Distributed Database research group

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Distributed Database research group
In the name of God

• Instructor Dr. M. Rahgouzar
• Samira Tasharofi
• Reza Basseda

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Outline

• Introduction
• Distributed Data Storage
• Distributed Transaction
• Commit Protocols
• Concurrency Control in Distributed Database
• Availability
• Distributed Query Processing
• Heterogeneous Distributed Databases
• Directory Systems
• Conclusion
• Acknowledgement

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Introduction

• Distributed computing
• Consists of a number of processing elements (not
  necessarily homogeneous ) that interconnected by
  a computer network and that co-operate in
  performing their assigned tasks
• Distributed Database
• Database whose relations reside on different
  sites
• Database some of whose relations are replicated
  at different sites
• Database whose relations are split between
  different sites

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Introduction

• Distributed database management system
• Software system that permits the management of
  distributed database system
• Advantages
• Local autonomy
• Improved performance (by proper fragmentation)
• Improved reliability/availability (by
  replication)
• Greater expandability
• Greater shareability

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Introduction (Cont.)

• Disadvantages
• Higher complexity
• Higher software and hardware cost
• Synchronization and co-ordination among the sites
• Higher maintenance overhead in case of
  replication
• Greater security problem

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Distributed Data Storage

• On Dynamic Fragmentation of Distributed Databases
  Using Partial Replication D. Pinto, G. Torres
  ,2002
• Fragmentation
• Three kinds of fragmentation Vertical
  Fragmentation, Horizontal Fragmentation and Mixed
  Fragmentation
• Solution
• horizontal fragmentation with partial replication
• RBy2(bound) Algorithm
• 1. For each query requested, a slave computer
  increments a counter (ctr) for the user that have
  made the request.
• 2. If ctr reaches bound number (parameter of this
  algorithm), then this computer is a candidate to
  have a set of records replicated and need to
  follow steps 3 and 4, else step 5.
• 3. Request the set of records that the user is
  asking for and save this information into the
  slave database.
• 4. Reset the user local counter to cero.
• 5. end
• Local access database before sending the query to
  the master computer

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Distributed Data Storage (Cont.)

• The algorithm is based on two techniques
• Slave-Master Search, to provide fast access to
  database on user queries
• Replication slave-master (two computers) in order
  to get availability
• Allows database availability even when the
  connection between slave and master database is
  broken
• Proved on local network
• Future Work
• Check how the algorithm works under dial-up
  connection

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Distributed Data Storage (Cont.)

• Transparent Data Relocation in Highly Available
  Distributed Systems,S. Voulgaris, M.V. Steen, A.
  Baggio, and G. Ballintjn, 2002
• Management issue of distributed services
  redistribution of non-replicated data among the
  servers comprising a distributed service
• Redistribute the data without disrupting the
  services availability
• Solution Base
• Shipping the data records that need to be
  relocated to their new hosting server
• Updating the servers mapping information to
  reflect the new configuration of the distributed
  service

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Distributed Data Storage (Cont.)

• Solution For a Single Redistribution
• Initialization
• Distribute new mapping M
• Record Relocation
• Termination
• Replace M with M
• Solution for Overlapping Redistributions
• Per-server Sequential Redistribution
• Using redistribution R2 after R1 completed
• Per-server Mixed but Ordered Redistributions
• The server ships each record as soon as possible,
  based on the virtual mapping with first
  preference
• Direct Shipping to Final Destination

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Distributed Data Storage (Cont.)

• Advantages
• low delays in the servicing of client requests
  during a configuration change
• Adding no significant processing requirement to
  the servers involved, and terminates in a timely
  fashion
• conceptual simplicity
• sequential concurrent versions

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Transaction Management

• Ensuring Relaxed Atomicity for Flexible
  Transaction in Multidatabase Systems, A.Zhang,
  M.Nodine, B.Bhargava, O.Bukhres, ACM
• Global transaction A set of sub transactions,
  where each sub transaction is a transaction
  accessing the data items at a single local site
• Flexible global transaction Specifying
  Definitions of
• Execution ordering dependencies between two sub
  transaction
• Alternating dependencies between two subsets of
  sub transactions
• Eliminate prepare-to-commit stage
• Sub transaction
• Retriable
• Compensable
• Pivot
• In global transactions at most one sub
  transaction can be pivot

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Transaction Management (Cont.)

