EEC 688/788 Secure and Dependable Computing

EEC 688/788Secure and Dependable Computing Lecture 13 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University wenbing@ieee.org

Outline Group communication systems Ordered multicast Techniques to implement ordered multicast Group membership service Agreed and safe delivery Checkpointing and recovery Reference: Reliable distributed systems, by K. P. Birman, Springer; Chapter 14-16 EEC688/788: Secure & Dependable Computing

Group Communication System Services provided by the GCS Membership service: who is up and who is down Deals with failure detection and more Reliable, ordered, multicast service FIFO, causal, total Virtual synchrony service Virtual synchrony synchronizes membership change with multicasts GCS is often used to build fault tolerant systems EEC688/788: Secure & Dependable Computing

Reliable Multicast Reliable multicast – the message is targeted to multiple receivers, and all receivers receive the message reliably Positive or negative acknowledgement Need to avoid ack/nack implosion Distinguish receiving from delivery! Application Delivering Middleware Receiving EEC688/788: Secure & Dependable Computing

Ordered Reliable Multicast Ordered reliable multicast – if many messages are multicast by many senders, in what order the messages are delivered at the receivers? First in first out (FIFO) Causal – the causal relationship among msgs preserved Total – all msgs are delivered at all receivers in the same order EEC688/788: Secure & Dependable Computing

FIFO Ordered Multicast FIFOor sender orderedmulticast: Messages are delivered in the order they were sent (by any single sender) a e p q r s b c d delivery of c to p is delayed until after b is delivered EEC688/788: Secure & Dependable Computing

Causally Ordered Multicast Causalor happens-beforeordering: If send(a)  send(b) then deliver(a) occurs before deliver(b) at common destinations a p q r s b EEC688/788: Secure & Dependable Computing

Causally Ordered Multicast Causalor happens-beforeordering: If send(a)  send(b) then deliver(a) occurs before deliver(b) at common destinations a p q r s b c delivery of c to p is delayed until after b is delivered EEC688/788: Secure & Dependable Computing

Causally Ordered Multicast Causalor happens-beforeordering: If send(a)  send(b) then deliver(a) occurs before deliver(b) at common destinations a e p q r s b c delivery of c to p is delayed until after b is delivered e is sent (causally) after b EEC688/788: Secure & Dependable Computing

Causally Ordered Multicast Causalor happens-beforeordering: If send(a)  send(b) then deliver(a) occurs before deliver(b) at common destinations a e p q r s b c d delivery of c to p is delayed until after b is delivered delivery of e to r is delayed until after b&c are delivered EEC688/788: Secure & Dependable Computing

Totally Ordered Multicast Totalordering: Messages are delivered in same order to all recipients (including the sender) a e p q r s b c d all deliver a, b, c, d, then e EEC688/788: Secure & Dependable Computing

Implementing Total Ordering Use a token that moves around Token has a sequence number When you hold the token you can send the next burst of multicasts Use a sequencer to order all multicast Message is first multicast to all, including the sequencer; then the sequencer determines the order for the message and informs all Or send to the sequencer and the sequencer multicast with total order information Each sender can take turn to serve as the sequencer EEC688/788: Secure & Dependable Computing

Group membership service Input: Process “join” events Process “leave” events Apparent failures Output: Membership views for group(s) to which those processes belong EEC688/788: Secure & Dependable Computing

Issues? The service itself needs to be fault-tolerant Otherwise our entire system could be crippled by a single failure! Hence Group Membership Service (GMS) must run some form of protocol (GMP) EEC688/788: Secure & Dependable Computing

Approach Assume that GMS has members {p,q,r} at time t Designate the “oldest” of these as the protocol “leader” To initiate a change in GMS membership, leader will run the GMP Others can’t run the GMP; they report events to the leader EEC688/788: Secure & Dependable Computing

GMP Example Example: Initially, GMS consists of {p,q,r} Then q is believed to have crashed p q r EEC688/788: Secure & Dependable Computing

Unreliable Failure Detection Recall that failures are hard to distinguish from network delay So we accept risk of mistake If p is running a protocol to exclude q because “q has failed”, all processes that hear from p will cut channels to q Avoids “messages from the dead” q must rejoin to participate in GMS again EEC688/788: Secure & Dependable Computing

Basic GMP Someone reports that “q has failed” Leader (process p) runs a 2-phase commit protocol Announces a “proposed new GMS view” Excludes q, or might add some members who are joining, or could do both at once Waits until a majority of members of current view have voted “ok” Then commits the change EEC688/788: Secure & Dependable Computing

GMP Example Proposes new view: {p,r} [-q] Needs majority consent: p itself, plus one more (“current” view had 3 members) Can add members at the same time Proposed V1 = {p,r} Commit V1 p q r OK V0 = {p,q,r} V1 = {p,r} EEC688/788: Secure & Dependable Computing

