Strabon

view README @ 1281:3ec1c2e8f6d3

updated README
author Konstantina Bereta <Konstantina.Bereta@di.uoa.gr>
date Sun Oct 20 12:23:58 2013 +0300 (2013-10-20)
parents da14a303bf2c
children b5fbf3cca0f7
line source
1 Strabon
2 -----------
4 Introduction
5 ============
6 Strabon is a fully implemented semantic geospatial database system that can be
7 used to store linked geospatial data expressed in RDF and query them using an
8 extension of SPARQL. Strabon supports spatial selections, spatial joins, a rich
9 set of spatial functions similar to those offered by geospatial relational
10 database systems, support for multiple Coordinate Reference Systems and widely
11 used serializations for geometric objects such as WKT and GML. Strabon is built
12 on top of the well-known RDF store Sesame and extends Sesame’s components to be
13 able to manage thematic and spatial data that are stored in PostGIS.
15 The development of Strabon started in the context of European FP7 project
16 SemsorGrid4Env (Semantic Sensor Grids for Rapid Application Development for
17 Environmental Management) [http://www.semsorgrid4env.eu/]. Starting September
18 2011, Strabon is being utilized and extended with new functionalities in the
19 FP7 project TELEIOS (Virtual Observatory Infrastructure for Earth Observation
20 Data) [http://www.earthobservatory.eu/] which our group leads.
22 The query language of Strabon is called stSPARQL. stSPARQL can be used to query
23 data represented in an extension of RDF called stRDF. stRDF and stSPARQL have
24 been designed for representing and querying geospatial data that changes over
25 time (e.g., the growth of a city over the years due to new developments).
27 Given the very close relationship between stSPARQL and GeoSPARQL which is a
28 recent OGC standard for an extension of SPARQL for querying geospatial metadata,
29 we recently provided support for the Core, Geometry and Geometry Topology
30 extension of GeoSPARQL.
33 Conformance to GeoSPARQL
34 ========================
35 Strabon implements the `Core', the `Topology Vocabulary Extension', the
36 `Geometry Extension', the `Geometry Topology Extension', and the `RDFS Entailment
37 Extension' except for Req. 25
38 (http://www.opengis.net/spec/geosparql/1.0/req/rdfs-entailment-extension/bgp-rdfs-ent).
40 With respect to GML, Strabon supports the GML Profile corresponding to Simple
41 Features, that is, GML Simple Features Profile 2.0.
44 Strabon Homepage
45 ================
46 The homepage of Strabon is at http://www.strabon.di.uoa.gr/.
49 Demo
50 ====
51 You can find a demo of the system Strabon at http://test.strabon.di.uoa.gr/NOA/.
54 How to build and run Strabon from command line
55 ==============================================
56 Assuming you have already downloaded Strabon and you are in the top-level
57 directory of Strabon, issue the following command to build it from command line:
59 $ mvn clean package
61 In order to run automatically the JUnit tests, pass the option `-DskipTests=false'
62 to the above command. The complete command is the following:
64 $ mvn -DskipTests=false clean package
66 In case of an error during building of Strabon and assuming that the error does
67 not come from the JUnit tests, please have a look at the `Known Issues' section
68 below. If none of the known issues of that section applies, please contact the
69 developers through the Strabon mailing-list or submit a corresponding bug
70 (see `Bugs' section below).
72 After you have successfully built Strabon, you have access to the following
73 components:
75 * Strabon Endpoint
77 This is a SPARQL endpoint for Strabon. It is distributed as a war file so
78 you may deploy it in a Tomcat container. You may find the war file under
79 directory `endpoint/target'.
81 * Strabon Endpoint (standalone)
83 This is a SPARQL endpoint for Strabon like the above one, but it differs
84 only in that it does not require the user to have already set up a Tomcat
85 container. The standalone Strabon Endpoint may be run by issuing the
86 following command:
88 $ java -jar endpoint-exec/target/strabon-endpoint-executable-${version}.jar
90 After issuing the above command, you may access the Strabon Endpoint at
91 the following URL: <http://localhost:8080/>.
93 Please see the page at <http://hg.strabon.di.uoa.gr/Strabon/rev/674f8f91162b>
94 to find out other options that you may pass to the Tomcat container that
95 will run by the above command.
