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Apache Accumulo User Manual: Writing Accumulo Clients

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Subsections

Writing Accumulo Clients

Running Client Code

There are multiple ways to run Java code that uses Accumulo. Below is a list of the different ways to execute client code.

Inorder to run client code written to run against Accumulo, you will need to include the jars that Accumulo depends on in your classpath. Accumulo client code depends on Hadoop and Zookeeper. For Hadoop add the hadoop core jar, all of the jars in the Hadoop lib directory, and the conf directory to the classpath. For Zookeeper 3.3 you only need to add the Zookeeper jar, and not what is in the Zookeeper lib directory. You can run the following command on a configured Accumulo system to see what its using for its classpath. 

$ACCUMULO_HOME/bin/accumulo classpath

Another option for running your code is to put a jar file in $ACCUMULO_HOME/lib/ext. After doing this you can use the accumulo script to execute your code. For example if you create a jar containing the class com.foo.Client and placed that in lib/ext, then you could use the command $ACCUMULO_HOME/bin/accumulo com.foo.Client to execute your code.

If you are writing map reduce job that access Accumulo, then you can use the bin/tool.sh script to run those jobs. See the map reduce example.

Connecting

All clients must first identify the Accumulo instance to which they will be communicating. Code to do this is as follows: 

String instanceName = "myinstance";
String zooServers = "zooserver-one,zooserver-two"
Instance inst = new ZooKeeperInstance(instanceName, zooServers);

Connector conn = inst.getConnector("user", "passwd");

Writing Data

Data are written to Accumulo by creating Mutation objects that represent all the changes to the columns of a single row. The changes are made atomically in the TabletServer. Clients then add Mutations to a BatchWriter which submits them to the appropriate TabletServers.

Mutations can be created thus: 

Text rowID = new Text("row1");
Text colFam = new Text("myColFam");
Text colQual = new Text("myColQual");
ColumnVisibility colVis = new ColumnVisibility("public");
long timestamp = System.currentTimeMillis();

Value value = new Value("myValue".getBytes());

Mutation mutation = new Mutation(rowID);
mutation.put(colFam, colQual, colVis, timestamp, value);

BatchWriter

The BatchWriter is highly optimized to send Mutations to multiple TabletServers and automatically batches Mutations destined for the same TabletServer to amortize network overhead. Care must be taken to avoid changing the contents of any Object passed to the BatchWriter since it keeps objects in memory while batching.

Mutations are added to a BatchWriter thus: 

long memBuf = 1000000L; // bytes to store before sending a batch
long timeout = 1000L; // milliseconds to wait before sending
int numThreads = 10;

BatchWriter writer =
    conn.createBatchWriter("table", memBuf, timeout, numThreads)

writer.add(mutation);

writer.close();

An example of using the batch writer can be found at
accumulo/docs/examples/README.batch

Reading Data

Accumulo is optimized to quickly retrieve the value associated with a given key, and to efficiently return ranges of consecutive keys and their associated values.

Scanner

To retrieve data, Clients use a Scanner, which provides acts like an Iterator over keys and values. Scanners can be configured to start and stop at particular keys, and to return a subset of the columns available. 

// specify which visibilities we are allowed to see
Authorizations auths = new Authorizations("public");

Scanner scan =
    conn.createScanner("table", auths);

scan.setRange(new Range("harry","john"));
scan.fetchFamily("attributes");

for(Entry<Key,Value> entry : scan) {
    String row = e.getKey().getRow();
    Value value = e.getValue();
}

Isolated Scanner

Accumulo supports the ability to present an isolated view of rows when scanning. There are three possible ways that a row could change in accumulo :

Isolation guarantees that either all or none of the changes made by these operations on a row are seen. Use the IsolatedScanner to obtain an isolated view of an accumulo table. When using the regular scanner it is possible to see a non isolated view of a row. For example if a mutation modifies three columns, it is possible that you will only see two of those modifications. With the isolated scanner either all three of the changes are seen or none.

The IsolatedScanner buffers rows on the client side so a large row will not crash a tablet server. By default rows are buffered in memory, but the user can easily supply their own buffer if they wish to buffer to disk when rows are large.

For an example, look at the following
src/examples/src/main/java/org/apache/accumulo/examples/isolation/InterferenceTest.java

BatchScanner

For some types of access, it is more efficient to retrieve several ranges simultaneously. This arises when accessing a set of rows that are not consecutive whose IDs have been retrieved from a secondary index, for example.

The BatchScanner is configured similarly to the Scanner; it can be configured to retrieve a subset of the columns available, but rather than passing a single Range, BatchScanners accept a set of Ranges. It is important to note that the keys returned by a BatchScanner are not in sorted order since the keys streamed are from multiple TabletServers in parallel. 

ArrayList<Range> ranges = new ArrayList<Range>();
// populate list of ranges ...

BatchScanner bscan =
    conn.createBatchScanner("table", auths, 10);

bscan.setRanges(ranges);
bscan.fetchFamily("attributes");

for(Entry<Key,Value> entry : scan)
    System.out.println(e.getValue());

An example of the BatchScanner can be found at
accumulo/docs/examples/README.batch

Next: Table Configuration Up: Apache Accumulo User Manual Version 1.4 Previous: Accumulo Shell Contents