diff --git a/src/model.ts b/src/model.ts
index 6bfd921..e3c0500 100644
--- a/src/model.ts
+++ b/src/model.ts
@@ -1,4 +1,5 @@
 import ModelController from "./controllers/modelController";
+import { creditHistories } from "./types/type";
 
 export default class Model {
   modelName: string;
@@ -8,53 +9,99 @@ export default class Model {
     this.modelName = modelName;
     this.modelController = ModelController.getInstance();
   }
+
   //TODO(woojae): login not Required
+  /**
+   * Check if model is running. It throws error when model is not running.
+   */
   async isRunning() {
     return await this.modelController.isRunning(this.modelName);
   }
 
+  /**
+   * Get model information. not implemented yet.
+   * @returns {string} Model information.
+   */
   //TODO(woojae): login not Required
   async getInformation() {
     return await this.modelController.getInformation(this.modelName);
   }
 
+  /**
+   * Calculate estimated cost for given request data.
+   * @param {string} rerquestData string data for request to model.
+   * @returns {number} Estimated cost.
+   */
   //TODO(woojae): login not Required
   async calculateCost (requestData: string) {
     return await this.modelController.calculateCost(this.modelName, requestData);
   }
 
+  /**
+   * Charge credit to model. 
+   * @param {number} amount Amount of credit to charge.
+   * @returns {string} Transaction hash.
+   */
   async chargeCredit(amount: number) {
     return await this.modelController.chargeCredit(this.modelName, amount);
   }
 
+  /**
+   * Withdraw credit from model.
+   * @param {number} amount Amount of credit to withdraw.
+   * @returns {string} Transaction hash.
+   */
   async withdrawCredit(amount: number) {
     return await this.modelController.withdrawCredit(this.modelName, amount);
   }
 
+  /**
+   * Get credit balance of model. 
+   * @returns {number} Amount of credit balance.
+   */
   async getCreditBalance() {
     return await this.modelController.getCreditBalance(this.modelName);
   }
 
+  /**
+   * Get credit history of model. 
+   * @returns {creditHistories} Histories of credit deposit and usage.
+   */
   async getCreditHistory() {
     return await this.modelController.getCreditHistory(this.modelName);
   }
 
+  /**
+   * Use model with given request data.
+   * @param {string} requestData String data for request to model. 
+   * @returns {string} Response data from model.
+   */
   async use(requestData: string) {
     return await this.modelController.use(this.modelName, requestData);
   }
 
+  /**
+   * Change status of AI model container to Running. Need to be admin. Not implemented yet.
+   */
   //NOTE(woojae): need admin
   async run() {
     await this.isAdmin();
     return await this.modelController.run(this.modelName);
   }
 
+  /**
+   * Change status of AI model container to Stopped. Need to be admin. Not implemented yet.
+   */
   //NOTE(woojae): need admin
   async stop() {
     await this.isAdmin();
     return await this.modelController.stop(this.modelName);
   }
 
+  /**
+   * Change model configuration. Need to be admin. Not implemented yet.
+   * @param {any} config Configuration to change. Not implemented yet.
+   */
   //NOTE(woojae): need admin
   async changeModelInfo(config: any) {
     await this.isAdmin();
diff --git a/src/types/type.ts b/src/types/type.ts
index c797106..72ed108 100644
--- a/src/types/type.ts
+++ b/src/types/type.ts
@@ -30,6 +30,17 @@ export enum HISTORY_TYPE {
   USAGE = "USAGE",
 }
 
+export type creditHistories = {
+  [timestamp: string]: creditHistory;
+}
+
+export type creditHistory = {
+  type: HISTORY_TYPE;
+  amount: number;
+  transferKey?: string;
+  requestTimestamp?: string;
+}
+
 export type opResult = {
   code: number;
   bandwidth_gas_amount: number;