97 SPECIAL NOTE: if you need to configure the connection details to the
98 underlying database, you may do so in two ways:
99 1. By modifying file `endpoint/WebContent/WEB-INF/connection.properties'
100 before building Strabon and executing the above command.
101 2. After executing the above command, by visiting the following page by a
102 browser: <http://localhost:8080/ChangeConnection>
104 * Strabon Endpoint Client
106 This is a Java client for interacting with Strabon Endpoint or any other
107 SPARQL endpoint. It is packaged as a jar file and may be found under
108 directory `endpoint-client/target/' with name
109 `strabon-endpoint-client-${version}.jar'. This jar contains any dependencies
110 to other code, so may copy and paste it to your project and start playing
111 with the code immediately.
113 * Strabon script
115 The `strabon' script is located under the `scripts/' directory and it is the
116 main command-line tool for interacting with Strabon. You may use it to store
117 RDF data with geospatial information or query/update it using one of
118 stSPARQL or GeoSPARQL query languages.
120 * Endpoint script
122 The `endpoint' script is located under the `scripts' directory and it is the
123 main command-line tool for interacting with a `Strabon Endpoint'. You may
124 use it to do any operation you would like to do with the `strabon' script
125 above, but in contrast to the `strabon' script you need to have access to a
126 Strabon endpoint. Of course, the `Strabon Endpoint Client' component above
127 can be used as well as a command-line tool. At the time of writing, the
128 `Strabon Endpoint Client' component supports only querying of RDF data with
129 geospatial information.
132 Getting Started
133 ===============
134 To get started with Strabon please have a look at the tutorial for the stRDF
135 data model and stSPARQL query language, the User Guide, and the Developer Guide.
137 stRDF and stSPARQL tutorial
138 http://www.strabon.di.uoa.gr/files/stSPARQL_tutorial.pdf
141 stSPARQL Reference
142 ~~~~~~~~~~~~~~~~~~
143 The reference for the spatial and temporal extension functions defined in
144 stSPARQL can be found at http://www.strabon.di.uoa.gr/stSPARQL#spatial and
145 http://www.strabon.di.uoa.gr/stSPARQL#temporals respectively.
148 User Guide
149 ~~~~~~~~~~
150 Assuming that you are familiar with Maven, the following steps need to be
151 followed in order to use Strabon using Eclipse:
153 1. Install PostgreSQL from http://www.postgresql.org/download/. At the time of
154 this writing the latest PostgreSQL version is 9.1.
155 2. Install PostGIS from http://postgis.refractions.net/download/. At the time of
156 this writing we have tested Strabon with PostGIS 1.5.3. For the
157 temporal features of stSPARQL download the PostgreSQL temporal extension
158 from here: https://github.com/jeff-davis/PostgreSQL-Temporal and follow
159 the installation guide.
160 3. Install Maven from http://maven.apache.org/download.html. At the time of this
161 writing the latest Maven version is 3.0.4.
162 4. Install Eclipse from http://www.eclipse.org/downloads/. At the time of this
163 writing the latest Eclipse version is 3.7.2.
164 5. Install the m2e plugin for Eclipse from http://www.eclipse.org/m2e/.
165 6. Install the MercurialEclipse plugin for Eclipse from
166 http://javaforge.com/project/HGE .
167 7. From Eclipse, go to File --> Import --> Mercurial --> Clone Existing
168 Mercurial Repository --> Next. In the URL textarea paste the following
169 URL: http://hg.strabon.di.uoa.gr/StrabonUser and then press Next --> Next -->
170 Finish. If you used the default settings, you should have a new project named
171 StrabonMain. Right click on the project and select Configure --> Convert to
172 Maven project. Eclipse will enable Maven dependency management for the
173 project, download any dependencies and build the project.
176 Storing stRDF graphs and evaluating stSPARQL queries
177 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
178 You can see some examples in the classes gr.uoa.di.strabon.example.PostgisExample
179 and gr.uoa.di.strabon.example.PostgisExample2.
