1. Implementation issues
Where to leave the data?
Where to process transactions and queries?

17. 2 x0 + 2 x x 0= 5220
2 x x x 225= 6020
2 x * x 0= 2468

22. Parallel & Distributed databases
Hardware
DBMS
Application
Distributed control
Application
DBMS
Hardware
Parallel algorithms & data structures
Hardware

23. Where to leave the data ?
Parallel systems
Scalable Distributed Data Structures
Dynamic Hash Table (P2P)

24. Introduction Parallel machines are quite common and affordable
Databases are growing increasingly large
large volumes of transaction data are collected and stored for later analysis.
multimedia objects like images are increasingly stored in databases
Large-scale parallel database systems increasingly used for:
storing large volumes of data
processing time-consuming decision-support queries
providing high throughput for transaction processing

25. Parallelism in Databases
Data can be partitioned across multiple disks for parallel I/O.
Individual relational operations (e.g., sort, join, aggregation) can be executed in parallel
data can be partitioned and each processor can work independently on its own partition.
Queries are expressed in high level language (SQL, translated to relational algebra)
makes parallelization easier.
Different queries can be run in parallel with each other. Concurrency control takes care of conflicts.
Thus, databases naturally lend themselves to parallelism.

26. I/O Parallelism
Reduce the time required to retrieve relations from disk by partitioning the relations on multiple disks.
Horizontal partitioning – tuples of a relation are divided among many disks such that each tuple resides on one disk.
Partitioning techniques (number of disks = n):
Round-robin:
Send the ith tuple inserted in the relation to disk i mod n.
Hash partitioning:
Choose one or more attributes as the partitioning attributes.
Choose hash function h with range 0…n - 1
Let i denote result of hash function h applied to the partitioning attribute value of a tuple. Send tuple to disk i.

27. I/O Parallelism (Cont.)
Partitioning techniques (cont.):
Range partitioning:
Choose an attribute as the partitioning attribute.
A partitioning vector [vo, v1, ..., vn-2] is chosen.
Let v be the partitioning attribute value of a tuple. Tuples such that vi  vi+1 go to disk I + 1. Tuples with v < v0 go to disk 0 and tuples with v  vn-2 go to disk n-1.
E.g., with a partitioning vector [5,11], a tuple with partitioning attribute value of 2 will go to disk 0, a tuple with value 8 will go to disk 1, while a tuple with value 20 will go to disk2.

28. Comparison of Partitioning Techniques
Evaluate how well partitioning techniques support the following types of data access:
1.Scanning the entire relation.
2.Locating a tuple associatively – point queries.
E.g., r.A = 25.
3.Locating all tuples such that the value of a given attribute lies within a specified range – range queries.
E.g., 10  r.A < 25.

29. Comparison of Partitioning Techniques (Cont.)
Round robin:
Advantages
Best suited for sequential scan of entire relation on each query.
All disks have almost an equal number of tuples; retrieval work is thus well balanced between disks.
Range queries are difficult to process
No clustering -- tuples are scattered across all disks

30. Comparison of Partitioning Techniques(Cont.)
Hash partitioning:
Good for sequential access
Assuming hash function is good, and partitioning attributes form a key, tuples will be equally distributed between disks
Retrieval work is then well balanced between disks.
Good for point queries on partitioning attribute
Can lookup single disk, leaving others available for answering other queries.
Index on partitioning attribute can be local to disk, making lookup and update more efficient
No clustering, so difficult to answer range queries

31. Comparison of Partitioning Techniques (Cont.)
Range partitioning:
Provides data clustering by partitioning attribute value.
Good for sequential access
Good for point queries on partitioning attribute: only one disk needs to be accessed.
For range queries on partitioning attribute, one to a few disks may need to be accessed
Remaining disks are available for other queries.
Good if result tuples are from one to a few blocks.
If many blocks are to be fetched, they are still fetched from one to a few disks, and potential parallelism in disk access is wasted
Example of execution skew.

32. Partitioning a Relation across Disks
If a relation contains only a few tuples which will fit into a single disk block, then assign the relation to a single disk.
Large relations are preferably partitioned across all the available disks.
If a relation consists of m disk blocks and there are n disks available in the system, then the relation should be allocated min(m,n) disks.

33. Handling of Skew
The distribution of tuples to disks may be skewed — that is, some disks have many tuples, while others may have fewer tuples.
Types of skew:
Attribute-value skew.
Some values appear in the partitioning attributes of many tuples; all the tuples with the same value for the partitioning attribute end up in the same partition.
Can occur with range-partitioning and hash-partitioning.
Partition skew.
With range-partitioning, badly chosen partition vector may assign too many tuples to some partitions and too few to others.
Less likely with hash-partitioning if a good hash-function is chosen.

