Transactions

1. Transactions Transactionsare communications with ACID property: • Atomicity: all or nothing • Consistency: interleaving results in serial execution in some order • Isolation: Partial results are not visible outside • Durability: After committing, the results are permanent CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

2. Two-phase Commit Protocol Participant: • Prepare to commit the transaction by writing every update in activity log. • Write a precommit message in the activity log. Wait for request to vote from coordinator. • Wait for commit message from the coordinator. If received, commit the transaction. If abort message is received, abort the transaction. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

3. Two-phase Commit Protocol (Recovery) Coordinator: • If the processor crashes, it will check the activity log for the transaction. • If the precommit message is not in the log, abort the transaction. • If the commit message is not in the log, retake the vote. • If the commit message is there in the log, finish the transaction. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

4. Distributed Mutual Exclusion • Contention based Lamport’s algorithm, Ricart & Agrawala’s algorithm, Voting algorithm • Token based Ring structure, tree structure, broadcast structure CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

5. Lamport's Algorithm Every process maintains a queue of pending requests for entering critical section ordered according to the logical time-stamp. Requestor: • Send a request to every process. • After all replies are collected, enter its own request in its own queue • If own request is at the head of the queue, enter critical section. • Upon exiting the critical section, send a release message to every process. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

6. Lamport's Algorithm Other processes: • After receiving a request, send a reply and enter the request in the queue • After receiving release message, remove the corresponding request from the queue. • If own request is at the head of the queue, enter critical section. Problems: • 3(N-1) messages per requests • Multiple points of failure CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

7. Ricart & Agrawal's Algorithm Requestor: • Send a request to all other process. • Enter critical section upon receipt of reply from all processes. Other Processes: • Upon receipt of a request check whether it has any older (by logical clock) pending request of its own or whether it is executing in critical section. • If neither of the above conditions hold, send reply. Otherwise delay the reply message till both the conditions are false. Problems: • 2(N-1) message exchange per request. • Multiple points of failure. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

8. Voting Scheme Requestor: • Send a request to all other process. • Enter critical section once REPLY from a majority is received • Broadcast RELEASE upon exit from the critical section. Other processes: • REPLY to a request if no REPLY has been sent. Otherwise, hold the request in a queue. • If a REPLY has been sent, do not send another REPLY till the RELEASE is received. Observations: • No single point of failure, but possibility of deadlock. • O(N) messages per request. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

9. Voting Scheme (Improved-1) Deadlock prevention method is used by ensuring no hold-and-wait Requestor: • Send a request with time stamp to all other process. • Enter critical section once REPLY from a majority is received • Return the REPLY message upon receipt of an INQUIRY if not currently executing in the critical section. • Broadcast RELEASE upon exit from the critical section. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

10. Voting Scheme (Improved-1) Other processes: • If no REPLY has been sent, REPLY to any REQUEST. • If a REPLY has been sent but the RELEASE has not been received, compare the time stamp of the new REQUEST with the time stamp of the replied REQUEST. If the new REQUEST has an earlier time stamp, try to retract the old REPLY sending INQUIRY message. If the older REPLY is returned, send REPLY to the new REQUEST. Otherwise, wait for the RELEASE. Observation: • High (O(N)) messages per critical section entry. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

11. Voting Scheme (Improved-2) To reduce message overhead, the set of (N) processes is divided into N sets S1 to Sn such that Si I Sk is non-null for all i and k. Si is called the quorum of process i. Requestor: • Need to secure the REPLY messages from all members of its quorum only. Observations: • It is possible to reduce number of messages significantly. • Depending on the structure of the quorum and the exact algorithm, single point of failure may exist. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

12. Token based Mutual Exclusion Control-token circulates in the system in some fixed order. Possession of the token grants permission to enter critical section. • Ring structure – simple, deadlock-free, fair. The token circulates even in the absence of any request (unnecessary traffic). Long path (O(N)) – the wait for token may be high. • Tree structure • Broadcast structure CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

13. Token circulation in tree The processes are organized in a logical tree structure, each node pointing to its parent. Further, each node maintains a FIFO list of token requesting neighbors. Each node has a variable Tokenholder initialized to false for everybody except for the first token holder (token generator). Entry section: If not Tokenholder If the request queue empty request token from parent; put itself in request queue; block self until Tokenholder is true; CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

