Stream Mining - Department of Information Technology

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Data Stream Mining
Tore Risch
Dept. of information technology
Uppsala University
Sweden
2014-05-14
New applications
• Data comes as huge data streams, e.g.:
-
Satellite data
Scientific instruments, e.g. colliders
Social networks
Stock data
Process industry
Traffic control
Patient monitoring
Enormous data growth
•
•
•
•
•
Read landmark article in Economist 2010-02-27:
http://www.economist.com/node/15557443/
The traditional Moore’s law:
– Processor speed doubles every 1.5 years
Current data growth rate significantly higher
– Data grows 10-fold every 5 year, which about the same as Moore’s law
Major opportunities:
– spot business trends
– prevent diseases
– combat crime
– scientific discoveries, the 4th paradigm
(http://research.microsoft.com/en-us/collaboration/fourthparadigm/)
– data-centered economy
Major challenges:
– Information overload
– Scalable data processing, ‘Bigdata management’
– Data security
– Data privacy
Too much data to store on disk
īƒžNeed to mine streaming data
Mining a swift data river
Cover, Economist 14 pages thematic issue 2010-02-27
Mining streams vs. databases
• Data streams dynamic and infinite in size
– Data is continuously generated and changing
– Live streams may have no upper limit
– A live stream, can be read only once (‘Just One
Look’)
– The stream rate may be very high
• Traditional data mining does not work for
streaming data:
– Regular data mining based on finite data collections
stored in files or databases
– Regular data mining done in batch:
• read (access) data in collection several times
• analyze accessed data
• store results in database (or file)
– For example, store for each data object what cluster it belongs to
Data stream mining vs.
traditional data mining
• Live streams grow continuously
– The system cannot read streams many times as with traditional
data mining
• Live streams mining must be done on-line
– Not traditional batch processing
• Live streams require main memory processing
– To keep up with very high data flow rates (speed)
• Live streams must be mined with limited memory
– Not load-and-analyze as traditional data mining
– Iterative processing of statistics
• Live streams must keep up with very high data flow volumes
– Approximate mining algorithms
– Parallel on-line computations
Streams vs. regular data
• Data stream mining requires iterative aggregation:
– Read data tuples iteratively from input stream(s)
• E.g. measured values
• Do not store read data in database
– Analyze read tuples incrementally
– Keep partial results in main memory
• E.g. sum and count
– Continuosly emit incremental analyzed resuls
• E.g. running average by dividing sum with count
• Result of data stream mining is derived stream
– Continuously produced by emit
– Can be stored as continuously changing database
Streams vs. regular data
• Stream processing should keep up with data
flow
– Make computations so fast that they keep up with the
flow (on average)
– Should not be catastrophic if if miner cannot keep up
with the flow:
• Drop input data values
• Use approximate computations such as sampling
• Asynchronous logging in files often possible
– At least of processed (reduced) streaming data
– At least during limited time (log files)
Requirements for Stream Mining
• Single scan of data,
– because of very large or infinite size of streams.
– because it may be impossible or very expensive to reread the
stream for the computation
• Limited memory and CPU usage
– because the processing should be done in main memory despite
the very large stream volume
• Compact continuously evolving representation of mined
data
– It is not possible to store the mined data in database as with
traditional data mining
– A compact main memory representation of mined data needed
Requirements for Stream Mining
• Allow for continuous evolution of mined data
– Traditional batch mining static mined data
– Continuous mining makes mined data into a stream
too
=>Concept drift
• Often mining over different kinds of windows of
streams
– E.g. sliding or tumbling windows
– Windows of limited size
– Often only statistics summaries needed (synopses,
sketches)
Data Stream Management
Systems (DSMS)
• DataBase Management System (DBMS)
– General purpose software to handle large volume
persistent data (usually on disk)
– Important tool for traditional datamining
• Data Stream Management System (DSMS)
– General purpose software to handle large volume
data streams (often transient data)
– Important tool for data stream mining
Data Base Management System
SQL Queries
DBMS
Query Processor
Data Manager
Meta –
data
Stored
Data
Data Stream Management System
Continuous Queries (CQs)
DSMS
Query Processor
Data
streams
Data & Stream Manager
Meta –
data
Stored
Data
Data
streams
Stream windows
• Limited size section of stream stored temporarily in
DSMS
– ’Regular’ database queries can be made over these windows
• Need window operator to chop stream into segments
• Window size (sz) based on:
– Number of elements, a counting window
• E.g. last 10 elements
– i.e. windows has fixed size of 10 elements
– A time window
