diff --git a/MastodonSDK/Sources/MastodonSDK/API/Error/Mastodon+API+Error+MastodonAPIError.swift b/MastodonSDK/Sources/MastodonSDK/API/Error/Mastodon+API+Error+MastodonAPIError.swift
new file mode 100644
index 000000000..47449e79e
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/API/Error/Mastodon+API+Error+MastodonAPIError.swift
@@ -0,0 +1,18 @@
+//
+//  Mastodon+API+Error+MastodonAPIError.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/27.
+//
+
+import Foundation
+
+extension Mastodon.API.Error {
+    public enum MastodonAPIError: Swift.Error {
+        case generic(errorResponse: Mastodon.Response.ErrorResponse)
+        
+        init(errorResponse: Mastodon.Response.ErrorResponse) {
+            self = .generic(errorResponse: errorResponse)
+        }
+    }
+}
diff --git a/MastodonSDK/Sources/MastodonSDK/API/Error/Mastodon+API+Error.swift b/MastodonSDK/Sources/MastodonSDK/API/Error/Mastodon+API+Error.swift
new file mode 100644
index 000000000..ad87fe8f3
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/API/Error/Mastodon+API+Error.swift
@@ -0,0 +1,36 @@
+//
+//  Mastodon+API+Error.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/26.
+//
+
+import Foundation
+import enum NIOHTTP1.HTTPResponseStatus
+
+extension Mastodon.API {
+    public struct Error: Swift.Error {
+        
+        public var httpResponseStatus: HTTPResponseStatus
+        public var mastodonAPIError: MastodonAPIError?
+        
+        init(
+            httpResponseStatus: HTTPResponseStatus,
+            mastodonAPIError: Mastodon.API.Error.MastodonAPIError?
+        ) {
+            self.httpResponseStatus = httpResponseStatus
+            self.mastodonAPIError = mastodonAPIError
+        }
+        
+        init(
+            httpResponseStatus: HTTPResponseStatus,
+            errorResponse: Mastodon.Response.ErrorResponse
+        ) {
+            self.init(
+                httpResponseStatus: httpResponseStatus,
+                mastodonAPIError: MastodonAPIError(errorResponse: errorResponse)
+            )
+        }
+        
+    }
+}
diff --git a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+App.swift b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+App.swift
index 6985bbd5c..78fd2a39f 100644
--- a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+App.swift
+++ b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+App.swift
@@ -16,17 +16,27 @@ extension Mastodon.API.App {
     
     public static func create(
         session: URLSession,
+        domain: String,
         query: CreateQuery
     ) -> AnyPublisher<Mastodon.Response.Content<Mastodon.API.App.Application>, Error>  {
-        fatalError()
+        let request = Mastodon.API.request(
+            url: appEndpointURL(domain: domain),
+            query: query,
+            authorization: nil
+        )
+        return session.dataTaskPublisher(for: request)
+            .tryMap { data, response in
+                let value = try Mastodon.API.decode(type: Application.self, from: data, response: response)
+                return Mastodon.Response.Content(value: value, response: response)
+            }
+            .eraseToAnyPublisher()
     }
-    
 
 }
 
 extension Mastodon.API.App {
     
-    struct Application: Codable {
+    public struct Application: Codable {
 
         public let id: String
 
@@ -49,12 +59,19 @@ extension Mastodon.API.App {
         }
     }
     
-    struct CreateQuery {
+    public struct CreateQuery: Codable, PostQuery {
         public let clientName: String
         public let redirectURIs: String
         public let scopes: String?
         public let website: String?
         
+        enum CodingKeys: String, CodingKey {
+            case clientName = "client_name"
+            case redirectURIs = "redirect_uris"
+            case scopes
+            case website
+        }
+        
         public init(
             clientName: String,
             redirectURIs: String = "urn:ietf:wg:oauth:2.0:oob",
@@ -67,19 +84,8 @@ extension Mastodon.API.App {
             self.website = website
         }
         
