SIP & SDP – Session Description Protocol

Protokołu SIP jest protokołem sygnalizacyjnym używanym do inicjowania sesji multimedialnych. SIP umożliwiają realizacje negocjacji charakterystyki sesji oraz jej aktualizacje ale sam w sobie nie zawiera mechanizmów do opisu kształtu sesji, przenoszonej za pomocą protkołu RTP. W tym celu SIP wykorzystuje protokół SDP (Session Descriptions Protocol)
wchodzący bezpośrednio w skład komunikatu. SDP zawiera informacje o rodzaju
mediów, kodekach i ich parametrach, adresach IP, kierunku strumieniów, dostępnym pasmie itp.

Opis sesji:

  • v= (protocol version) – wersja protokołu -> “0”
  • o= (originator and session identifier) – zrodlo i identyfikator sesji -> “dowolna_nazwa id_sesji wersja_sesji IN (IP4|IP6) adresIP_zródła_sesji”
  • s= (session subject) – temat sesjii -> “-“
  • i=* (session information) – opis sesji -> nie używany
  • u=* (URI of description) – uri dodatkowe opisu sesji -> nie używany
  • e=* (email address) – adres email osoby odpowiedzialnej za sesje -> nie używany
  • p=* (phone number) – numer telefoniczny osoby odpowiedzialnej za sesje -> nie używany
  • c=* (connection information) – opis połaczenia, nie wymagany jeśli obecny dla każdego strumienia mediów -> “IN (IP4|IP6) adresIP”
  • b=* (zero or more bandwidth information lines) – sugerowane pasmo -> nie używany
  • One or more time descriptions
  • z=* (time zone adjustments)- definicja strefy czasu -> nie używany
  • k=* (encryption key) – klucz szyfrujący -> nie używane
  • a=* (zero or more session attribute lines) – atrybuty: sendonly – jeśli strona tylko chce wysyłać media, recvonly – jeśli strona chce tylko odbierać media, inactive – bez mediów, sendrecv – jeśli strona chce wysyłać i odbierać media
  • Zero or more media descriptions

Opis Czasu:

  • t= (time the session is active) – czas sesji -> “0 0”
  • r=* (zero or more repeat times) – cykliczność sesji -> nie używany

Opis Mediów:

  • m= (media name and transport address) – opis mediów -> “(audio|video|text) RTP/AVP opis danych medialnych”
  • i=* (media title) – opis mediów -> nie używany
  • c=* (connection information) – opis połaczenia, nie wymagany jeśli obecny w opisie sesji, nadpisuje wartość z opisu sesji -> “IN (IP4|IP6) AdresIP”
  • b=* (zero or more bandwidth information lines) – sugerowane pasmo -> nie używany
  • k=* (encryption key) – klucz szyfrujący -> nie używane
  • a=* (zero or more media attribute lines)  – atrybuty strumienia, nadpisują atrybuty zdefiniowane w opisie sesji: sendonly – jeśli strona tylko chce wysyłać media, recvonly
    – jeśli strona chce tylko odbierać media, inactive – bez mediów,
    sendrecv – jeśli strona chce wysyłać i odbierać media, rtcp – port dla rtcp jesli nie, ptime – dlugosc mediów w sekunadach w przesylanym pakiecie, rtpmap – mapuje numer typu zawartosci do konkretnego kodeka i jego czestotliwosci, fmtp – umożliwia mapowanie parametrow tak aby sdp nie musialo tego rozumiec
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JBoss Envers and Spring transaction managers

I've stumbled upon a bug with my configuration for JBoss Envers today, despite having integration tests all over the application. I have to admit, it casted a dark shadow of doubt about the value of all the tests for a moment. I've been practicing TDD since 2005, and frankly speaking, I should have been smarter than that.

