Async script result

This page describes StackState version 4.1.

The StackState 4.1 version range is End of Life (EOL) and no longer supported. We encourage customers still running the 4.1 version range to upgrade to a more recent release.

Go to the documentation for the latest StackState release.

Most API functions execute asynchronously. That means they will not directly return results. Some functions are dependent on the network or other resources in order to complete, therefore they can not immediately return results. Such asynchronous functions return an AsyncScriptResult. The concept of an AsyncScriptResult is modelled after how promises in Javascript work.

Working with AsyncScriptResult.then

If the result of your script returns is an AsyncScriptResult StackState will automatically wait for the actual result to resolve. If however you want to continue your script with the resolved result of an AsyncScriptResult you must use the .then method.

The .then method expects a Groovy closure. The closure will execute as soon as the result is received. This lambda function can work with the result and return either a new AsyncScriptResult or a simple (synchronous) result.

For example:

asyncScriptResult = ScriptApi.asyncFn()
asyncScriptResult.then { result -> result.toString() }

The Groovy script above can be shortened to:

ScriptApi.asyncFn().then { it.toString() }

The it keyword is default Groovy keyword that you do not need to define a variable in which you receive your result. You might see this being used in our examples.

Chaining

Multiple asynchronous script results can be chained together. This is useful for combining for example the results of topology with telemetry.

For example:

ScriptApi.asyncFn1()
  .then {  ScriptApi.asyncFn2(it)  }
  .then {  ScriptApi.asyncFn3(it)  }

Is equivalent to:

ScriptApi.asyncFn1()
  .then {  ScriptApi.asyncFn2(it).then { ScriptApi.asyncFn3(it) }  }

The above means that the results of asyncFn1 are passed to asyncFn2, then the results of asyncFn2 in turn are passed to asyncFn3.

Flattening

Since version 1.15 arrays of AsyncScriptResult are automatically flattened when returned from a .then call. For example:

ScriptApi.asyncFn1().then  {
  [ScriptApi.asyncFn2(), ScriptApi.asyncFn3()]
}

will return an array of both the result of asyncFn2 and asyncFn3.

Reducing with thenInject

Since version 4.1.0 arrays of AsyncScriptResult can be automatically reduced when returned. For example:

ScriptApi.asyncFn1().thenInject([])  { accumulator, element ->
  accumulator + element
}

Suppose that asyncFn1 returns a list, then subsequent thenInject call can accumulate the result, in this case using summation.

In particular this call can be interesting for the cases where an accumulating operation returns the AsyncScriptResult. See example below:

ScriptApi.asyncFn1().thenInject([])  { accumulator, element ->
  ScriptApi.asyncFn2(element)
    .then { result -> accumulator + result }
}

Handling Exceptions

It is sometimes necessary to handle exceptions raised during execution of AsyncScriptResult. This can be achieved using catchError function available from version 4.0. For example:

ScriptApi.asyncFn1().catchError { ex ->
  // Do something with the exception
  ex.getMessage()
}

Any result returned by the closure passed to catchError gets automatically flattened just like .then call.