14. Token circulation in tree Exit section: If the request queue is not empty parent = dequeue(request queue); send token to parent; set Tokenholder to false; if the request queue is still not empty, request token from parent; CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

15. Token circulation in tree Upon receipt of a request: If Tokenholder If in critical section put the requestor in the queue else parent = requestor; Tokenholder = false; send token to parent; endif else if the queue is empty send a request to the parent; put the requestor in queue; CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

16. Token circulation in tree Upon receipt of a token: Parent = Dequeue(request queue); if self is the parent Tokenholder = true else send token to the parent; if the queue is not empty request token from parent; Note that all requests/token receipts must be processed in FIFO order and the routines must execute atomically. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

17. Token Circulation by Broadcast Suzuki/Kasami Algorithm Data Structure: The token contains Token vector T(.) – number of completion of the critical section for every process. Request queue Q(.) – queue of requesting processes. Every process (i) maintains the following seq_no – how many times i requested critical section. Si(.) – the highest sequence number from every process i heard of. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

18. Suzuki/Kasami Algorithm Entry Section (process i): Broadcast a REQUEST message stamped with seq_no. Enter critical section after receiving token. Exit Section (process i): Update the token vector T by setting T(i) to Si(i). If process k is not in request queue Q and there are pending requests from k (Si(k)>T(k)), append process k to Q. If Q is non-empty, remove the first entry from Q and send the token to the process indicated by the top entry. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

19. Suzuki/Kasami Algorithm Processing a REQUEST (process j): Set Sj(k) to max(Sj(k), seq_no) after receiving a REQUEST from process k. If holds an idle token, send it to k. Requires broadcast. Therefore message overhead is high. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

20. Election Algorithms Used to elect a centralized controller if the older one fails. The election of a new controller helps to alleviate some of the problems arising from having a single point of failure. • Two types of election algorithm • Extrema- finding – based on global priority • Preference-based – based on local preferences. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

21. Election Algorithms Election algorithms and mutual-exclusion algorithms are similar in the sense that they try to find one process that will be leader or enter the critical section. However, there are significant differences between two. • Leader election is one time. A process may yield to another process. In mutual exclusion algorithm, a process competes until it succeeds. • In leader election starvation is not an issue. • Once the leader is elected, all other processes must know who is the leader. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

22. Election Algorithms • Election algorithms are designed on the basis of the logical topology of the group of the processes. • Complete topology • Ring • Tree CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

23. Complete Topology Assumptions: • A process may reach all other processes in one logical hop. • Communication is reliable, only processes may fail. • Time to handle a message is bounded. • Process faults are benign, i.e., a faulty process stops to generate messages. In other words, a process never generates confusing wrong messages. • Upon recovery, a process knows that it experienced a failure. • A failed process may rejoin the group upon recovery. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

24. Complete Topology Our algorithm is an extrema-finding algorithm. It is called the bully algorithm. Upon detecting that the leader has failed, a process sends an inquiry message to all higher-priority processes. If no reply is received within the time-out period, the initiator sends an enter-election message to all lower priority processes. After the initiator receives a reply from all lower-priority processes or after the time-out, it declares itself as new leader and broadcasts that information to all processes. If a process receives an inquiry message from a lower priority process, it send inquiry message to all higher priority process and enters election. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

25. Logical Ring Topology Our algorithm is an extrema-finding algorithm. The processes are organized in a logical ring. The initiator sends a message along ring with its own id. Requesting everybody to elect it the leader. A process compares its own priority with that of the id. on an incoming message. If its own priority is higher, and it never forwarded any other messages, replace the message with its own id. and forward it. If its own priority is lower, forward the message. If the message carries its own id. then elect itself the leader and forward the election message. Message complexity O(N2) in the worst-case, O(N) in the best-case. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan

26. Logical Tree Topology There are two types of election algorithms those use the logical tree topology. • The algorithm runs after the spanning tree is constructed. • The algorithm runs before the spanning tree is constructed and constructs the spanning tree in the process. CSC 8420 Advanced Operating Systems Georgia State University Yi Pan