How to add pgp public keys for mongo respository

While installing mongo db, if you are trying to get pgp key using
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10

you might receive following error :

Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver keyserver.ubuntu.com --recv 7F0CEB10
gpg: requesting key 7F0CEB10 from hkp server keyserver.ubuntu.com
gpgkeys: HTTP fetch error 7: couldn't connect to host
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0

You can fix this by storing pgp public key written in http://www.mongodb.org/display/DOCS/Ubuntu+and+Debian+packages into a file say mongo.key and then execute "sudo apt-key add mongo.key" and then proceed with "sudo apt-get update"

Store ajax json response into javascript variable using jquery

If you wish to make an ajax call, and assign response json into a variable directly...here is a small hack :

function getJson(url) {
 return JSON.parse($.ajax({
     type: 'GET',
     url: url,
     dataType: 'json',
     global: false,
     async:false,
     success: function(data) {
         return data;
     }
 }).responseText);
}

var myJsonObj = getJson('myjsonurl');

Umarshalling JSON and XML

If you want to unmarshall json or xml inputstream to Java object, here are the functions :

public static <T> T unmarshalXML(InputStream is, Class<T> c)
   throws JAXBException {
  JAXBContext jc = JAXBContext.newInstance(c);
  Unmarshaller u = jc.createUnmarshaller();
  T response = (T) u.unmarshal(is);
  return response;
 }

 public static <T> T unmarshalJSON(InputStream is, Class<T> c)
   throws JAXBException, IOException, JSONException, XMLStreamException {
  JAXBContext jc = JAXBContext.newInstance(c);
  Unmarshaller u = jc.createUnmarshaller();
  String sJson = IOUtils.toString(is);
  JSONObject obj = new JSONObject(sJson);
  Configuration config = new Configuration();
  MappedNamespaceConvention con = new MappedNamespaceConvention(config);
  XMLStreamReader xmlStreamReader = new MappedXMLStreamReader(obj, con);
  T response = (T) u.unmarshal(xmlStreamReader);
  return response;
 }

Installing ReviewBoard plugin in Eclipse IDE

ReviewBoard (www.reviewboard.org/) is a nice code review web tool. ereviewboard (http://marketplace.eclipse.org/content/ereviewboard) is the corresponding plugin for Eclipse IDE. Below are the steps mentioned to integrate it with Eclipse IDE :

• Goto Help->Install New software
• Use URL http://rombert.github.com/ereviewboard/update/ and install "Mylyn Reviews Connector: ReviewBoard" and "Mylyn Reviews Connector: Review Board Subeclipse integration" (there are 3 connectors available, choose the appropriate one...here i am assuming subversion)
• If subeclipse is not installed, use url http://subclipse.tigris.org/update_1.6.x
• After installation, Open the task repositories view by navigating to Window -> Show View -> Other -> Mylyn -> Task Repositories
• Click the "Add Task Repository" button located in the view's toolbar.
• Select reviewboard and enter server as your reviewboard weburl (ex : http://192.168.x.x), username, password (Save password) and finish.
• Now, Right-click on a Project and select Team -> Create Review Request will post to reviewboard (if you dont see such a option trying restarting eclipse)

Some helpful links :
http://help.eclipse.org/galileo/index.jsp?topic=/org.eclipse.mylyn.help.ui/userguide/Task-Repositories.html
https://github.com/rombert/ereviewboard/wiki/Subclipse-integration

Remarks :
This tool can only post diff's of 1 project to reviewboard. Multiple project diff are not supported

Listing all entities in a JPA

Sometimes it may be a usecase scenario to find whether a particular class is an entity managed by persistence context. If you have entityManager or entityManagerFactory you can easily do that :

 Metamodel meta = entityManagerFactory.getMetamodel();
 // or
 Metamodel meta = entityManager.getEntityManagerFactory().getMetamodel();

 // to iterate over all classes
 for (EntityType<?> e : meta.getEntities()) {
  // get entity class
  Class c = e.getJavaType();
  // get entity name as string
  String entityName = e.getName(); //or c.getName()
 }

 // test a particular class is entity
 // will throw java.lang.IllegalArgumentException if not an entity
 meta.entity(inputClass);

Curious Case in MYSQL : Lock wait timeout exceeded on INSERT

Sounds strange, that how can a insert be locked or timed out . I had a innodb table with very frequent inserts, updated and deletes. After every few minutes, one of the inserts got timed out (50 sec is default value for innodb_lock_wait_timeout). My understanding was that timeouts happen when some other thread/transaction holds a exclusive record lock (select .. from update) for a long time. So how can a non-existent new row be already locked. I do not have a proper answer to this.

What solved my problem was dropping index and foreign key mapping, which were luckily irrelevant. Random guess is that innodb locks a range of index on insert. If you have an answer do let me know !

Avoiding dirty reads in a concurrent environment

Recently, I was facing issue of handling concurrent access and modifications to database using JPA. Most of the time inserts where happening based on dirty reads or unique key violations were happening, thereby making the data inconsistent. So, to overcome this, I used 2 separate things. Firstly, delegated few tasks to central db, by using before insert triggers (need to refresh the entity after persist in case of pre-insert trigger). Secondly, introduced version based modifications. Here is a nice article which explains this : http://java.dzone.com/articles/jpa-20-concurrency-and-locking.

One can use application based optimistic locks or db row level based pessimistic locks. While using locks you need to handle errors like lock timeout or optimistic lock exceptions and do multiple retries with some random short sleeps. It is important to remember that for most of the exceptions, jpa marks the transaction for rollbackonly, so one needs to begin a fresh transaction after exception, to have proper commits.