【一】代碼下載
https://github.com/tensorflow/tensorflow/releases/
PS：本次源碼分析採用1.11版本
【二】Session簡介
在TensorFlow中，session是溝通tf的橋樑，模型的訓練、推理，都需要通過session，session持有graph的引用。tf支持單機與分佈式，因此session也分爲單機版與分佈式版。
【三】session類圖
Session ::tensorflow\core\public\session.h
DirectSession ::tensorflow\core\common_runtime\direct_session.h
GrpcSession ::tensorflow\core\distributed_runtime\rpc\grpc_session.h
class DirectSession : public Session --->單機版session
class GrpcSession : public Session ---> 分佈式版session
【四】session創建
tf對外提供了一套C API，負責給client語言使用。在tensorflow\c\c_api.h中，session的創建流程我們從這裏開始。
TF_NewSession
NewSession （tensorflow\core\public\session.h）
1. NewSession實現 tensorflow\core\common_runtime\session.cc
Status NewSession(const SessionOptions& options, Session** out_session) {
SessionFactory* factory;
Status s = SessionFactory::GetFactory(options, &factory); // 通過SessionOptions來選擇不同的factory
s = factory->NewSession(options, out_session); // 調用對應的factory來創建對應的session，也就是創建DirectSession 還GrpcSession
}
2. 關於SessionOptions
這裏看一下該定義，省略掉其他成員，僅僅看看target，註釋中說，如果target爲空，則創建DirectSession，如果target以 'grpc’開頭，則創建GrpcSession
/// Configuration information for a Session.
struct SessionOptions {
/// \brief The TensorFlow runtime to connect to.
///
/// If 'target' is empty or unspecified, the local TensorFlow runtime
/// implementation will be used. Otherwise, the TensorFlow engine
/// defined by 'target' will be used to perform all computations.
///
/// "target" can be either a single entry or a comma separated list
/// of entries. Each entry is a resolvable address of the
/// following format:
/// local
/// ip:port
/// host:port
/// ... other system-specific formats to identify tasks and jobs ...
///
/// NOTE: at the moment 'local' maps to an in-process service-based
/// runtime.
///
/// Upon creation, a single session affines itself to one of the
/// remote processes, with possible load balancing choices when the
/// "target" resolves to a list of possible processes.
///
/// If the session disconnects from the remote process during its
/// lifetime, session calls may fail immediately.
string target;
};
3. SessionFactory::GetFactory
這裏的關鍵在於：session_factory.second->AcceptsOptions(options)
依據選項來，這裏實際上是調用對應的factory方法
DirectSessionFactory
bool AcceptsOptions(const SessionOptions& options) override {
return options.target.empty(); // target爲空則爲true
}
GrpcSessionFactory
bool AcceptsOptions(const SessionOptions& options) override {
return str_util::StartsWith(options.target, kSchemePrefix); // const char* const kSchemePrefix = "grpc://"; target爲grpc開頭
}
class GrpcSessionFactory : public SessionFactory
class DirectSessionFactory : public SessionFactory
【五】總結
session的創建通過option（可以理解爲配置）來選擇性創建，而session的真正創建由sessionfactory來完成，這裏採用了工廠設計模式，將各個factory通過註冊的方式註冊到sessionfactory中，這樣在系統啓動後，這些factory就可以用了，新增一個factory也很容易，且不影響上層代碼。