181 Tuning PostgreSQL
182 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
183 The default settings of Postgres are rather conservative. As a result, parameter
184 tuning is neccessary for speeding up Postgres, therefore Strabon. If you are
185 using Strabon to compare its performance against your implementation of
186 stSPARQL/GeoSPARQL, you are *strongly* encouraged to contact us using the Strabon
187 Users mailing list for assistance on tuning Postgres.
189 You can follow the instructions below for tuning a Postgres server running on an
190 Ubuntu machine that is dedicated to PostgreSQL and Strabon.
192 1. Append the following text at the end of postgresql.conf.
193 *Uncomment* the appropriate lines.
195 ### RAM
196 ## 4 GB of RAM
197 #shared_buffers = 3GB
198 #effective_cache_size = 3GB
199 #maintenance_work_mem = 1GB
200 #work_mem = 2GB
201 ## 8 GB of RAM
202 #shared_buffers = 5GB
203 #effective_cache_size = 6GB
204 #maintenance_work_mem = 2GB
205 #work_mem = 5GB
206 ## 16 GB of RAM
207 #shared_buffers = 10GB
208 #effective_cache_size = 14GB
209 #maintenance_work_mem = 4GB
210 #work_mem = 10GB
211 ## 24 GB of RAM
212 #shared_buffers = 16GB
213 #effective_cache_size = 22GB
214 #maintenance_work_mem = 6GB
215 #work_mem = 15GB
216 ## 48 GB of RAM
217 #shared_buffers = 32GB
218 #effective_cache_size = 46GB
219 #maintenance_work_mem = 8GB
220 #work_mem = 30GB
221 ## 64 GB of RAM
222 # contact us to find out!
223 ### HD
224 ## RAID with ordinary 7.200 disks
225 #random_page_cost = 3.5 #3.0-3.5
226 ## High-End NAS/SAN
227 #random_page_cost = 2 #1.5-2.5
228 ## Amazon EBS/Heroku
229 #random_page_cost = 1.3 #1.1-2.0
230 ## SSD array
231 #random_page_cost = 2.0 #1.5-2.5
232 ### Planner options
233 # Increase the following values in order to avoid using the GEQO planner.
234 # Small values (<8) reduce planning time but may produce inferior query plans
235 #
236 geqo_threshold = 15 # keep this value larger that the following two parameters
237 from_collapse_limit = 14
238 join_collapse_limit = 14
239 ### Misc
240 default_statistics_target = 10000
241 constraint_exclusion = on
242 checkpoint_completion_target = 0.9
243 wal_buffers = 32MB
244 checkpoint_segments = 64
245 ### Connections
246 max_connections = 10
248 2. Append the following lines at the end of /etc/sysctl.conf
249 *Uncomment* the appropriate lines.
251 ## 4 GB of RAM
252 #kernel.shmmax = 3758096384
253 #kernel.shmall = 3758096384
254 #kernel.shmmni = 4096
255 ## 8 GB of RAM
256 #kernel.shmmax = 5905580032
257 #kernel.shmall = 5905580032
258 #kernel.shmmni = 4096
259 ## 16 GB of RAM
260 #kernel.shmmax = 11274289152
261 #kernel.shmall = 11274289152
262 #kernel.shmmni = 4096
263 ## 24 GB of RAM
264 #kernel.shmmax = 17716740096
265 #kernel.shmall = 17716740096
266 #kernel.shmmni = 4096
267 ## 48 GB of RAM
268 #kernel.shmmax = 35433480192
269 #kernel.shmall = 35433480192
270 #kernel.shmmni = 4224
271 ## 64 GB of RAM
272 # contact us to find out!
274 3. Apply all changes by executing
276 $ sudo sysctl -p
277 $ sudo /etc/init.d/postgresql restart
279 4. Prepare for the next run by issuing the command
281 $ sudo -u postgres psql -c 'VACUUM ANALYZE;' db
283 or
285 $ psql -c 'VACUUM ANALYZE;' db
287 where db is the name of the Postgres database that Strabon will use.
290 Developer Guide
291 ~~~~~~~~~~~~~~~
292 Assuming that you are familiar with Maven, the following steps need to be
293 followed in order to use Strabon using Eclipse:
295 1. Install PostgreSQL from http://www.postgresql.org/download/. At the time of
296 this writing the latest PostgreSQL version is 9.1.
