Lecture 12 Synchronization

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Lecture 12Synchronization
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Summary so far

• A distributed system is
• a collection of independent computers that
  appears to its users as a single coherent system
• Components need to
• Communicate
• Cooperate gt support needed
• Naming enables some resource sharing
• Synchronization

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Last time

• Physical clocks
• Two applications
• Provide at-most-once semantics
• Global Positioning Systems
• Logical clocks
• Where only ordering of events matters
• Other coordination primitives
• Leader election How do I choose a coordinator?
• Mutual exclusion

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Efficient at-most-once message delivery

• Issues
• 1 How long to maintain transaction data?
• 2 How to deal with server failures? (Minimize
  state that is persistently stored)

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Efficient at-most-once message delivery (II)

• Issue1 How long to maintain transaction data?
• Solution
• Client
• Sends transaction id and physical timestamp
• Client (or network) may resend messages
• Server Discards messages with duplicate id and
  messages that have been generates too far in the
  past
• Mechanism
• Maintains G Tcurrent - MaxLifeTime -
  MaxClockSkew
• Discards messages with timestamps older than G
• Ignores (or delays) message that arrive in the
  future
• (Maintains transaction data only for the interval
  G--Tnow

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Efficient at-most-once message delivery (III)

• Issue 2 What to persistently store across server
  failures?
• Solution
• Current Time (CT) is written to disk every ?T
• At recovery Gfailure is recomputed after a crash
  from saved CT
• After recovery
• discard messages with timestamp older than
  Gfailure ?T
• Process messages with timestamp newer than
  Gfailure ?T
• Note the formulas above ignore clock skew!

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Uses of (synchronized) physical clocks

• NTP
• Using physical clocks to implement at-most-once
  semantics
• Global Positioning Systems

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GPS Global Positioning Systems (1)

• Basic idea Estimate signal propagation time
  between satellite and receiver to estimate
  distance
• Principle
• Problem Assumes that the clocks of the
  satellites and receiver are accurate and
  synchronized
• The receivers clock is definitely out of synch
  with the satellite

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GPS Global Positioning Systems (2)

• Xr, Yr, Zr, are unknown coordinates of the
  receiver.
• Ti is the timestamp on a message from satellite i
• ?Ii (Tnow Ti) is the measured delay of the
  message sent by satellite i.
• Distance to satellite i can be estimated in two
  ways
• Propagation time di c x ?Ii
• Real distance
• 3 satellites? 3 equations in 3 unknowns
• So far I assumed receiver clock is synchronized!
• What if it needs to be adjusted?
• ?I (Tnow Ti) ?r
• collect one more measurement frm one more
  satellite!

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Computing position in wired networks

• Observation a node P needs at least k 1
  landmarks to compute its own position in a
  k-dimensional space.
• Consider two-dimensional case
• Solution P needs to solve three
• equations in two unknowns (xP,yP)

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Computing Position (cont)

• Problems
• measured latencies to landmarks fluctuate
• computed distances will not
• even be consistent
• Solution Measure latencies to L landmarks and
  let each node P minimize
• where denotes the actual distance to
  landmark bi, given a computed coordinate for P.

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Logical clocks -- Time Revisited

• Whats important?
• What precise time an event occurred?
• The order in which events occur?

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Happens-before relation

• Problem We first need to introduce a notion of
  ordering before we can order anything.
• The happened-before relation on the set of events
  in a distributed system
• if a and b in the same process, and a occurs
  before b,
• then a ? b
• if a is an event of sending a message by a
  process, and b
• receiving same message by another
  process then a ? b
• Property transitive
• Notation a ? b, when all participants agree that
  b occurs after a.
• Two events are concurrent if nothing can be said
  about the order in which they happened (partial
  order)

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Logical clocks

• Problem How do we maintain a global view on the
  systems behavior that is consistent with the
  happened-before relation?
• Solution attach a timestamp C(e) to each event
  e, satisfying the following properties
• P1 If a and b are two events in the same
  process, and a?b, then we demand that C(a) lt
  C(b).
• P2 If a corresponds to sending a message m, and
  b to the receipt of that message, then also C(a)
  lt C(b).
• Note C must only increase
• Problem Need to attach timestamps to all events
  in the system (consistent with the rules above)
  when theres no global clock
• maintain a consistent set of logical clocks, one
  per process.

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Logical clocks (cont) -- Lamports Approach

• Solution Each process Pi maintains a local
  counter Ci and adjusts this counter according to
  the following rules
• (1) For any two successive events that take place
  within process Pi, the counter Ci is incremented
  by 1.
• (2) Each time a message m is sent by process Pi
  the message receives a timestamp ts(m) Ci
• (3) Whenever a message m is received by a process
  Pj, Pj adjusts its local counter Cj to maxCj,
  ts(m) then executes step 1 before passing m to
  the application.
• Property P1 is satisfied by (1)
• Property P2 by (2) and (3).
• Note it can still occur that two events happen
  at the same time. Avoid this by breaking ties
  through process IDs.

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Example
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Architectural view

• Middleware layer in charge of
• Stamping messages with clock times, reading
  timestamps
• Local management of logical clocks
• Message ordering (if necessary)

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Totally ordered group communication

• Example
• Initial state 100 account balance
• Update 1 add 100
• Update 2 add 1 monthly interest
• Q Whats the result if updates are performed in
  different order at the two replica?

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Totally ordered group communication (cont)

• Solution
• Each message is timestamped with local logical
  time then multicasted
• When multicasted, also message logically sent to
  the sender and queued using timestamp order
• When receiving, the middleware layer
• Adds message to queue
• Acknowledges (using multicst) the message
• Delivers from queue to application only when all
  acks are received

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Totally Ordered Multicast Algorithm

• Process Pi sends timestamped message msgi to all
  others. The message itself is put in a local
  queue queuei.
• Any incoming message at Pk is queued in queuek,
  according to its timestamp, and acknowledged to
  every other process.
• Pk passes a message msgi to its application if
• msgi is at the head of queuek
• for each process Px, there is a message msgx in
  queuek with a larger timestamp.
• Note We are assuming that communication is
  reliable and FIFO ordered.
• Guarantee all multicasted messages in the same
  order at all destination.
• Nothing is guaranteed about the actual order!

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So far

• Physical clocks
• Two applications
• Provide at-most-once semantics
• Global Positioning Systems
• Logical clocks
• Where only ordering of events matters