CSE 5306 Distributed Systems Synchronization 1
Synchronization An important issue in distributed system is how processes cooperate and synchronize with one another Cooperation is partially supported by naming, which allow them to share resources Example of synchronization Access to shared resources Agreement on the ordering of events Will discuss Synchronization based on actual time Synchronization based on relative orders Algorithms for election of synchronization coordinators 2
Clock Synchronization When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time 3
Physical Clock All computers have a circuit to keep track of time using a quartz crystal However, quartz crystals at different computers often run at a slightly different speed This leads to the so called clock skew between different machines Some systems (e.g., real-time systems) need external physical clock Solar day: interval between two consecutive noons Solar day varies due to many reasons International atomic time (TAI): transitions of cesium 133 atom Cannot be directly used as everyday clock. TAI second < Solar second Solution: leap second whenever the difference is 800msec -> UTC 4
Global Positioning System Used to locate a physical point on earth Need at least 3 satellites to measure: Longitude, Latitude, and Altitude (height) Example: computing a position in a 2D space 5
GPS Challenges Clock skew complicates the GPS localization The receiver s clock is generally not well synchronized with that of a satellite E.g. 1 sec of clock offset could lead to 300,000 kilometers error in distance estimation Other source of errors The position of satellite is not known precisely The receivers clock has a finite accuracy The signal propagation speed is not constant Earth is not a perfect sphere - need further correction 6
Clock Synchronization Algorithms The goal of synchronization is to keep all machines synchronized to an external reference clock, or just keep all machines together as well as possible The relation between clock time and UTC when clocks tick at different rates 7
Network Time Protocol (NTP) Pairwise clock synchronization e.g., a client synchronize its clock with a server What is the clock offset? 8
The Berkeley Algorithm Goal: just keep all machine together Steps The time daemon tell all machines its time Other machines answer how far ahead or behind The time daemon computes the average and tell other how to adjust 9
Clock Sync. in Wireless Networks In traditional distributed systems, we can deploy many time servers that can easily contact each other for efficient information dissemination However, in wireless networks, communication becomes expensive and unreliable RBS (Reference Broadcast Synchronization) is a clock synchronization protocol where a sender broadcasts a reference message that will allow its receivers to adjust their clocks 10
Reference Broadcast Synchronization To estimate the mutual, relative clock offset, two nodes exchange the time when they receive the same broadcast the difference is the offset in one broadcast The average of M offsets is then used as the result However, offset increases over time due to clock skew (how to overcome?) 11
Logical Clocks In many applications, what matters is not the real time It is the order of the events For the algorithms that synchronize the order of events, the clocks are often refereed to as logical clocks Example: Lamport s logical clock, which defines the happen-before relation If a and b are events in the same process, and a occurs before b, then a b is true. If a is the event of a message being sent by one process, and b is the event of the message being received by another process, then a b 12
Lamport s Logical Clocks Three processes, each with its own clock. The clocks run at different rates. Lamport s algorithm corrects the clock 13
Lamport s Algorithm Each process P i maintains a counter C i 1. Before executing an event P i executes C i C i + 1. 2. When process P i sends a message m to P j, it sets m s timestamp ts (m) equal to C i after having executed the previous step. 3. Upon the receipt of a message m, process P j adjusts its own local counter as C j max{c j, ts (m)}, after which it then executes the first step and delivers the message to the application. 14
Application of Lamport s Algorithm Updating a replicated database and leaving it in an inconsistent state. 15
Totally Ordered Multicasting Apply Lamport s algorithm Every message is timestamped and the local counter is adjusted according to every message Each update triggers a multicast to all servers Each server multicasts an acknowledgement for every received update request Pass the message to the application only when the message is at the head of the queue all acknowledgements of this message has been received The above steps guarantees that the messages are in the same order at every server, assuming message transmission is reliable 16
Problem with Lamport s Algorithm Lamport s algorithm guarantees that if event a happened before event b, then we have C(a)<C(b) However, this does not mean that C(a)<C(b) implies event a happened before event b 17
Vector Clocks (1) Vector clocks are constructed by letting each process P i maintain a vector VC i with the following two properties: 1. VC i [ i ] is the number of events that have occurred so far at P i. VC i [ i ] is the local logical clock at process P i. 2. If VC i [ j ] = k then P i knows that k events have occurred at P j. It is thus P i s knowledge of the local time at P j. 18
Vector Clocks (2) Steps carried out to accomplish the property 2 of previous slide Before executing an event P i executes VC i [i] VC i [i]+1 When process P i sends a message m to P j, it sets m s (vector) timestamp ts(m) equal to VC i after having executed the previous step. Upon the receipt of a message m, process P j adjusts its own vector by setting VC j [k] max{vc j [k], ts(m[k]} for each k, after which it executes the first step and delivers the message to the application. 19
