Real Time Databases

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Real Time Databases
Group Members Gajendran Mahendran
Kajitha Balasundaram
Vithiya Perampalam
Suganthini Nanthanan Group Number 13 Date
March 26, 2008
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Outline

• Introduction
• Conventional Vs. Real-Time Databases
• Preservation of Data Consistency
• Timing Constraints Deadlines
• Future of Real-Time Databases
• Q A

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Conventional Databases

• Designed for ensuring that data integrity is
  maintained for permanent/persistent data and that
  concurrent transaction execution is correct.
• ACID Properties
• Cannot deal with changes that are very rapid and
  dynamic as they contain persistent data mostly
  UNAFFECTED by time

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Real-Time Database Systems (RTDBS)

• Consists of two areas Real-Time Systems
  Database Systems
• Real-Time Systems (RTSs)
• Timing requirements for task execution times
• Processing data items whose values and validity
  change in time
• Database Systems (DBSs)
• Data volumes processed by the systems -gt
  organized mechanism for storing, managing and
  retrieving information
• Requirement Concerns durability, security and
  consistency of the processed data
• RTDBS is the INTEGRATION of features of RTSs and
  DBSs

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RTDBS - II

• RTDBS is designed for
• Processing transactions having timing constraints
  associated with them
• Accessing data items whose values and validity
  change in time
• Supporting data descriptions, data correctness
  and integrity maintenance methods, efficient data
  access and management techniques
• Guaranteeing the correct execution of
  transactions in spite of concurrency and
  failures!!!
• Timeliness is more important than correctness
• Correctness can be traded for timeliness by
  relaxing the ACID properties.

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Objectives of RTDBS

• Objectives
• Gathering data from the environment, processing
  it in the context of information acquired in the
  past, for providing timely and temporally correct
  response
• Correctness Requirements
• Time constraints on transactions
• How fast a system responds to a request
• Time constraints on data
• How fresh the data read is
• Database consistency constraints

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Why Time Constraints on Transactions?

• Dictate the behavior of the environment ?
  specification of rates and times of I/O of the
  system
• Requirements on reaction times ? basically
  dictating the responsiveness of the system
• Need to maintain temporal consistency of data
• A transaction value depends on completion time!!!

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Types of Time Constraints

• Based on type of time constraints
• Periodic
• - Every 10 secs Sample wind velocity
• - Every 20 secs Update robot position
• Aperiodic
• - If temperature gt 1000
• within 10 secs add coolant to reactor
• Based on Value
• Hard must execute before deadline
• Firm abort if not completed by deadline
• Soft diminished value if completed after deadline

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Deadlines Timing Constraint Violations

• Three categories of time constraint violations
  can be distinguished

• Soft Deadline Result useful even after deadline,
  though value is reduced
• Firm Deadline No value after deadline, but no
  penalty is accrued
• Hard Deadline Penalty is accrued - negative,
  leading to a catastrophe

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Examples of Time Constraints using
Event-Condition-Action (ECA) Rules

• Time constraints can be specified using ECA
  rules
• ON (10 seconds after initiating landing
  preparations)
• IF (steps not completed)
• DO (within 5 seconds abort landing)
• ON (deadline of object recognition)
• IF (action not completed)
• DO (increase importance, adjust deadlines)

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Real-Time (Temporal) Data

• Arrives from continuously changing environment
• Data items reflect the state of the environment
• Data from sensors - e.g., temperature and
  pressure
• Derived data - e.g., rate of reaction
• Input to actuators - e.g., amount of chemicals,
  coolant
• Archival data - e.g., history of (interactions
  with) environment
• Has observed time and validity interval
• Users of temporal data need to see temporally
  valid views of the data (state of the
  environment)
• When must the data be temporally consistent?
• ideally, at all times
• in practice, only when they are used by
  transactions!!!

