cellular networks and mobile computing coms 6998-8, spring 2012

Cellular Networks and Mobile ComputingCOMS 6998-8, Spring 2012

Instructor: Li Erran Li ([email protected])

http://www.cs.columbia.edu/~coms6998-8/3/5/2012: Mobile Cloud Platform Services

http://www.cs.columbia.edu/~coms6998-8/




Cellular Networks and Mobile Computing (COMS 6998-8)

2

Announcements

• iOS assignment 2 and Android assignment 1 are due March 19th

• More mobile security papers in syllabus – Please email me if you want to present one of

them instead of the originally assigned• Windows Phones are available for project use– On loan from Microsoft, please take good care of

them

3/5/12


3

Mobile Cloud Platform Services

• Syncing and storage service (iCloud)• Proxy service (Kindle Split Browser)• Speech to text/text to speech service• Natural language processing service (open Siri

API for 3rd party applications in the future)• Push notification service• Track service (supporting location based

services)

3/5/12


4

Outline

• Speech to text service demo• Push notification service– Apple push notification service– Google C2DM (not covered in this lecture)– Thialfi: reliable push notification system

• Track service

3/5/12


5

Apple Push Notification Architecture Overview

• iOS device maintains a persistent TCP connection to a Apple Push Notification Server(APNS)

3/5/12

A push notification from a provider to a client application

Multi-providers to multiple devices


6

Apple Push Notification Architecture Overview (Cont’d)

• What if devices uninstalled the app?– Feedback service

• Providers poll to obtain list of device tokens for their applications

• What if devices are offline?– QoS service

• QoS stores the notification• It retains only the last notification received from a provider• When the offline device reconnects, the QoS forwards the

stored notification to the device• QoS retains a notification for a limited period before deleting it

3/5/12


7

Push Notification

• Push notification– Delivery is best effort and is not guaranteed– Max size is 256 bytes– Providers compose a JSON dictionary object • This dictionary must contain another dictionary

identified by the key aps– Action:• An alert message to display to the user• A number to badge the application icon with• A sound to play

3/5/12


8

Device Token• Device token is analogous to a phone number

– Contains information that enables APNs to locate the device– Client app needs to provide the token to its provider– Device token should be requested and passed to providers every time your application

launches

3/5/12


9

Apple Push Notification Programming Example

• Provisioning: https://developer.apple.com/ios/manage/provisioningprofiles/howto.action– Generate Certification Signing Request (CSR) using Keychain

Access• Save to disk: PushChat.certSigningRequest• Export the private key as “PushChatKey.p12” and enter a passphrase

– Make an App ID in iOS Provisioning Portal• Check the Enable for Apple Push Notification service box• click on the Configure button for the Development Push SSL Certificate• click Download to get the certificate – it is named

“aps_development.cer”3/5/12

https://developer.apple.com/ios/manage/provisioningprofiles/howto.action




10

Apple Push Notification Programming Example (Cont’d)

• Client code1. - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary

*)launchOptions2. {3. // Let the device know we want to receive push notifications4. [[UIApplication sharedApplication] registerForRemoteNotificationTypes:5. (UIRemoteNotificationTypeBadge | UIRemoteNotificationTypeSound |

UIRemoteNotificationTypeAlert)];6. 7. return YES;8. }

9. - (void)application:(UIApplication*)application didReceiveRemoteNotification:(NSDictionary*)userInfo10. {//userInfo contains the notification11. NSLog(@"Received notification: %@", userInfo);12. }

13. - (void)application:(UIApplication*)application didRegisterForRemoteNotificationsWithDeviceToken:(NSData*)deviceToken

14. {15. NSLog(@"My token is: %@", deviceToken);16. }

3/5/12


11

Apple Push Notification Programming Example (Cont’d)

• Server code1. $devicetoken ='f05571e4be60a4e11524d76e4366862128f430522fb470c46fc6810fffb07af7’;2. // Put your private key's passphrase here:3. $passphrase = 'PushChat';4. // Put your alert message here:5. $message = 'Erran: my first push notification!';

6. $ctx = stream_context_create();7. Stream_context_set_option($ctx, 'ssl', 'local_cert', 'ck.pem');8. stream_context_set_option($ctx, 'ssl', 'passphrase', $passphrase);

9. // Open a connection to the APNS server10. $fp = stream_socket_client(11. 'ssl://gateway.sandbox.push.apple.com:2195', $err,12. $errstr, 60, STREAM_CLIENT_CONNECT|STREAM_CLIENT_PERSISTENT, $ctx);

13. if (!$fp)14. exit("Failed to connect: $err $errstr" . PHP_EOL);

15. echo 'Connected to APNS' . PHP_EOL;

