# Design Doc: The Client Library of Parameter Server

For an overview of trainer's role, please refer to [distributed training design doc](README.md). In this design doc, we will discuss the parameter server's client library, which will manage communication with parameter servers. The library will be implemented in [Go](https://golang.org/) and made available as a static or dynamic library with a C header file.

## C Interface

```c
#define PADDLE_ELEMENT_TYPE_INT32   0
#define PADDLE_ELEMENT_TYPE_UINT32  1
#define PADDLE_ELEMENT_TYPE_INT64   2
#define PADDLE_ELEMENT_TYPE_UINT64  3
#define PADDLE_ELEMENT_TYPE_FLOAT32 4
#define PADDLE_ELEMENT_TYPE_FLOAT64 5

typedef struct {
  char* name;
  int   element_type;
  void* content;
  int   content_len;
} paddle_parameter, paddle_gradient;

typedef struct paddle_pserver_client paddle_pserver_client;

paddle_pserver_client* paddle_new_pserver_client();
void paddle_pserver_client_release(paddle_pserver_client* client);

/**
 * @brief paddle_begin_init_params begins to initialize parameters on
 * parameter servers.
 *
 * paddle_begin_init_params will be called from multiple trainers,
 * only one trainer will be selected to initialize the parameters on
 * parameter servers. Other trainers will be blocked until the
 * initialization is done, and they need to get the initialized
 * parameters from parameter servers using @paddle_get_params.
 *
 * @param config_proto serialized parameter server configuration in
 * Protocol Buffers format.
 * @return 1 if the trainer is selected to initialize parameter
 * servers, otherwise 0.
 */
int paddle_begin_init_params(paddle_pserver_client* client, const char* config_proto);

/**
 * @brief paddle_init_param initializes the parameter on parameter
 * servers.
 *
 * @param param the parameter to initialize.
 * @return 0 if successful, otherwise -1. On failure, the trainer
 * needs to restart the entire initialization process (starting from
 * @paddle_begin_init_param). Or simply exit the program and wait for
 * the cluster management system to restart the trainer.
 */
int paddle_init_param(paddle_pserver_client* client, paddle_parameter params);

/**
 * @brief paddle_finish_init_params tells parameter servers client has
 * sent all parameters to parameter servers as initialization.
 *
 * @return 0 if successful, otherwise -1. On failure, the trainer
 * needs to restart the entire initialization process (starting from
 * @paddle_begin_init_param). Or simply exit the program and wait for
 * the cluster management system to restart the trainer.
 */
int paddle_finish_init_params(paddle_pserver_client* client);

/**
 * @brief paddle_send_grads sends gradients to parameter servers for
 * updating parameters.
 *
 * @param grads the array of gradients to send.
 * @param total the total number of gradient inside the gradient array.
 * @param learning_rate the learning rate for the gradients.
 * @return 0 if successful, otherwise -1.
 */
int paddle_send_grads(paddle_pserver_client* client, const paddle_gradient* grads, int total, double learning_rate);

/**
 * @brief paddle_set_params sets parameters to parameter servers.
 *
 * @param params the array of parameters to set to parameter servers.
 * @param total the total number of parameters inside the parameter
 * array.
 * @return 0 if successful, otherwise -1.
 */
int paddle_set_params(paddle_pserver_client* client, const paddle_parameter* params, int total);

/**
 * @brief paddle_get_params gets parameters from parameter servers.
 *
 * @param names the array of names of the parameters to get.
 * @param dst the destination array of parameters to save to.
 * @param total the total number of parameters to get.
 * @return 0 if successful, otherwise -1.
 */
int paddle_get_params(paddle_pserver_client* client, const char** names, paddle_parameter* dst, int total);

/**
 * @brief paddle_save_model indicates parameters to save the parameter
 * to the given path
 *
 * @param path the path to save parameters.
 * @return 0 if successful, otherwise -1.
 */
int paddle_save_model(paddle_pserver_client* client, const char* path);
```