Spring cloud stream transform expression syntax

Question

I am experimenting with Spring Cloud Stream using the local Spring Cloud Dataflow server and the shell. I've gotten simple examples like this

dataflow:>stream create --name test --definition "time --trigger.time-unit=SECONDS | log" --deploy

working correctly, in that I see a message in the log once per second.

2017-08-09 12:51:30,602  INFO -kafka-listener-1 log-sink:202 - 08/09/17 12:51:30
2017-08-09 12:51:31,603  INFO -kafka-listener-1 log-sink:202 - 08/09/17 12:51:31
2017-08-09 12:51:32,605  INFO -kafka-listener-1 log-sink:202 - 08/09/17 12:51:32
.... more log msgs ....

Now I'm trying to expand this example to insert a transform. Instead of the time, I want to see 'hello world' in the log once per second, and I can't figure out how to specify the expression properly.

I've tried this:

dataflow:>stream create --name test --definition "time --trigger.time-unit=SECONDS | transform --transformer.expression='hello world' | log" --deploy

and this (note addition of '#{}'):

dataflow:>stream create --name test --definition "time --trigger.time-unit=SECONDS | transform --transformer.expression=#{'hello world'} | log" --deploy

But I continue to get an error in the transform log:

Failed to convert property value of type 'java.lang.String' 
  to required type 'org.springframework.expression.Expression' for 
  property 'expression'; nested exception is 
  org.springframework.core.convert.ConverterNotFoundException: No converter 
  found capable of converting from type [java.lang.String] to type 
  [org.springframework.expression.Expression]

I have read the Spring Expression Language docs (which I'm not finding helpful since the examples are all in terms of Java code). I've also looked at some of the Spring Cloud Stream test code for examples.

What am I missing?

Once I get 'hello world' working, I'd like to echo a property out of the local Dataflow Server's application.yml file too; suggestions welcome!

---

Update: I followed the quick-start docs for the 1.3.0.M1 milestone. The apps are loaded with

dataflow:>app import --uri http://bit-ly/Bacon-RELEASE-stream-applications-kafka-10-maven 

just as documented. (Note, replaced bit.ly with bit-ly for purposes of this post since StackOverflow doesn't like bit.ly URLs.) When I hit the bit.ly URL directly in my browser and download the file, I see this:

source.file=maven://org.springframework.cloud.stream.app:file-source-kafka-10:1.2.0.RELEASE
source.file.metadata=maven://org.springframework.cloud.stream.app:file-source-kafka-10:jar:metadata:1.2.0.RELEASE
source.ftp=maven://org.springframework.cloud.stream.app:ftp-source-kafka-10:1.2.0.RELEASE
....

What URL should I use in order to download the 1.3.0.M1 apps? Is it a problem that the 1.2.0 apps don't appear to work with the 1.3.0.M1 server?

Answer 1

See the documentation 'White space and quote rules'.

To insert a literal expression from the shell you need \"'hello'\" or '''hello'''.

Answer 2

Looks like you use some old versions for SCDF and all those out-of-the-box apps.

All the apps has something like this in the transform:

@EnableBinding(Processor.class)
@EnableConfigurationProperties(TransformProcessorProperties.class)
public class TransformProcessorConfiguration {

Where @EnableBinding comes from Spring Cloud Stream with the:

@Configuration
@Import({ BindingServiceConfiguration.class, BindingBeansRegistrar.class, BinderFactoryConfiguration.class,
        SpelExpressionConverterConfiguration.class })
@EnableIntegration
public @interface EnableBinding {

Pay attention to the SpelExpressionConverterConfiguration. Exactly this one is responsible for converting strings to the Expression properties.

So, be sure that you use the latest Bacon SCSt Apps: http://docs.spring.io/spring-cloud-stream-app-starters/docs/Bacon.RELEASE/reference/html/