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Fault Tolerant Distributed Computing system. Fundamentals . What is fault? A fault is a blemish, weakness, or shortcoming of a particular hardware or software component. Fault, error and failures Why fault tolerant ? Availability, reliability, dependability, …

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fundamentals
Fundamentals
  • What is fault?
    • A fault is a blemish, weakness, or shortcoming of a particular hardware or software component.
    • Fault, error and failures
  • Why fault tolerant?
    • Availability, reliability, dependability, …
  • How to provide fault tolerance ?
    • Replication
    • Checkpointing and message logging
    • Hybrid
message logging
Message Logging
  • Tolerate crash failures
  • Each process periodically records its local state and log messages received after
    • Once a crashed process recovers, its state must be consistent with the states of other processes
    • Orphan processes
      • surviving processes whose states are inconsistent with the recovered state of a crashed process
    • Message Logging protocols guarantee that upon recovery no processes are orphan processes
message logging protocols
Message logging protocols
  • Pessimistic Message Logging
      • avoid creation of orphans during execution
      • no process p sends a message m until it knows that all messages delivered before sending m are logged; quick recovery
      • Can block a process for each message it receives - slows down throughput
      • allows processes to communicate only from recoverable states; synchronously log to stable storage any information that may be needed for recovery before allowing process to communicate
message logging1
Message Logging
  • Optimistic Message Logging
      • take appropriate actions during recovery to eliminate all orphans
      • Better performance during failure-free runs
      • allows processes to communicate from non-recoverable states; failures may cause these states to be permanently unrecoverable, forcing rollback of any process that depends on such states
causal message logging
Causal Message Logging
  • Causal Message Logging
      • no orphans when failures happen and do not block processes when failures do not occur.
      • Weaken condition imposed by pessimistic protocols
      • Allow possibility that the state from which a process communicates is unrecoverable because of a failure, but only if it does not affect consistency.
      • Append to all communication information needed to recover state from which communication originates - this is replicated in memory of processes that causally depend on the originating state.
kan a reliable distributed object system
KAN – A Reliable Distributed Object System
  • Developed at UC Santa Barbara
  • Project Goal:
    • Language support for parallelism and distribution
    • Transparent location/migration/replication
    • Optimized method invocation
    • Fault-tolerance
    • Composition and proof reuse
system description
System Description

Kan source

Kan Compiler

Java bytecode + Kan run-time libraries

JVM

JVM

JVM

UNIX sockets

fault tolerance in kan
Fault Tolerance in Kan
  • Log-based forward recovery scheme:
    • Log of recovery information for a node is maintained externally on other nodes.
    • The failed nodes are recovered to their pre-failure states, and the correct nodes keep their states at the time of the failures.
  • Only consider node crash failures.
    • Processor stops taking steps and failures are eventually detected.
basic architecture of the fault tolerance scheme

