ANAPSID: An Adaptive Query Processing Engine for SPARQL Endpoints

ANAPSID: An Adaptive Query Processing Engine for SPARQL Endpoints
ANAPSID: An Adaptive Query Processing Engine for SPARQL Endpoints
Bibliographical Metadata
Subject:	Adaptive Query Processing
Keywords:	Adaptive Query Processing, ANAPSID, Linked Data
Year:	2011
Authors:	Maribel Acosta, Maria-Esther Vidal, Tomas Lampo, Julio Castillo
Venue	ISWC
Content Metadata
Problem:	SPARQL Query Federation
Approach:	Querying Distributed RDF Data Sources,
Implementation:	ANAPSID
Evaluation:	Execution time evaluation

Abstract

Following the design rules of Linked Data, the number of available SPARQL endpoints that support remote query processing is quickly growing; however, because of the lack of adaptivity, query executions may frequently be unsuccessful. First, fixed plans identified following the traditional optimize-then execute paradigm, may timeout as a consequence of endpoint availability. Second, because blocking operators are usually implemented, endpoint query engines are not able to incrementally produce results, and may become blocked if data sources stop sending data. We present ANAPSID, an adaptive query engine for SPARQL endpoints that adapts query execution schedulers to data availability and run-time conditions. ANAPSID provides physical SPARQL operators that detect when a source becomes blocked or data traÆc is bursty, and opportunistically, the operators produce results as quickly as data arrives from the sources. Additionally, ANAPSID operators implement main memory replacement policies to move previously computed matches to secondary memory avoiding duplicates. We compared ANAPSID performance with respect to RDF stores and endpoints, and observed that ANAPSID speeds up execution time, in some cases, in more than one order of magnitude.

Conclusion

We have defined ANAPSID, an adaptive query processing engine for RDF Linked Data accessible through SPARQL endpoints. ANAPSID provides a set of physical operators and an execution engine able to adapt the query execution to the availability of the endpoints and to hide delays from users. Reported experimental results suggest that our proposed techniques reduce execution times and are able to produce answers when other engines fail. Also, depending on the selectivity of the join operator and the data transfer delays, ANAPSID operators may overcome state-of-the-art Symmetric Hash Join operators. In the future, we plan to extend ANAPSID with more powerful and lightweight operators like Eddy and MJoin, which are able to route received responses through different operators and adapt the execution to unpredictable delays by changing the order in which each data item is routed.

Future work

In the future we plan to extend ANAPSID with more powerful and lightweight operators like Eddy and MJoin, which are able to route received responses through different operators, and adapt the execution to unpredictable delays by changing the order in which each data item is routed.

Approach

Positive Aspects: - decompose the query into simple sub-plans that can be executed by the remote endpoints. - propose a set of physical operators that gather data generated by the endpoints, and quickly produce responses. - an execution engine able to adapt the query execution to the availability of the endpoints and to hide delays from users.

Negative Aspects: -

Limitations: -

Challenges: Query Decomposition, Query Optimization, and Query adaptation.

Proposes Algorithm: -

Methodology: Lightweight wrappers translate SPARQL sub-queries into calls to endpoints as well as convert endpoint answers into ANAPSID internal structures. Mediators maintain information about endpoint capabilities, statistics that describe their content and performance, and the ontology used to describe the data accessible through the endpoint. The Local As View (LAV) approach is used to describe endpoints in terms of the ontology used in the endpoint dataset. Further, mediators implement query rewriting techniques, decompose queries into sub-queries against the endpoints, and gather data retrieved from the contacted endpoints. Currently, only SPARQL queries comprised of joins are considered; however, the rewriting techniques have been extended to consider all SPARQL operators, but this piece of work is out of the scope of this paper. Finally, mediators hide delays, and produce answers as quickly as data arrives.

Requirements: {{{Requirements}}}

Limitations: -

Implementations

Download-page: https://github.com/anapsid/anapsid

Access API: -

Information Representation: RDF

Data Catalogue: -

Runs on OS: Linux CentOS

Vendor: -

Uses Framework: Twisted Network framework

Has Documentation URL: https://github.com/anapsid/anapsid

Programming Language: Python 2.6.5

Version: 1

Platform: -

Toolbox: -

GUI: No

Research Problem

Subproblem of: query processing on Linked Data

RelatedProblem: decompose queries into sub-queries that can be executed by the selected endpoints

Motivation: distrinution of RDF datastores

Evaluation

Experiment Setup: We empirically analyze the performance of the proposed query processing techniques and report on the execution time of plans comprised of ANAPSID operators versus queries posed against SPARQL endpoints, and state-of-the-art RDF engines. Three sets of queries were considered (Table of Figure 5(b)); each sub-query was executed as a query against its corresponding endpoint. Benchmark 1 is a set of 10 queries against LinkedSensorData-blizzards; each query can be grouped into 4 or 5 sub-queries. Benchmark 2 is a set of 10 queries over linkedCT with 3 or 4 subqueries. Benchmark 3 is a set of 10 queries with 4 or 5 sub-queries executed against linkedCT and DBPedia endpoints. Experiments were executed on a Linux CentOS machine with an Intel Pentium Core2 Duo 3.0 GHz and 8GB RAM.

Evaluation Method : report on the execution time of plans comprised of ANAPSID operators versus queries posed against SPARQL endpoints, and state-of-the-art RDF engines

Hypothesis: -

Description: We report on runtime performance, which corresponds to the user time produced by the _ 􀀀_ command of the Unix operation system. Experiments in RDF-3X were run in both cold and warm caches; to run cold cache, we executed the same query five times by dropping the cache just before running the first iteration of the query. Each query executed by ANAPSID and SPARQL endpoints was run ten times, and we report on the average time.

Dimensions: Performance

Benchmark used: LinkedSensorData-blizzards, linkedCT and DBPedia (english articles)

Results: We observe that SHJ and ANAPSID operators are able to produce the first tuple faster than ARQ or Hash join, even in an ideal scenario with no delays; further, ARQ performance is clearly aff_ected by data transfer distribution and its execution time can be almost two orders of magnitude greater than the time of SHJ or ANAPSID. We notice that SHJ and ANAPSID are competitive, this is because the number of intermediate results is very small, and the benefits of the RJTs cannot be exploited. This suggests that the performance of ANAPSID operators depends on the selectivity of the join operator and the data transfer delays.