浅读TiKV源码：Coprocessor

rust
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#[async_trait(?Send)]
pub trait RawStorage {
    /// Retrieves the value for a given key from the storage on the current
    /// node. Returns [`Option::None`] if the key is not present in the
    /// database.
    async fn get(&self, key: Key) -> PluginResult<Option<Value>>;

    /// Same as [`RawStorage::get()`], but retrieves values for multiple keys at
    /// once.
    async fn batch_get(&self, keys: Vec<Key>) -> PluginResult<Vec<KvPair>>;

    /// Same as [`RawStorage::get()`], but accepts a `key_range` such that
    /// values for keys in `[key_range.start, key_range.end)` are retrieved.
    /// The upper bound of the `key_range` is exclusive.
    async fn scan(&self, key_range: Range<Key>) -> PluginResult<Vec<Value>>;

    /// Inserts a new key-value pair into the storage on the current node.
    async fn put(&self, key: Key, value: Value) -> PluginResult<()>;

    /// Same as [`RawStorage::put()`], but inserts multiple key-value pairs at
    /// once.
    async fn batch_put(&self, kv_pairs: Vec<KvPair>) -> PluginResult<()>;

    /// Deletes a key-value pair from the storage on the current node given a
    /// `key`. Returns [`Result::Ok]` if the key was successfully deleted.
    async fn delete(&self, key: Key) -> PluginResult<()>;

    /// Same as [`RawStorage::delete()`], but deletes multiple key-value pairs
    /// at once.
    async fn batch_delete(&self, keys: Vec<Key>) -> PluginResult<()>;

    /// Same as [`RawStorage::delete()`], but deletes multiple key-values pairs
    /// at once given a `key_range`. All records with keys in
    /// `[key_range.start, key_range.end)` will be deleted. The upper bound
    /// of the `key_range` is exclusive.
    async fn delete_range(&self, key_range: Range<Key>) -> PluginResult<()>;
}

rust
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pub trait Store: Send {
    /// The scanner type returned by `scanner()`.
    type Scanner: Scanner;

    /// Fetch the provided key.
    fn get(&self, key: &Key, statistics: &mut Statistics) -> Result<Option<Value>>;

    /// Re-use last cursor to incrementally (if possible) fetch the provided
    /// key.
    fn incremental_get(&mut self, key: &Key) -> Result<Option<Value>>;

    /// Take the statistics. Currently only available for `incremental_get`.
    fn incremental_get_take_statistics(&mut self) -> Statistics;

    /// Whether there was data > ts during previous incremental gets.
    fn incremental_get_met_newer_ts_data(&self) -> NewerTsCheckState;

    /// Fetch the provided set of keys.
    fn batch_get(
        &self,
        keys: &[Key],
        statistics: &mut Vec<Statistics>,
    ) -> Result<Vec<Result<Option<Value>>>>;

    /// Retrieve a scanner over the bounds.
    fn scanner(
        &self,
        desc: bool,
        key_only: bool,
        check_has_newer_ts_data: bool,
        lower_bound: Option<Key>,
        upper_bound: Option<Key>,
    ) -> Result<Self::Scanner>;
}

vbnet
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## Future work

### Official coprocessor

Provide an official coprocessor plugin which defines common primitives like `bool`, `f32`, `UTF8String`, `Array<T>` along with codec for them, moreover, implements simple scanning expressions or even aggregations.

Two reasons for that:

1. To provide an example showing how to build a coprocessor plugin for TiKV and guaranteed to be always up-to-date.

2. To comfort simple use cases to TiKV, so as to avoid forcing the users to develop a new plugin at the beginning.

### Transaction mode

Transaction mode is way more complicated than the RawKV mode. The current picture of txn plugin is an 'interactive' component with the client, which means, the client and the coprocessor will talk to each others during the request being processed. This is especially for resolving locks and coordinating the transaction among multiple coprocessors in multiple TiKV nodes.

### Static mode

Discussed in the section above.

### Migrate `tidb_query`

When static plugin and txn plugin is implemented, we can move the coprocessor specific to TiDB out of TiKV, and the repository of TiDB might be a good new home.