Logical Node y

Logical Node x

Failure handler

Fault Detector

Request handler

Communication Layer

Basic Architecture of the Fault Tolerance Scheme

Physical Node i

External

Log

IP Address

Network

logical ring
Logical Ring
  • Use logical ring to minimize the need for global synchronization and recovery.
  • The ring is only used for logging (remote method invocations).
  • Two parts:
    • Static part containing the active correct nodes. It has a leader and a sense of direction: upstream and downstream.
    • Dynamic part containing nodes that trying to join the ring
  • A logical node is logged at the next T physical nodes in the ring, where T is the maximum number of nodes failures to tolerate.
logical ring maintenance
Logical Ring Maintenance
  • Each node participating in the protocol maintains a variables:
    • Failedi(j): true if i has detected the failure of j
    • Mapi(x): the physical node on which logical node x resides
    • Leaderi: i’s view of the leader of the ring
    • Viewi: i’s view of the logical ring (membership and order)
    • Pendingi: the set of physical nodes that i suspects of failing
    • Recovery_counti: the number of logical nodes that need to be recovered
    • Readyi: records whether I is active.
      • Initial set of ready nodes; new nodes become ready when they are linked into the ring.
failure handling
Failure Handling
  • When node i is informed of failure of node j:
    • If every node upstream of i has failed, then I must become new leader. It remaps all logical nodes from the upstream physical nodes, and informs the other correct nodes by sending a remap message. It then recovers the logical nodes.
    • If the leader has failed but there is some upstream node k that will become the new leader, then just update the map and leader variables to reflect the new situation
    • If the failed node j is upstream of i, then just update map. If I is the next downstream node from j, also recover the logical nodes from j.
    • If j is downstream of i and there is some node k downstream of j, then just update map.
    • If j is downstream of I and there is no node downstream of j, then wait for the leader to update map.
    • If i is the leader and must recover j, then change map, send a remap message to change the correct nodes’ maps, and recover all logical nodes that are mapped locally
physical node and leader recovery
Physical Node and Leader Recovery
  • When a physical node comes back up:
    • It sends a join message to the leader.
    • The leader tries to link this node in the ring:
      • Acquire <-> Grant
      • Add, Ack_add
      • Release
  • When the leader fails, the next downstream node in the ring becomes the new leader.
slide15
AQuA
  • Adaptive Quality of Service Availability
  • Developed in UIUC and BBN.
  • Goal:
    • Allow distributed applications to request and obtain a desired level of availability.
  • Fault tolerance
    • replication
    • reliable messaging
features of aqua
Features of AQuA
  • Uses the QuO runtime to process and make availability requests.
  • Proteus dependability manager to configure the system in response to faults and availability requests.
  • Ensemble to provide group communication services.
  • Provide CORBA interface to application objects using the AQuA gateway.
proteus functionality
Proteus functionality
  • How to provide fault tolerance for appl.
    • Style of replication (active, passive)
    • voting algorithm to use
    • degree of replication
    • type of faults to tolerate (crash, value or time)
    • location of replicas
  • How to implement chosen ft scheme
    • dynamic configuration modification
    • start/kill replicas, activate/deactivate monitors,voters
group structure
Group structure
  • For reliable mcast and pt-to-pt. Comm
    • Replication groups
    • Connection groups
    • Proteus Communication Service Group for replicated proteus manager
      • replicas and objects that communicate with the manager
      • e.g. notification of view change, new QuO request
      • ensure that all replica managers receive same info
    • Point-to-point groups
      • proteus manager to object factory
fault model detection and handling
Fault Model, detection and Handling
  • Object Fault Model:
    • Object crash failure - occurs when object stops sending out messages; internal state is lost
      • crash failure of an object is due to the crash of at lease one element composing the object
    • Value faults - message arrives in time with wrong content (caused by application or QuO runtime)
      • Detected by voter
    • Time faults
      • Detected by monitor
    • Leaders report fault to Proteus; Proteus will kill objects with fault if necessary, and generate new objects
egida
Egida
  • Developed in UT, Austin
  • An object-oriented, extensible toolkit for low-overhead fault-tolerance
  • Provides a library of objects that can be used to composelog-basedrollback recovery protocols.
      • Specification language to express arbitrary rollback-recovery protocols
log based rollback recovery
Log-based Rollback Recovery
  • Checkpointing
    • independent, coordinated, induced by specific patterns of communication
  • Message Logging
    • Pessimistic, optimistic, causal
core building blocks
Core Building Blocks
  • Almost all the log-based rollback recovery protocols share event-driven structures
  • The common events are:
    • Non-deterministic events
      • Orphans, determinant
    • Dependency-generating events
    • Output-commit events
    • Checkpointing events
    • Failure-detection events
a grammar for specifying rollback recovery protocols
A grammar for specifying rollback-recovery protocols

Protocol := <non-det-event-stmt>* <output-commit-event-stmt>*

<dep-gen-event-stmt> <ckpt-stmt>op t <recovery-stmt>op t

<non-det-event-stmt> := <event> : determinant : <determinant-structure>

<Log <event-info-list><how-to-log>on <stable-storage>>opt

<output-commit-event-stmt> := <output-commit-proto> output commit on < event-list>

<event> := send | receive | read | write

<determinant-structure> := {source, sesn, dest, dest}

<output-commit-proto> := independent | co-ordinated

<how-to-log> := synchronously | asynchronously

<stable-storage> := local disk | volatile memory of self

egida modules
Egida Modules
  • EventHandler
  • Determinant
  • HowToOutputCommit
  • LogEventDeterminant
  • LogEventInfo
  • HowToLog
  • WhereToLog
  • StableStorage
  • VolatileStorage
  • Checkpointing