Archive for the 'Construct internals' Category

Speaking Construct Protocols

Published
by
graeme
on July 9, 2008
in Construct internals
. 0 Comments

All messages that are exchanged between Construct services and applications take the following format: [3 bytes][10 bytes][payload]

  • The first 3 bytes correspond to the message type identifier (see below).

The next 10 bytes correspond to the length of the message payload in bytes. For example, a message with a 64 byte payload would be written as 0000000064.

  • Finally, the message payload consists of a number of bytes indicated by the previous part of the message. The contents of the payload are protocol specific.

The message identifier codes used are as follows:

The protocol identifier for querying QUERY 200
The protocol identifier used for responding to queries QUERY_RESPONSE 210
The protocol identifier used for sending rdf statements to the data port RDF_ADD 100
The protocol identifier used for responding to an add rdf statements request RDF_ADD_RESPONSE 110
The protocol identifier used for sending a service descriptor SERVICE_DESCRIPTOR_RESPONSE 310

When most service send a response to the client, the payload also takes the form of a code:

The operation succeeded OK 600
An error occured during the operation ERROR 610
The service code was unrecognised UNKNOWN 650

Working with the discovery service

Open a connection to the host/port given in the bonjour resolution (there is no need to send any data). You will be sent an XML descriptor file of the form -

  1. <services>
  2. <servicecomponentdescriptor>
  3. <name>Construct DataPort</name>
  4. <description>Raw data port: Connect via a socket and send N-TRIPLE RDF strings. Response string will be ok or error if it fails.</description>
  5. <host>erdinger</host>
  6. <port>3528</port>
  7. <misc>See example applications.</misc>
  8. </servicecomponentdescriptor>
  9. <servicecomponentdescriptor>
  10. <name>Construct QueryService</name>
  11. <description>The query service: Connect via a socket and send SPARQL queries. Response string will be a SPARQL result set in RDF.</description>
  12. <host>erdinger</host>
  13. <port>3531</port>
  14. <misc>See example applications.</misc>
  15. </servicecomponentdescriptor>
  16. </services>

This provides you with all the information required to open a connection to the data port or query service directly.

Working with the data port

Use the protocol described above. This is an example of adding a single line:

  • 1000000000043<http://hello> <http://construct> "world" .

NOTE: The data RDF Triple must have the trailing full stop . Example responses might be:

  • 1100000000003600 (OK)
  • 1100000000003610 (ERROR)
  • 1100000000003650 (UNKNOWN)

Working with the query service

The point of contact for application developers to Construct is the Query Service. It takes a SPARQL query as an input. This query is run on the data store (a list of RDF triples). If the query is valid and answerable, a string of data will be returned. This section will try to help you write a SPARQL query, and make sense of the returned information.

Writing a SPARQL query

The presence of a query service implies that there must be something to query. The data in Construct is stored in the data store. All the data is represented in RDF triples. Here’s a few examples of these: For readability, we will replace the URIs with the prefixes “sighting:” and “person:”. NOTE: The prefixes in the query(below) is valid SPARQL syntax.

  1. PREFIX: sighting:http://srg.ucd.ie/construct/sighting#
  2. PREFIX: person:http://www.pervasive-ontologies.org/ontologies/context/person.owl#
  3. <sighting:Waldo109> <sighting:person> <person:Waldo>
  4. <sighting:Waldo109> <sighing:computer> <http://srg.cs.ucd.ie/construct/computer/waldo.ucd.ie>
  5. <sighting:Waldo109> <sighting:time> <2006-10-04T13:17:59Z>
  6. <sighitng:Waldo109> <sightingstatus> <Active>

These four statements represent a sighting from a computer activity sensor.

The Query

To retrieve information from construct, a query must be written in the SPARQL format. This has a similar “Select X From Y Where Z” form to an SQL query. Here is an example query which relates to the RDF Triples above. It finds all the times of sightings of Waldo(note the time triple above).

  1. String personQuery =  “PREFIX sighting:<http://srg.ucd.ie/construct/sighting#> “
  2. + “PREFIX user:<http://www.pervasive-ontologies.org/ontologies/context/person.owl#> “
  3. + “SELECT ?time “
  4. + “WHERE {”
  5. + “?sighting sighting:person user:Waldo . “
  6. + “?sighting sighting:status sighting:Active . “
  7. + “?sighting sighting:time ?time”
  8. + “}”;

For a more comprehensive tutorial on SPARQL, visit the website at: http://www.w3.org/TR/rdf-sparql-query/

In order to send a query to the query service we must use the protocol described above. Below is an example.

  • 200[payload length][SPARQL QUERY]
The Response

You will be sent back a string of the form

  • 210[payload length][SPARQL result set in RDF form]
  • OR 2100000000003610 (if an error occured)

This String is a SPARQL ResultSet. This is a valid RDF string, represented in N-TRIPLE format. This means that way in which it is processed is dependent on the RDF parsing capabilities of the language used. For example, in java, a new Jena model (or indeed a ResultSet object) can be created from it, and it can be then easily traversed.

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Installing Bonjour for *nix Users

Published
by
graeme
on July 9, 2008
in Advanced and Construct internals
. 0 Comments

Every time Ubuntu’s Synaptic Package Manager updates the Avahi layer for Bonjour support it stomps on my Apple Bonjour install and breaks my Construct install. My guess is that the update puts Avahi first in the list of Bonjours to run when Construct starts. So I get this error:

*** WARNING *** The programme ‘java’ uses the Apple Bonjour compatiblity layer of Avahi.

*** WARNING *** Please fix your application to use the native API of Avahi!

*** WARNING *** For more information see

*** WARNING *** The programme ‘java’ called ‘DNSServiceQueryRecord()’ which is not supported (or only supported partially) in the Apple Bonjour compatiblity layer of Avahi.

