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Recipe: Shared Counter

Description​

Manages a shared integer. All clients watching the same path will have the up-to-date value of the shared integer (considering ZK's normal consistency guarantees).

Participating Classes​

  • SharedCount
  • SharedCountReader
  • SharedCountListener

Creating a SharedCounter​

// Parameters:
// client - the client
// path - the shared path - i.e. where the shared count is stored
// seedValue - the initial value for the count iff the path has not yet been created
public SharedCount(CuratorFramework client, String path, int seedValue);

General Usage​

SharedCounts must be started:

count.start();

When you are through with the instance, you should call close:

count.close();
// Return the current value of the count
int getCount();
// Add a listener for changes to the count
void addListener(SharedCountListener listener);
// Change the shared count value irrespective of its previous state
public void setCount(int newCount);
// Changes the shared count only if its value has not changed since this client last read it. If the count
// has changed, the value is not set and this client's view of the value is updated. i.e. if the count is
// not successful you can get the updated value by calling getCount().
// Parameters:
// newCount - the new value to attempt
// Returns:
// true if the change attempt was successful, false if not. If the change was not successful, getCount()
// will return the updated value
public boolean trySetCount(int newCount);

Error Handling​

The SharedCountListener class extends ConnectionStateListener. When the SharedCount is started, it adds the listener to the Curator instance. Users of the SharedCount must pay attention to any connection state changes.

If the SUSPENDED state is reported, the instance must assume that, until it receives a RECONNECTED state, the count is no longer accurate and isn't being updated. If the LOST state is reported, the count is permanently down.