dart vm send back stream from isolate

Question

there is similar question (how to process a HTTP stream with Dart) about processing streams in dart2js. I am focused on vm.

I can read from spec that:

The content of message can be: primitive values (null, num, bool, double, String), instances of SendPort, and lists and maps whose elements are any of these. List and maps are also allowed to be cyclic.

In the special circumstances when two isolates share the same code and are running in the same process (e.g. isolates created via spawn), it is also possible to send object instances (which would be copied in the process). This is currently only supported by the dartvm. For now, the dart2js compiler only supports the restricted messages described above.

I ve learnt I cannot send to isolates and back following objects: HttpRequest and HttpResponse objects, I cannot send streams.

Q: I cannot really understand how should I process big chunk of data in isolate and then send it back to main isolate and in turn it can be send back to client.

Normally if I want to read a file. I can obtain a stream, apply transforms and then pipe a stream to http response. what is best practice to do that using isolates?

thanks for help!

Answer 1

I have provided an example but Ill give a quick overview on how you can achieve this although it may not necessarily be the best practise - its simply the only way I personally know how.

We are given a method called Isolate.spawnUri(Uri uri, List<String> args, dynamic message)

The message parameter can hold any of the values the extract in your first post mentions. What we want to do is in the main thread create a new ReceivePort and listen for incoming data, then we want to spawn an isolate with the message as the ReceivePorts .sendPort.

The isolate should then create its own ReceivePort and use the message value to send back its own sendPort and listen on its receive port. What this essentially does is creates a 2 way communication. Allowing us to keep the isolate alive and send work back and forth.

Any response from your isolate will come through your original receive port and any data you want to send to the isolate will be through the send port the isolate just sent back.

You can then directly send the stream data to the isolate for it to process at will and it can send back data as its made (or you can just close the ports after isolate has sent its response - up to you how you want to go about it).

I have provided an example on GitHub: https://github.com/TomCaserta/ExampleIsolate

Please note, if you want your isolates to work in dart2js its important that you run dart2js on each of your isolate files via the following command:

dart2js -o <filename>.dart.js <filename>.dart