297 2. Install PostGIS from http://postgis.refractions.net/download/. At the time of
298 this writing we have tested Strabon with PostGIS 1.5.3. For the
299 temporal features of stSPARQL download the PostgreSQL temporal extension
300 from here: https://github.com/jeff-davis/PostgreSQL-Temporal and follow
301 the installation guide.
302 3. Install Maven from http://maven.apache.org/download.html. At the time of this
303 writing the latest Maven version is 3.0.4.
304 4. Install Eclipse from http://www.eclipse.org/downloads/. At the time of this
305 writing the latest Eclipse version is 3.7.2.
306 5. Install the m2e plugin for Eclipse from http://www.eclipse.org/m2e/.
307 6. Install the MercurialEclipse plugin for Eclipse from
308 http://javaforge.com/project/HGE.
309 7. From Eclipse, go to File --> Import --> Mercurial --> Clone Existing
310 Mercurial Repository --> Next. In the URL textarea paste the following
311 URL: http://hg.strabon.di.uoa.gr/Strabon and then press Next --> Next -->
312 Finish. If you used the default settings, you should have a new project named
313 StrabonMain. Right click on the project and select Configure --> Convert to
314 Maven project. Eclipse will enable Maven dependency management for the
315 project, download any dependencies and build the project.
318 Tester Guide
319 ~~~~~~~~~~~~
320 Assuming again that you are familiar with Maven and Junit these are the steps
321 you need to follow to test the functionality of Strabon:
324 * Using Eclipse
326 If you want to create a new test:
328 1. Import Strabon into Eclipse as explained in the Developer Guide.
329 2. Go to strabon-testsuite project.
330 3. Create a new folder (Recommended folder name: <test's name>) and place inside the following files:
331 3.1. An ntriples or nquads file with the test dataset (with .nt or .nq extension).
332 3.2. Pairs of files with sparql test queries and expected test results in xml format.
333 Notice that each pair must have the same name and .rq extension for the queryFile and .srx extension for the resultsFile.
334 4. Create a test class that extends TemplateTest class.
335 5. If you have followed the recommendations the test is ready. If you have different names or location for your
336 files, insert them explicitly in the constructor of the class. WARNING: All prefixes must be placed in file "prefixes" so that
337 every time a namespace changes, we have to change it just once.
339 If you want to run a test:
341 1. Right-click on the test class.
342 2. Select "Run as JUnit Test".
343 3. Database properties are retrieved from database.properties file. If you want, you can change a property
344 "on the fly" with an environment variable.
347 * Command Line
349 If you want to run all the tests:
351 1. Go to Strabon directory (root directory of all the subprojects).
352 2. Run "mvn test -DskipTests=false".
353 3. Optionally you can pass an environment variable with "-DvariableName=variableValue".
356 Storing stRDF graphs and evaluating stSPARQL queries
357 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
358 You can see some examples in the classes
359 eu.earthobservatory.runtime.postgis.StoreOp and
360 eu.earthobservatory.runtime.postgis.QueryOp.
363 Download
364 ========
365 You can download the source code of the latest version of Strabon by accessing
366 our public mercurial repository located at http://hg.strabon.di.uoa.gr/Strabon.
367 You can find more information on how to use and extend Strabon at the Getting
368 Started section.
371 Publications
372 ============
373 You can learn about stRDF data model and stSPARQL query language employed in
374 Strabon by reading our tutorial under the Getting Started section and/or the
375 publications given on this page.
377 The current versions of stRDF and stSPARQL which are based on OGC standards are
378 presented in the following documents:
379 * K. Kyzirakos, M. Karpathiotakis, and M. Koubarakis. Strabon: A Semantic
380 Geospatial DBMS. In Internatioanl Semantic Web Conference (ISWC'12). Boston,
381 USA, November 11-15, 2012.
382 [pdf: http://strabon.di.uoa.gr/files/strabon-iswc.pdf]
384 * Manolis Koubarakis, Kostis Kyzirakos, Babis Nikolaou, Michael Sioutis, and
385 Stavros Vassos. A data model and query language for an extension of RDF with
386 time and space. Deliverable D2.1, European ICT project TELEIOS, 2011.
