1. Taming Cloud APIs with Swift
Tim Burks, Google
Swift Cloud Workshop No. 3
February 23, 2018

2. How to make Swift a first-class
language in the cloud:
1. Run Swift in containers.
2. Call (and serve) cloud APIs.

5. 10 billion+
API calls
every second

6. Protocol Buffers are a language-neutral, platform-neutral, extensible
mechanism for serializing structured data.

7. “Protocol Buffers” means several things
1. A serialization mechanism
2. An interface description language
3. A methodology

8. Protocol Buffer Serialization
It’s just a stream of bytes
[field_number<<3 + wire_type] [length if necessary] [data]...
$ hexdump /tmp/request.bin
0000000 0a 05 68 65 6c 6c 6f
0a is “0000 1010”, so
field_number = 1 and wire_type = 2

9. Protocol Buffers aren’t just for networking
class UserData {
static let sharedInstance = UserData()
public var user : Models_User
init() {
// read info from UserDefaults
if let userdata = UserDefaults.standard.data(forKey:"user") {
do {
user = try Models_User(serializedData:userdata)
} catch {
user = Models_User()
}
} else {
user = Models_User()
}
}
func save() {
DispatchQueue.main.async {
do {
try UserDefaults.standard.set(self.user.serializedData(), forKey:"user")
} catch (let error) {
print("SAVE ERROR (error)")
}
}
}

10. A Data Definition Language
syntax = "proto3";
package models;
message PlantList {
string id = 1;
string name = 2;
repeated PlantListItem plants = 11;
}
message PlantListItem {
string id = 1;
string botanical_name = 2;
}
message User {
string id = 1;
string name = 2;
repeated PlantList plantLists = 3;
}

11. A Methodology
% protoc models.proto --swift_out=.
#
# This runs a plugin called protoc-gen-swift
#
# The plugin generates a Swift source file that implements
# the data structures defined in models.proto and code
# for reading and writing them as serialized bytes.
#

12. Interface Builder for Data
message Person {
string name = 1;
int32 id = 2;
string email = 3;
message PhoneNumber {
string number = 1;
}
repeated PhoneNumber phone = 4;
}
Interface Builder: Developers specify
their interfaces using a special tool,
tooling compiles and integrates that into
their apps.
Protocol Buffers: Developers specify
their data structures using a special
language, tooling compiles and
integrates that into their apps.

13. Toolkit and the next generation of Google APIs

16. echo.proto
package echo;
service Echo {
// Immediately returns an echo of a request.
rpc Get(EchoRequest) returns (EchoResponse) {}
// Splits a request into words and returns each word in a stream of messages.
rpc Expand(EchoRequest) returns (stream EchoResponse) {}
// Collects a stream of messages and returns them concatenated when the caller closes.
rpc Collect(stream EchoRequest) returns (EchoResponse) {}
// Streams back messages as they are received in an input stream.
rpc Update(stream EchoRequest) returns (stream EchoResponse) {}
}
message EchoRequest {
// The text of a message to be echoed.
string text = 1;
}
message EchoResponse {
// The text of an echo response.
string text = 1;
}

17. Try it! https://goo.gl/ux4Txh

18. EchoService.swift (1/3)
class EchoProvider : Echo_EchoProvider {
// get returns requests as they were received.
func get(request : Echo_EchoRequest, session : Echo_EchoGetSession) throws -> Echo_EchoResponse {
var response = Echo_EchoResponse()
response.text = "Swift echo get: " + request.text
return response
}
// expand splits a request into words and returns each word in a separate message.
func expand(request : Echo_EchoRequest, session : Echo_EchoExpandSession) throws -> Void {
let parts = request.text.components(separatedBy: " ")
var i = 0
for part in parts {
var response = Echo_EchoResponse()
response.text = "Swift echo expand ((i)): (part)"
try session.send(response)
i += 1
sleep(1)
}
}

19. EchoService.swift (2/3)
// collect collects a sequence of messages and returns them concatenated when the caller closes.
func collect(session : Echo_EchoCollectSession) throws -> Void {
var parts : [String] = []
while true {
do {
let request = try session.receive()
parts.append(request.text)
} catch Echo_EchoServerError.endOfStream {
break
} catch (let error) {
print("(error)")
}
}
var response = Echo_EchoResponse()
response.text = "Swift echo collect: " + parts.joined(separator: " ")
try session.sendAndClose(response)
}

