best use of NSUrlConnection when getting multiple json objects that depend on the previous

Question

what I am doing is I am querying an API to search for articles in various data bases. There are multiple steps involved, each returns a json object. Each step involves a NSUrlConnection with different query strings to the API

step 1: returns json object indicating status of query & record set ID.

step 2: takes record set id from step 1 and returns list of databases that are valid for querying

step 3: queries each database that was ready from step 2 and gets json data array that has results

I am confused as to the best way of going about this. Is it better to use one nsurlconnection and reopen that connection in connection did finish loading based on which step I am in. Or is it better to open a new connection at the end of each subsequent connection?

Answer 1

A couple of observations

1. Network latency:

   The key phenomenon that we need to be sensitive to here (and it sounds like you are) is network latency. Too often we test our apps in an idea scenario (on simulator with high speed internet access, or on device connected to wifi). But when you use an app in a real-world scenario, network latency can seriously impact performance and you'll want to architect a solution that minimizes this.

2. Simulating sub-optimal, real-world network situations:

   By the way, if you're not doing it already, I'd suggest you install the "Network Link Conditioner" which is part of the "Hardware IO Tools" (available from the "Xcode" menu, choose "Open Developer Tool" - "More Developer Tools"). If you install the "Network Link Conditioner", you can then have your simulator simulate a variety of network experiences (e.g. Good 3G connection, Poor Edge connection, etc.).

3. Minimize network requests:

   Anyway, I'd try to figure out how to minimize separate requests that are dependent upon the previous one. For example, I see step 1 and step 2 and wonder if you could merge those two into a single JSON request. Perhaps that's not possible, but hopefully you get the idea. You want to reduce the number of separate requests that have to happen sequentially.

   I'd also look at step 3, and those look like they have to be dependent upon step 2, but perhaps you can run a couple of those step 3 requests concurrently, reducing the latency effect there.

4. Implementation:

   In terms of how this would be implemented, I personally use a concurrent NSOperationQueue with some reasonable maxConcurrentOperationCount setting (e.g. 4 or 5, enough to enjoy concurrency and reduce latency, but not so many as to tax either the device or the server) and submit network operations. In this case, you'll probably submit step 1, with a completion operation that will submit step 2, with a completion operation that will submit a series of step 3 requests and these step 3 requests might run concurrent.

   In terms of how to make a good network operation object, I might suggest using something like AFNetworking, which already has a decent network operation object (including one that parses JSON), so maybe you can start there.

   In terms of re-using a NSURLConnection, generally its one connection per request. If I have had an app that wanted to have a lengthy exchange of messages with a server (e.g. a chat like service where you want the server to be able to send a message to the client whenever it wants, such as in a chat service), I've done a sockets implementation, but that doesn't seem like the right architecture here.

Answer 2

I would dismiss the first connection and create a new one for each connection.

Just, don't ask me why.

BTW, I would understand the question if this was about reusing vs. creating new objects in some performance sensitive context like scrolling through a table or animations or if it is just about of 10 thousands of iterations where it happens. But you are talking about 3 objects to either create new or reuse the old one. What is the gain of even thinking about it?