[GitHub] milleruntime commented on a change in pull request #48: Added conditional writer to tour

[GitBox <gi...@apache.org>]

milleruntime commented on a change in pull request #48: Added conditional writer to tour
URL: https://github.com/apache/accumulo-website/pull/48#discussion_r155819409
 
 

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 File path: tour/conditional-writer.md
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 @@ -1,3 +1,119 @@
 ---
 title: Conditional Writer
 ---
+
+When read-modify-write operations run concurrently, its possible changes made
+by some operations will be overwritten by others. The following sequence of
+events shows an example of this.
+
+ 1. Thread 0 sets the key `id0001:location:home` to `1007 Mountain Dr, Gotham, New York`
+ 2. Thread 1 reads `id0001:location:home`
+ 3. Thread 2 reads `id0001:location:home`
+ 4. Thread 1 replaces `Dr` with `Drive`
+ 5. Thread 2 replaces `New York` with `NY`
+ 6. Thread 1 sets key `id0001:location:home` to `1007 Mountain Drive, Gotham, New York`
+ 7. Thread 2 sets key `id0001:location:home` to `1007 Mountain Dr, Gotham, NY`
+
+In this situation the changes made by Thread 1 are lost, ending up with `1007
+Mountain Dr, Gotham, NY` instead of `1007 Mountain Drive, Gotham, NY`.  To
+correctly handle this, Accumulo offers the [ConditionalWriter].  The
+ConditionalWriter atomically checks conditions on a row and only applies a
+mutation when all are satisfied.
+
+## Exercise
+
+The following code simulates the concurrency situation above.  Because it uses
+a BatchWriter it will lose modifications.
+
+```java
+  static String getAddress(Connector conn, String id) {
+    // The IsolatedScanner ensures partial changes to a row are not seen
+    try (Scanner scanner = new IsolatedScanner(conn.createScanner("GothamPD", Authorizations.EMPTY))) {
+      scanner.setRange(Range.exact(id, "location", "home"));
+      for (Entry<Key,Value> entry : scanner) {
+        return entry.getValue().toString();
+      }
+      return null;
+    } catch (TableNotFoundException e) {
+      throw new RuntimeException(e);
+    }
+  }
+
+  static boolean setAddress(Connector conn, String id, String expectedAddr, String newAddr) {
+    try (BatchWriter writer = conn.createBatchWriter("GothamPD", new BatchWriterConfig())) {
+      Mutation mutation = new Mutation(id);
+      mutation.put("location", "home", newAddr);
+      writer.addMutation(mutation);
+      return true;
+    } catch (Exception e) {
+      throw new RuntimeException(e);
+    }
+  }
+
+  public static Future<Void> modifyAddress(Connector conn, String id, Function<String,String> modifier) {
+    return CompletableFuture.runAsync(() -> {
+      String currAddr, newAddr;
+      do {
+        currAddr = getAddress(conn, id);
+        newAddr = modifier.apply(currAddr);
+        System.out.printf("Thread %3d attempting change %20s -> %-20s\n",
+            Thread.currentThread().getId(), "'"+currAddr+"'", "'"+newAddr+"'");
+      } while (!setAddress(conn, id, currAddr, newAddr));
+    });
+  }
+
+  static void exercise(MiniAccumuloCluster mac) throws Exception {
+    Connector conn = mac.getConnector("root", "tourguide");
+    conn.tableOperations().create("GothamPD");
+
+    String id = "id0001";
+
+    setAddress(conn, id, null, "  1007 Mountain Dr, Gotham, New York  ");
+
+    // create async operation to trim whitespace
+    Future<Void> future1 = modifyAddress(conn, id, String::trim);
+
+    // create async operation to replace Dr with Drive
+    Future<Void> future2 = modifyAddress(conn, id, addr -> addr.replace("Dr", "Drive"));
+
+    // create async operation to replace New York with NY
+    Future<Void> future3 = modifyAddress(conn, id, addr -> addr.replace("New York", "NY"));
+
+    // wait for async operations to complete
+    future1.get();
+    future2.get();
+    future3.get();
+
+    // print the address stored in Accumulo
+    System.out.println("Final address : '"+getAddress(conn, id)+"'");
+  }
+```
+
+The following is one of a few possible outputs.  Notice that only the
+modification of `New York` to `NY` shows up in the final output.  The other
+modifications were lost.
+
+```
+Thread  38 attempting change '  1007 Mountain Dr, Gotham, New York  ' -> '  1007 Mountain Drive, Gotham, New York  '
+Thread  39 attempting change '  1007 Mountain Dr, Gotham, New York  ' -> '  1007 Mountain Dr, Gotham, NY  '
+Thread  37 attempting change '  1007 Mountain Dr, Gotham, New York  ' -> '1007 Mountain Dr, Gotham, New York'
+Final address : '  1007 Mountain Dr, Gotham, NY  '
+```
+
+To fix this example, make the following changes in `setAddress()` to use a
+ConditionalWriter.
+
+ * Call [createConditionalWriter] instead of creating a batch writer
+ * Create a [Condition] for the column 'location:home'.  If `expectedAddr` is not null, then pass it to [setValue].  A condition with no value set means that column is expected to absent.
 
 Review comment:
   An example on how to create a Condition would be helpful.  I found this tricky never having used it.  I kept looking for a method to set the logic vs explicitly performing the checks in java.

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