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R-GMA – Architecture and Query Mediation 24/4/2003

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  1. R-GMA – Architecture and Query Mediation 24/4/2003 Werner Nutt (Heriot-Watt University) <w.nutt@hw.ac.uk>

  2. Contributors • Rob Byrom RAL • Andy Cooke Heriot-Watt • Roney Cordenonsi QMUL • Abdeslem Djaoui RAL • Laurence Field PPARC • Steve Fisher RAL • Alasdair Gray Heriot-Watt • Steve Hicks RAL • Jason Leake RAL • Lisha Ma Heriot-Watt • James Magowan IBM-UK • Werner Nutt Heriot-Watt • Norbert Podhorszki SZTAKI • Manish Soni PPARC • Paul Taylor IBM-UK • Antony Wilson PPARC R-GMA - Architecture and Query Mediation

  3. Grid Monitoring: Where are the Concepts? 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

  4. The Grid Monitoring Problem 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

  5. Monitoring Data Come in two Kinds 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


  6. Examples of Monitoring Queries • “Show me the (average) cpu-load of computers at Heriot-Watt!” • “Between which nodes was yesterdaythe average transportation time for 1 MB packets higher than than 0.… seconds?” • For every node N, how many computers connected to N have currentlya cpu-load of no “ more than 30%?” R-GMA - Architecture and Query Mediation

  7. Stream Queries can have Various Temporal Interpretations 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 • from now ?(Continuous Query) • up until now ?(History Query) • right now ?(Latest Snapshot Query) Today: Queries can be “flagged” with their type R-GMA - Architecture and Query Mediation

  8. Advanced Queries: Mixing Temporal Query Types • “Which connections have currentlya 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

  9. Architecture Approach 1: A Monitoring Data Warehouse 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

  10. DirectoryService find/register Consumer Monitoring-Application Producer Sensor Data Base Approach 2: Monitoring with a “Multi-agent System” The Grid Monitoring Architecture (GMA) of the Global Grid Forumdistinguishes between: • Consumers of information • Producers of information • Directory Service • Producers register their supply • Consumers register their demand Directory Service mediatesbetween producers and consumers R-GMA - Architecture and Query Mediation

  11. Questions about GMA: • Which kinds of producers and consumers are there? • In which language do producers register their supplyand 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

  12. DB Query DB-Producer Stream Producer Consumer Views on S Registry V1V2...Vn V Sensor Global Schema S R-GMA: A Virtual Monitoring Data Warehouse • Language of producers and consumers: relational queries (SQL) • Vocabulary: Relations in a global schema • Consumer: poses queries over global schema • Producer: • has a type(stream p., database p.) • publishes relationsR1,…,Rk • for every R, registers a simple view V on the global schema R-GMA - Architecture and Query Mediation

  13. Primary Producers 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

  14. ProducerServlet ConsumerServlet Producer Consumer IIIIIIII... IIIIIIII... Queue Queue Communication Modes of Stream Producers Stream Producers may offer two communication modes for continuous queries: • lossless (… but tuples could become stale) • lossy (… but tuples are fresh) Today: R-GMA’s StreamProducer’s are resilient and support lossless communication R-GMA - Architecture and Query Mediation

  15. Republishers Publish Query Answers Archiver: shows the history of a stream. Stream Republisher: enables • merging, • thinning, • summarising of streams … R-GMA - Architecture and Query Mediation

  16. Republishers in R-GMA Today 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

  17. Which View should a Republisher Register? 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

  18. What is the Meaning of a Query in R-GMA? 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 virtualdatabase 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

  19. In R-GMA Query Answering Needs Mediation SupposeP1, P2produce for tt (Transport Time) P1:… WHERE src = hw P2:… 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 P1.tt WHERE pcktSize > 10 UNION SELECT * FROM P2.tt Today: R-GMA’smediator handles simple queries like the one above R-GMA - Architecture and Query Mediation

  20. Global and Local Consumers • Global consumers pose queries over global relations SELECT * FROM tt WHERE pcktSize > 10 , which are translated into queries over local relations SELECT * FROM P1.tt WHERE pcktSize > 10 UNION SELECT * FROM P2.tt • Local consumerspose 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

  21. How does the Mediator Find Suitable Producers? P1, P2, P3produce for tt (Transport Time) P1:… src = hw P2:… src = ral AND pcktSize > 20 P3:… src = ral AND method = ping Q: SELECT * FROM tt WHERE src = ral AND method = ping We see: P1 is not suitable for Q, but P2 and P3 are. Why? src = hwANDsrc = ral AND method = ping is never true src = ral AND pcktSize > 20AND… is sometimes true Satisfiability Test! Today:implemented R-GMA - Architecture and Query Mediation

  22. … So Which Producers Should the Mediator Ask? P2:… src = ral AND pcktSize > 20 P3:… 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 > 20ANDsrc = ral AND method = ping is true, then src = ral AND method = pingANDsrc = ral AND method = ping is true. Hence, R-GMA only needs to askP3 Entailment Test! Today:not implemented R-GMA - Architecture and Query Mediation

  23. … But What Did the Producers Promise? P registers view V Does P promise • someof V ? (sound description) • allof V? (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

  24. … Why May a Producer not be Complete? • The language of views is more restricted than the language of queriesHence: 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

  25. Keys in the Global Schema tt(src, dest, method, pcktSize, timestamp, time) Intuitively, tthas the primary key (src, dest, method, pcktSize, timestamp). We need to know the primary keys • to understand the global schema • to answer latestsnapshot 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

  26. Summary (1) Types of Stream Queries • continuous vs. history vs. latest snapshot Producers • primary producers vs. republishers • DBproducers: publish database • stream producers: lossless vs. lossy communication modes • republishers:materialised views vs. archivers vs. stream republishers R-GMA - Architecture and Query Mediation

  27. Summary (2) Global Schema • primary keys Consumers • global vs. local consumers Mediator • translates global query into local queries • applies SatisfiabilityTest to find suitable producers Query Planning • EntailmentTest • sound vs. sound and complete producers R-GMA - Architecture and Query Mediation