16. // Create the payload body17. $body['aps'] = array(18. 'alert' => $message,19. 'sound' => 'default'20. );

21. // Encode the payload as JSON22. $payload = json_encode($body);

23. // Build the binary notification24. $msg = chr(0) . pack('n', 32) . pack('H*', $deviceToken) . pack('n', strlen($payload)) . $payload;

25. // Send it to the server26. $result = fwrite($fp, $msg, strlen($msg));

27. if (!$result)28. echo 'Message not delivered' . PHP_EOL;29. else30. echo 'Message successfully delivered' . PHP_EOL;

31. // Close the connection to the server32. fclose($fp);

3/5/12


12

Push Notification Programming Example (Cont’d)

• Demo

3/5/12

Thialfi: A Client Notification Servicefor Internet-Scale Applications

Atul Adya, Gregory Cooper, Daniel Myers, Michael Piatek

Google Seattle


14

A Case for NotificationsProblem: Ensuring cached data is fresh across

users and devices

Courtesy: Adya et al.3/5/12


15

Common Application Patterns

• Clients poll to detect changes– Simple and reliable, but slow and inefficient

• Push updates to the client– Fast but complex– Add backup polling to get reliability– Tail latencies can be high: masks bugs– Application-specific protocol

sacrifice reliability

3/5/12 Courtesy: Adya et al.


16

Solution: Thialfi

• Scalable: tracks millions of clients and objects• Fast: notifies clients in less than a second• Reliable: even when entire data centers fail• Easy to use: deployed in Chrome Sync, Contacts,

Google Plus



17

Thialfi Outline

• Thialfi’s abstraction: reliable signaling

• Delivering notifications in the common case

• Detecting and recovering from failures

• Evaluation and experience



18

Thialfi Overview

Thialfi client library

Register X Notify X

ClientData center

X: C1, C2

Client C1 Client C2

Thialfi Service

Update XRegister

Register

Update XApplication backend

Notify X Notify X



19

Thialfi Abstraction

• Objects have unique IDs and version numbers, monotonically increasing on every update

• Delivery guarantee– Registered clients learn latest version number– Reliable signal only: cached object ID X at version Y



20

Why Signal, Not Data?

• Developers want reliable, in-order data delivery

• Adds complexity to Thialfi and application, e.g.,– Hard state, arbitrary buffering– Offline applications flooded with data on wakeup

• For most applications, reliable signal is enough– Invoke polling path on signal: simplifies integration



21

API Without Failure Recovery

Thialfi Service Publish(objectId, version)

ClientLibrary

Register(objectId)Unregister(objectId)

Notify(objectId, version)



22

Thialfi Outline







23

Architecture

ClientBigtable

• Matcher: Object ID registered clients, version• Registrar: Client ID registered objects, notifications

Client

Registrar

MatcherObjectBigtable

Data center

Notifications Application Backend

Registrations, notifications,acknowledgments

Client library


• Each server handles a contiguous range of keys, • Each server maintains an in-memory version• Bigtable: log structured, fast write


24

C1: x, v7C2: x, v7C1: x, v5C2: x,

x: v5; C1, C2x: v7; C1, C2x: v7; C1, C2

x

Life of a Notification

ClientBigtable

C1: x, v7

C2: x, v7

Notify: x, v7

Client C2


Data center

Publish(x, v7)x, v7

Ack: x, v7

Registrar



25

Thialfi Outline







26

Data center lossServer state loss/schema migrationPartial storage unavailability

Possible Failures

ClientLibrary

ClientBigtable Registrar


ClientBigtable Registrar


. . .

Data center 1 Data center nThialfi Service

ClientStore

Client restartClient state loss

Publish Feed

Network failures



27

Failures Addressed by Thialfi

• Client restart• Client state loss• Network failures• Partial storage unavailability• Server state loss / schema migration• Publish feed loss• Data center outage



28

Main Principle: No Hard State

• Thialfi remains correct even if all state is lost– All registrations– All object versions

• Detect and reconstruct after failures using:– ReissueRegistrations() client event– Registration Sync Protocol– NotifyUnknown() client event



29

Recovering Client Registrations

Registrar


x

y

x yReissueRegistrations()

Register(x); Register(y)

ReissueRegistrations: Not a burden for applications– Application stores objects in its cache, or – Object list is implicit, e.g., bookmarks for user X



30

Registrar


Register: x, y

Syncing Client Registrations

x

y

Hash(x, y)x y

• Goal: Keep client-registrar registration state in sync• Every message contains hash of registered objects• Registrar initiates protocol when detects out-of-sync• Allows simpler reasoning of registration state

Reg syncHash(x, y)


Merkle tree for syncing large number of objects


31

Recovering From Lost Versions

• Versions may be lost, e.g. schema migration

• Refreshing from backend requires tight coupling

• Inform client with NotifyUnknown(objectId) – Client must refresh, regardless of its current state



32

Thialfi Outline







33

Notification Latency Breakdown

Notification latency (ms)0

100

200

300

Matcher to Registrar RPC (Batched)

Matcher Bigtable Read

Matcher Bigtable Write (Batched)

Bridge to Matcher RPC (Batched)

App Backend to Bridge

Batching accounts for significant fraction of latency3/5/12 Courtesy: Adya et al.


