WP3
R-GMA – Architecture
and Query Mediation
24/4/2003
Werner Nutt (Heriot-Watt University)
<w.nutt@hw.ac.uk>
Contributors
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Rob Byrom
Andy Cooke
Roney Cordenonsi
Abdeslem Djaoui
Laurence Field
Steve Fisher
Alasdair Gray
Steve Hicks
Jason Leake
Lisha Ma
James Magowan
Werner Nutt
Norbert Podhorszki
Manish Soni
Paul Taylor
Antony Wilson
R-GMA - Architecture and Query Mediation
RAL
Heriot-Watt
QMUL
RAL
PPARC
RAL
Heriot-Watt
RAL
RAL
Heriot-Watt
IBM-UK
Heriot-Watt
SZTAKI
PPARC
IBM-UK
PPARC
Werner Nutt - 24/4/2003
WP3
2
Grid Monitoring:
Where are the Concepts?
WP3
There are two styles of talking about the Grid:
– General metaphors (virtual organisations, services,…)
– Low-level technicalities and jargon
(LDAP, XML, SOAP, OGSA, OGSI, ...)
What is missing
– Clear definitions of the problems
– intuitive concepts for solving them
Needed for communication with both,
users and developers
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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The Grid Monitoring Problem
WP3
In a Grid we have
– Computers
– Storage elements
– Network nodes and connections
– Application programmes, …
Monitoring:
– What is the current state of the system?
– How did the system behave in the past ?
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
4
Monitoring Data
Come in two Kinds
WP3
A Grid monitoring system makes available
two kinds of data
• static data “pools”, e.g., databases on
– network topology, nodes connected
– applications available (versions, licences, ...)
• “streams” of data, e.g.,
– sensor data (cpu load, network traffic, ...)
Data streams may give rise to data pools if they are archived
Today: R-GMA is tailored towards streams,
but not pools
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Examples of
Monitoring Queries
WP3
• “Show me the (average) cpu-load
of computers at Heriot-Watt!”
• “Between which nodes was yesterday
the average transportation time for 1 MB packets
higher than than 0.… seconds?”
• For every node N, how many computers
connected to N have currently
a cpu-load of no “ more than 30%?”
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Stream Queries can have
Various Temporal Interpretations
WP3
Consider a query over the relation “Transport Time”
tt(src, dest, pcktSize, method, timestamp, time)
SELECT * FROM tt
WHERE src = ral AND dest = bologna
What is meant? Measurements
(Continuous Query)
– from now ?
– up until now ?
(History Query)
– right now ?
(Latest Snapshot Query)
Today: Queries can be “flagged” with their type
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Advanced Queries:
Mixing Temporal Query Types
WP3
• “Which connections have currently
a transportation time
that is higher than last week's average?”
(latest snapshot and history)
• “Show me the cpu load of those machines
where it is lower than yesterday's load average!”
(continuous and history)
We do not intend to support such queries by R-GMA!
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Architecture Approach 1:
A Monitoring Data Warehouse
WP3
Idea:
– store all data about the Grid status into a huge
database
– and query it
Not realistic:
• Loading takes time
• Data occupy space
• Connections to the warehouse may fail
• Often monitoring data flow as data streams, and
queries ask for data streams as output
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Approach 2: Monitoring with a
“Multi-agent System”
WP3
The Grid Monitoring Architecture (GMA) of the Global Grid Forum
distinguishes between:
• Consumers of information
• Producers of information
Consumer
find/
register
• Directory Service
– Producers register their
supply
– Consumers register their
demand
Sensor
Directory
Service
Producer
Data Base
MonitoringApplication
Directory Service mediates between producers and consumers
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Questions about GMA:
WP3
• Which kinds of producers and consumers are there?
• In which language do producers register their supply
and consumers their demand ?
• What is the meaning of a registration?
• How does a consumer find suitable producers?
And how does a producer find suitable consumers?
• Producers have different capabilities to answer queries
(e.g. selections, joins, …).
Which of them should they register?
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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R-GMA: A Virtual Monitoring
Data Warehouse
• Language of producers and consumers:
relational queries (SQL)
• Vocabulary: Relations in a global schema
Consumer
• Consumer: poses queries over
global schema
Query
Global Schema S
DB-Producer
DB
V1
V2
.
.
.
Vn
Views
on S
WP3
V
Registry
R-GMA - Architecture and Query Mediation
• Producer: – has a type
(stream p., database p.)
Stream
Producer
Sensor
– publishes relations
R1, … ,Rk
– for every R, registers a
simple view V on the
global schema
Werner Nutt - 24/4/2003
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Primary Producers
WP3
Database producer
• supports queries over fixed set of tuples (static queries)
• can be used to publish a database
Stream producer
• supports queries over changing set of tuples
(continuous queries)
• supports “latest snapshot queries”
– offers up-to-date values for each primary key
Today: DatabaseProducer’s and StreamProducer’s
in R-GMA are different from the above!
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Communication Modes of
Stream Producers
WP3
Stream Producers may offer two communication modes
for continuous queries:
– lossless (… but tuples could become stale)
– lossy
(… but tuples are fresh)
Producer
Producer
Servlet
Consumer
Servlet
IIIIIIII...