• Semi-atomicity in Flexible Transaction Weaker
  than atomicity in global transaction
• All its sub transaction in one lt-rpo commit and
  all attempted sub transactions not in the
  committed lt-rpo are either aborted or have their
  effects undone
• No partial effects of its sub transactions remain
  permanent in local database
• Ensuring semi-atomicity
• Using retry and compensetable techniques
• Flexible transaction Advantages
• Enhances the scope of global transaction
  management beyond that offered by the traditional
  global transaction model
• Blocking that caused by two-phase commit can be
  prevented

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Transaction Management (Cont.)

• Global scheduling for flexible Transactions in
  Heterogeneous distributed Database Systems, A.
  Zhang, M. Nodine, B. Bhargava
• Global Serializability
• If the projection of committed local, flexible
  and surplus transactions is conflict equivalent
  to some serial execution of these transactions
• Compensation-interference free
• For any sub transaction tj which is serialized
  between a subtransaction ti and its compensating
  transaction cti in S, WC(ti) /\ AC(tj) 0

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Transaction Management (Cont.)

• F-serializability
• Prevents the flexible transactions which are
  serialized between a flexible transaction and its
  compensating sub transactions to affect any data
  items that have been updated by flexible
  transaction (global serializable and compensation
  free)
• Avoids unnecessary abort or compensation
• Scheduling Protocols
• Stored Sub transaction Execution Graph (SSEG)
• Avoid cascading abort

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Commit Protocols

• A Two-Phase Commit Protocol for Mobile Wireless
  Environment, N. Nouali, A. Doucet, H. Drias, 2005
• IF the traditional 2PC is executed in mobile
  environment, disconnections will increase the
  number of, may be wrong, abortion decisions of
  transaction because if a FH tries to communicate
  with it a disconnected MH this will cause a
  failure
• Disconnections are not exceptions but rather are
  part of the normal mode of operation, so they
  should not be treated as failures

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Commit Protocols (Cont.)

• The case of mobile client and fixed servers,
  Fixed Coordinator
• To mitigate the unforeseeable breakdowns, the
  client must force-write the identity and location
  information of the coordinator (commit-BS) just
  before sending the commit-request
• Only one force-write is needed to record the
  coordinator information during the entire
  execution of Atomic commitment Protocol (ACP)

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Commit Protocols (Cont.)

• The case of mobile client and mobile servers
• The participant-agent is responsible of
  transmitting the result to the participant at
  reconnection time and also of keeping logs and
  eventually recovering in the case of failure
• When participant registers to a new BS, the
  participant MH (or mobile participant) informs
  its participant-agent about its new location
• Workload is shifted to the fixed part of the
  network thus preserving processing power and
  communication resources and minimizing traffic
  cost over the wireless links

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Commit Protocols (Cont.)

• Reducing the Latency of Non-Blocking Commitment
  using Optimism and Replication, R.J.Peris,
  M.P.Martinez,G.Alonso, S.Arevalo, 2001
• 2Phase Commit Blocking
• 3Phase Commit
• Sending too much messages
• Low performance
• Flushing log records adds to the overall latency
  as messages cannot be sent or responded to before
  writing to the log
• Delay is reduced by allowing sites to send
  messages instead of flushing log records

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Commit Protocols (Cont.)

• Solution
• To use the main memory of a replicated group as
  stable memory instead of a mirrored log with
  careful writes
• The group of the participating transaction
  managers (the TM group)
• A replicated group providing the commit service
  that acts as coordinator (the CS group)
• A participant does not wait to flush its log,
  instead it uniformly multicasts its vote together
  with its log entry

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Commit Protocols (Cont.)

• If the message corresponds to the last vote, and
  all were yes votes, the transaction is
  optimistically committed, and the fact is
  communicated to the TM group
• The optimistic commit changes the locks held by
  the transaction to opt-mode
• Do not allow to the holding transaction to
  commit until the transaction that released them,
  definitively commits

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Commit Protocols (Cont.)

• The message is optimistically delivered right
  away without waiting for the stabilization of the
  message (e.g., waiting for the message to be
  received by all the members of the group)

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Commit Protocols (Cont.)

• The PROMPT Real-Time Commit Protocol,
  J.H.Harista, K.Ramamritham, R.Gupta,IEEE,1999
• Distributed commit processing can have
  considerably more effect than distributed data
  processing on real-time performance
• PROMPT
• New commit protocol in real-time distributed
  transactions
• Preventing borrowers from continuing to execute
  if their associated lenders had not been received
  their decisions was addressed by incorporating an
  additional bit and message that informed the
  master about the borrowing state and the
  completion of borrowing by a cohort

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Commit Protocols (Cont.)