34

Thialfi Usage by ApplicationsApplication Language Network

ChannelClient Lines of Code(Semi-colons)

Chrome Sync C++ XMPP 535Contacts JavaScript Hanging GET 40

Google+ JavaScript Hanging GET 80Android Application Java C2DM +

Standard GET300

Google BlackBerry Java RPC 340



35

Some Lessons Learned

• Add complexity at the server, not the client– Deploy at server: minutes. Upgrade clients: years+

• Asynchronous events, not callbacks– Spontaneous events occur: need to handle them

• Initial applications have few objects per client– Earlier use of polling forces such a model



36

Thialfi Summary

• Fast, scalable notification service• Reliable even when data centers fail• Two key ideas simplify failure handling– Deliver a reliable signal, not data– No hard state: reconstruct after failure

• Deployed in Chrome Sync, Contacts, Google+



37

Outline

• Speech to text service demo• Push notification service– Apple push notification service– Google C2DM(not covered in this lecture)– Thialfi: reliable push notification system

• Track service

3/5/12


38

Location-Based Applications• Many phones already have the ability to

determine their own location– GPS, cell tower triangulation, or proximity to WiFi

hotspots

• Many mobile applications use location information

3/5/12 Courtesy: Maya et al.


39

TrackTime-ordered sequence of location readings

Latitude: 37.4013Longitude: -122.0730Time: 07/08/10 08:46:45.125



40

Application: Personalized Driving Directions

Goal: Find directions to new gymTake US-101 North



41

A Taxonomy of ApplicationsPersonal Social

Current location

Driving directions, Nearby restaurants

Friend finder, Crowd scenes

Past locations

Personal travel journal, Geocoded photos

Post-it notes, Recommendations

Tracks Personalized Driving Directions, Track-Based Search

Ride sharing, Discovery, Urban sensing

Class of applications enabled by StarTrack



42

StarTrack System

ST Client

Insertion Application

Location Manager

• Retrieval• Manipulation• Comparison …

Application

ST Client

• InsertionST Server

ST Server

ST Server


http://www.google.com/imgres?imgurl=http://www.iconarchive.com/icons/icons-land/vista-hardware-devices/256/Computer-icon.png&imgrefurl=http://www.iconarchive.com/category/business/vista-hardware-devices-icons-by-icons-land.html&usg=__Ni7PshA-i9QGgAwmDI_3y4j0VPU=&h=256&w=256&sz=48&hl=en&start=6&zoom=1&um=1&itbs=1&tbnid=iU9O3OB53R_79M:&tbnh=111&tbnw=111&prev=/images?q=computer+icon&um=1&hl=en&sa=N&rls=com.microsoft:en-us:IE-SearchBox&tbs=isch:1

http://www.google.com/imgres?imgurl=http://www.iconarchive.com/icons/icons-land/vista-hardware-devices/256/Computer-icon.png&imgrefurl=http://www.iconarchive.com/category/business/vista-hardware-devices-icons-by-icons-land.html&usg=__Ni7PshA-i9QGgAwmDI_3y4j0VPU=&h=256&w=256&sz=48&hl=en&start=6&zoom=1&um=1&itbs=1&tbnid=iU9O3OB53R_79M:&tbnh=111&tbnw=111&prev=/images?q=computer+icon&um=1&hl=en&sa=N&rls=com.microsoft:en-us:IE-SearchBox&tbs=isch:1


43

System Challenges

1. Handling error-prone tracks

2. Flexible programming interface

3. Efficient implementation of operations on tracks

4. Scalability and fault tolerance3/5/12 Courtesy: Maya et al.


44

Challenges of Using Raw Tracks

Advantages of Canonicalization:– More efficient retrieval and comparison operations– Enables StarTrack to maintain a list of non-duplicate tracks



45

StarTrack API

Track Collections (TC): Abstract grouping of tracks– Programming Convenience– Implementation Efficiency• Prevent unnecessary client-server message exchanges− Enable delayed evaluation− Enable caching and use of in-memory data structures

Pre-filter tracks Manipulate tracks Fetch tracks



46

StarTrack API: Track Collections

TC JoinTrackCollections (TC tCs[], bool removeDuplicates) TC SortTracks (TC tC, SortAttribute attr) TC TakeTracks(TC tC, int count) TC GetSimilarTracks (TC tC, Track refTrack, float simThreshold) TC GetPassByTracks (TC tC, Area[] areas) TC GetCommonSegments(TC tC, float freqThreshold)