• E.g. elements last second
– i.e. windows contains all event processed during the last second
– A landmark window
• All events from time t0 in window
– c.f. growing log file
– A decaying window
• Decrease importance of measurement by multiplying with factor l
• Remove when importance below threshold
Stream windows
• Windows may also have stride (str)
– Rule for how fast they move forward,
• E.g. 10 elements for a 10 element counting window
– A tumbling window
• E.g. 2 elements for a 10 element counting window
– A sliding windows
• E.g. 100 elements for a 10 element counting window
– A sampling window
• Windows need not always be materialized
– E.g often sufficient to keep statistics materialized
Continuous (standing) queries over
streams from expressways
Schema for stream CarLocStr of tuples:
CarLocStr(car_id,
/* unique car identifier
*/
speed,
/* speed of the car
*/
exp_way,
/* expressway: 0..10
*/
lane,
/* lane: 0,1,2,3
*/
dir,
/* direction: 0(east), 1(west) */
x-pos);
/* coordinate in express way */
CQL query to continuously get the cars in a window of the stream every 30 seconds:
SELECT DISTINCT car_id
FROM CarLocStr [RANGE 30 SECONDS];
Get the average speed of vehicles per expressway, direction, segment each 5 minutes:
SELECT exp_way, dir, seg, AVG(speed) as speed,
FROM CarSegStr [RANGE 5 MINUTES]
GROUP BY exp_way, dir, seg;
•
http://www.cs.brandeis.edu/~linearroad/
Denstream
• Streamed DBScan
• Published: 2006 SIAM Conf. on Data Mining
(http://user.it.uu.se/~torer/DenStream.pdf)
• Regular DBScan:
– DBScan saves cluster memberships of static database per
member object in database by scanning database looking for
pairs of objects close to each other
– Database accessed many times
– For scalable processing a spatial index must be used to index
points in hyperspace and answer nearest-neighbor queries
Denstream
• Denstream
–
–
–
–
One pass processing
Limited memory
Evolving clustering => not static cluster membership
Indefinite stream => store cluster memberships not
stored in database
– No assumption of number of clusters
• clusters fade in and fade out
– Clusters of arbitrary shape allowed
– Good at handling outliers
Core micro-clusters
• Core point: ‘anchor’ in cluster of other points
• Core micro-cluster: An area covering points
close to (epsilon similar) a core point
• Cluster defined as set of micro-clusters
Potential micro-clusters
•
Outlier o-micro-cluster
– New point not included in any micro-cluster
•
Potential p-micro-cluster
– Several clustered points not large enough to form a
micro-cluster
•
When new data point arrives:
1. Try to merge with nearest p-micro-cluster
2. Try to merge with nearest o-micro-cluster
•
If so convert o-micro-cluster to p-micro-cluster
3. Otherwise make new o-micro-cluster
Decaying p-micro-cluster windows
• Maintain weight Cp per p-micro-cluster
• Periodically (each Tp time period)
decrease weight exponentially by
multiplying old weight with l
• Weight lower than threshold => delete,
i.e. decaying window
• Decaying window of micro clusters
Dealing with outliers
•
o-micro-clusters important for forming new
evoving p-micro-clusters
īƒž Keep o-micro-clusters around
•
Keeping all o-micro-clusters may be expensive
īƒž Delete o-micro-cluster by special exponential
pruning rule (decaying window)
•
Decaying window method proven to make #
micro-clusters grow logarithmically with stream
size
– Good, but not sufficient for indefinite stream
– Shown to grow very slowly though
Growth of micro-clusters
Forming c-micro-cluster sets
• Regularly (e.g. each time period) the user
demands forming current c-micro-clusters
from the current p-micro-clusters
• Done by running regular DBSCAN over
the p-micro-clusters
– Center of each p-micro-cluster regarded as
point
– Close when p-micro-clusters intersect
=> Clusters formed
Bloom-filters
• Problem: Testing membership in extremely large
data sets
– E.g. all non-spam e-mail addresses
• No false negatives, i.e. if address is in set then
OK guaranteed
• Few false positives allowed, i.e. a small number
of spams may sneek through
See http://infolab.stanford.edu/~ullman/mmds/ch4.pdf
section 4.3.2
Bloom-filters
• Main idea:
– Assume bitmap B of objects of size s
– Hash each object x to h in [1,s]
– Set bit B[h]
• Smaller than sorted table in memory:
– 109 email addresses of 40 bytes => 40 GByte if set to
be stored sorted in memory
• Would be expensive to extend
– Bitmap could have e.g. 109/8= 125MBytes
• May have false positives
– Since hash function not perfect
Lowering false positives
• Small bitmap => many false positives
• Idea, hash with several independent hash
functions h1(x), h2(x) and set bits
correspondingly (logical OR)
• For each new x check that all hi(x) are set
– If so => match
– Chance of false positives decrease exponentially with
number of hi
• Assumes independent hi(x)
– hi(x) and hj(x) no common factors if i ≠ j
Books
- Anand Rajaraman & J.Ullman: Mining of
Massive Datasets
http://infolab.stanford.edu/~ullman/mmds.html
- L. Golab and T. Özsu: Issues in Stream
Data Management, SIGMOD Records,
32(2), June 2003,
http://www.acm.org/sigmod/record/issues/
0306/1.golab-ozsu1.pdf
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