• Features
• Controlled optimistic access to uncommitted data
• Reduce data inaccessibility and priority
  inversion
• Active abort
• Cohorts inform the master as soon as they decide
  to abort locally
• Silent kill
• Aborts due to deadline misses that occur before
  the master has initiated the commit protocol are
  implemented silently
• Healthy-Lending
• Health factor deadline of transaction
• Using HF to decide whether transaction can lend
  its data (best choice)

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Commit Protocols (Cont.)

• One-Phase Real-Time Commit Protocols, P.Saha,
  1999
• Comparing one-phase protocols (e.g. EP) and
  PROMPT, the best-performing two-phase real-time
  commit protocol
• For parallel distributed transaction, EP
  outperforms PROMPT
• For sequential distributed transaction, EP
  perform rather poorly
• For high workload cases EP performs better
• Future Works
• Addressing the security considerations in
  Multi-Level Secure (MLS) distributed RTDBS
• Combination of EP and PROMPT

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Concurrency Control in Distributed Databases

• Distributed Concurrency Control Performance A
  Study of Algorithms, Distribution, and
  Replication
• Michael J. Carey Miron Livny
• It express Distributed Concurrency Control
  Algorithms and evaluate their performance in some
  of conditions. At the start, It describe
  Concurrency Control classic algorithms such as
  2PL, Wound-Wait, Basic Timestamp ordering and
  discussed on the structure of distributed
  concurrency control algorithm. Then it suggest a
  basic model for DDB and has some experiments with
  those algorithm and evaluate the model.

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Concurrency Control in Distributed Databases
(Cont.)

• Concurrency Control in Distributed Database
  Systems
• PHILIP A. BERNSTEIN NATHAN GOODMAN
• It explain mythological proofs for Distributed
  Concurrency Control Algorithms and theoretically
  evaluate them. It define serializablity in DDB
  and define a formal language to formulate
  transactions in DDB. Then it express each of
  classic algorithms in his formal language and
  prove their correctness and completeness.

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Concurrency Control in Distributed Databases
(Cont.)

• Concurrency Control in Distributed
  Object-Oriented Database Systems
• Kjetil Nørvag Olav Sandsta and Kjell
  Bratbergsengen
• The simulation results in this paper is a
  comparison of performance and response times for
  two concurrency control algorithms, timestamp
  ordering and two-phase locking. The simulations
  have been run with different number of nodes,
  network types, data declustering and workloads.
  The results show that for a mix of small and long
  transactions, the throughput is significantly
  higher for a system with a timestamp ordering
  scheduler than for a system with a two-phase
  locking scheduler.
• Implementing Atomic Actions on Decentralized Data
• DAVID P. REED
• Its a general survey on Concurrency Control in
  DDB and describe classic method.

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Concurrency Control in Distributed Databases
(Cont.)

• Dynamic Voting Algorithms for Maintaining the
  Consistency of a Replicated Database
• SUSHIL JAJODIA DAVID MUTCHLER
• The best known pessimistic algorithm, voting, is
  a static algorithm, meaning that all potential
  distinguished partitions can be listed in
  advance. It presents a dynamic extension of
  voting called dynamic voting. This algorithm
  permits updates in a partition provided it
  contains more than half of the up-to-date copies
  of the replicated file. It also presents an
  extension of dynamic voting called dynamic voting
  with linearly ordered copies (abbreviated as
  dynamic-linear). These algorithms are dynamic
  because the order in which past distinguished
  partitions were created plays a role in the
  selection of the next distinguished partition.

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Concurrency Control in Distributed Databases
(Cont.)

• Deadlock Detection in Distributed Databases
• EDGAR KNAPP
• This paper is concerned only with the aspect of
  deadlock detection. Recent developments in the
  area of distributed deadlock detection algorithms
  are surveyed, with a special emphasis on their
  relation to distributed DBSs. The paper
  introduces a uniform framework for the discussion
  of these algorithms. The abstraction achieved
  this way allows us to talk about the algorithms
  in terms of the underlying theoretical concepts,
  instead of just giving a phenomenon-logical
  description of the workings of the algorithms.

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Concurrency Control in Distributed Databases
(Cont.)

• MODELS OF A VERY LARGE DISTRIBUTED DATABASE
• Mark Blakey
• The best known pessimistic algorithm, voting, is
  a static algorithm, meaning that all potential
  distinguished partitions can be listed in
  advance. It presents a dynamic extension of
  voting called dynamic voting. This algorithm
  permits updates in a partition provided it
  contains more than half of the up-to-date copies
  of the replicated file. It also presents an
  extension of dynamic voting called dynamic voting
  with linearly ordered copies (abbreviated as
  dynamic-linear). These algorithms are dynamic
  because the order in which past distinguished
  partitions were created plays a role in the
  selection of the next distinguished partition.