34. Handling Skew in Range-Partitioning
To create a balanced partitioning vector (assuming partitioning attribute forms a key of the relation):
Sort the relation on the partitioning attribute.
Construct the partition vector by scanning the relation in sorted order as follows.
After every 1/nth of the relation has been read, the value of the partitioning attribute of the next tuple is added to the partition vector.
n denotes the number of partitions to be constructed.
Duplicate entries or imbalances can result if duplicates are present in partitioning attributes.
Alternative technique based on histograms used in practice

35. Handling Skew using Histograms
Balanced partitioning vector can be constructed from histogram in a relatively straightforward fashion
Assume uniform distribution within each range of the histogram
Histogram can be constructed by scanning relation, or sampling (blocks containing) tuples of the relation

36. Handling Skew Using Virtual Processor Partitioning
Skew in range partitioning can be handled elegantly using virtual processor partitioning:
create a large number of partitions (say 10 to 20 times the number of processors)
Assign virtual processors to partitions either in round-robin fashion or based on estimated cost of processing each virtual partition
Basic idea:
If any normal partition would have been skewed, it is very likely the skew is spread over a number of virtual partitions
Skewed virtual partitions get spread across a number of processors, so work gets distributed evenly!

37. Scalable Distributed Data Structures
The leading researcher is Withold Litwin

38. Multicomputers need data structures and file systems
Why SDDSs
Multicomputers need data structures and file systems
Trivial extensions of traditional structures are not best
hot-spots
scalability
parallel queries
distributed and autonomous clients
distributed RAM & distance to data

39. What is an SDDS ? Data are structured
records with keys  objects with an OID
more semantics than in Unix flat-file model
abstraction popular with applications
allows for parallel scans
function shipping
Data are on servers
always available for access
Overflowing servers split into new servers
appended to the file without informing the clients
Queries come from multiple autonomous clients
available for access only on their initiative
no synchronous updates on the clients
There is no centralized directory for access computations

40. What is an SDDS ? Clients do not make same error twice
Clients can make addressing errors
Clients have less or more adequate image of the actual file structure
Servers are able to forward the queries to the correct address
perhaps in several messages
Servers may send Image Adjustment Messages
Clients do not make same error twice
See the SDDS talk for more on it
Or the LH* ACM-TODS paper (Dec. 96)

41. An SDDS
growth through splits under inserts
Servers
Clients

42. An SDDS
growth through splits under inserts
Servers
Clients

43. An SDDS
growth through splits under inserts
Servers
Clients

44. An SDDS
growth through splits under inserts
Servers
Clients

45. An SDDS
growth through splits under inserts
Servers
Clients

46. An SDDS
Clients

47. An SDDS
Clients

48. An SDDS
IAM
Clients

49. An SDDS
Clients

50. An SDDS
Clients

51. DS SDDS Classics Hash m-d trees 1-d tree LH* DDH Breitbart & al
Known SDDSs DS
SDDS
(1993)
Classics
Hash
m-d trees
1-d tree
k-RP*
dPi-tree
Nardelli-tree
LH*
DDH
Breitbart & al
RP*
Kroll & Widmayer
Breitbart & Vingralek
H-Avail.
LH*m, LH*g
LH*SA
Security
s-availability
LH*s
LH*RS

52. LH* (A classic) Allows for the primary key (OID) based hash files
generalizes the LH addressing schema
variants used in Netscape products, LH-Server, Unify, Frontpage, IIS, MsExchange...
Typical load factor %
In practice, at most 2 forwarding messages
regardless of the size of the file
In general, 1 m/insert and 2 m/search on the average
4 messages in the worst case
Search time of 1 ms (10 Mb/s net), of 150 s (100 Mb/s net) and of 30 s (Gb/s net)

53. High-availability LH* schemes
In a large multicomputer, it is unlikely that all servers are up
Consider the probability that a bucket is up is 99 %
bucket is unavailable 3 days per year
If one stores every key in only 1 bucket
case of typical SDDSs, LH* included
Then file reliability : probability that n-bucket file is entirely up is:
37 % for n = 100
0 % for n = 1000
Acceptable for yourself ?

54. High-availability LH* schemes
Using 2 buckets to store a key, one may expect the reliability of:
99 % for n = 100
91 % for n = 1000
High-availability files
make data available despite unavailability of some servers
RAIDx, LSA, EvenOdd, DATUM...
High-availability SDDS
make sense
are the only way to reliable large SDDS files

55. P2P datastructures

56. Chord lookup algorithm properties
Interface: lookup(key)  IP address
Efficient: O(log N) messages per lookup
N is the total number of servers
Scalable: O(log N) state per node
Robust: survives massive failures
Simple to analyze

57. Chord Hashes a Key to its Successor
Key ID Node ID
N10
K5, K10
K100
N100
Circular
ID Space
N32
K11, K30
K65, K70
N80
N60
K33, K40, K52
Successor: node with next highest ID

58. Lookups find the ID’s predecessor Correct if successors are correct
Basic Lookup N5
N10
N110
“Where is key 50?”
N20
N99
“Key 50 is
At N60”
N32
N40
N80
N60
Lookups find the ID’s predecessor
Correct if successors are correct

59. Successor Lists Ensure Robust Lookup
10, 20, 32 N5
20, 32, 40
N10
5, 10, 20
N110
32, 40, 60
N20
110, 5, 10
N99
40, 60, 80
N32
N40
60, 80, 99
All r successors have to fail before we have a problem.
List ensures we find actual current successor.
99, 110, 5
N80
N60
80, 99, 110
Each node remembers r successors
Lookup can skip over dead nodes to find blocks

60. Chord “Finger Table” Accelerates Lookups
1/8
1/16
1/32
1/64
Log(N) means we don’t have to track all nodes. Easy to maintain finger info.
1/128
N80

61. Chord lookups take O(log N) hops
Lookup(K19)
Maybe note that fingers point to the first relevant node.
N80
N60