Test: add example of singlethreaded kv-store

[drmingdrmer]

Changelog

Test: add example of singlethreaded kv-store

This example examples/raft-kv-memstore-singlethreaded illustrates
building a single threaded Openraft application.

By enabling feature flag singlethreaded, Openraft does not require the
data types to be Send, which enables using Rc in place of Arc to
gain a performance improvement.

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This change is 

[drmingdrmer]

@wvwwvwwv @schreter
Please help on reviewing this example of a single-threaded application to determine if it meets the requirements of your environment.

[schreter]

On the first look, it looks good, thanks. I didn't do a very thorough review, though, since I suppose it's mostly copy&paste from another example anyway :-).

Some minor comments below.

Reviewed 14 of 14 files at r1, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @ariesdevil, @drmingdrmer, and @lichuang)

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examples/raft-kv-memstore-singlethreaded/src/network.rs line 25 at r1 (raw file):

// NOTE: This could be implemented also on `Arc<ExampleNetwork>`, but since it's empty, implemented
// directly.

?

Suggestion:

// NOTE: This could be implemented also on `Rc<ExampleNetwork>`, but since it's empty, implemented
// directly.

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examples/raft-kv-memstore-singlethreaded/src/router.rs line 17 at r1 (raw file):

#[derive(Default)]
pub struct Router {
    pub targets: Rc<Mutex<BTreeMap<NodeId, RequestTx>>>,

Since it's all running in a single thread, why the lock? A RefCell or so would be sufficient, right?

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examples/raft-kv-memstore-singlethreaded/src/store.rs line 106 at r1 (raw file):

    pub state_machine: RwLock<StateMachineData>,

    snapshot_idx: Arc<Mutex<u64>>,

Same here - a Rc<Cell<u64>> or so should be sufficient, right?

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examples/raft-kv-memstore-singlethreaded/tests/cluster/test_cluster.rs line 27 at r1 (raw file):

    let backtrace = {
        format!("{:?}", Backtrace::force_capture())
        // #[cfg(feature = "bt")]

Why the commented-out code?

[drmingdrmer]

Thank you. The following fixup commit replaced these multi-threaded primitives.

Reviewed 6 of 6 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ariesdevil and @lichuang)

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examples/raft-kv-memstore-singlethreaded/tests/cluster/test_cluster.rs line 27 at r1 (raw file):

Previously, schreter wrote…

Why the commented-out code?

Not used codes. Removed:(

[schreter]

Reviewed 6 of 6 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

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examples/raft-kv-memstore-singlethreaded/src/store.rs line 104 at r2 (raw file):

    pub state_machine: RefCell<StateMachineData>,

    snapshot_idx: RefCell<u64>,

BTW, any Copy values can be also stored as Cell, which incurs less overhead (especially, no need for runtime checking). Then, you can use simple get()/set() interface to read/write the cell.

[drmingdrmer]

Reviewable status: complete! all files reviewed, all discussions resolved

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examples/raft-kv-memstore-singlethreaded/src/store.rs line 104 at r2 (raw file):

Previously, schreter wrote…

BTW, any Copy values can be also stored as Cell, which incurs less overhead (especially, no need for runtime checking). Then, you can use simple get()/set() interface to read/write the cell.

Thank you for the suggestion.

I have given thought to employing Cell in the implementation. However, for the sake of simplicity in this example application, I'm aiming to minimize the number of types used. :)