Session

与spark类型，Datafusion程序执行期间也是使用Session作为统一的状态管理。

使用Session trait定义了行为。

#[async_trait]  
pub trait Session: Send + Sync {  
    /// Return the session ID  
    fn session_id(&self) -> &str;  
  
    /// Return the [`SessionConfig`]  
    fn config(&self) -> &SessionConfig;  
  
    /// return the [`ConfigOptions`]  
    fn config_options(&self) -> &ConfigOptions {  
        self.config().options()  
    }  
  
    async fn create_physical_plan(  
        &self,  
        logical_plan: &LogicalPlan,  
    ) -> Result<Arc<dyn ExecutionPlan>>;  
  
    fn create_physical_expr(  
        &self,  
        expr: Expr,  
        df_schema: &DFSchema,  
    ) -> Result<Arc<dyn PhysicalExpr>>;  
  
    /// Return reference to scalar_functions  
    fn scalar_functions(&self) -> &HashMap<String, Arc<ScalarUDF>>;  
  
    /// Return reference to aggregate_functions  
    fn aggregate_functions(&self) -> &HashMap<String, Arc<AggregateUDF>>;  
  
    /// Return reference to window functions  
    fn window_functions(&self) -> &HashMap<String, Arc<WindowUDF>>;  
  
    /// Return the runtime env  
    fn runtime_env(&self) -> &Arc<RuntimeEnv>;  
  
    /// Return the execution properties  
    fn execution_props(&self) -> &ExecutionProps;  
  
    fn as_any(&self) -> &dyn Any;  
  
    /// Return the table options  
    fn table_options(&self) -> &TableOptions;  
  
    /// return the TableOptions options with its extensions  
    fn default_table_options(&self) -> TableOptions {  
        self.table_options()  
            .combine_with_session_config(self.config_options())  
    }  
  
    /// Returns a mutable reference to [`TableOptions`]  
    fn table_options_mut(&mut self) -> &mut TableOptions;  
  
    /// Get a new TaskContext to run in this session  
    fn task_ctx(&self) -> Arc<TaskContext>;  
}

使用SessionStore结构体作为外部使用的统一对象，其实也就是对Session实现对象的简单封装

type SessionRefLock = Arc<Mutex<Option<Weak<RwLock<dyn Session>>>>>;  
/// The state store that stores the reference of the runtime session state.  
#[derive(Debug)]  
pub struct SessionStore {  
    session: SessionRefLock,  
}

而实现了Session trait的结构就是SessionState，这里包含一个会话进行计算所需的全部环境、状态信息，是一个核心结构。

从Session trait定义的方法当中就可以看出由几个比较重要的运行状态

• RuntimeEnv
• TaskContext 其中RuntimeEnv主要是程序运行过程中的资源管理，在RuntimeEnv当中说明。本模块下主要以TaskContext作为切入点，来进一步了解计算任务的执行过程。