, flexibility, and variety. However, they also pose many challenges, such as consistency, reliability, security, and compatibility.


Conclusion




In this article, we have discussed the principles of distributed database systems and how a solution manual can help students and instructors learn and teach this topic. We have covered the main aspects of distributed data management, such as design, query processing, transaction management, and advanced topics. We have also provided some examples and exercises from the solution manual to illustrate the concepts and techniques.


Distributed database systems are an important and relevant topic in the field of computer science and information technology. They enable us to manage large-scale and complex data in a distributed and parallel manner. They also enable us to cope with the challenges and opportunities of emerging applications and technologies, such as cloud computing, social media, web data management, big data platforms, NoSQL systems, and blockchain.


FAQs




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