20. EchoService.swift (3/3)
// update streams back messages as they are received in an input stream.
func update(session : Echo_EchoUpdateSession) throws -> Void {
var count = 0
while true {
do {
let request = try session.receive()
count += 1
var response = Echo_EchoResponse()
response.text = "Swift echo update ((count)): (request.text)"
try session.send(response)
} catch Echo_EchoServerError.endOfStream {
break
} catch (let error) {
print("(error)")
}
}
try session.close()
}
}

21. main.swift (1/3)
// Unary
if client == "get" {
var requestMessage = Echo_EchoRequest()
requestMessage.text = message
print("Sending: " + requestMessage.text)
let responseMessage = try service.get(requestMessage)
print("get received: " + responseMessage.text)
}
// Server streaming
if client == "expand" {
var requestMessage = Echo_EchoRequest()
requestMessage.text = message
print("Sending: " + requestMessage.text)
let expandCall = try service.expand(requestMessage) {result in }
var running = true
while running {
do {
let responseMessage = try expandCall.receive()
print("Received: (responseMessage.text)")
} catch Echo_EchoClientError.endOfStream {
print("expand closed")
running = false
}
}
}

22. main.swift (2/3)
// Client streaming
if client == "collect" {
let collectCall = try service.collect() {result in }
let parts = message.components(separatedBy:" ")
for part in parts {
var requestMessage = Echo_EchoRequest()
requestMessage.text = part
print("Sending: " + part)
try collectCall.send(requestMessage) {error in print(error)}
sleep(1)
}
let responseMessage = try collectCall.closeAndReceive()
print("Received: (responseMessage.text)")
}

23. main.swift (3/3)
// Bidirectional streaming
if client == "update" {
let updateCall = try service.update() {result in}
DispatchQueue.global().async {
var running = true
while running {
do {
let responseMessage = try updateCall.receive()
print("Received: (responseMessage.text)")
} catch Echo_EchoClientError.endOfStream {
print("update closed")
latch.signal()
break
} catch (let error) {
print("error: (error)")
}
}
}
...
...
let parts = message.components(separatedBy:" ")
for part in parts {
var requestMessage = Echo_EchoRequest()
requestMessage.text = part
print("Sending: " + requestMessage.text)
try updateCall.send(requestMessage) {error in print(error)}
sleep(1)
}
try updateCall.closeSend()
// Wait for the call to complete.
latch.wait()
}
}

24. The Authorization Problem
How do we tell an API server that it’s ok to
respond to our requests?
(hint: github.com/google/auth-library-swift)

25. Authorization
We need to provide a token:
var request = URLRequest(url:url)
request.httpMethod = method
request.httpBody = ...
// add any needed request headers
request.setValue(authorization, forHTTPHeaderField:"Authorization")
How do we get that?

26. OAuth2 Authorization Flow
For an implementation in Swift, see BrowserTokenProvider.swift
Client Service
Authorization URL
Sign-In Page (HTML)
Human Confirmation
Browser Redirect w/ Code
Token Request (Code)
Token
Web
Browser

27. Aside: Build an HTTP server with swift-server/http

28. Easier ways to get tokens (1 of 2)
If you’re running inside a VM on Google Cloud Platform, you can get a token
from the Google Cloud Metadata Service.
% curl http://metadata/computeMetadata/v1/instance/service-accounts/default/token
{"access_token":"ya29.GqUBUgXcBmIt7vfHsWJT4qVzdhWxwEb2f3tamcA6ykrIsEANZfQnoH0HDCBnlCztLw
cD47w7YENghIucNUIIypLId4C5dXc4H8D93e17MrSbGRe4ipfoQhxPCIhIU3KJsvFjel0HcN2iwf8xURv2z1lWiN
2jkZjzLiMRWPKfSvtBVzuWkIo5uZ5u25IXle3tJ4SICh0-516sU84DFu0wkPO-q1xGpiff","expires_in":179
9,"token_type":"Bearer"}
Then pass “Bearer “ + access_token as the Authorization header.
See GoogleCloudMetadataTokenProvider.swift.

29. Easier ways to get tokens (2 of 2)
If you’re calling a Google API from anywhere, you can use a Service Account.
1. Create and download the account credentials.
2. Create a JWT token and sign it with the account credentials.
3. POST the signed token to the Google Account Service and get a token!
See ServiceAccountTokenProvider.swift.

30. Toward Idiomatic Swift Clients: the Google Cloud Datastore API

31. https://github.com/grpc/grpc-swift
https://github.com/google/auth-library-swift