5

Let's say a client makes a GET call to the server to get all the followers of some user. now the client shows a list of all the followers but the only data the list needs is:

{"username" : "user", "thumbUrl" : "http:/www.example.com/photo/1", "age" : 78}

now the user can click on one of the followers and he can see more data about the follower he clicked.

My question: should i bring from advance all the followers data from the server (full User Object) vs bring only partial data and then make another call onDemand when the userclicks a follower. And more important is if do i really need to care about such optimizations?

assumptions:

  1. data is throttled (10 Objects per call to followers)
  2. size of each User Object is around 1kb, partial around 200 bytes
  3. User usually clicks on 5 followers for each 10 Objects.

Points of Interest:

  1. size saved: about 10kb - 2kb - 5kb = 3kb per bucket of 10 users. is it negligible at this age of the internet? would it matter if the size difference was 30kb?
  2. bucket size: I gave examples with low bucket size but let's sat my bucket size can get up to 2Mb. Does it matter if my bucket size is 2Mb with the full User data vs 400kb with partial call? Is it slower? (Assuming the User will click enough followers to make the size difference negligible
  3. Will welcome any other points of interest
5

It depends.

Basically, you have to look at what the expected latency of the connection is, what the bandwidth is, and what responsiveness you want.

As an example: Suppose the round trip latency from client to server is 100 msecs and the bandwidth is 8 mb/s. If you send the "full" data is 2Mb and the "partial" data is 400kb, then it will take 350 msecs to send the "full" record and 150 msecs to send a "partial" record. If you send partial records, then each click requires 110 msecs to retrieve results. Otherwise, each click is instant. So:

  • Full - First load: 350 msec, click: instant
  • Partial - First load: 150 msecs, click: 110 msecs

The key point is to understand that each call adds overhead. While it is very tempting to minimize the data transferred, this can actually make things slower, if it causes more round trips.

Of course, this is in and of itself misleading because network calls are variable. But personally with these numbers I'd be tempted to load up front.

But this is just a very high level analysis. Other things to consider:

  • This completely ignores the server side cost. How fast does it take the "full" data vs. the "partial"? Can you pull the "full" data, cache it for the future, and return the partial?
  • Code that gets the data in one block is likely to be simpler on both client and server, and therefore less buggy.
  • If you send partial data, you have to worry about what happens if records change between the first pull and the second.
  • To users, a single slow call followed by instant responsiveness feels "faster" than if every single click takes noticeable time. You want to pay attention to how fast users feel the system is as much as possible at the expensive of concrete measures of how fast the system actually is.

In general, you are better off minimizing the number of network calls rather than minimizing the amount of data transferred overall. But this cannot be a hard and fast rule because again, it really depends on both the expected bandwidth and the expected latency. I should note, though, that bandwidth is constantly improving while latency is unlikely to improve significantly over time.

  • thanks for your detailed answer. i agree with you on almost all your points. I don't think however that i need to worry about changes between the first and second call. the fact that i'm getting "fresh" data is always preferable so this is (maybe) an advantage even for two calls. i guess your last point is the most crucial because optimization in price of responsiveness does not mean a lot(-: – royB Jan 2 '15 at 22:08
1

You can define the resources and representations in the way that suits your needs. I usually use GET to provide a "summary" view of a collection (like ".../followers") as a resource that provides a list of summaries of the followers, including their ids (or URIs if I'm being good). This is usually sufficient for the client to provide a list that the user can browse, and then drill down into.

However, sometimes the client really does want a list of the complete things. It just depends on your usage. And you can always provide a query string parameter (e.g. "?full=true") to switch back and forth (although an accept header, and content-negotiation, might be a more "correct" but complicated way to do it).

(As for the performance & optimization aspect, it's hard to pin down specific criterion, because that's going to be based on lots of things like your hardware, where it's deployed, middleware in front of your service, etc. Just (obviously) sending back fewer bytes will be faster... beyond that, you can do some performance testing to see what the acceptable thresholds are.)

  • thanks for your answer. actually my implementation is really similar to yours (full=true). We did try to make a performance testing but it didn't give any valuable info regarding speed (unless the chunks where Hugh) – royB Jan 2 '15 at 22:14
  • the flip side to performance testing is run-time metrics. You might instrument your calls, and see what the latency is for actual requests/responses. Then you can make an optimization decision, and track the outcome, based on what users are seeing rather than what you or I worry about. – Rob Jan 2 '15 at 22:18

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