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Time Constraints on Data

• Data perceived by the controlling system must be
  consistent with the actual data
• how closely is the data read by a transaction
  models the environment?
• Requirements
• Timely monitoring of the environment
• Timely processing of sensed information
• Timely derivation of needed data
• Temporal Consistency Measurement
• absolute consistency freshness of data between
  actual state and its representation
• relative consistency correlation among data
  accessed by a transaction

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Time Constraints on Data - II

• Absolute consistency is measured by Absolute
  Validity Interval (AVI) state of environment and
  image in DB
• Given (value, avi, ts) ? current time ts lt
  avi
• Relative consistency is measured by Relative
  Validity Interval (RVI) data used to derive
  other data
• Given (value, avi, ts) and (value, avi, ts) ?
  ts ts lt rvi

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Time Constraints on Data - III

• Example
• Data object is specified by
• (value, absolute validity interval, time-stamp)
• Interested in temperature and pressure with RVI
  of 5
• Let current time 100
• temperature (347, 10, 95) and pressure (50,
  20, 98)
• ? temporally consistent
• temperature (347, 10, 98) and pressure (50,
  20, 91)
• ? temporally inconsistent

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Database Consistency

• Concurrency Control
• Control of interaction among concurrent
  transactions to preserve database consistency
• Coordinating read write transactions to shared
  data in DBSs
• Concurrency control protocols for DBS Vs. RTDBS

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Concurrency Control

• Popular Concurrency Control Serializability
• Sequence of DB Operations serial transaction
  schedule ? serializable
• Limits degree of multiprogramming
• Introduces blocking and restarts of transactions
• But data is short lived!!!

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Approaches to Concurrency

• Key Issue of System Degradation in RTDBSs
• aborts and restarts of transactions.
• caused by concurrency control protocols trying to
  resolve conflicts between transactions
• Lock-Based Protocols (i.e. 2PL)
• Optimistic Concurrency Control (OCC) Protocols

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Lock-Based Protocol

• Locks are used to synchronize concurrent actions
• Two-Phase Locking (2PL)
• all locking operations precedes the first unlock
  operation in the transaction
• expanding phase (locks are acquired)
• shrinking phase (locks are released)
• suffers from deadlock
• priority inversion
• UNSATISFACTORY for RTDBS

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Dead-Lock

• Goal Largest number of transactions can meet
  their deadline
• Abort transaction that has already passed
• Abort transaction with longest deadline
• Abort transaction that is least critical

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Priority Inversion

• High priority tasks are blocked by low priority
  tasks
• Impacts real-time scheduling algorithms
  (ineffective)
• Solutions
• Wait Promote
• High Priority (aborts)
• Leads to cyclic restart
• Conditional Restart (measure the slack time)
• No single strategy excels
• Depends on applications, avail. of resources
  cost of transaction restart!

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OCC Protocol - I

• 3 Phases
• 1. Read 2. Validate 3. Write
• Rules
• Given T1 be serialized before T2
• R/W rule Data Items to be written by T1 should
  not have already been read by T2
• W/W rule T1s write should not overwrite T2s
  writes

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OCC Protocol - Validation

• Check for Rule Violations
• T1 completes execution before T2 starts (no
  interleaving)
• W of T1 does not intersect R of T2 (R/W)
• T1 finishes W before T2 starts validation (W/W)
• Conflict Resolution if Rule is violated!
• ? Validation fails!
• ? (broadcasting commit, etc.)

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Applications

• military command and control
• Aerospace systems
• aircraft control
• traffic control
• telecommunication and computer network management
• telephone directory service systems
• computer integrated manufacturing (CIM)
• factory automation and robotics
• workflow systems
• medical monitoring
• stock arbitrage systems
• multimedia systems

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References

• References
• 1 Azer, B., Kwei-Jay, L., Sang, H.S.,
  Real-Time Database Systems Issues and
  Applications, Kluwer Academic Pub., 1997.
• 2 Kam-Yiu, L., Tei-Wei, L., Real-Time Database
  Systems Architecture and Techniques, Kluwer
  Academic Pub., 2001.
• 3 Norman, W.P., Active Rules in Database
  Systems, Springer., 1998.
• 4 Piotr Krzyzagorski., Concurrency Control in
  Real-Time Systems, 2005.
• http//www.edbt2000.uni-konstanz.de/phd-workshop/p
  apers/Krzyzagorski.pdf
• 5 Matthew, R.L., Young-Kuk, K., Sang, H.S.,
  Managing Contention and Timing Constraints in a
  Real-Time Database System, Univeristy of
  Virginia, Charlottesville, 1995 IEEE
• 6 http//ieeexplore.ieee.org/iel3/3569/10678/004
  95222.pdf

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Question Answer