Does Dio execute network requests in a separate isolate?

[tuanhwing]

Hi Dio maintainers 👋,

I recently had a technical interview and was told that Dio runs its network requests in a separate isolate. I would like to confirm whether this is true or not.

From my understanding:

• Dio uses Dart's HttpClient under the hood, which performs asynchronous non-blocking I/O on the main isolate
• Unless we explicitly use Isolate.spawn() or compute(), everything still runs on the main isolate

Can you please clarify:

1. Does Dio internally create a new isolate or use any kind of thread-like mechanism to perform network requests?
2. Is there any part of Dio that runs outside the main isolate by default?
3. Are there performance best practices when working with Dio + large JSON parsing?

Thank you so much! 🙏

[AlexV525]

The above statements are not 100% correct. Here is what Dio is doing (at this time, Dart 3.8):

1. Requests are computed and forwarded in the main isolate.
2. Responses are received and deserialized (streamed response) in the main isolate.
3. Regarding the structured data, Dio uses FusedTransformer to improve speed by default:
   a. If the user wants to consume ResponseType.stream or ResponseType.bytes, the response will be directly consolidated in the main thread.
   b. If the response is UTF-8-encoded JSON and no custom decoder is specified, the length of the content will be calculated, and using the compute isolate or the main isolate to read data according to the threshold. The default threshold is 50 KB.
4. Interceptors are all running in the main isolate unless user code specifies that.

[AlexV525]

See also: https://pub.dev/documentation/flutter_library/latest/flutter_library/FusedTransformer-class.html

[tuanhwing]

Thanks for the clear explanation!