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LH* RS P2P : A Scalable Distributed Data Structure for P2P Environment

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### LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

W. LITWIN

T. SCHWARZ

H.YAKOUBEN

Paris Dauphine University

Santa Clara University (USA)

Paris Dauphine University

Plan

- Objective
- Overview: SDDS & P2P
- LH*RSP2P
- Architecture
- Addressing
- Properties
- Churn Management
- Conclusion

2

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Objective

Very Large Scalable Files

High availabilityto deal with churn

At mostone forwarding message for key search or insert or scan

(fastest known performance)

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- A File of records identified by keys
- SDDS client nodes face the applications and send queries to SDDS server nodes
- No centralized addressing
- Servers contain application or parity data
- In buckets
- Overflowing servers split on new servers
- Servers do not notify clients about splits

4

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- Clients use images of the file state for addressing
- Key based
- Range queries
- Scans
- …
- Images get adjusted towards the file state during queries by Image Adjustment Messages
- Triggered by incorrect addressing by the client
- IAMs reflect the file evolution by splits or, rarely, merges.
- IAMs reflect also the location changes because of failures and recovery

5

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

SDDS Typology

VBI-Tree

CHORD...

BATON

Structured P2P Schemes

LH*, DDH, EH*,

LH*sa

LH*s

Alg. Sign…

IH*…

LH*m LH*g

Data Structures

SDDS(1993)

Classics

Tree

Hash

m-d Tree

1-dimensional

1-d Tree

d-dimensional

k-RP*, SD-Rtree, DRT*,

RP*,

High Availability

k-Availability

Security

LH*rs

6

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Growth through splits under inserts

New Peer

SDDS Expansion

Peer

Clients

7

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

SDDS Client Image Evolution

Clients

8

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- Available at CERIA site
- Announced at DbWorld
- Managing LH* RS and RP* files
- LH* implementaton based on LH*LH n scheme
- J. Karlsson’s Thesis & EDBT paper
- In distributed RAM
- Under Windows
- Over 1 gbs Ethernet
- Various functions
- Response time reaching 30 microsec
- Up to 300 times faster than disk files

9

- Autonomous nodes store and search data
- By flooding in early systems
- Freenet, Napster, Gnutella…
- Structured P2P reduce the flooding
- Using decentralized data structures
- Distributed Hash Table (DHT) especially
- Few folks know the concept is due to B. Devine
- FODO 93
- Chord, P-tree, VBI, Baton…
- Structured P2P schemes are specific SDDS schemes

10

Server Part

LH*RS

Client

LH*RS

DB

j

Candidate Peer

Client &

Spare Storage

Candidate Peer

i’

n’

Client Part

LH*P2P Peer

LH*RSP2P- Architecture based on LH*RS
- ACM –TODS, Oct. 05)

LH*RSP2P Peer

LH*RS

Client

LH*RS

PB

Pupils

Pupil’s IP = Its key C

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- Global Addressing Rule
- a hi(C ) ; /* a is the address of peer destination of the key C*/
- if a < n then a hi+1(C ) ;/* (i, n) state of an SDDS file, they are only known to the file coordinator node

hi (C ) = C mod 2i

- Client Address Calculus
- a’ hi’(C ) ; /* a’ is the address of peer destination of the key C*/
- if a’ < n’ then a hi’+1(C ) ;

12

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- File starts with i = 0 and n = 0 and a single data bucket 0
- Every bucketkeeps the bucket levelj of hash function hi last used to split, j = 0 initially.
- Overflowing bucket m alerts the coordinator
- Coordinator notifies bucket n to split
- Usually n ≠ m
- It sends the address of the pupil ready for the new bucket n + 2i
- It also sends the addresses of the buckets that have been created since bucket n lastsplit

13

- Bucket n applies hi + 1
- About half of keys migrates to new bucket n + 2i
- About half of pupils migrate as well
- Bucket n andthenew one set j = j + 1
- Coordinator performs
- n = n + 1 if n = 2i then i = i + 1 and n = 0
- Resulting address space growth, starting from i = 0 and n = 0:
- 0 1
- 0 2, 1 3 ;
- 0 4, 1 5, 2 6, 3 7
- 0 8, 1 9, 2 10, 3 11, 412, 513…7 15

14

Peer & Pupil Image Adjustment After Peer Split

i’ = j - 1 ; /* j value after the split

n‘ = a + 1 /* ais the splitting bucket ; n = a + 1

if n’ = 2i’ then i’ = j + 1 ; n’ = 0 ;

- The adjustment concerns
- Splitting peer a
- New peer a’ = a + 2i
- Every pupil of a and of a’

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

j=2

j=2

j=2

j=2

j=2

j=2

i’=1

n’=0

i’=1

n’=1

i’=2

n’=0

i’=1

n’=1

i’=2

n’=0

i’=1

n’=1

i’=1

n’=1

P2

P0

P1

P0

P1

P2

P3

i=1

n=1

i=2

n=0

CP

Coordinator Peer (CP)

Before splitting

After splitting

Example

i’= j =1;

n’= m+1= 1+1;

If n’=21thenn’=0; i’= i’+1

and

(i’, n’)= (2,0)

