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13.10.2 The checkpointing protocol use none none implement todevelop none on noneqr code library for Next (see A none for none lgorithm 13.4) we describe the Helary Mostefaoui Netzer Raynal communication-induced checkpointing protocol, which takes as few forced checkpoints as possible and also ensures that no local checkpoint is useless..

PLANET Checkpointing and rollback recovery Procedure t none for none ake-checkpoint: k do sent_toi k := false end do; k do min_toi k := + end do; k = i do takeni k := true end do; clocki i = clocki i + 1; Save the current local state with a copy of clocki i ; /* let Ci x denote this checkpoint. We have Ci x t = clocki i */ ckpti i = ckpti i + 1; (S0) initialization: k do clocki k = 0; ckpti k = 0 end do; takeni i := false; take_checkpoint; (S1) When Pi sends a message to Pk : if sent_toi [k] then sent_toi [k] := true; min_toi [k]:= clocki [i] end if; Send (m, clocki , ckpti , takeni ) to Pk ; (S2) When Pi receives (m, clocki i, ckpti , takeni ) from Pj : /* m clock j is the Lamport s timestamp of m (i.e.

, m t) */ if ( k : sent_toi k m clock j > min_toi k m clock j > max clocki k m clock k m ckpt i = ckpti i m taken i then take_checkpoint /*forced checkpoint */ end if; clocki i = max clocki i m clock j ; /* update of the scalar clock lci clocki i */ k = i do clocki k = max clocki k m clock k ; case m ckpt k < ckpti k skip m ckpt k > ckpti k ckpti k = m ckpt k ; takeni k = m taken k m ckpt k < ckpti k takeni k = takeni k m taken k end case end do deliver (m);. Algorithm 13.4 protocol [16]. The Helary Mostefaoui Netzer Raynal communication-induced checkpointing The protoco l is executed by each process Pi . S0, S1, and S2 describe the initialization, the statements executed by Pi when it sends a message, and statements it executes when it receives a message, respectively. The.

13.11 summary procedure t ake-checkpoint is called each time Pi takes a checkpoint (basic or forced). The protocol uses the following additional data structure: every process Pi maintains an array clocki 1 n , where clocki j denotes the highest value of lcj known to Pi . clocki 1 n is initialized to (0, 0, ,0) and is updated as follows: When a process Pi takes a (basic or forced) checkpoint, it increases clocki i by 1.

When Pi sends a message m, the current value of clocki is sent on the message. Let m clock be the timestamp associated with a message m. When a process Pi receives a message m from Pj , it updates its clock as follows: clocki i = max clocki i m clock j k = i clocki k = max clocki k m clock k Note that clocki i is lci , so we do not need to keep lci .

Helary et al. [15] showed that given a local checkpoint Ci x with timestamp a, the checkpoint can be associated with the consistent global checkpoint it belongs to using the following result: Theorem 13.5 Let a be a Lamport timestamp and Ca be a global checkpoint, {C1 x1 , C2 x2 , Cn xn ,}.

If k, Ck xk is the last checkpoint of Pk such that Ck xk .t a, then Ca is a consistent global checkpoint. Proof For a proof, the readers are referred to the original source [15, 16].

This result implies that given a local checkpoint at a process, it is easy to determine what local checkpoints at other processes form a consistent global checkpoint with it. This result has a strong implications on the recovery from a failure..

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