About Jaql

Jaql is primarily a query language for JavaScript Object Notation (JSON), but it supports more than just JSON. It allows you to process both structured and nontraditional data and was donated by IBM to the open source community (just one of many contributions IBM has made to open source).

Specifically, Jaql allows you to select, join, group, and filter data that is stored in HDFS, much like a blend of Pig and Hive. Jaql’s query language was inspired by many programming and query languages, including Lisp, SQL, XQuery, and Pig. Jaql is a functional, declarative query language that is designed to process large data sets. For parallelism, Jaql rewrites high-level queries, when appropriate, into “low-level” queries consisting of MapReduce jobs.


What is JSON

Before we get into the Jaql language, let’s first look at the popular data interchange format known as JSON, so that we can build our Jaql examples on top of it. Application developers are moving in large numbers towards JSON as their choice for a data interchange format, because it’s easy for humans to read, and because of its structure, it’s easy for applications to parse or generate.

JSON is built on top of two types of structures. The first is a collection of name/value pairs (which, as you learned earlier in the “The Basics of MapReduce” section, makes it ideal for data manipulation in Hadoop, which works on key/value pairs). These name/value pairs can represent anything since they are simply text strings (and subsequently fit well into existing models) that could represent a record in a database, an object, an associative array, and more. The second JSON structure is the ability to create an ordered list of values much like an array, list, or sequence you might have in your existing applications. An object in JSON is represented as {string:value}, where an array can be simply represented by [ value, value, … ], where value can be a string, number, another JSON object, or another JSON array.

Both Jaql and JSON are record-oriented models, and thus fit together perfectly. Note that JSON is not the only format that Jaql supports—in fact, Jaql is extremely flexible and can support many semi-structured data sources such as XML, CSV, flat files, and more. However, in consideration of the space we have, we’ll use the JSON example above in the following Jaql queries. As you will see from this section, Jaql looks very similar to Pig but also has some similarity to SQL.


What is a Jaql Query

Much like a MapReduce job is a flow of data, Jaql can be thought of as a pipeline of data flowing from a source, through a set of various operators, and out into a sink (a destination). The operand used to signify flow from one operand to another is an arrow: ->. Unlike SQL, where the output comes first (for example, the SELECT list), in Jaql, the operations listed are in natural order, where you specify the source, followed by the various operators you want to use to manipulate the data, and finally the sink.

What is Jaql?

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