387 [pdf: http://strabon.di.uoa.gr/files/deliv2-1-re-revised.pdf]
389 The temporal features of stRDF and stSPARQL are presented in the following
390 document:
391 * K. Bereta, P. Smeros and M. Koubarakis. Representing and Querying the
392 Valid Time of Triples for Linked Geospatial Data. In the 10th Extended
393 Semantic Web Conference (ESWC 2013). Montpellier, France. May 26-30, 2013.
394 [pdf: http://www.strabon.di.uoa.gr/files/eswc2013.pdf]
396 The initial versions of stRDF and stSPARQL that are based on constraint
397 databases are presented in the following publications:
398 * Manolis Koubarakis and Kostis Kyzirakos. Modeling and Querying Metadata in
399 the Semantic Sensor Web: the Model stRDF and the Query Language stSPARQL.
400 In 7th Extended Semantic Web Conference (ESWC 2010). Heraklion, Crete,
401 30 May - 03 June, 2010.
402 [pdf: http://strabon.di.uoa.gr/files/stSPARQL.pdf]
404 * Kostis Kyzirakos, Manos Karpathiotakis and Manolis Koubarakis. Developing
405 Registries for the Semantic Sensor Web using stRDF and stSPARQL (short
406 paper).
407 In Proceedings of 3rd International workshop on Semantic Sensor Networks
408 2010, in conjunction with ISWC 2010, November 2010, Shanghai, China.
409 [pdf: http://strabon.di.uoa.gr/files/strabon.pdf]
411 Applications of stRDF, stSPARQL, and the system Strabon are described here:
412 * Alasdair J. G. Gray, Raúl García-Castro, Kostis Kyzirakos, Manos
413 Karpathiotakis, Jean-Paul Calbimonte, Kevin Page, Jason Sadler, Alex
414 Frazer, Ixent Galpin, Alvaro A. A. Fernandes, Norman W. Paton, Oscar
415 Corcho, Manolis Koubarakis, David De Roure, Kirk Martinez and Asunción
416 Gómez-Pérez. A Semantically Enabled Service Architecture for Mashups over
417 Streaming and Stored Data. In 8th Extended Semantic Web Conference (ESWC
418 2011). Heraklion, Crete, May 20 - June 2, 2011.
419 [pdf: http://strabon.di.uoa.gr/files/Gray2011Architecture.pdf]
421 * A.J.G. Gray, J. Sadler, O. Kit, K. Kyzirakos, M. Karpathiotakis, J.-P.
422 Calbimonte, K. Page, R. García-Castro, A. Frazer, I. Galpin, A.A.A.
423 Fernandes, N.W. Paton, O. Corcho, M. Koubarakis, D.D. Roure, K. Martinez,
424 A. Gómez-Pérez. A Semantic Sensor Web for Environmental Decision Support
425 Applications. Sensors. 11, 8855-8887.
426 [pdf: http://strabon.di.uoa.gr/files/sensors-11-08855.pdf]
428 Coming up soon:
429 Strabon will soon support an extension of RDF for incomplete geospatial
430 information. The following publication gives a preview of the relevant research
431 problems:
432 * M. Koubarakis, K. Kyzirakos, M. Karpathiotakis, C. Nikolaou, M. Sioutis,
433 S. Vassos, D. Michail, T. Herekakis, C. Kontoes and I. Papoutsis. Challenges
434 for Qualitative Spatial Reasoning in Linked Geospatial Data. In Proceedings
435 of IJCAI 2011 Workshop on Benchmarks and Applications of Spatial Reasoning,
436 Barcelona, Spain.
437 [pdf: http://www.earthobservatory.eu/publications/SciQL_ADASS2011.pdf]