My fault was simple. I've started using Envers the right way, with exploratory tests and a prototype. Then I've deleted the prototype and created some integration tests using in-memory H2 that looked more or less like this example:

public void savingAndUpdatingPersonShouldCreateTwoHistoricalVersions() {
    Person person = createAndSavePerson();
    String oldFirstName = person.getFirstName();
    String newFirstName = oldFirstName + "NEW";

    updatePersonWithNewName(person, newFirstName);

    verifyTwoHistoricalVersionsWereSaved(oldFirstName, newFirstName);

private Person createAndSavePerson() {
    Transaction transaction = session.beginTransaction();
    Person person = PersonFactory.createPerson();;
    return person;

private void updatePersonWithNewName(Person person, String newName) {
    Transaction transaction = session.beginTransaction();

private void verifyTwoHistoricalVersionsWereSaved(String oldFirstName, String newFirstName) {
    List<Object[]> personRevisions = getPersonRevisions();
    assertEquals(2, personRevisions.size());
    assertEquals(oldFirstName, ((Person)personRevisions.get(0)[0]).getFirstName());
    assertEquals(newFirstName, ((Person)personRevisions.get(1)[0]).getFirstName());

private List<Object[]> getPersonRevisions() {
    Transaction transaction = session.beginTransaction();
    AuditReader auditReader = AuditReaderFactory.get(session);
    List<Object[]> personRevisions = auditReader.createQuery()
            .forRevisionsOfEntity(Person.class, false, true)
    return personRevisions;

Because Envers inserts audit data when the transaction is commited (in a new temporary session), I thought I have to create and commit the transaction manually. And that is true to some point.

My fault was that I didn't have an end-to-end integration/acceptance test, that would call to entry point of the application (in this case a service which is called by GWT via RPC), because then I'd notice, that the Spring @Transactional annotation, and calling transaction.commit() are two, very different things.

Spring @Transactional annotation will use a transaction manager configured for the application. Envers on the other hand is used by subscribing a listener to hibernate's SessionFactory like this:

<bean id="sessionFactory" class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean" >        
 <property name="eventListeners">
     <map key-type="java.lang.String" value-type="org.hibernate.event.EventListeners">
         <entry key="post-insert" value-ref="auditEventListener"/>
         <entry key="post-update" value-ref="auditEventListener"/>
         <entry key="post-delete" value-ref="auditEventListener"/>
         <entry key="pre-collection-update" value-ref="auditEventListener"/>
         <entry key="pre-collection-remove" value-ref="auditEventListener"/>
         <entry key="post-collection-recreate" value-ref="auditEventListener"/>

<bean id="auditEventListener" class="org.hibernate.envers.event.AuditEventListener" />

Envers creates and collects something called AuditWorkUnits whenever you update/delete/insert audited entities, but audit tables are not populated until something calls AuditProcess.beforeCompletion, which makes sense. If you are using org.hibernate.transaction.JDBCTransaction manually, this is called on commit() when notifying all subscribed javax.transaction.Synchronization objects (and enver's AuditProcess is one of them).

The problem was, that I used a wrong transaction manager.

<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager" >
    <property name="dataSource" ref="dataSource"/>

This transaction manager doesn't know anything about hibernate and doesn't use org.hibernate.transaction.JDBCTransaction. While Synchronization is an interface from javax.transaction package, DataSourceTransactionManager doesn't use it (maybe because of simplicity, I didn't dig deep enough in org.springframework.jdbc.datasource), and thus Envers works fine except not pushing the data to the database.

Which is the whole point of using Envers.

Use right tools for the task, they say. The whole problem is solved by using a transaction manager that is well aware of hibernate underneath.

<bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager" >
    <property name="sessionFactory" ref="sessionFactory"/>

Lesson learned: always make sure your acceptance tests are testing the right thing. If there is a doubt about the value of your tests, you just don't have enough of them,

Context menu or Action buttons ?

Recently I was drawn into one of those UI "religious" disputes that has no easy answers and usually both sides are right. One of our web developers was trying out new web tech (with pretty rich widget library) and started to question himself about some basic usability decisions. The low level problem in this case is usually brought to "which widget should I use ?". I'm not fond of bringing the usability problems to questions: Should I use Tabs over Menu ? Or should I use Context menu instead of buttons panel ? But sometimes if time is crucial factor and other usability levels are by default not addressed at all - better developer that asks those basic questions than developer that do not question himself at all.