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From Apache Wiki <wikidi...@apache.org>
Subject [Hadoop Wiki] Trivial Update of "ZooKeeper/Tutorial" by VladimirTretyakov
Date Mon, 26 Sep 2011 07:22:30 GMT
Dear Wiki user,

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The "ZooKeeper/Tutorial" page has been changed by VladimirTretyakov:
http://wiki.apache.org/hadoop/ZooKeeper/Tutorial?action=diff&rev1=4&rev2=5

Comment:
Improove understanding how mutex uses (as object to notify about changes) to code example.

  = Programming with ZooKeeper - A quick tutorial =
- 
  In this tutorial, we show simple implementations of barriers and producer-consumer queues
using !ZooKeeper. We call the respective classes Barrier and Queue. These examples assume
that you have at least one !ZooKeeper server running.
  
  Both primitives use the following common excerpt of code:
  
  {{{
+ public class SyncPrimitive implements Watcher {
      static ZooKeeper zk = null;
      static Integer mutex;
  
      String root;
  
-     SyncPrimitive(String address) 
+     SyncPrimitive(String address)
      throws KeeperException, IOException {
          if(zk == null){
                  System.out.println("Starting ZK:");
@@ -22, +22 @@

          }
      }
  
+     @Override
      synchronized public void process(WatcherEvent event) {
          synchronized (mutex) {
              mutex.notify();
          }
      }
- 
  }}}
- 
  Both classes extend !SyncPrimitive. In this way, we execute steps that are common to all
primitives in the constructor of !SyncPrimitive. To keep the examples simple, we create a
!ZooKeeper object the first time we instantiate either a barrier object or a queue object,
and we declare a static variable that is a reference to this object. The subsequent instances
of Barrier and Queue check whether a !ZooKeeper object exists. Alternatively, we could have
the application creating a !ZooKeeper object and passing it to the constructor of Barrier
and Queue.
  
  We use the process() method to process notifications triggered due to watches. In the following
discussion, we present code that sets watches. A watch is internal structure that enables
!ZooKeeper to notify a client of a change to a node. For example, if a client is waiting for
other clients to leave a barrier, then it can set a watch and wait for modifications to a
particular node, which can indicate that it is the end of the wait. This point becomes clear
once we go over the examples.
@@ -39, +38 @@

  
  In this example, each process instantiates a Barrier object, and its constructor takes as
parameters:
  
-     * The address of a !ZooKeeper server (e.g., "zoo1.foo.com:2181");
+  * The address of a !ZooKeeper server (e.g., "zoo1.foo.com:2181");
-     * The path of the barrier node on !ZooKeeper (e.g., "/b1");
+  * The path of the barrier node on !ZooKeeper (e.g., "/b1");
-     * The size of the group of processes.
+  * The size of the group of processes.
- 
  
  The constructor of Barrier passes the address of the Zookeeper server to the constructor
of the parent class. The parent class creates a !ZooKeeper instance if one does not exist.
The constructor of Barrier then creates a barrier node on !ZooKeeper, which is the parent
node of all process nodes, and we call root (obs: this is not the !ZooKeeper root "/").
  
@@ -72, +70 @@

              name = new String(InetAddress.getLocalHost().getCanonicalHostName().toString());
          }
  }}}
- 
  To enter the barrier, a process calls enter(). The process creates a node under the root
to represent it, using its host name to form the node name. It then wait until enough processes
have entered the barrier. A process does it by checking the number of children the root node
has with "getChildren()", and waiting for notifications in the case it does not have enough.
To receive a notification when there is a change to the root node, a process has to set a
watch, and does it through the call to "getChildren()". In the code, we have that "getChildren()"
has two parameters. The first one states the node to read from, and the second is a boolean
flag that enables the process to set a watch. In the code the flag is true.
- 
  
  {{{
      /**
@@ -101, +97 @@

              }
          }
  }}}
- 
  Note that enter() throws both !KeeperException and !InterruptedException, so it is reponsibility
of the application to catch and handle such exceptions.
- 
  
  Once the computation is finished, a process calls leave() to leave the barrier. First it
deletes its corresponding node, and then it gets the children of the root node. If there is
at least one child, then it waits for a notification (obs: note that the second parameter
of the call to getChildren() is true, meaning that !ZooKeeper has to set a watch on the the
root node). Upon reception of a notification, it checks once more whether the root node has
any child.
  
@@ -130, +124 @@

                  }
          }
  }}}
- 
  == Producer-Consumer Queues ==
- 
  A producer-consumer queue is a distributed data structure that group of processes use to
generate and consume items. Producer processes create new elements and add them to the queue.
Consumer processes remove elements from the list, and process them. In this implementation,
the elements are simple integers. The queue is represented by a root node, and to add an element
to the queue, a producer process creates a new node, a child of the root node.
  
  The following excerpt of code corresponds to the constructor of the object. As with Barrier
objects, it first calls the constructor of the parent class, !SyncPrimitive, that creates
a !ZooKeeper object if one doesn't exist. It then verifies if the root node of the queue exists,
and creates if it doesn't.
@@ -157, +149 @@

              }
          }
  }}}
- 
  A producer process calls "produce()" to add an element to the queue, and passes an integer
as an argument. To add an element to the queue, the method creates a new node using "create()",
and uses the SEQUENCE flag to instruct !ZooKeeper to append the value of the sequencer counter
associated to the root node. In this way, we impose a total order on the elements of the queue,
thus guaranteeing that the oldest element of the queue is the next one consumed.
- 
  
  {{{
      /**
@@ -182, +172 @@

              return true;
          }
  }}}
- 
  To consume an element, a consumer process obtains the children of the root node, reads the
node with smallest counter value, and returns the element. Note that if there is a conflict,
then one of the two contending processes won't be able to delete the node and the delete operation
will throw an exception.
  
  A call to getChildren() returns the list of children in lexicographic order. As lexicographic
order does not necessary follow the numerical order of the counter values, we need to decide
which element is the smallest. To decide which one has the smallest counter value, we traverse
the list, and remove the prefix "element" from each one.
  
  {{{
- 
       /**
           * Remove first element from the queue.
           *

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