The custom export client is declared as a Java subclass extending ExportClientBase. Within the subclass you can override the configure() method, which receives any properties defined in the export connection configuration. The subclass must also declare the constructExportDecoder() method, which in turn generates a subclass of ExportDecoderBase. This extension of ExportDecoderBase is the class that does the actual work at run time and must override the primary methods sourceNoLongerAdvertised(), onBlockStart(), processRow(), and onBlockCompletion. Figure 9.1, “Structure of the Custom Export Class” illustrates the structure of the custom client.

Figure 9.1. Structure of the Custom Export Class

At run time, VoltDB passes data to the export client in blocks that are roughly 2MB in size but do not align with transactions. A block is guaranteed to contain complete rows — that is, no single SQL INSERT to an export stream is split across blocks. The handoff from the internal VoltDB producer to the custom export client follows a simple pattern:

producer -> client.onBlockStart
foreach row in block:
    producer -> client.processRow
producer -> client.onBlockCompletion

Each time the pattern executes, it runs within a single thread. Therefore, it is not necessary to synchronize accesses to the data structures used in client.onBlockStart, client.processRow, and client.onBlockCompletion unless they are used in other threads as well.

For each row of data, the processRow() method is called. The object passed in as an argument contains a full description of the row, including the column names, types, and values, which your client code can iterate over. For example:

public boolean processRow(ExportRow row) 
               throws RestartBlockException {

   for (int i =0; i < row.values.length; i++) {
      String column_name = row.names.get(i);
      Object column_value = row.values[i];
          // do work . . .
   }

Note that each row starts with six columns of metadata, including the transaction ID and timestamp. If you do not need this information, you can skip the first six columns. Also, within each block of export data, the schema is constant. However, it is possible for the schema to change between blocks (if a schema change is applied to the database while export is active). The custom client can evaluate the ExportRow object passed into the onBlockStart() or processRow() method to recognize any changes to the schema and configure the downstream system as necessary to accept the new data.

As a rule, the custom client must balance the requirements to execute and return control quickly, so as not to block other export threads (since connectors share a limited thread pool), against the need to ensure that the individual rows are accepted and acknowledged by the downstream system before onBlockCompletion() returns. If the client fails at onBlockStart, processRow or onBlockCompletion, the export client must throw a RestartBlockException to prevent VoltDB from acknowledging (ACKing) and dropping the export data from its durability control. This point deserves repeating: if the custom ExportClient runs onBlockStart, processRow and onBlockCompletion without throwing the correct exception, VoltDB assumes the data is remotely durable and that the VoltDB database can discard that export block.

The ExportClient must not return from onBlockCompletion until it ensures the downstream target acknowledges receipt of the data.

Properties, set in the deployment file as part of the export configuration, let you pass information to the export connector. For example, if the user needs to pass the export connector a file location or IP address for the export target. What properties are necessary or supported is up to you as the author of the export client to decide.

The properties specified in the deployment file are passed to the export client as a Properties object argument to the configure() method every time the connector starts. That is, whenever the database starts with the connector enabled or whenever the schema or deployment is modified (for example, by a voltadmin update command).

The configure() method can either iterate over the Properties object or it can look for specific entries as needed. For example:

public void configure(Properties config) throws Exception {
  
        // Check for specific property value
    if config.containsKey("filename") {
       filename = config.getProperty("filename");
     }   
}

Once your export client code is complete, you need to compile, package, and install the connector on the appropriate VoltDB servers. You compile the export client like other Java methods. Be sure to include the VoltDB server jar file in the classpath. For example, if VoltDB is installed in a directory called voltdb in your home directory, the command could be:

$ javac -cp "$HOME/voltdb/voltdb/*:./" -d obj \
   org.voltdb.exportclient/MyExportClient.java

After compiling the source code, you must package the resulting class into a JAR file, like so:

$ jar cvf myexportclient.jar -C obj .

Finally you must install the JAR file in the lib/extension folder where VoltDB is installed on all servers in the cluster that will be running the export client. For, example, if you are running a single node cluster on the current node, where VoltDB has been installed as $HOME/voltdb, you can copy the JAR file with the following command:

$ cp myexportclient.jar $HOME/voltdb/lib/extension/

Once your custom export client is installed you can configure and start it. Custom export clients are configured like any other export connector, by adding a <configure> section to <export> in the deployment file (or configuring it interactively in the Volt Management Center). For custom clients, you declare the connector type as "custom" and add the exportconnectorclass attribute specifying the connector's Java classpath. For example:

<export>
   <configuration enabled="true" target="myspecial" type="custom" 
    exportconnectorclass="org.voltdb.exportclient.MyExportClient" >
     <property name="filename">myexportfile.txt</property>
  </configuration>
</export>

Any properties listed in the <configuration> ("filename" in this example) are passed to the custom export client as arguments to the configure() method, as described in Section 9.1.2, “How to Use Custom Properties to Configure the Client”. See the chapter on "Importing and Exporting Live Data" in the Using VoltDB manual for more information on configuring export connectors.