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

a’ hj (C ) ;

if a’= a then exit /* Bucket a is the correct one

else send C to bucket a/* Forwarding to bucket a’

exit;

- Simpler and faster than for LH*
- As only one forwarding is possible

17

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- IAM comes from the correct bucket
- Bucket a is the forwardingone
- Bucket level j is that of the correct bucket
- 0f the forwarding one as well

i’ j - 1, n’ a + 1 ;

if n’ >2i’then n’ 0 ; i’ i’ + 1 ;

- Same algorithm as for the adjustment of the local client and of pupils after a split

18

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

j=4

i’=3

n’=2

i’=3

n’=2

P9

P1

9

Peer Image Adjustment by IAM

Checking and forward the key using A2

9

IAM

a = 1

j = 4

Pairs

j=4

j=3

i’=3

n’=1

i’=2

n’=1

i =3

n=2

P0

P4

PC

19

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

j=4

i’=3

n’=2

i’=3

n’=2

P1

P9

9

Peer Image Adjustment by IAM

9

IAM

a = 1

j = 4

Pairs

j=4

j=3

i’=3

n’=1

i’= 3

n’= 2

i =3

n=2

P0

P4

PC

20

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

j=3

j=2

j=3

j=3

j=3

i’=2

n’=3

i’=2

n’=1

i’=1

n’=1

i’=2

n’=2

i’=2

n’=3

P0

P2

P5

P6

i’=2

n’=1

i=2

n=2

i’=0

n’=0

PC

Candidate Peer

Pupil

LH*RSP2P

TUTOR, Update Pupil

- Example of the File Expansion

Assign a Tutor for Candidate Peer:

LH-hash of its IP Address

i=2

n=3

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- The maximal number of forwarding messages for the key search is one.
- The maximal number of rounds for the scan search can be two.
- The worst case addressing performance of LH*RSP2P as defined by Property 1 is the fastest possible for any SDDS or a practical structured P2P addressing scheme.

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- Case 1 : i’ = i andn’ < n
- Peer a addresses peer a’, using its image (i’,n’) from last split
- No IAM came since.

j = i’+1

j = i’

a

2i’

0

n

a’

n+2i’

a+2i’

No forwarding

23

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

j = i’

a

2i’

0

n

n+2i’

a’

a+2i’

Proof Property 1

- Case 1 : i’ = i and n’ < n
- Peer a addresses peer a’, using its image (i’,n’) from last split
- No IAM came since.

Forwarding possible for any address a’ between (a, n)

24

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

j = i’+1

j = i’

n

a

2i’

0

n+2i’+1

2i’+1

a’

Proof Property 1

- Case 2 : i = i’ + 1 and n < n’
- Peer a addresses peer a’, using its image (i’,n’) from last split
- No IAM came since.

Forwarding possible for any address a’ beyond [n, a]

25

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- Peer a sends the scan to all buckets in its image
- Including its image (i’, n’)
- Receiving peer a’ can have bucket level j as in the image
- j (a) = j’ (a)
- No forwarding of the scan
- Or, bucket a’ split
- Once and only once
- j (a) = j’ (a) + 1
- See the figs for the key address calculus

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- Peer a’ forwards the scan to its (only) child
- No child can have a child
- Peer a would first need to split again as well
- Every peer gets thus the scan and only once
- There at worst two rounds

27

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

- The only faster worst case performance is zero forwarding messages
- Every split has to be notified then to every peer
- It would be against the scalability goal of every SDDS & structured P2P scheme

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Bucket reliability group with k paritybucketsprotect against up to k bucket failures per group

Data Record

Parity Record

5

4

3

2

1

0

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

Tutoring records

Rank

Data Peer

Parity Peer

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Peer leaves with notice

Say that’s OK

j

j

j

… …

Coordinator Peer

i’,n’

i’,n’

i’,n’

i’,n’

Pm

P0

Pl

Candidate Peer

Notification

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Peer leaves without notice or fails

LH*RS Bucket Recovery

j

j

j

Forward

Coordinator Peer

i’,n’

i’,n’

i’,n’

i’,n’

Pl-1

Pm

Pl

Parity Peer

Query

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Peer leaves without notice or fails

LH*RS Bucket Recovery

j

j

j

i’,n’

Pl

Coordinator Peer

i’,n’

i’,n’

i’,n’

Pl-1

Pm

Parity Peer

Answer

32

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

j

Sure Search : Protects against outdated server read (transient communication or peer failure)

i’,n’

j

j

j

Pl

Coordinator Peer

i’,n’

i’,n’

i’,n’

i’,n’

Pl-1

Pm

Pl

Parity Peer

Answer

Query

33

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Conclusion

LH*RSP2P require at most one forward message when addressing error occur

Is the fastest known SDDS and P2P key based addressing algorithm

Protects efficiently against churn

Allows to manage very large scalable files

Should have numerous applications

Google ??

34

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

Current & Future Work

- Implementation of peer node architecture and of tutoring functions
- Using existing LH*RS prototype
- Created by Rim Moussa & shown at VLDB 2004
- Performance Analysis
- Variants

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

END

Thank you for Your Attention

Work partly funded by the IST eGov-Bus project

36

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

LH*RSP2P: A Scalable Distributed Data Structure for P2P Environment

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