Time and Global States Part 3

Time and Global StatesPart 3 ECEN5053 Software Engineering of Distributed Systems University of Colorado, Boulder

Topics • Clock synchronization • Logical clocks • Vector timestamps • Global State ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Vector Timestamps • With Lamport logical clocks, nothing can be said about the relationship between a and b simply by comparing their timestamps C(a) and C(b). • Just because C(a) < C(b), doesn’t mean a happened before b (remember concurrent events) • Consider network newswhere processes post articles and react to posted articles • Postings are multicast to all members • Want reactions delivered after associated postings ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Will totally-ordered multicasting work? • That scheme does not mean that if msg B is delivered after msg A, B is a reaction to msg A. They may be completely independent. • What’s missing? • If causal relationships are maintained within a group of processes, then receipt of a reaction to an article should always follow the receipt of the article. • If two items are independent, their order of delivery should not matter at all ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Vector Timestamps capture causality • VT(a) < VT(b) means event a causally precedes event b. • Let each process Pi maintain vector Vi such that • Vi[i] is the number of events that have occurred so far at Pi • If Vi[j] = x then Pi knows that x events have occurred at Pj • We increment Vi[i] at the occurrence of each new event that happens at process Pi • Piggyback vectors with msgs that are sent. When Pi sends msg m, it sends its current vector along as a timestamp vt. ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

For example • P1 V1 = [3,1,2] • P2 V2 = [2,4,1] • P3 V3 = [0,2,5] • With a new event at P2, V2[2] = V2[2] + 1 • If P3 receives a msg a from P1, V3[1] = vt(a)[1] + 1 where vt(a) = V1 ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Receiver thus learns the number of events that have occurred at Pi • Receiver is also told how many events at other processes have taken place before Pi sent message m. • timestamp vt of m tells the receiver how many events in other processes have preceded m and on which m may causally depend • When Pj receives m, it adjusts its own vector by setting each entry Vj[k] to max{Vj[k], vt[k]} • The vector now reflects the # of msgs that Pj must receive to have at least seen the same msgs that preceded the sending of m. • Vj[i] is incremented by 1 representing the event of receiving msg m as the next message from Pi ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

When are messages delivered? • Vector timestamps are used to deliver msgs when no causality constraints are violated. • When process Pi posts an article, it multicasts that article as a msg a with timestamp vt(a) set equal to Vi. • When another process Pj receives a, it will have adjusted its own vector such that Vj[i] > vt(a)[i] • Now suppose Pj posts a reaction by multicasting msg r with timestamp vt(r) equal to Vj. vt(r)[i] > vt(a)[i]. • Both msg a and msg r will arrive at Pk in some order ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

When receiving r, Pk inspects timestamp vt(r) and decides to postpone delivery until all msgs that causally precede r have been received as well. • In particular, r is delivered only if the following conditions are met • vt(r)[j] = Vk[j] + 1 • vt(r)[i] <= Vk [i] for all i not equal to j • says r is the next msg Pk was expecting from Pj • says Pk has seen no msg not seen by Pj when Pj sent r. In particular, Pk has already seen message a. ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Controversy • There has been some debate about • whether support for totally-ordered and causally-ordered multicasting should be provided as part of the message-communication layer or • whether applications should handle ordering • Comm layer doesn’t know what it contains, only potential causality • 2 msgs from same sender will always be marked as causally related even if they are not • Application developer may not want to think about it ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Global State

Global state of a distributed system • Local state of each process and • The messages that are currently in transit (sent but not received) ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Who cares, globally speaking? • When it is known that local computations have stopped and that there are no more messages in transit, the system has obviously entered a state in which no more progress will be made. • deadlocked? • correctly terminated? ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

How to record the global state • Distributed snapshot • reflects a state in which the distributed system might have been • reflects a consistent global state • If we have recorded that process P has received a msg from another process Q, then we should also have recorded that process Q had actually sent the msg • The reverse condition (Q has sent a msg that P has not yet received) is allowed. ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Cut! • A cut represents the last event that has been recorded for each of several processes. • All recorded msg receipts have a corresponding recorded send event • An inconsistent cut would have a receipt of a msg but no corresponding send event ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

The algorithm (Chandy & Lamport) • Assume the distributed system can be represented as a collection of processes connected to each other through uni-directional point-to-point communication channels. • Any process may initiate the algorithm. • P records its own local state • It sends a marker along each of its outgoing channels, indicating that the receiver should participate in recording the global state • ... ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Chandy & Lamport algorithm, cont • When process Q receives the marker through an incoming channel C, its action depends on whether or not it has already saved its local state • If it has not • it first records its local state and also sends a marker along its own outgoing channels • If it has • the marker on channel C is an indicator that Q should record the state of thechannel, namely, the sequence of messages received by Q since the last time it recorded its own local state and before it received the marker. ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Chandy & Lamport algorithm, cont • A process has finished its part of the algorithm when it has received a marker along each of its incoming channels and processed each one. • Its recorded local state as well as the state it recorded for each incoming channel, can be collected and sent to the process that initiated the snapshot • The initiator can subsequently analyze the current state • Meanwhile, the distributed system as a whole can continue to run normally ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Photo album • Because any process can initiate the algorithm, the construction of several snapshots may be in progress at the same time • A marker is tagged with the identifier and possibly also a version number of the process that initiated the snapshot • Only after a process has received that marker through each of its incoming channels, can it finish its part in the construction of that marker’s associated snapshot ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Application of a snapshot

Termination Detection • If a process Q receives the marker requesting a snapshot for the first time, • considers the process that sent that marker as its predecessor • When Q completes its part of the snapshot, it sends its predecessor a DONE msg. • By recursion, when the initiator of the distributed snapshot has received a DONE msg from all of its successors, it knows the snapshot has been completely taken ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

What if msgs still in transit? • A snapshot may show a global state in which msgs are still in transit • Suppose a process records that it had rec’d msgs along one of its incoming channels • between the point where it had recorded its local state • and the point where it received the marker through that channel • Cannot conclude the distributed computation is completed • Termination requires a snapshot in which all channels are empty ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Modified algorithm • When a process Q finishes its part of a snapshot, it either returns DONE or CONTINUE to its predecessor • A DONE msg is returned only when • All of Q’s successors have returned a DONE msg • Q has not received any msg between the point it recorded its own local state and the point it had received the marker along each of its incoming channels • In all other cases, Q sends a CONTINUE msg to its predecessor ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Modified algorithm, continued • The original initiator of the snapshot will either receive at least one CONTINUE or only DONE msgs from its successors • When only DONE messages are received, it is known that no regular msgs are in transit • Conclusion? The computation has terminated. • If a CONTINUE appears, P initiates another snapshot and continues to do so until only DONE msgs are returned. (There are lots of other algorithms, too.) ECEN5053 SW Eng of Distributed Systems, Time and Global States, Univ of Colorado

Time and Global States Part 3