DSL Reference

Introduction

Spring XD provides a DSL for defining a stream. Over time the DSL is likely to evolve significantly as it gains the ability to define more and more sophisticated streams as well as the steps of a batch job.

Pipes and filters

A simple linear stream consists of a sequence of modules. Typically an Input Source, (optional) Processing Steps, and an Output Sink. As a simple example consider the collection of data from an HTTP Source writing to a File Sink. Using the DSL the stream description is:

http | file

A stream that involves some processing:

http | filter | transform | file

The modules in a stream definition are connected together using the pipe symbol |.

Module parameters

Each module may take parameters. The parameters supported by a module are defined by the module implementation. As an example the http source module exposes port setting which allows the data ingestion port to be changed from the default value.

http --port=1337

It is only necessary to quote parameter values if they contain spaces or the | character. Here the transform processor module is being passed a SpEL expression that will be applied to any data it encounters:

transform --expression='new StringBuilder(payload).reverse()'

If the parameter value needs to embed a single quote, use two single quotes:

// Query is: Select * from /Customers where name='Smith'
scan --query='Select * from /Customers where name=''Smith'''

Tap

A Tap can be used to "listen in" to data from another stream and process the data in a separate stream. A tap can consume data from any point along the target stream’s processing pipeline. The format of tap is:

tap <stream>.<module>

For example, here is a stream called 'mystream':

source | filter | transform | sink

The output of the filter step can be tapped:

tap mystream.filter | sink2

The Spring XD M2 release will provide a DSL for non-linear flows, e.g. a directed graph.