mndag 13 maj 13 OVERVIEW Fail-recovery Precedence (1,N) Logged - - PowerPoint PPT Presentation

FAIL-RECOVERY & BYZANTINE REGISTERS

måndag 13 maj 13

OVERVIEW

Fail-recovery Precedence (1,N) Logged register Byzantine (1,N) Safe (1,N) Regular x2 (1,N) Atomic

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FAIL-RECOVERY

Process can fail before completion.

• precedes o’ if
• 1. o completes before the invocation of o’; OR

2.o and o’ are invoked by the same process and o’ is invoked after o.

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FAIL-RECOVERY

Correct process = fails & recovers and then “never” fails Page 171

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FAIL-RECOVERY

If an operation does not complete, it appears as if it never happened. After a crash, we can restart the operation.

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Stable storage! Modify “Majority Voting” -> regular logged register.

FAIL-RECOVERY - LOGGING

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Pages 174 -175

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Stubborn links -> termination. reading & writing no avoid duplication events. Assumption: majority correct. “Read-Impose Write-Majority” -> atomic “Read-Impose Consult-Majority” -> (N,N) atomic.

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BYZANTINE SAFE REGISTER

No concurrency! One correct writer! Page 176

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Page 178 byzhighestval removes all values that occur less than f times. No such values, use default value. Hence only safe.

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BYZANTINE SAFE REGISTER

Only works for N > 4f Byzantine masking qourum: More than (N+2f)/2 After write: more than (N+2f)/2 - f correct & informed processes. Less than N - (N+2f)/2 uninformed processes. In read: (N+2f)/2 + 1 (received values) - (N - (N+2f)/2 -1) (uninformed) - f (faulty) > f values from informed processes.

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BYZANTINE REGULAR REGISTER

Same properties as before: regular means either current

• r previous value.

One writer w. Digital signatures! N < 3f

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Page 181 faulty may not return latest value

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RECAP: BYZANTINE QUORUM

N- f > (N + f)/2 <=> N> 3f. More than (N + f)/2 - f = (N -f)/2 correct processes. (N -f)/2 x 2 = N-f. Two quorums have more than N-f correct processes. At least one is in both!

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NO SIGNATURES!

“Read-Impose Write-Majority” + digital signatures => Byzantine atomic (1, N) register. But: we will do it the hard way! No signatures! Regular Atomic

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BYZANTINE REGULAR REGISTER - AGAIN

Only problem with concurrency! Each process store both old and new value. Write twice! It’s only safe to erase the old value when majority has the new. If either the old or new value occurs more than f times -> authentic! Take the value (old or new) with largest timestamp (selectedmax).

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Pages 184-185 Prewrite with a larger timestamp -> update! N-f > (N + f)/2

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Trigger new reads if too few good answers. Wait. Many writes -> no quorum. Read concurrent with infinity many writes, no

• termination. Finite-write termination.

Either updated timestamp or same value If more than f occurrences of old

• r new value (authentic) and a

quorum, then selectmax ts.

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BYZANTINE ATOMIC REGISTER - AGAIN

(1,N) Could use “Read-Impose Write-Majority” + signatures ... Instead ... introduce listeners. Atomic -> ordering.

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Pages 190 - 191

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Have to get the same value from other processes. When rewritten, forward value to concurrent read. Have to clean up answers[ts] somehow.

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Pages 191 -192

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HAPPY ENDING?

Multiple writers? Yes, according to the book. Extend Listener-algorithm. How?

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