-        var queryItems: [URLQueryItem]? {
-            var items: [URLQueryItem] = []
-            items.append(URLQueryItem(name: "client_name", value: clientName))
-            items.append(URLQueryItem(name: "redirect_uris", value: redirectURIs))
-            scopes.flatMap {
-                items.append(URLQueryItem(name: "scopes", value: $0))
-            }
-            website.flatMap {
-                items.append(URLQueryItem(name: "website", value: $0))
-            }
-            
-            guard !items.isEmpty else { return nil }
-            return items
+        var body: Data? {
+            return try? Mastodon.API.encoder.encode(self)
         }
     }
     
diff --git a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Error.swift b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Error.swift
deleted file mode 100644
index 7a6c5e899..000000000
--- a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Error.swift
+++ /dev/null
@@ -1,22 +0,0 @@
-//
-//  Mastodon+API+Error.swift
-//  
-//
-//  Created by MainasuK Cirno on 2021/1/26.
-//
-
-import Foundation
-
-extension Mastodon.API.Error {
-    
-    struct ErrorResponse: Codable {
-        let error: String
-        let errorDescription: String?
-
-        enum CodingKeys: String, CodingKey {
-            case error
-            case errorDescription = "error_description"
-        }
-    }
-    
-}
diff --git a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+OAuth.swift b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+OAuth.swift
new file mode 100644
index 000000000..88461f995
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+OAuth.swift
@@ -0,0 +1,18 @@
+//
+//  Mastodon+API+OAuth.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/27.
+//
+
+import Foundation
+
+extension Mastodon.API.OAuth {
+
+    public static let authorizationField = "Authorization"
+
+    public struct Authorization {
+        public let accessToken: String
+    }
+
+}
diff --git a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Timeline.swift b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Timeline.swift
new file mode 100644
index 000000000..944aad7ec
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API+Timeline.swift
@@ -0,0 +1,91 @@
+//
+//  Mastodon+API+Timeline.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/27.
+//
+
+import Foundation
+import Combine
+
+extension Mastodon.API.Timeline {
+    
+    static func publicTimelineEndpointURL(domain: String) -> URL {
+        return Mastodon.API.endpointURL(domain: domain).appendingPathComponent("timelines/public")
+    }
+    
+    public static func create(
+        session: URLSession,
+        domain: String,
+        query: PublicTimelineQuery
+    ) -> AnyPublisher<Mastodon.Response.Content<[Mastodon.Entity.Toot]>, Error>  {
+        let request = Mastodon.API.request(
+            url: publicTimelineEndpointURL(domain: domain),
+            query: query,
+            authorization: nil
+        )
+        return session.dataTaskPublisher(for: request)
+            .tryMap { data, response in
+                let value = try Mastodon.API.decode(type: [Mastodon.Entity.Toot].self, from: data, response: response)
+                return Mastodon.Response.Content(value: value, response: response)
+            }
+            .eraseToAnyPublisher()
+    }
+    
+}
+
+extension Mastodon.API.Timeline {
+    public struct PublicTimelineQuery: Codable, GetQuery {
+        
+        public let local: Bool?
+        public let remote: Bool?
+        public let onlyMedia: Bool?
+        public let maxID: Mastodon.Entity.Toot.ID?
+        public let sinceID: Mastodon.Entity.Toot.ID?
+        public let minID: Mastodon.Entity.Toot.ID?
+        public let limit: Int?
+    
+        public init(local: Bool?, remote: Bool?, onlyMedia: Bool?, maxID: Mastodon.Entity.Toot.ID?, sinceID: Mastodon.Entity.Toot.ID?, minID: Mastodon.Entity.Toot.ID?, limit: Int?) {
+            self.local = local
+            self.remote = remote
+            self.onlyMedia = onlyMedia
+            self.maxID = maxID
+            self.sinceID = sinceID
+            self.minID = minID
+            self.limit = limit
+        }
+        
+        var queryItems: [URLQueryItem]? {
+            var items: [URLQueryItem] = []
+            local.flatMap {
+                items.append(URLQueryItem(name: "local", value: $0.queryItemValue))
+            }
+            remote.flatMap {
+                items.append(URLQueryItem(name: "remote", value: $0.queryItemValue))
+            }
+            onlyMedia.flatMap {
+                items.append(URLQueryItem(name: "only_media", value: $0.queryItemValue))
+            }
+            maxID.flatMap {
+                items.append(URLQueryItem(name: "max_id", value: $0))
+            }
+            sinceID.flatMap {
+                items.append(URLQueryItem(name: "since_id", value: $0))
+            }
+            minID.flatMap {
+                items.append(URLQueryItem(name: "min_id", value: $0))
+            }
+            limit.flatMap {
+                items.append(URLQueryItem(name: "limit", value: String($0)))
+            }
+            guard !items.isEmpty else { return nil }
+            return items
+        }
+    }
+}
+
+extension Bool {
+    var queryItemValue: String {
+        return self ? "true" : "false"
+    }
+}
diff --git a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API.swift b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API.swift
index b21c44cd0..e8ea282f8 100644
--- a/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API.swift
+++ b/MastodonSDK/Sources/MastodonSDK/API/Mastodon+API.swift
@@ -6,36 +6,107 @@
 //
 
