CS603 Data Replication: Advanced

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CS603Data Replication Advanced

• March 1, 2002

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Data ReplicationWhat havent we Covered?

• Transparent replication possible
• Maintain serializability, ACID properties
• Implemented in real systems
• But poses performance problems
• Delays on failure
• Doesnt support mobility/disconnected operation
• What if we dont need transparent replication?
• Transaction serializability not required
• COTS DBMSs provide alternatives
• But how do we make sense of them!

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Problem Formalisms for Relaxed consistency

• Goal Relaxed consistency constraints
• Meet application needs
• Outperform true transparent replication
• How do we ensure constraints meet needs?
• Formalisms to describe application needs
• Methods to prove constraints adequate

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Quasi-Copies(Alonso, Barbará, Garcia-Molina 90)

• Data Caching
• Each site keeps copy of data likely to be used
  locally
• Propagation cost of writes high
• User-Defined Cache
• Controlled Divergence
• Weak consistency constraints
• Bounds on the differences between copies
• User defines constraints

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Tradeoffs Involved

• Cost of transmission
• Delay
• Failure
• Vs.
• Cost of complexity
• Defining semantics
• Getting it wrong
• Cost of consistency checks
• Processing to see if copy meets constraints

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Assumptions

• Read-only copies
• Updates sent to master copy
• E.g., ORACLE Materialized View
• User Specified Coherency
• Strict limits
• Hints
• Example Stock Purchase
• Place order based on delayed price
• Limit order to ensure price paid okay

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Quasi-Caching Definition

• Central node C
• Set of objects O
• Object x ? O has values (attributes)
• Changes result in versions x(t), latest is v(x)
• Set of nodes N (C ? N)
• x Quasi-copy of x ? O
• xj copy at j ? N
• Conditions
• Selection
• Coherency
• Access to x on j translates to xj if it exists

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Selection Conditions

• Identification clause
• Select/Project Query
• SELECT NAME, PRICE
• FROM STOCKS
• WHERE TYPE Chemical Company
• Modifier Clause
• Add / drop from cache
• Compulsory or advisory cache
• Static / Dynamic As new objects meet the
  identification clause, are they cached?
• Triggering delay on dynamic

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Coherency Conditions

• Default (always enforced) Value was true once
• ?t 0 ?t0 s.t. 0 t0 t and x(t)x(t0)
• Delay W(x,a) Max time lag
• ?t 0 ?k s.t. 0 k a and x(t)x(t - k)
• Version V(x) Number of updates
• ?t 0 ?k, t0 s.t. 0 k ß, 0 t0 t,v(x(t))
  v(x(t0))k, x(t) x(t0)
• Periodic P(x) Time for refresh
• ?t 0 ?n s.t. n 0, anß t lt a(n1)ß,x(t)
  x(anß)

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Coherency Constraints (cont.)

• Arithmetic A(x) Bounded Difference
• ?t 0, x(t) - x(t) lt e
• ?t 0, ( x(t) - x(t) ) / x(t) lt e
• Combine conditions with logical operators
• Multi-object conditions
• Consistency conditions on a group
• Order of application in a group

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Implementation

• Transmission Delays and Failures
• C(xj) really C(xj) ? W(xj) d ? C_Failed(j)
• What to Propagate from Server
• Data New values
• Invalidation Message
• Version number message (without data)
• Implicit Invalidation (time expired message)

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Implementation (cont.)

• When to Propagate
• Last minute When condition violated
• Immediately When update occurs
• Early
• Delayed Update Dont update central site until
  conditions satisfied
• Collapsing conditions
• Server checks conditions
• How to check efficiently?
• Load balancing
• Can conditions be checked at nodes?

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Performance

• Performance Model
• Query processing time at node
• Update time at node
• Query processing time at central site
• Update time at central site
• Assumes central site 20 speed of node

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Model Parameters
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Results