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Designing an API for mobile - Lessons we've learned at SocialRadar. Given at API Craft DMV meetup in April 2014.
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Victor Quinn @victorquinn
Lead Node.js Engineer
Designing an API for MobileAPI Craft DMV Meetup, April 28 2014
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!This will be pretty fast-paced
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Special considerations for a Mobile API
• Speed really matters • Battery usage - minimize
• Maximizing network utilization • Response size matters • Minimize number of requests
• Stateless
Speed: Problems• Only one thing on screen at a time, perception change
• On desktop other things can distract • Best Case: ~400ms latency for round trip on cell network
• We tested at SocialRadar using a basic ping no-op endpoint
• Verizon LTE, iPhone 5S • Device doesn’t maintain active connection • Average case much worse, ~1000ms round trip
Ideal round tripiPhone 5S on Verizon LTE
User's Phone API
200ms
200ms
200ms
Total round trip time 800ms
200ms
bit of hand waving, I’m not a network expert
Likely round tripiPhone 4S on Verizon 3G
User's Phone API
500ms
500ms
200ms
Total round trip time 1600ms
400ms
Speed: Solutions• Cache anything humanly possible • Respond as quickly as possible, deferring
anything that doesn’t need to be done immediately • Queue most things • Make sure your API can handle high concurrency
(we settled on Node.js to accomplish this, WhatsApp uses Erlang)
Minimize battery usage
!• Brief note about cell battery and network
usage (more) • Response size matters • Minimize number of requests
Maximize network utilization
Response Size: Problems• At SocialRadar, we tested responses of different sizes • Optimal was largest response that would fit into a
single TCP packet on the cell network. • Around 128KB. Differs based on client device and
network • Any longer would jump response time significantly
Response Size: Solutions• Paging sizes — largest
that fit within TCP packet window • Contrast with
Desktop, paging more dependent on server response time and UI
User's Phone API
200ms
200ms
200ms
Total round trip time 800ms
200ms
Page size of 20 users
To retrieve 100 users, ~4s
Paging Size: 20 (Ideal)
Paging Size: 50 (Ideal)
User's Phone API
200ms
200ms
500ms
Total round trip time 1100ms
200ms
Page size of 50 users
To retrieve 100 users, ~2.2s
Paging Size: 20 (Likely)
User's Phone API
500ms
500ms
200ms
Total round trip time 1600ms
400ms
To retrieve 100 users, ~8s
Paging Size: 50 (Likely)
User's Phone API
500ms
500ms
500ms
Total round trip time 1900ms
400ms
To retrieve 100 users, ~3.8s
Number of Requests: Problems• Traditionally parallelized, not (well) on mobile • Battery life is of great concern and every network
request eats into that • Spinning up the cell modem, making request,
waiting for it drains the battery • So if you make 5 requests instead of 1, draining
battery, leaving user waiting
Number of Requests: Solutions
• Paging size as mentioned above, try to maximize response that will fit in a packet to minimize the number of requests • So one request with 20KB much better than 5
with 4KB each • Batch multiple requests into a single one
Batch Request Library
• Finding no great existing batch request library for Node.js, we rolled our own open source library
• http://batch-request.socialradar.com • Allows you to send a single request that represents
multiple • No need to compromise on RESTful principles and
move to a SOA • Handles complex dependencies
Batch Request Library (2)1. npm install batch-request 2. Add one line of code to your API:
app.post('/batch', batch.validate, batch);!3. Send batch request as single POST:
{ "myRequest1": { "method": "GET", "uri": "http://api.mysite.com/users/1/first_name" }, "myRequest2": { "method": "GET", "uri": "http://api.mysite.com/users/1/email" } }
Batch Request Library (3)4. Receive single response:
{ "myRequest1": { “statusCode": 200, “body": “Victor”, “headers”: {…} }, "myRequest2": { “statusCode": 200, “body": “[email protected]”, “headers”: {…} } }
When you pull to refresh, Batch Request to:
• Update user location with
latest GPS reading • Retrieve current City/State • Retrieve most recent stat
numbers • Get some user lists to
cache them
Stateless• Cell networks are spotty
• For performance, want load balance among multiple servers
• Solution: Assign access token to device rather than relying on sessions for user access
• We have an access token which is an encrypted string containing user_id and some other stuff
Victor Quinn @victorquinn
Lead Node.js Engineer !
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