Fluo Tour: Observer Example

Tour page 18 of 26

The following code shows how to setup and trigger an observer. The observer is triggered when the column obs:data is changed.

  public static final Column OBSERVED_COL = new Column("obs", "data");
  public static final Column INVERT_COL = new Column("inv", "data");

  // This class is responsible for registering Observers for all observed columns.
  public static class MyObserverProvider implements ObserverProvider {

    public void provide(Registry obsRegistry, Context ctx) {

      // Observer is a functional interface allowing Observers to be lambdas
      Observer invObserver = (tx, row, col) -> {
        Bytes value = tx.get(row, col);
        tx.set(value, INVERT_COL, row);

      // Register an observer to process notifications for the column obs:data
      obsRegistry.forColumn(OBSERVED_COL, NotificationType.STRONG).useObserver(invObserver);


  private static void preInit(FluoConfiguration fluoConfig) {
    // Configure ObserverProvider before initialization. Workers will instantiate this class and use
    // it to create Observers.

  private static void exercise(MiniFluo mini, FluoClient client) {
    try (Transaction tx1 = client.newTransaction()) {
      tx1.set("kerbalnaut0001", OBSERVED_COL, "Jebediah");

    try (Transaction tx2 = client.newTransaction()) {
      tx2.set("kerbalnaut0002", OBSERVED_COL, "Bill");


    try (Snapshot snap = client.newSnapshot()) {

The code above prints :

Bill inv data  kerbalnaut0002
Jebediah inv data  kerbalnaut0001
kerbalnaut0001 obs data  Jebediah
kerbalnaut0002 obs data  Bill

The following events happen when this code is run.

  • tx1 modifies kerbalnaut0001:obs:data causing MyObserver to run later on that row+column.
  • tx2 modifies kerbalnaut0002:obs:data causing MyObserver to run later on that row+column.
  • Later MyObserver is run and passed row+column kerbalnaut0001:obs:data
  • Later MyObserver is run and passed row+column kerbalnaut0002:obs:data

Observers are run in the background by Fluo threads. Fluo also creates the transaction passed to an Observer and commits it. The transaction does not need to call commit and can not, the TransactionBase type passed to an Observer does not have a commit method. The framework handles committing because it retries in case of a commit exception.

Since observers are run in the background, you never know when they will run. For testing purposes MiniFluo provides the waitForObservers() method that is called above. This method waits for all notifications to be processed by observers.

There is no stand alone exercise for the Observer. Hands on experience with it can be obtained by completing the word count exercise which is the next step in the tour.

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