Special Concerns? What if someone doesn’t respond? P can tolerate failures of a minority of members of the current view New first-round “overlaps” its commit: “Commit that q has left. Propose add s and drop r” P must wait if it can’t contact a majority Avoids risk of partitioning EEC688/788: Secure & Dependable Computing

What If Leader Fails? Here we do a 3-phase protocol New leader identifies itself based on age ranking (oldest surviving process) It runs an inquiry phase “The adored leader has died. Did he say anything to you before passing away?” Note that this causes participants to cut connections to the adored previous leader Then run normal 2-phase protocol but “terminate” any interrupted view changes leader had initiated EEC688/788: Secure & Dependable Computing

GMP Example New leader first sends an inquiry Then proposes new view: {r,s} [-p] Needs majority consent: q itself, plus one more (“current” view had 3 members) Again, can add members at the same time p Inquire [-p] Proposed V1 = {r,s} Commit V1 q r OK: nothing was pending OK V0 = {p,q,r} V1 = {r,s} EEC688/788: Secure & Dependable Computing

Safe and Agreed Delivery For totally ordered reliable multicast, there are two delivery policies Safe delivery: a message is delivered only when all correct processes have received it Agreed delivery: a message is delivered as long as it is the next message in total order EEC688/788: Secure & Dependable Computing

Safe and Agreed Delivery Safe delivery guarantees the uniformity of multicast: If a message is delivered to any process, it is delivered by all correct processes Agreed delivery does not: It is possible that a message is delivered in one (or more) process, but is not delivered by some correct process EEC688/788: Secure & Dependable Computing

Checkpointing and Recovery • Faults occur over time. How to ensure a fault tolerant system remain operational for extensive period of time? • Recover failed replicas, or replace failed replicas with new one => Recovery is needed • How to recover a failed replica or install a new replica? • Checkpointing a correct replica and transfer the state to the recovering replica EEC688/788: Secure & Dependable Computing

Checkpointing Checkpointing: the act of taking a snapshot of an entity so that we can restore it later A replica is a process running in an operating system. The state of a process Processes' memory, stack and registers Threads Open or mmap'ed files Current working directory Interprocess communication: Semaphores, shared memory, pipes, sockets Dynamic Load Libraries … EEC688/788: Secure & Dependable Computing

Checkpointing Many tools are available to perform checkpointing transparently or semi-transparently http://www.checkpointing.org/ Condor, libckpt, etc. Checkpoints taken in general are not portable Checkpoint size might be big EEC688/788: Secure & Dependable Computing

Checkpointing of Application State Sometimes it is more efficient to save and store the application state only Checkpoints can be very portable and compact in size class Counter { int counter; Counter(int initVal) { counter = initVal; } void increment() {counter++; } void decrement() {counter--; } void setState(int c) {counter = c; } int getState() { return counter;}|} EEC688/788: Secure & Dependable Computing

Logging Logging of messages Checkpointing in general is expensive Logging of messages is cheaper => we can periodically do checkpointing, or do checkpointing on demand and log all messages in between Logging of other non-deterministic activities Access order to shared data EEC688/788: Secure & Dependable Computing

Roll-Forward Recovery With replication in space, it is possible to recover a fault while the system is progressing ahead Roll-forward recovery is made possible by Checkpointing of replica state Logging of incoming messages Reliable, totally ordered group communication system EEC688/788: Secure & Dependable Computing

Roll-Forward Recovery We want to ensure the newly admitted replica to have a consistent state with others when it starts Steps of adding a new replica into a group (with on-demand checkpointing) A recovered (or a new) replica joins a group A join message is multicast in total order On receiving the join message, it is put into incoming message queue and wait for processing When the join message is at the head of the queue, a checkpoint is taken and it is transferred to the new replica EEC688/788: Secure & Dependable Computing

Roll-Forward Recovery At the new replica, it starts queueing messages after it receives the join messages (sent by itself) When the checkpoint is received by the new replica, its state is restored using the received checkpoint (the checkpoint is delivered out of order!) The queued messages are delivered in order, at the new replica Other replicas do not stop and wait for the new replica Steps of adding a new replica into a group with periodic checkpointing is similar EEC688/788: Secure & Dependable Computing

Steps of Roll-Forward Recovery EEC688/788: Secure & Dependable Computing

Roll-backward Recovery • Roll-backward recovery is used for systems relying on replication in time for fault tolerance • When a failure occurs, roll back using the most recent checkpoint (and retry) EEC688/788: Secure & Dependable Computing

Roll-backward Recovery in a Distributed System • Performing roll-backward recovery in a distributed system is non-trivial • Need to solve the distributed snapshot problem • It is easy to perform a local checkpoint of a process, but in a distributed system, when one process rolls back, other processes must also roll back to a consistent state EEC688/788: Secure & Dependable Computing

EEC 688/788 Secure and Dependable Computing