Rewrite net::client::redundant
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The new implementation is significantly simpler and shorter. There are
two important changes to discuss:
- The 'Transport' type and worker task have been replaced by a simple
concurrent data structure within 'Connection'. This eliminates an
unnecessary synchronization point for performing requests: now, many
requests can be launched from multiple threads simultaneously.
- The 'QueryState' based state machine has been replaced with a simple
async function definition, that iterates through transports and then
falls back to waiting on all running connections. It is far easier
to understand the control flow here.
- Instead of 'tokio::select!{}' in a loop, we use 'buffer_unordered'
to dynamically collect ongoing requests (which we produce following
timeouts).
- Instead of tracking IDs for transports, we wrap them in 'Arc's and
then give out references to them. Transport statistics can then be
updated directly, without external communication.
- Instead of 'QueryState::Report()', we use a drop guard in the
underlying request futures. This directly writes to a statistics
structure that is shared with all threads.
The end result is a smaller, more readable codebase. 'Connection::new'
no longer returns a separate 'Transport' type. 'Connection::add' has
become a synchronous, infallible function. Multiple requests can be
performed in parallel, without having to request data from a task.
This removes a layer of vtables that may not be necessary in some applications. However, it complicates the typing -- we'll see whether it's worthwhile.
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This is a low-priority PR for simplifying the
Redundantclient transport. I've managed to replace a large and difficult state machine with a much simpler control flow, making good use of thefutures_utilcrate. The module has become significantly smaller and easier to understand.