delta_scheduler

This repository holds Receiver and Scheduler class, that can be used to send/receive files through channels.

General architecture

Scheduler

A scheduler sends a file as chunks going through multiple channels.

It uses the selective repeat ARQ technique by sending file chunks in different channels, and retrying if a chunk isn't acknowledged by sending channel in a given time.

This package provides a Scheduler abstract class that must be implemented with the channel-selection policy of your choice.

Receiver

Receiver is plugged to the same channels as the scheduler, and after collecting all file chunks, it rebuilds the file.

Data exchange sequence

To avoid the struggle of manually configuring data channels on receiving end, we can use bootstrap channels: they will send configuration information to the receiver, which will use it to open data channels, being then ready to receive file chunks.

Implementation

Scheduler implementation

To implement your scheduler, you must extend the Scheduler class, which contains only one method for you to implement:

/// This lets Scheduler instances implement their own messages sending policy.
/// 
/// The implementation should send all messages' content, by calling the 
/// sendMessage method; it can also check for any resubmission timer presence, 
/// to avoid finishing execution while some messages have not been acknowledged.
Future<void> sendMessages(
  List<VeniceMessage> messages,
  List<Channel> channels,
  Map<int, CancelableOperation> resubmissionTimers);

Your sendMessages implementation must send all messages through available channels; the resubmissionTimers variable holds delays until a chunk is considered as not-transmitted, and must be sent again; timers handling is done by the Scheduler class itself (i.e. don't touch it), but you can still check its content (like tests/mock/scheduler/MockScheduler does) to ensure all chunks have been transmitted successfully.

Getting Started

# Install dependencies.
flutter pub get

# Run tests.
flutter test

File sending log example

Here is a file sending log using one channel:

# File chunks are sent sequentially.
[Scheduler] Sending chunk n°0.
[Scheduler] Sending chunk n°1.
[Scheduler] Sending chunk n°2.
[Scheduler] Sending chunk n°3.
[Scheduler] Sending chunk n°4.
[Scheduler] Sending chunk n°5.
[Scheduler] Sending chunk n°6.
[Scheduler] Sending chunk n°7.
[Scheduler] Sending chunk n°8.

# Some are acknowledged.
[Scheduler] Chunk n°7 was acknowledged.
[Scheduler] Chunk n°8 was acknowledged.
[Scheduler] Chunk n°5 was acknowledged.

# Some time out.
[Scheduler] Chunk n°0 was not acknowledged in time, resending.
[Scheduler] Chunk n°4 was acknowledged.
[Scheduler] Chunk n°1 was not acknowledged in time, resending.
[Scheduler] Chunk n°2 was not acknowledged in time, resending.
[Scheduler] Chunk n°3 was not acknowledged in time, resending.
[Scheduler] Chunk n°6 was not acknowledged in time, resending.

# Chunks which timed out are sent anew.
[Scheduler] Sending chunk n°6.
[Scheduler] Sending chunk n°3.
[Scheduler] Sending chunk n°2.
[Scheduler] Sending chunk n°1.
[Scheduler] Sending chunk n°0.
[Scheduler] Chunk n°2 was acknowledged.
[Scheduler] Chunk n°0 was acknowledged.
[Scheduler] Chunk n°1 was acknowledged.
[Scheduler] Chunk n°6 was acknowledged.
[Scheduler] Chunk n°3 was acknowledged.

# File sending ends when all chunks have been acknowledged.
[Scheduler] Finished dispatching all chunks to channels.