 import Foundation
-import NIOHTTP1
+import enum NIOHTTP1.HTTPResponseStatus
 
-public extension Mastodon.API {
+extension Mastodon.API {
         
     static let timeoutInterval: TimeInterval = 10
+    static let httpHeaderDateFormatter = ISO8601DateFormatter()
+    static let encoder: JSONEncoder = {
+        let encoder = JSONEncoder()
+        encoder.dateEncodingStrategy = .iso8601
+        return encoder
+    }()
     static let decoder: JSONDecoder = {
         let decoder = JSONDecoder()
         decoder.dateDecodingStrategy = .iso8601
         
         return decoder
     }()
-    static let httpHeaderDateFormatter = ISO8601DateFormatter()
-    
-}
-
-extension Mastodon.API {
-    enum Error { }
-    enum App { }
-}
-
-extension Mastodon.API {
     
     static func endpointURL(domain: String) -> URL {
         return URL(string: "https://" + domain + "/api/v1/")!
     }
     
+}
+
+extension Mastodon.API {
+    public enum App { }
+    public enum OAuth { }
+    public enum Timeline { }
+}
+
+extension Mastodon.API {
+    
     static func request(
-        url: URL
+        url: URL,
+        query: GetQuery,
+        authorization: OAuth.Authorization?
     ) -> URLRequest {
-        fatalError()
+        var components = URLComponents(string: url.absoluteString)!
+        components.queryItems = query.queryItems
+        
+        let requestURL = components.url!
+        var request = URLRequest(
+            url: requestURL,
+            cachePolicy: .reloadIgnoringLocalAndRemoteCacheData,
+            timeoutInterval: Mastodon.API.timeoutInterval
+        )
+        if let authorization = authorization {
+            request.setValue(
+                "Bearer \(authorization.accessToken)",
+                forHTTPHeaderField: Mastodon.API.OAuth.authorizationField
+            )
+        }
+        request.httpMethod = "GET"
+        return request
+    }
+    
+    static func request(
+        url: URL,
+        query: PostQuery,
+        authorization: OAuth.Authorization?
+    ) -> URLRequest {
+        let components = URLComponents(string: url.absoluteString)!
+        let requestURL = components.url!
+        var request = URLRequest(
+            url: requestURL,
+            cachePolicy: .reloadIgnoringLocalAndRemoteCacheData,
+            timeoutInterval: Mastodon.API.timeoutInterval
+        )
+        request.setValue("application/json; charset=utf-8", forHTTPHeaderField: "Content-Type")
+        request.httpBody = query.body
+        if let authorization = authorization {
+            request.setValue(
+                "Bearer \(authorization.accessToken)",
+                forHTTPHeaderField: Mastodon.API.OAuth.authorizationField
+            )
+        }
+        request.httpMethod = "POST"
+        return request
+    }
+    
+    static func decode<T>(type: T.Type, from data: Data, response: URLResponse) throws -> T where T : Decodable {
+        // decode data then decode error if could
+        do {
+            return try Mastodon.API.decoder.decode(type, from: data)
+        } catch let decodeError {
+            #if DEBUG
+            debugPrint(decodeError)
+            #endif
+            
+            guard let httpURLResponse = response as? HTTPURLResponse else {
+                assertionFailure()
+                throw decodeError
+            }
+            
+            let httpResponseStatus = HTTPResponseStatus(statusCode: httpURLResponse.statusCode)
+            if let errorResponse = try? Mastodon.API.decoder.decode(Mastodon.Response.ErrorResponse.self, from: data) {
+                throw Mastodon.API.Error(httpResponseStatus: httpResponseStatus, errorResponse: errorResponse)
+            }
+            
+            throw Mastodon.API.Error(httpResponseStatus: httpResponseStatus, mastodonAPIError: nil)
+        }
     }
     