439 * C. Nikolaou and M. Koubarakis: "Querying Linked Geospatial Data with
440 Incomplete Information". In 5th International Terra Cognita Workshop -
441 Foundations, Technologies and Applications of the Geospatial Web. In
442 conjunction with the 11th International Semantic Web Conference, Boston,
443 USA, November, 2012.
444 [http://www.earthobservatory.eu/publications/iswc-workshop.pdf]
447 Contributors
448 ============
449 The system Strabon has been developed by the following members of our team:
451 * Manos Karpathiotakis <mk@di.uoa.gr>
452 * Kostis Kyzirakos <kkyzir@di.uoa.gr>
453 * Manolis Koubarakis <koubarak@di.uoa.gr>
454 * Giorgos Garbis <ggarbis@di.uoa.gr>
455 * Konstantina Bereta <konstantina.bereta@di.uoa.gr>
456 * Charalampos Nikolaou <charnik@di.uoa.gr>
457 * Stella Gianakopoulou <sgian@di.uoa.gr>
458 * Panagiotis Smeros <psmeros@di.uoa.gr>
459 * Kallirroi Dogani <kallirroi@di.uoa.gr>
462 Mailing-list
463 ============
464 Currently, we maintain the following mailing lists:
466 * Strabon-users, is used as a communication channel for Strabon users.
467 To subscribe to the mailing-list, please visit page
468 http://cgi.di.uoa.gr/~mailman/listinfo/strabon-users. To post e-mails
469 to Strabon-users mailing-list, write to strabon-users@di.uoa.gr.
471 * Strabon-devel, is used as a communication channel with the developers
472 of Strabon. To subscribe to the mailing-list, please visit page
473 http://cgi.di.uoa.gr/~mailman/listinfo/strabon-devel. To post e-mails
474 to Strabon-devel mailing-list, write to strabon-devel@di.uoa.gr.
477 Bugs
478 ====
479 Please report bugs to http://bug.strabon.di.uoa.gr/report or
480 the Strabon-devel mailing-list Strabon-devel@di.uoa.gr.
483 Known Issues
484 ============
485 * By default, Tomcat uses ISO-8859-1 character encoding when decoding URLs received
486 from a browser. This can cause problems when encoding is UTF-8, and you are using
487 international characters. In order to fix this, edit conf/server.xml and find the
488 line where the Connector is defined. Add the parameter URIEncoding and set it to
489 UTF-8. For example:
491 <Connector port="8080" protocol="HTTP/1.1"
492 connectionTimeout="20000"
493 URIEncoding="UTF-8"
494 redirectPort="8443" />
496 * Building and executing any maven goals fails for maven versions <3.0 due to a
497 dependency to the `shade' plugin that is available only for maven version 3.0
498 (http://maven.apache.org/plugins/maven-shade-plugin/). In such systems, you may
499 disable execution of this plugin by setting the environmental variable
500 `shade.skip'. For example, to build Strabon using maven version 2.0 you may
501 execute the following command:
503 $ mvn clean package -Dshade.skip
505 * When using MonetDB as a backend, the following source code of MonetDB must be
506 used:
507 https://hg.strabon.di.uoa.gr/MonetDB/
510 License
511 =======
512 This Source Code Form is subject to the terms of the Mozilla Public
513 License, v. 2.0. If a copy of the MPL was not distributed with this
514 file, You can obtain one at http://mozilla.org/MPL/2.0/.
516 Copyright (C) 2010, 2011, 2012, 2013 Pyravlos Team
518 http://www.strabon.di.uoa.gr/
521 How to apply the license
522 ========================
523 * In the beginning of script files (after the shell directive) paste the
524 following statement:
525 #
526 # This Source Code Form is subject to the terms of the Mozilla Public
527 # License, v. 2.0. If a copy of the MPL was not distributed with this
528 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
529 #
530 # Copyright (C) 2010, 2011, 2012, 2013 Pyravlos Team
531 #
532 # http://www.strabon.di.uoa.gr/
533 #
535 * In the beginning of Java source code files paste the following statement:
536 /**
537 * This Source Code Form is subject to the terms of the Mozilla Public
538 * License, v. 2.0. If a copy of the MPL was not distributed with this
539 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
540 *
541 * Copyright (C) 2010, 2011, 2012, 2013 Pyravlos Team
542 *
543 * http://www.strabon.di.uoa.gr/
544 */
546 * In the beginning of HTML/XML files paste the following statement:
547 <!-- This Source Code Form is subject to the terms of the Mozilla Public
548 - License, v. 2.0. If a copy of the MPL was not distributed with this
549 - file, You can obtain one at http://mozilla.org/MPL/2.0/.
550 -
551 - Copyright (C) 2010, 2011, 2012, 2013 Pyravlos Team
552 -
553 - http://www.strabon.di.uoa.gr/
554 -->