IIIIIIII...
Queue
Queue
Consumer
Today: R-GMA’s StreamProducer’s are resilient and
support lossless communication
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Republishers Publish
Query Answers
WP3
into database
into stream
Static Query
Materialised View
--
Continuous
Query
Archiver
Stream
Republisher
Archiver: shows the history of a stream.
Stream Republisher: enables
– merging,
– thinning,
– summarising of streams …
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Republishers in R-GMA Today
WP3
Republishers are called “archivers”
(although some of them don't archive anything)
An archiver (= republisher)
• is defined by a query
• consumes only from “stream producers”
• publishes the query result according to its type, using
– a “stream producer”, or
– a “latest snapshot producer”, or
– a “database producer”
(which keeps an archive)
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Which View should a
Republisher Register?
WP3
Problem:
Republishers may compute complex queries
… but complex views would confuse the “mediator”!
Ideas:
– register a simplified view for a complex query
– register a new table
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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What is the Meaning of
a Query in R-GMA?
WP3
Assumption: the views of (primary) producers are
selections on a single relation, i.e., queries of the form
SELECT *
FROM
cpu_load
WHERE machine_id = ‘AB123’ AND loc = ‘hw’
(each producer contributes its parts of a relation)
• The virtual database contains
the union of the data of all the primary producers
• Conceptually, a query is evaluated
over the entire virtual db
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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In R-GMA Query Answering
Needs Mediation
WP3
Suppose P1, P2 produce for tt (Transport Time)
P1:
P2:
… WHERE src = hw
… WHERE src = ral AND pcktSize > 20
A global consumer poses its query over global relations
SELECT * FROM
tt
WHERE
pcktSize > 10
A mediator translates this into queries over local relations
SELECT * FROM
UNION
SELECT * FROM
P1.tt
WHERE
pcktSize > 10
P2.tt
Today: R-GMA’s mediator handles simple queries like the one above
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Global and Local Consumers
•
WP3
Global consumers pose queries over global relations
SELECT * FROM
tt
WHERE
pcktSize > 10 ,
which are translated into queries over local relations
SELECT * FROM
UNION
SELECT * FROM
•
P1.tt
WHERE
pcktSize > 10
P2.tt
Local consumers pose queries over local relations directly
SELECT * FROM P1.tt WHERE method = ping
Today: a consumer can be global or local,
but local relations cannot be referred to explicitly
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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How does the Mediator Find
Suitable Producers?
WP3
P1, P2, P3 produce for tt (Transport Time)
P1:
P2:
P3:
Q:
… src = hw
… src = ral AND pcktSize > 20
… src = ral AND method = ping
SELECT * FROM tt WHERE src = ral AND method = ping
We see: P1 is not suitable for Q, but P2 and P3 are.
Why?
src = hw AND src = ral AND method = ping
src = ral AND pcktSize > 20 AND …
is never true
is sometimes true
Satisfiability Test!
Today: implemented
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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… So Which Producers Should
the Mediator Ask?
P2:
P3:
WP3
… src = ral AND pcktSize > 20
… src = ral AND method = ping
Q:
SELECT * FROM tt WHERE
src = ral AND method = ping
All answers to Q returned by P2 are also returned by P3 :
whenever
src = ral AND pcktSize > 20 AND src = ral AND method = ping
is true, then
src = ral AND method = ping AND src = ral AND method = ping
is true.
Hence, R-GMA only needs to ask P3
Entailment Test!
Today: not implemented
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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… But What Did the
Producers Promise?
WP3
P registers view V
Does P promise
– some of V ?
– all of V?
(sound description)
(sound and complete description)
• The Entailment Test only makes sense when the
registered views are sound and complete descriptions
• Producers should register completeness flags
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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… Why May a Producer
not be Complete?
WP3
• The language of views is more restricted than the
language of queries
Hence: republishers may be unable to say exactly
what they publish
• Archivers may archive in lossy mode
• Producers may lose tuples
• A producer may not know everything
about the real world
Open to debate
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Keys in the Global Schema
WP3
tt(src, dest, method, pcktSize, timestamp, time)
Intuitively, tt has the primary key
(src, dest, method, pcktSize, timestamp).
We need to know the primary keys
• to understand the global schema
• to answer latest snapshot queries
But can we enforce them?
Sometimes, they hold globally if they hold locally !
Today: global tables have keys, which
are used to keep a latest snapshot cache
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Summary (1)
WP3
Types of Stream Queries
• continuous vs. history vs. latest snapshot
Producers
• primary producers vs. republishers
• DB producers: publish database
• stream producers: lossless vs.
lossy communication modes
• republishers: materialised views vs. archivers
vs. stream republishers
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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Summary (2)
WP3
Global Schema
• primary keys
Consumers
• global vs. local consumers
Mediator
• translates global query into local queries
• applies Satisfiability Test to find suitable producers
Query Planning
• Entailment Test
• sound vs. sound and complete producers
R-GMA - Architecture and Query Mediation
Werner Nutt - 24/4/2003
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