keith-turner closed pull request #102: Document exact deletes
URL: https://github.com/apache/accumulo-website/pull/102
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diff --git a/_docs-2-0/development/exact-deletes.md
b/_docs-2-0/development/exact-deletes.md
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+---
+title: Exact Deletes
+category: development
+order: 10
+---
+
+### Overview
+
+In Accumulo's [data model][dm] each row column can have multiple versions and
+each version has a timestamp. By default, deletes remove every version in a row
+column that is less than or equal to a timestamp.
+
+### Example
+
+The following example shows the default delete behavior.
+
+```java
+ Connector conn = getConnector();
+ NewTableConfiguration ntc = new NewTableConfiguration();
+
+ // set wether table has exact deletes, this defaults to false if not set
+ ntc.setExactDeleteEnabled(false);
+
+ // only allow deletes with a timestamp
+ ntc.setProperties(Collections.singletonMap("table.constraint.1",
+ "org.apache.accumulo.core.constraints.NoTimestampDeleteConstraint"));
+
+ // disable versioning iterator that only shows the latest version of a
column
+ ntc.withoutDefaultIterators();
+ conn.tableOperations().create("example1", ntc);
+
+ // insert five versions for the row '02498' and column 'addr:hist'
+ try (BatchWriter writer = conn.createBatchWriter("example1")) {
+ Mutation mutation = new Mutation("02498");
+ mutation.put("addr", "hist", 5L, "5677 Aspen Ave, Springfield, AK");
+ mutation.put("addr", "hist", 4L, "3455 Birch Blvd, Springfield, BC");
+ mutation.put("addr", "hist", 3L, "1111 Lychee Lane, Springfield, LA");
+ mutation.put("addr", "hist", 2L, "1234 Dogwood Dr, Springfield, DE");
+ mutation.put("addr", "hist", 1L, "6651 Willow Way, Springfield, WY");
+ writer.addMutation(mutation);
+ }
+
+ // print if exact deletes are enabled for the table
+ System.out.println("Is exact delete enabled : "
+ + conn.tableOperations().isExactDeleteEnabled("example1"));
+
+ System.out.println("\nBefore delete : ");
+ Scanner scanner = conn.createScanner("example1", Authorizations.EMPTY);
+ for (Entry<Key,Value> e : scanner)
+ System.out.println(e.getKey().getTimestamp() + " : " + e.getValue());
+
+ // write out a delete with a timestamp of 3
+ try (BatchWriter writer = conn.createBatchWriter("example1")) {
+ Mutation mutation = new Mutation("02498");
+ mutation.putDelete("addr", "hist", 3L);
+ writer.addMutation(mutation);
+ }
+
+ System.out.println("\nAfter delete : ");
+ for (Entry<Key,Value> e : scanner)
+ System.out.println(e.getKey().getTimestamp() + " : " + e.getValue());
+
+ scanner.close();
+```
+
+Below is the output from running the example. Notice the delete with timestamp
+3 removed versions 3,2,and 1.
+
+```
+Is exact delete enabled : false
+
+Before delete :
+ 5 : 5677 Aspen Ave, Springfield, AK
+ 4 : 3455 Birch Blvd, Springfield, BC
+ 3 : 1111 Lychee Lane, Springfield, LA
+ 2 : 1234 Dogwood Dr, Springfield, DE
+ 1 : 6651 Willow Way, Springfield, WY
+
+After delete :
+ 5 : 5677 Aspen Ave, Springfield, AK
+ 4 : 3455 Birch Blvd, Springfield, BC
+```
+
+The behavior of deletes can be changed at table creation time to only delete
+the exact version specified by the delete timestamp. This is done by changing
+the following line in the example.
+
+```java
+ ntc.setExactDeleteEnabled(false);
+```
+
+Change the above line to :
+
+```java
+ ntc.setExactDeleteEnabled(true);
+```
+
+Below is output from running the example with this change. Notice that only
+version 3 was deleted and not versions 1 and 2.
+
+```
+Is exact delete enabled : true
+
+Before delete :
+ 5 : 5677 Aspen Ave, Springfield, AK
+ 4 : 3455 Birch Blvd, Springfield, BC
+ 3 : 1111 Lychee Lane, Springfield, LA
+ 2 : 1234 Dogwood Dr, Springfield, DE
+ 1 : 6651 Willow Way, Springfield, WY
+
+After delete :
+ 5 : 5677 Aspen Ave, Springfield, AK
+ 4 : 3455 Birch Blvd, Springfield, BC
+ 2 : 1234 Dogwood Dr, Springfield, DE
+ 1 : 6651 Willow Way, Springfield, WY
+```
+
+### System timestamp for deletes
+
+Deletes can be added to a mutation with no timestamp. When this is done
+Accumulo chooses an ever increasing timestamp for the delete. When using exact
+deletes, its likely the system chosen timestamp would never correspond to any
+existing versions. Therefore not setting the timestamp is likely an error. To
+detect this error a table constraint can be set that only allows deletes with a
+timestamp. This constraint causes deletes without a timestamp to fail to write.
+The example above enabled this constraint with the following lines.
+
+```java
+ ntc.setProperties(Collections.singletonMap("table.constraint.1",
+ "org.apache.accumulo.core.constraints.NoTimestampDeleteConstraint"));
+```
+
+[dm]: {% durl getting-started/design#data-model %}
+
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