In iOS 6 Apple also implemented the ability to debug websites via USB & your desktop browser like Chrome for Android. With this new feature it is easy to debug such problems.

Here you see a screenshot which shows this XHR POST request (grey marked) with new tool:

The marked post request looks like an uncached request which goes to the remote server. But that’s not the truth. The request doesn’t contact the remote server – it only returns some old cached data. So don’t believe in the new remote debugging tool. Here it shows fictional rows/data.

Here you see the request in detail:

The response date in the header is some minutes old and also the data shown is not up-to-date.

The problem occurs in Safari browser and in the UIWebView. And it seems that the caching only occurs in the loading phase of a website. Perhaps Apple tried to improve the loading time of some sites but have forgotten that it is not allowed to cache HTTP POST requests.

Solution (Server-Side)

It works correctly if you add the header fields you normally use on HTTP GET requests to avoid caching.

Cache-Control: no-cache
Expires: Thu, 01 Jan 1970 00:00:00 GMT

At Up to Eleven we are currently using MongoDB 2.0 in a sharded setup. Where each shard has 3 replica sets.

The biggest collection stores messages to a specific address with a read flag, status and a message date.

For the web we need an overview of this messages per address (conversation). Therefore we currently use a mapreduce command,  because in a sharded setup the group function is not available.

With release of MongoDB 2.2 their are now 2 new options for this problem – It’s now possible to run MapReduce commands with JsMode on shards and their also is the brand new aggregation framework.

In the following article the options are compared to each other.

Schema

Document

{
  "_id" : NumberLong("1073741825094"), // combination of message id & user id
  "u" : 250, // user id
  "a" : "test", // address
  "b" : "Hello, what are you doing right now?", // body
  "i" : true, // incoming flag
  "r" : true, // read flag
  "s" : 0, // status
  "d" : ISODate("2011-06-06T18:49:55Z") // date
}

{
  "v" : 1,
  "key" : {
    "_id" : 1
  },
  "ns" : "test.user_messages",
  "name" : "_id_"
},
{
  "v" : 1,
  "key" : {
    "u" : 1,
    "a" : 1,
    "d" : -1
  },
  "ns" : "test.user_messages",
  "name" : "conversation"
}

Tests

MapReduce

db.runCommand({
  "mapreduce": "user_messages",
  "map": "function(){
     emit(this.a, {
       count:1,
       unread:this.r ? 0 : 1,
       unsent:this.s==5 ? 1 : 0,
       messageId:this._id,
       date:this.d,
       body:this.b
    });
  }",
  "reduce": "function(address,values) {
    var result = { 
      count:0,
      unread:0,
      unsent:0,
      messageId:0,
      date:0,
      body:''
    };
    values.forEach(function(value) {
      result.count += 1;
      if (value.unread > 0) result.unread += 1;
      if (value.unsent > 0) result.unsent += 1;
      if (value.messageId > result.messageId) result.messageId = value.messageId;
      if (value.dateSent >= result.dateSent) {
        result.message = value.message;
        result.dateSent = value.dateSent;
      }
    });
    return result;
  }",
  "verbose" : true,
  "out" : { "inline" : 1 },
  "query" : { "u" : 1 },
}

time: 38 s 718 ms

MapReduce with JsMode

db.runCommand({
  "mapreduce": "user_messages",
  "map": "function(){
     emit(this.a, {
       count:1,
       unread:this.r ? 0 : 1,
       unsent:this.s==5 ? 1 : 0,
       messageId:this._id,
       date:this.d,
       body:this.b
    });
  }",
  "reduce": "function(address,values) {
    var result = { 
      count:0,
      unread:0,
      unsent:0,
      messageId:0,
      date:0,
      body:''
    };
    values.forEach(function(value) {
      result.count += 1;
      if (value.unread > 0) result.unread += 1;
      if (value.unsent > 0) result.unsent += 1;
      if (value.messageId > result.messageId) result.messageId = value.messageId;
      if (value.dateSent >= result.dateSent) {
        result.message = value.message;
        result.dateSent = value.dateSent;
      }
    });
    return result;
  }",
  "verbose" : true,
  "out" : { "inline" : 1 },
  "query" : { "u" : 1 },
  "jsMode" : true
}

time: 22 s 237 ms

Aggregation Framework

db.user_messages.aggregate(
 { $match: { u:1 } },
 { $sort: { a:1, d:-1 } },
 { $group: {
     _id: "$a",
     count: { $sum : 1 },
     unread: { $sum : { $cond : [ "$r", 0 , 1]} },
     unsent: { $sum : { $cond : [ { $eq : [ "$s", 5 ] }, 1, 0 ] } },
     messageId: { $max : "$_id" }, 
     date: { $max : "$d" },
     body: { $first : "$b" },
 } }
);

time: 6 s 835 ms

Conclusion