Mutual Exclusion Concurrent access may corrupt the resource or make it inconsistent Token-based approach for mutual exclusion only 1 token is passed around in the system Process can only access when it has the token Easy to avoid starvation and deadlock However, situation becomes complicated if token is lost Permission-based approach for mutual exclusion A process has to get permission before accessing a resource Grant permission to only one process at any time 20
A Centralized Algorithm Three steps: (a) Process 1 asks the coordinator for resource. Permission is granted (b) Process 2 asks the coordinator for resource. The coordinator does not reply (c) When process 1 releases the resource, it notifies the coordinator. The coordinator then grants permission to process 2 Easy to implement, but has the single point of failure problem 21
A Distributed Algorithm When a process wants to access a shared resource, it builds a messages containing: Name of the resource, Its process number, and the current time Then, sends the message to all other processes, even to itself Three different cases If the receiver (of a message) is not accessing the resource and does not want to access it, it sends back an OK message to the sender. If the receiver already has access to the resource, it simply does not reply. Instead, it queues the request. If the receiver wants to access the resource as well but has not yet done so, it compares the timestamp of the incoming message with the one contained in the message that it has sent everyone. The lowest one wins. When a process receives OK from all other processes, it starts access 22
An Example (a) Two processes want to access a shared resource at the same moment (b) Process 0 has the lowest timestamp, so it wins. (c) When process 0 is done, it sends an OK also, so 2 can now go ahead. 23
Problems in the Distributed Algorithm Any process fails, the algorithm fail worse than the centralized algorithm Each process has to maintain a list of all other processes process addition, leaving, and crashing Every process needs to do the same amount of work as the coordinator in the centralized algorithm Improvements A majority voting, e.g., as long as you get more than half votes, you can access the resource The algorithm is still slow, expensive, and not robust Distributed algorithms are not always the best option 24
A Token Ring Algorithm When a ring is initialized, process 0 is given a token. Token is passed from k to k+1 (modulo the ring size) in a pointto-point message. Ordering is logical, usually based on the process number or other means When a process acquires the token from its neighbor, it checks to see if needs to access the shared resource. If yes, go ahead with the resource, and then release the resource and pass the token when it finishes If not, pass the token immediately to the next one Each process only needs to know who is next inline Problem: if the token is lost, it is hard to detect 25
Election Algorithms The main objective select one process as the coordinator to perform some special roles such as synchronization Examples of election algorithms The bully algorithm the ring algorithm Election algorithms for wireless networks 26
The Bully Algorithm Assume every process knows the process numbers of all other processes But do not know which ones are dead or alive When any process, P, notices the coordinator is no longer responding to requests, it initiates an election. P sends an ELECTION message to all processes with higher numbers. If no one responds, process P wins the election and becomes the coordinator. If one of the processes with higher numbers answers, it takes over. Process P s job is done. 27
An Example (1) The bully election algorithm. (a) Process 4 holds an election. (b) Processes 5 and 6 respond, telling 4 to stop. (c) Now 5 and 6 each hold an election. 28
An Example (2) The bully election algorithm. (d) Process 6 tells 5 to stop. (e) Process 6 wins and tells everyone. 29
The Ring Algorithm No token is used in the algorithm Every process knows the ring structure i.e., how processes are arranged Whenever a process notices that the coordinator is not functioning It builds an ELECTION message, add its process number in it, and pass it to the next alive process one in the ring Whoever receives the election message will add its process number in it and pass it to the next alive process Eventually, the message will circuit back to the original process that initiated the ELECTION message This process will converts it to a COORDINATOR message and notify other processes that the one with the highest number in the list wins 30
An Example Election algorithm using a ring when two nodes initiated the ELECTION at the same time 31
Election in Wireless Networks Bully or Ring algorithms are not realistic in wireless environments since they assume message passing is reliable which is not true in wireless environments Our goal is to select the best node as the coordinator (or leader) not just any node e.g. best capacity based on remaining battery To elect a leader, any node, called source, can initiate an election by sending an ELECTION message to its neighbors A tree is build during the propagation of this election message The best eligible node is passed from leafs to root 32
An Example (1) Election algorithm in a wireless network, with node a as the source. (a) Initial network. (b) (e) The build-tree phase 33
An Example (2) Election algorithm in a wireless network, with node a as the source. (a) Initial network. (b) (e) The build-tree phase 34
An Example (3) Election algorithm in a wireless network, with node a as the source. (a) Initial network. (b) (e) The build-tree phase 35