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Revisiting failure detectors

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Title: Concurrent Reading and Writing using Mobile Agents Author: Sukumar Ghosh Last modified by: Sukumar Ghosh Created Date: 11/1/2002 2:53:35 AM – PowerPoint PPT presentation

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Title: Revisiting failure detectors


1
Revisiting failure detectors
  • Some of you asked questions about implementing
  • consensus using S - how does it differ from
  • reaching consensus using P. Here it is.
  • Recall the definition of S (strong) FD
  • Strong completeness weak accuracy

2
Consensus using S
  • Program for process p
  • Vp (?,?, .. ?) Vpp input of p Dp Vp
  • (Phase 1) Same as phase 1 of consensus with P
  • (Phase 2)
  • send (Vp, p) to all
  • receive (Dq, q) from all q, or q is a
    suspect
  • k 1
  • do k ? n ?
  • if ?Vqk Vpp ? ? ? Vqk ? ? Vpk
    Dpk ? fi
  • od
  • (Phase 3)
  • Decide on the first element Vp j Vp j ? ?

3
Example
0 1 2 3 4
0 1 2 3 4
1, 4
Never suspected
? - ? ? -
? - - ? -
0
? ? - ? -
? - - ? -
2, 4
1
? - - ? -
? ? ? ? -
4
2
? - - ? -
2, 4
? ? - ? -
3
crashed
4
V after Phase 2
V after Phase 1
List of suspects
4
Atomic Commit Protocols
  • Network of servers
  • The initiator of a transaction is called the
    coordinator,
  • and the remianing servers are participants

S1
Servers may crash
S3
S2
5
Requirements of Atomic Commit Protocols
S1
  • Network of servers
  • Termination. All non-faulty servers must
    eventually reach an irrevocable decision.
  • Agreement. If any server decides to commit, then
    every server must have voted to commit.
  • Validity. If all servers vote commit and there is
    no failure, then all servers must commit.

Servers may crash
S3
S2
6
One-phase Commit
server
participant
Commit / abort
server
server
client
participant
coordinator
server
participant
If a participant deadlocks or faces a problem
then the coordinator may never be able to find
it. Too simplistic.
7
Two-phase commit (2PC)
  • Phase 1 The coordinator sends VOTE to the
    participants. and receive yes / no from them.
  • Phase 2
  • if ?server j vote(j) yes ? multicast COMMIT to
    all severs
  • ? ? server j vote (j) no ? multicast ABORT
    to all servers
  • fi
  • What if failures occur?

8
Failure scenarios in 2PC
  • (Phase 1)
  • Fault Coordinator did not receive YES / NO
  • OR
  • Participant did not receive VOTE
  • Solution Broadcast ABORT
  • Abort local transactions

9
Failure scenarios in 2PC
  • (Phase 2)
  • (Fault) A participant does not receive a COMMIT
    or ABORT message from the coordinator
  • (it may be the case that the coordinator crashed
    after sending ABORT or COMIT to a fraction of the
    servers), then it remains undecided, until the
    coordinator is repaired and reinstalled into the
    system.
  • This blocking is a known weakness of 2PC.

10
Coping with blocking in 2PC
  • A non-faulty participant can ask other
    participants about
  • what message (COMMIT or ABORT) did they receive
    from
  • the coordinator, and take appropriate actions.
  • But what if no non-faulty participant received
    anything?
  • Who knows if the coordinator committed or aborted
    the
  • local transaction before crashing? Continue to
    wait

11
Non-blocking Atomic Commit
  • A blocking protocol has the potential to prevent
    non-faulty participants from reaching a final
    decision.
  • A solution to the atomic commitment problem is
    called non-blocking, if in spite of server
    crashes, every non-faulty participant eventually
    decides.
  • One solution is to impose the requirement of
    uniform agreement

12
Uniform agreement
  • If any participant (faulty or not) delivers a
    message m
  • (commit or abort) then all correct processes
    eventually
  • deliver m.
  • To implement uniform agreement, no server should
    deliver a COMMIT or ABORT message until it has
    relayed it to all other servers.
  • If a process times out in phase 2, then it
    decides abort.

13
Recovery Stable storage
Creates the illusion of an incorruptible storage,
even if a writer or a disk crashes at any time.
The implementation Uses at least two independent
disks.
A0
A1
inspect
update
14
Stable storage
  • To write, do the following
  • copy on disk A0
  • record timestamp T0
  • compute checksum S0
  • copy on disk A1
  • record timestamp T1
  • compute checksum S1
  • Readers check four cases
  • Both checksums OK and T1gtT0
  • Both checksums OK and T1ltT0
  • Checksum on A1 wrong
  • Checksum on A2 wrong
  • (Which copy to accept in each case?)

A0
update
inspect
A1
15
Checkpointing
  • Mechanism for (backward) error recovery.
    Transaction states are periodically stored on
    stable storages. Following a failure, the
    transaction rolls back to the nearest checkpoint.
  • Independent (unsynchronized) or coordinated
    (synchronized) checkpointing

16
Classification of checkpointing
Coordinated Checkpointing takes a consistent
snapshot. Has some overhead. Uncoordinated
checkpointing apparently has no overhead. But it
may have some efficiency problems.
17
Checkpointing (continued)
  • Some actions can be reversed, but some cannot be
    reversed (like dispensing cash from an ATM
    machine, printing a document etc).
  • Such actions are logged, and during replay, logs
    substitute real actions.

18
Group Communication
  • Group oriented activities are steadily
    increasing.
  • There are many types of groups
  • ? Open and Closed groups
  • ? Peer-to-peer and hierarchical groups

19
Major issues
  • Atomic multicast
  • Ordered multicast
  • Dynamic groups
  • Failure handling

20
Atomic multicast
  • A multicast is called atomic, when the message is
    delivered to every correct (i.e. functioning)
    member, or to no member at all.
  • Sometimes, certain features available in the
    infrastructure of a distributed system simplify
    the implementation of multicast. Examples are (1)
    multicast on an ethernet LAN (2) IP multicast

21
Basic vs. reliable multicast
  • Basic multicast does not consider crash failures.
  • Reliable multicast does.
  • Three criteria for basic multicast
  • Liveness. Each process must receive every
    message
  • Integrity. No spurious message received
  • No duplicate. Accepts exactly one copy of a
    message

22
Reliable atomic multicast
  • Senders program Receivers program
  • i0 if m is new ?
  • do i ? n ? accept it
  • send message to i multicast m
  • i i1 ? m is duplicate ? discard m
  • od fi

Tolerates process crashes.
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