*** WARNING *** Please fix your application to use the native API of Avahi!

*** WARNING *** For more information see

One way to address this is just to turn off the Avahi warning. Set an environmental variable list this export AVAHI_COMPAT_NOWARN=1 This isn’t a fix though it is just hiding the warning.

The best I have figured out so far is just to reinstall Bonjour:

Instructions for Installing Apple’s Bonjour on *nix

  1. Download the latest version of the Bonjour source code here.
  2. Decompress the downloaded archive
  3. cd to the mDNSPosix directory below the mDNSResponder-xxx directory (where xxx is the Bonjour version number).
  4. Edit Makefile and change line JDK = /usr/jdk to make it point at your Java installation (e.g., /usr/lib/jvm/java-6-sun)
  5. Type sudo make os=linux Java
  6. Type sudo make os=linux install — if you have problems with this, see below. Most likely you will need to install a specific version of gcc
  7. Copy the java specific files to your jre lib’s ext directory (cp build/prod/* /path/to/jre/lib/ext/)
  8. Open up /etc/nsswitch.conf and ensure that the ‘mdns’ switch appears on the “hosts:” line. My “hosts:” line looks like this:
hosts:          files dns mdns

Remember, whenever you use a new jre, make sure to include dns_sd.jar (Bonjour JAR file) in your jre/lib/ext directory

Problems with the install

Problems with stdlib.h

Lorcan had some problems with this install on Ubuntu (Edgy). He says: when running sudo make os=linux Java I got a lot of errors, starting with ../mDNSShared/dnssd_clientlib.c:71:20: error: stdlib.h: No such file or directory.

I installed the libc6-dev package. When I tried again everything worked fine

Problems with __stack_chk_fail_local

I had some major problems with this install on Ubuntu (Edgy), specifically when using gcc4.1 (to find out which version of gcc you are using type gcc --version).

lorcan@comp:~/Desktop/mDNSResponder-107.6/mDNSPosix$ sudo make os=linux install
Stopping Apple Darwin Multicast DNS / DNS Service Discovery daemon: mdnsd.
cp build/prod/mdnsd /usr/sbin/mdnsd
/usr/sbin/mdnsd installed
cp mdnsd.sh /etc/init.d/mdns
chmod ugo+x /etc/init.d/mdns
/etc/init.d/mdns start
Starting Apple Darwin Multicast DNS / DNS Service Discovery daemon: mdnsd.
ln -s -f /etc/init.d/mdns /etc/rc2.d/S52mdns
ln -s -f /etc/init.d/mdns /etc/rc3.d/S52mdns
ln -s -f /etc/init.d/mdns /etc/rc4.d/S52mdns
ln -s -f /etc/init.d/mdns /etc/rc5.d/S52mdns
ln -s -f /etc/init.d/mdns /etc/rc0.d/K16mdns
ln -s -f /etc/init.d/mdns /etc/rc6.d/K16mdns
/etc/init.d/mdns installed
cp build/prod/libdns_sd.so /usr/lib/libdns_sd.so.1
ln -s -f /usr/lib/libdns_sd.so.1 /usr/lib/libdns_sd.so
/usr/lib/libdns_sd.so.1 /usr/include/dns_sd.h installed
/usr/share/man/man8/mdnsd.8 installed
make[1]: Entering directory `/home/lorcan/Desktop/mDNSResponder-107.6/Clients’
mkdir build
cc dns-sd.c -L../mDNSPosix/build/prod/ -ldns_sd -I../mDNSShared -o build/dns-sd
/usr/bin/ld: build/dns-sd: hidden symbol `__stack_chk_fail_local’ in /usr/lib/libc_nonshared.a(stack_chk_fail_local.oS) is referenced by DSO
/usr/bin/ld: final link failed: Nonrepresentable section on output
collect2: ld returned 1 exit status
make[1]: *** [build/dns-sd] Error 1
make[1]: Leaving directory `/home/lorcan/Desktop/mDNSResponder-107.6/Clients’

make: *** [../Clients/build/dns-sd] Error 2

Solution:
If the version of gcc is not 4.0.x Follow these instructions (EXACTLY!!!) to correct the problem:

  1. Ensure that gcc version 4.0.x is installed. This is a little tricky as the synaptic package manager stopped supporting this version of gcc since edgy. You’ll have to download the following packages:  gcc-4.0-base, cpp-4.0 , gcc-4.0
  2. Open the terminal and cd to where you saved the package.
  3. Do the following for each of the three packages: dpkg -i PACKAGE.deb Install them in the same order the are above. gcc base first, then cpp, then gcc — This should put gcc-4.0 into your /usr/bin directory.
  4. cd /usr/bin
  5. sudo mv gcc gcc-backup
  6. sudo ln -s gcc-4.0 gcc

Now completely rerun the instructions above (at the top of this page) to install Bonjour (you must completely rerun them from step 2). When installation is complete continue these instructions.

  1. sudo cd /usr/bin
  2. sudo rm gcc
  3. sudo mv gcc-backup gcc

Now gcc should point to the original version of gcc (i.e. that version that you were using before you installed Bonjour).

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HTTP Port - Part 2

Published
by
steve
on July 9, 2008
in Construct internals
. 0 Comments

In part 1 of this post I described the new HTTP Port in Construct. In this part of the post I’ll explain how to write a web form and style the return values.

SPARQL Query

You can send a query to Construct with GET or POST. Here is how we do it in HTML with GET:

  1. <form method=“GET” action=“http://duvel.ucd.ie:88