There can be use cases where none of the inbuilt data types match your requirement or a custom data type optimized for your use case may perform better than a Hadoop built-in data type. In such scenarios, we can easily write a custom Writable data type by implementing the org.apache.hadoop.io.Writable interface to define the serialization format of your data type. The Writable interface-based types can be used as value types in Hadoop MapReduce computations.

In this recipe, we implement a sample Hadoop Writable data type for HTTP server log entries. For the purpose of this sample, we consider that a log entry consists of the five fields: request host, timestamp, request URL, response size, and the HTTP status code. The following is a sample log entry:

192.168.0.2 - - [01/Jul/1995:00:00:01 -0400] "GET /history/apollo/ HTTP/1.0" 200 6245

You can download a sample HTTP server log dataset from ftp://ita.ee.lbl.gov/traces/NASA_access_log_Jul95.gz...