Feature: feature flag generic-snapshot-data

[drmingdrmer]

Changelog

Feature: feature flag generic-snapshot-data

Add feature flag generic-snapshot-data: when enabled, SnapshotData
does not have AsyncSeek + AsyncRead + AsyncWrite bound.
This enables application to define their own snapshot format.

If this feature flag is not eabled, no changes are required for application to upgrade Openraft.

On the sending end(leader that sends snapshot to follower):

• Without generic-snapshot-data: RaftNetwork::snapshot()
  provides a default implementation that invokes the chunk based API
  RaftNetwork::install_snapshot() for transmit.

• With generic-snapshot-data enabled: RaftNetwork::snapshot() must be
  implemented to provide application customized snapshot transmission.
  Application does not need to implement RaftNetwork::install_snapshot().

On the receiving end(follower):

• Raft::install_snapshot() is available only when
  generic-snapshot-data is disabled.

Add an example examples/raft-kv-memstore-generic-snapshot-data with
generic-snapshot-data enabled.
In this example snapshot is transmitted without fragmentation, i.e., via
RaftNetwork::snapshot(). The chunk based API
RaftNetwork::install_snapshot() is not used.
In a production scenario, a snapshot can be transmitted in arbitrary
manner.

• Fix: Refactor snapshot API #606
• Fix: Consider refactoring Snapshot type bound. #209

---

This change is 

[schreter]

Thanks for the PR.

general-snapshot-data

I'd suggest generic-snapshot-data, that fits it better, I think.

I didn't read all the code, but the general direction seems to fit. It will show whether there are issues once it's actually used in our code :-).

When it's merged, I'll try to integrate it with our snapshot soon (if I find time - I have to prepare a presentation and demo to our management in the next two weeks; but at the latest some time after that).

Reviewed 22 of 26 files at r1, 4 of 4 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ariesdevil, @drmingdrmer, and @lichuang)

---

openraft/Cargo.toml line 97 at r2 (raw file):

# Enable this feature flag to eliminate the `AsyncRead + AsyncWrite + AsyncSeek + Unpin` bound from `RaftTypeConfig::SnapshotData`.
# Enabling this feature allows applications to use a custom snapshot data format and transport fragmentation, diverging from the default implementation which typically relies on a single-file structure .

Maybe reflow the comment? The lines are quite long.

Code quote:

# Enable this feature flag to eliminate the `AsyncRead + AsyncWrite + AsyncSeek + Unpin` bound from `RaftTypeConfig::SnapshotData`.
# Enabling this feature allows applications to use a custom snapshot data format and transport fragmentation, diverging from the default implementation which typically relies on a single-file structure .

---

openraft/src/docs/feature_flags/feature-flags.md line 0 at r2 (raw file):
Also here, the lines are quite long.

BTW, we use dprint (via cargo dprint and also as a VS Code plugin to format on save) to format and reflow .md files to 100 chars (doesn't work for Rust comments, unfortunately).

What IDE are you using? Or just a text-based editor?

[drmingdrmer]

Make sense!

Look forward to hearing from you.

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ariesdevil, @lichuang, and @schreter)

---

openraft/src/docs/feature_flags/feature-flags.md line at r2 (raw file):

Previously, schreter wrote…

Also here, the lines are quite long.

BTW, we use dprint (via cargo dprint and also as a VS Code plugin to format on save) to format and reflow .md files to 100 chars (doesn't work for Rust comments, unfortunately).

What IDE are you using? Or just a text-based editor?

I use both vim and jetbrains Clion.

In vim, in normal mode gq wraps current line and in visual mode gq wraps selected lines. This works fine in most scenario. gq is syntax sensitive, it handles comment head such as # or // correctly.

In vim :help gq explains it:

In Clion, the action I bind to wrap line is Apply Structural Wrapping, which wraps rust doc comment correctly.

You can bind this action to a keystroke in Clion Keymap setting, or(as I do) bind it to an ideaVim(enabling vim functions in Clion) keystroke in .ideavimrc:

vmap Q     <Action>(Grazie.Pro.Structural.Wrap)

With this setting, in Clion press Q after selecting several lines of comment, it will wrap and reformat it.

[schreter]

Reviewed 21 of 21 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ariesdevil, @drmingdrmer, and @lichuang)

---

openraft/src/docs/feature_flags/feature-flags.md line at r2 (raw file):

Previously, drmingdrmer (张炎泼) wrote…

I use both vim and jetbrains Clion.

In vim, in normal mode gq wraps current line and in visual mode gq wraps selected lines. This works fine in most scenario. gq is syntax sensitive, it handles comment head such as # or // correctly.

In vim :help gq explains it:

In Clion, the action I bind to wrap line is Apply Structural Wrapping, which wraps rust doc comment correctly.

You can bind this action to a keystroke in Clion Keymap setting, or(as I do) bind it to an ideaVim(enabling vim functions in Clion) keystroke in .ideavimrc:

vmap Q     <Action>(Grazie.Pro.Structural.Wrap)

With this setting, in Clion press Q after selecting several lines of comment, it will wrap and reformat it.

Well, we are not allowed to use Clion at SAP, but VS Code works also quite well for Rust development (with rust-analyzer).