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Concurrency Control in Distributed Databases
(Cont.)

• Performance Study of a Centralized Concurrency
  Control Algorithm for Distributed Database
  Systems using SIMULA
• K. Viswanathan Iyer  L. M. Patnaik
• One objective of this paper is to elaborate the
  simulation methodology using SIMULA. Detailed
  studies have been carried out on a centralized CC
  algorithm and its modified version. The results
  compare well with a previously reported study on
  these algorithms. Here, additional results
  concerning the update intensiveness of
  transactions and the degree of conflict are
  obtained. The degree of conflict is
  quantitatively measured and it is seen to be a
  useful performance index. It seems that, It is
  going to formulate the effectiveness of
  Concurrency Control Algorithm and it focused on
  the behavior of a class of performance index.

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Availability

• Maintaining Availability in Partitioned
  Replicated Databases
• AMR EL ABBADI SAM TOUEG
• It describes a new replica control protocol that
  allows the accessing of data in spite of site
  failures and network partitioning. It claims that
  this protocol provides the database designer with
  a large degree of flexibility in deciding the
  degree of data availability, as well as the cost
  of accessing data.
• Providing High Availability Using Lazy
  Replication
• RIVKA LADIN BARBARA LISKOV SANJAY GHEMAWAT
• This paper describes a new technique that
  supports causal order. An operation call is
  executed at just one replica updating of other
  replicas happens by lazy exchange of gossip
  messageshence the name lazy replication. The
  replicated service continues to provide service
  in spite of node failures and network partitions.

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Distributed Query Processing

• Query Brokers for Distributed And Flexible Query
  Evaluation
• Tuyet-Trinh yu Christine Collet
• This paper provides an approach for designing
  query processor of a DDB by using hierarchical
  mediators and using query brokers which translate
  a global query in DDB context to local queries
• Query Decomposition, Optimization and Processing
  in Multidatabase Systems
• Cem Evrendilek Asuman Dogac
• This paper suggest an approach to decomposing
  queries in a optimized manner. In this way, we
  need to dynamically calculate cost of query
  processing in every sites for all of the sub
  queries and using this factors in calculating
  minimum cost.

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Distributed Query Processing (Cont.)

• Dynamically Distributed Query Evaluation
• Trevor Jim Dan Suciu
• This paper provides an approach for evaluation of
  queries over the web and a directory system
  dynamically. It provides a language for
  explaining information requirements over a multi
  database system. It uses this language for
  defining a DDB and its queries in a formal way.
  So it suggest an algorithm for dynamic query
  evaluation in DDB and by this logic it proves its
  algorithms correctness.

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Distributed Query Processing (Cont.)

• Database Connectivity Using an Agent-Based
  Mediator System
• Larry M. Stephenes Michael N Huhns
• This paper provides an Agent-Based approach for
  managing a DDB. It uses Agents as proactive
  components which include KB about system and have
  reaction to topology changes to manage query
  processing and concurrency control , . It uses
  KQML and a specific coordination strategy for
  this system.

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Distributed Query Processing (Cont.)

• Optimizing Equijoin Queries In Distributed
  Databases Where Relations Are t-lash Partitioned
• DENNIS SHASHA TSONG-LI WANG
• It studies the optimization problem that arises
  when the query processor must repartition the
  relations and intermediate results participating
  in a multi join query. Using estimates of the
  sizes of intermediate relations, it shows (1)
  optimum solutions for closed chain queries (2)
  the NP-completeness of the optimization problem
  for star, tree, and general graph queries and
  (3) effective heuristics for these hard cases.

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Conclusion

• DDB is a mature topic and many model provided for
  expressing its approaches such as concurrency
  control, availability ,
• Various models using to prove correctness of its
  approaches in concurrency controls ,
• It is faced with many of DB problems in a new
  viewpoint because of its distribution
• Legacy system and wrapper design to

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Conclusion (Cont.)

• High availability by increasing replication vs.
  performance of transaction management and
  concurrency control A trade off
• DDB system Concurrency control open problems
• Improving recent approaches to increase
  availability with performance
• Improving distributed query evaluation over large
  distributions

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Conclusion (Cont.)

• Tuple routing over a distribution and data
  distribution with high performance and
  performance evaluation factors in DDB
• Using autonomous components to manage a DDB and
  using AI in peer-to-peer DDB

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Acknowledgement
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References

• Silbershots et al , Database System Concepts
  4th edition , McGraw-Hill, 2002