mikewalch commented on a change in pull request #48: Added conditional writer
to tour
URL: https://github.com/apache/accumulo-website/pull/48#discussion_r155815228
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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) {
Review comment:
More imports are needed at top of file. We could move to wild card imports.
Below worked for me. However, this should be tested with other exercises.
```java
// Classes you will use along the tour
import java.util.*;
import java.nio.file.*;
import java.util.concurrent.*;
import java.util.function.*;
import org.apache.accumulo.core.client.*;
import org.apache.accumulo.core.client.Scanner;
import org.apache.accumulo.core.data.*;
import org.apache.accumulo.core.security.*;
import org.apache.accumulo.minicluster.MiniAccumuloCluster;
import org.apache.hadoop.io.Text;
```
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