什么是session-local与thread-local

百度内的检索程序大量地使用了thread-local storage (缩写TLS)，有些是为了缓存频繁访问的对象以避免反复创建，有些则是为了在全局函数间隐式地传递状态。你应当尽量避免后者，这样的函数难以测试，不设置thread-local变量甚至无法运行。session-local与thread-local是brpc解决thread-local问题的两种机制，实际上还有一种，可自行查阅官网文档。这两种机制其实就是提供了一个保存rpc请求过程中的用户关注数据的保存、处理接口。

session-local

session-local data与一次server端RPC绑定: 从进入service回调开始，到调用server端的done结束，不管该service是同步还是异步处理。 session-local data会尽量被重用，在server停止前不会被删除。

设置ServerOptions.session_local_data_factory后访问Controller.session_local_data()即可获得session-local数据。若没有设置，Controller.session_local_data()总是返回NULL。

若ServerOptions.reserved_session_local_data大于0，Server会在提供服务前就创建这么多个数据。

struct MySessionLocalData {MySessionLocalData() : x(123) {}int x;
};class EchoServiceImpl : public example::EchoService {
public:...void Echo(google::protobuf::RpcController* cntl_base,const example::EchoRequest* request,example::EchoResponse* response,google::protobuf::Closure* done) {...brpc::Controller* cntl = static_cast<brpc::Controller*>(cntl_base);// Get the session-local data which is created by ServerOptions.session_local_data_factory// and reused between different RPC.MySessionLocalData* sd = static_cast<MySessionLocalData*>(cntl->session_local_data());if (sd == NULL) {cntl->SetFailed("Require ServerOptions.session_local_data_factory to be set with a correctly implemented instance");return;}...

struct ServerOptions {...// The factory to create/destroy data attached to each RPC session.// If this field is NULL, Controller::session_local_data() is always NULL.// NOT owned by Server and must be valid when Server is running.// Default: NULLconst DataFactory* session_local_data_factory;// Prepare so many session-local data before server starts, so that calls// to Controller::session_local_data() get data directly rather than// calling session_local_data_factory->Create() at first time. Useful when// Create() is slow, otherwise the RPC session may be blocked by the// creation of data and not served within timeout.// Default: 0size_t reserved_session_local_data;
};

session_local_data_factory的类型为DataFactory，我们需要实现其中的CreateData和DestroyData。
brpc的DataFactory源码：

class DataFactory {
public:virtual ~DataFactory() {}// Implement this method to create a piece of data.// Notice that this method is const.// Returns the data, NULL on error.virtual void* CreateData() const = 0;// Implement this method to destroy a piece of data that was created// by Create().// Notice that this method is const.virtual void DestroyData(void*) const = 0;
};

实际上只是实现了一个抽象工厂的基类，对于产品族、具体工厂类都没有定义，这些是由具体业务去实现的。
注意：CreateData和DestroyData会被多个线程同时调用，**必须线程安全。**通常我们会在继承DataFactory的相关操作中添加锁保障线程安全。后面会有例子讲到。

class MySessionLocalDataFactory : public brpc::DataFactory {
public:void* CreateData() const {return new MySessionLocalData;}  void DestroyData(void* d) const {delete static_cast<MySessionLocalData*>(d);}  
};MySessionLocalDataFactory g_session_local_data_factory;int main(int argc, char* argv[]) {...brpc::Server server;brpc::ServerOptions options;...options.session_local_data_factory = &g_session_local_data_factory;...

我们需要将具体产品类的对象指针传入DestroyData以便brpc能帮我们回收资源，这里的资源回收是无需我们人工干预的，brpc已经帮我们进行管理：
如下源码：
在server start的时候就会根据传入的tls参数来进行启动一个sever bthread，随后在server结束时我们传入的具体session local数据会被抽象DestroyData指针调用destroy掉。

 // Init _keytable_pool always. If the server was stopped before, the pool// should be destroyed in Join()._keytable_pool = new bthread_keytable_pool_t;if (bthread_keytable_pool_init(_keytable_pool) != 0) {LOG(ERROR) << "Fail to init _keytable_pool";delete _keytable_pool;_keytable_pool = NULL;return -1;}if (_options.thread_local_data_factory) {_tl_options.thread_local_data_factory = _options.thread_local_data_factory;if (bthread_key_create2(&_tl_options.<