 }
diff --git a/MastodonSDK/Sources/MastodonSDK/Entity/Mastodon+Entity+Toot.swift b/MastodonSDK/Sources/MastodonSDK/Entity/Mastodon+Entity+Toot.swift
new file mode 100644
index 000000000..3bf52991f
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/Entity/Mastodon+Entity+Toot.swift
@@ -0,0 +1,34 @@
+//
+//  Mastodon+Entity+Toot.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/27.
+//
+
+import Foundation
+
+extension Mastodon.Entity {
+    public struct Toot: Codable {
+        
+        public typealias ID = String
+
+        public let id: ID
+
+        public let createdAt: Date
+        public let content: String
+        public let account: User
+        
+        public let language: String
+        public let visibility: String
+        
+        enum CodingKeys: String, CodingKey {
+            case id
+            case createdAt = "created_at"
+            case content
+            case account
+            case language
+            case visibility
+        }
+        
+    }
+}
diff --git a/MastodonSDK/Sources/MastodonSDK/Entity/Mastodon+Entity+User.swift b/MastodonSDK/Sources/MastodonSDK/Entity/Mastodon+Entity+User.swift
new file mode 100644
index 000000000..276ca8e4e
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/Entity/Mastodon+Entity+User.swift
@@ -0,0 +1,31 @@
+//
+//  Mastodon+Entity+User.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/27.
+//
+
+import Foundation
+
+extension Mastodon.Entity {
+    public struct User: Codable {
+        
+        public typealias ID = String
+
+        public let id: ID
+
+        public let username: Date
+        public let acct: String
+        public let displayName: String?
+        public let avatar: String?
+        
+        enum CodingKeys: String, CodingKey {
+            case id
+            case username
+            case acct
+            case displayName = "display_name"
+            case avatar
+        }
+        
+    }
+}
diff --git a/MastodonSDK/Sources/MastodonSDK/Protocol/Query.swift b/MastodonSDK/Sources/MastodonSDK/Protocol/Query.swift
new file mode 100644
index 000000000..e95b9141d
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/Protocol/Query.swift
@@ -0,0 +1,16 @@
+//
+//  Query.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/27.
+//
+
+import Foundation
+
+protocol GetQuery {
+    var queryItems: [URLQueryItem]? { get }
+}
+
+protocol PostQuery {
+    var body: Data? { get }
+}
diff --git a/MastodonSDK/Sources/MastodonSDK/Response/Mastodon+Response+ErrorResponse.swift b/MastodonSDK/Sources/MastodonSDK/Response/Mastodon+Response+ErrorResponse.swift
new file mode 100644
index 000000000..48519a5b6
--- /dev/null
+++ b/MastodonSDK/Sources/MastodonSDK/Response/Mastodon+Response+ErrorResponse.swift
@@ -0,0 +1,20 @@
+//
+//  Mastodon+Response+ErrorResponse.swift
+//  
+//
+//  Created by MainasuK Cirno on 2021/1/27.
+//
+
+import Foundation
+
+extension Mastodon.Response {
+    public struct ErrorResponse: Codable {
+        public let error: String
+        public let errorDescription: String?
+
+        enum CodingKeys: String, CodingKey {
+            case error
+            case errorDescription = "error_description"
+        }
+    }
+}
diff --git a/MastodonSDK/Tests/MastodonSDKTests/MastodonSDKTests.swift b/MastodonSDK/Tests/MastodonSDKTests/MastodonSDKTests.swift
index 73ff16ee8..db8f0d315 100644
--- a/MastodonSDK/Tests/MastodonSDKTests/MastodonSDKTests.swift
+++ b/MastodonSDK/Tests/MastodonSDKTests/MastodonSDKTests.swift
@@ -16,11 +16,19 @@ final class MastodonSDKTests: XCTestCase {
             clientName: "XCTest",
             website: nil
         )
-        Mastodon.API.App.create(session: session, query: query)
+        Mastodon.API.App.create(session: session, domain: domain, query: query)
             .receive(on: DispatchQueue.main)
             .sink { completion in
-                
+                switch completion {
+                case .failure(let error):
+                    XCTFail(error.localizedDescription)
+                case .finished:
+                    break
+                }
             } receiveValue: { response in
+                XCTAssertEqual(response.value.name, "XCTest")
+                XCTAssertEqual(response.value.website, nil)
+                XCTAssertEqual(response.value.redirectURI, "urn:ietf:wg:oauth:2.0:oob")
                 theExpectation.fulfill()
             }
             .store(in: &disposeBag)
@@ -28,6 +36,4 @@ final class MastodonSDKTests: XCTestCase {
         wait(for: [theExpectation], timeout: 10.0)
     }
     
-    
-    
 }