Track[] GetTracks (TC tC, int start, int count)

Manipulation

Retrieval

Creation TC MakeCollection(GroupCriteria criteria, bool removeDuplicates)



47

API Usage: Ride-Sharing Application// get user’s most popular track in the morning

TC myTC = MakeCollection(“name = Maya”, [0800 1000], true);TC myPopTC = SortTracks(myTC, FREQ);Track track = GetTracks(myPopTC, 0, 1);

// find tracks of all fellow employeesTC msTC = MakeCollection(“name.Employer = MS”, [0800 1000], true);

// pick tracks from the community most similar to user’s popular trackTC similarTC = GetSimilarTracks(msTC, track, 0.8);Track[] similarTracks = GetTracks(similarTC, 0, 20);

// Verify if each track is frequently traveled by its respective ownerUser[] result = FindOwnersOfFrequentTracks(similarTracks);



48

Efficient Implementation of Operations

• StarTrack exploits redundancy in tracks for efficient retrieval from database– Set of non-duplicate tracks per user– Separate table of unique coordinates

• StarTrack builds specialized in-memory data-structures to accelerate the evaluation of some operations– Quad-Trees for geographic range searches– Track Trees for similarity searches



49

S1-4

S5 S6-7

Track Similarity

Tracks A, B

s1

s2

s3

s4

s5

Track D

s8

s9

Track C

s6 s7

Limited database support for computing track similarity

SIM (A ,C )=¿S1−4∨ ¿|S1−4|+|S5|+¿S6−7∨¿¿

¿

SIM (A , B)=¿S1−5∨ ¿|S1−5|

¿



50

Track Tree

s1 s2 s3 s4 s5 s6 s7 s8 s9

S1-2 S6-7 S8-9

S1-3

S1-4

S1-5Tracks A, B

s1

s2

s3

s4

s5

Track Ds8

s9

Track Cs6 s7 1) Create leaf nodes for all

segments

2) Merge nodes based on # of tracks that go through adjacent segments



51

Evaluation• Performance of our Track Tree approach• Performance of 2 sample applications– Ride-sharing– Personalized Driving Directions

• Configuration– Synthetically generated tracks– Up to 9 StarTrack Servers + 3 Database Servers– Server Configuration:

• 2.6 GHz AMD Opteron Quad-Core Processors• 16 GB RAM



52

Evaluation: Track Tree• Evaluation of GetSimilarTracks

• Alternative approaches:– Database filteringPre-filter tracks that intersect ref track at database– In-memory filteringPre-filter tracks that intersect ref track in memory – In-memory brute forceCompute similarity between each track and ref track in memory



53

Get Similar Tracks – Query Time

0 10 20 30 40 50 60 70 80 90 1000.1

1

10

100

1000

10000

Number of tracks (thousands)

Que

ry T

ime

(ms)

Track Tree

In-Memory Filtering

In-Memory Brute Force

Database Filtering



54

Track Tree Construction Costs

0 20 40 60 80 1000

30

60

90

120

150

180

Number of Tracks (thousands)

Time (s)

Memory (MB)



55

Performance of ApplicationsRide Sharing

- Track Collection on multiple users- Calls to GetSimilarTracks- 30 requests/s at about 170 ms

Personalized Driving Directions

- Track Collection for single user at a time- Calls to GetCommonSegments- 30 requests/s at about 100 ms (uncached)- 250 requests/s at about 55 ms (cached)

0 10 20 30 400

100200300400500600

Request Rate (per second)

Resp

onse

Tim

e (m

s)

150 175 200 225 2500

20406080

100120

Request Rate (per second)

Resp

onse

Tim

e (m

s)



56

Related Work• Management of tracks has been studied by the database

community– Storage of tracks as 3-dimensional objects – Specialized indexing schemes (Quad-Trees, R-Trees, etc.)

• CarTel Project (MIT) – Provides an infrastructure for collecting traces, relying on a relational database using spatial queries

• Access and sharing of data in StarTrack is similar to that provided by social networks, where users’ data is shared by applications; Similar access control policies could be employed to ensure privacy in StarTrack.



57

Summary

• StarTrack is a scalable service designed to manage tracks and facilitate the construction of track-based applications

• Important Design Features– Canonicalization of Tracks– API based on Track Collections– Use of Novel Data Structures



58

Questions?

3/5/12

cellular networks and mobile computing coms 6998-8, spring 2012

Documents

notification payload

stored notification

mobile computing coms

mobile computingcoms

services3512cellular

it3512cellular networks

thmore mobile security

aps dictionary