1. Distributed Systems Foundations Lecture 1

2. Main Characteristics of Distributed Systems Independent processors, sites, processes Message passing No shared memory No shared clock Independent failure modes CS 2712

3. Distributed System Models Synchronous System: Known bounds on times for message transmission, processing, bounds on local clock drifts, etc. – Can use timeouts Asynchronous System: No known bounds on times for message transmission, processing, bounds on local clock drifts, etc. – More realistic, practical, but no timeout. CS 2713

4. CAUSALITY AND TIME CS 2714

5. What is a Distributed System? A simple model of a distributed system proposed by Lamport in a landmark 1978 paper: “Time, Clocks and the Ordering of Events in a Distributed System” Communications of the ACM CS 2715

6. What is a Distributed System? A set of processes that communicate using message passing. A process is a sequence of events 3 kinds of events: – Local events – Send events – Receive events Local events on a process for a total order. CS 2716

7. Example of a Distributed System CS 2717

8. Happens Before or Causal Order on Events Event e happens before (causally precedes) event f, denoted e → f if: 1.The same process executes e before f ; or 2.e is send(m) and f is receive(m); or 3.Exists h so that e → h and h → f We define concurrent, e || f, as: ¬(e → f  f → e) CS 2718

9. Lamport Logical Clocks Assign “clock” value to each event such that – if a  b then clock(a) < clock(b) Assign each process a clock “counter”. – Clock must be incremented between any two events in the same process – Each message carries the sender’s clock value When a message arrives set local clock to: – max(local value, message timestamp + 1) – This clock forms a partial order. If a total order is needed use process ids to break ties—totally ordered Lamport (or Logical) Clocks CS 2719

10. Example of a Logical Clock CS 27110

11. Vector clocks 1.Vector initialized to 0 at each process V i [j] = 0 for i, j =1, …, N 2.Process increments its element of the vector in local vector before event: V i [i] = V i [i] +1 3.Piggyback V i with every message sent from process P i 4.When P j receives message, compares vectors element by element and sets local vector to higher of two values V j [k] = max(V j [k], V i [k]) for k=1, …, N CS 27111

12. Comparing vector timestamps Define V = V’ iff V [i ] = V’[i ] for i = 1 … N V  V’ iff V [i ]  V’[i ] for i = 1 … N For any two events e, e’ e  e’ if and only if V(e) < V(e’) Two events are concurrent if neither V(e)  V(e’) nor V(e’)  V(e) CS 27112

13. Vector Clock Example CS 27113

14. Clock Synchronization Assume each process p has a physical clock C p. Assume an accurate time source providing Universal Coordinated Time (UCT)—atomic clock or GPS. Clock Synchronization: – Keep all clocks in a distributed system close to each other, ie, synchronized. If UCT is t, and value of clock at process p is C p (t) then ideally, C p (t) = t, ie, dC p (t)/dt = 1. CS 27114

15. Clock Synchronization The internal timer causes an interrupt h times a second. When interrupt occurs, clock is incremented, and clock keeps times since it was initialized. In reality, timers have drift ρ therefore: 1- ρ =< dC p (t)/dt =< 1+ ρ Two clocks can drift 2 ρ Δt after Δt. If we want clocks not to differ by more than δ, then resynch at least every δ/2ρ secs

16. Cristian’s Clock Synch Algorithm [Distributed Computing 1989] A processor p requests time from UTC. UTC sends back current time t. What does p set local clock to? t + T round /2 T round = t receive –t send. Assume minimum time is min. Error: +/- [T round /2 –min] Time must be monotonic. What if t is less than current time at p? CS 27116