add doc file about use of mongo (by cms+lms)

doc/mongo.md

+# Notes on using mongodb backed LMS and CMS
+
+These are some random notes for developers, on how things are stored in mongodb, and how to debug mongodb data.
+
+## Databases
+
+Two mongodb databases are used:
+
+- xmodule: stores module definitions and metadata (modulestore)
+- xcontent: stores filesystem content, like PDF files
+
+modulestore documents are stored with an _id which has fields like this:
+
+    {"_id": {"tag":"i4x","org":"HarvardX","course":"CS50x","category":"chapter","name":"Week_1","revision":null}}
+
+## Document fields
+
+### Problems
+
+Here is an example showing the fields available in problem documents:
+
+    {
+	"_id" : {
+		"tag" : "i4x",
+		"org" : "MITx",
+		"course" : "6.00x",
+		"category" : "problem",
+		"name" : "ps03:ps03-Hangman_part_2_The_Game",
+		"revision" : null
+	},
+	"definition" : {
+		"data" : " ..."
+	},
+	"metadata" : {
+		"display_name" : "Hangman Part 2: The Game",
+		"attempts" : "30",
+		"title" : "Hangman, Part 2",
+		"data_dir" : "6.00x",
+		"type" : "lecture"
+	}
+    }
+
+## Sample interaction with mongodb
+
+1. "mongo"
+2. "use xmodule"
+3. "show collections" should give "modulestore" and "system.indexes"
+4. 'db.modulestore.find( {"_id.org": "MITx"} )' will produce a list of all MITx course documents
+5. 'db.modulestore.find( {"_id.org": "MITx", "_id.category": "problem"} )' will produce a list of all problems in MITx courses
+
+Example query for finding all files with "image" in the filename:
+
+- use xcontent
+- db.fs.files.find({'filename': /image/ } )
+- db.fs.files.find({'filename': /image/ } ).count()
+
+## Debugging the mongodb contents
+
+A convenient tool is http://phpmoadmin.com/  (needs php)
+
+Under ubuntu, do:
+
+  - apt-get install php5-fpm php-pear
+  - pecl install mongo
+  - edit /etc/php5/fpm/php.ini to add "extension=mongo.so" 
+  - /etc/init.d/php5-fpm restart
+
+and also setup nginx to run php through fastcgi.
+
+## Backing up mongodb
+
+- mogodump  (dumps all dbs)
+- mongodump --collection modulestore --db xmodule (dumps just xmodule/modulestore)
+- mongodump  -d xmodule -q '{"_id.org": "MITx"}' (dumps just MITx documents in xmodule)
+- mongodump -q '{"_id.org": "MITx"}' (dumps all MITx documents)
+
+## Deleting course content
+
+Use "remove" instead of "find":
+
+- db.modulestore.remove( {"_id.course": "8.01greytak"})
+
+## Finding useful information from the mongodb modulestore
+
+- Organizations
+
+    > db.modulestore.distinct( "_id.org")
+    [ "HarvardX", "MITx", "edX", "edx" ]
+
+- Courses
+
+    > db.modulestore.distinct( "_id.course")
+    [
+    	"CS50x",
+    	"PH207x",
+    	"3.091x",
+    	"6.002x",
+    	"6.00x",
+    	"8.01esg",
+    	"8.01rq_MW",
+    	"8.02teal",
+    	"8.02x",
+    	"edx4edx",
+    	"toy",
+    	"templates"
+    ]
+
+- Find a problem which has the word "quantum" in its definition
+
+    db.modulestore.findOne( {"definition.data":/quantum/})n
+
+- Find Location for all problems with the word "quantum" in its definition
+
+    db.modulestore.find( {"definition.data":/quantum/}, {'_id':1})
+
+- Number of problems in each course
+
+    db.runCommand({
+      mapreduce: "modulestore",
+      query: { '_id.category': 'problem' },
+      map: function(){ emit(this._id.course, {count:1}); },
+      reduce: function(key, values){
+                  var result = {count:0};
+     	      values.forEach(function(value) {
+    	          result.count += value.count;
+    	      });
+    	      return result;
+    	   },
+      out: 'pbyc',
+      verbose: true
+    });
+
+    produces:
+
+    > db.pbyc.find()
+    { "_id" : "3.091x", "value" : { "count" : 184 } }
+    { "_id" : "6.002x", "value" : { "count" : 176 } }
+    { "_id" : "6.00x", "value" : { "count" : 147 } }
+    { "_id" : "8.01esg", "value" : { "count" : 184 } }
+    { "_id" : "8.01rq_MW", "value" : { "count" : 73 } }
+    { "_id" : "8.02teal", "value" : { "count" : 5 } }
+    { "_id" : "8.02x", "value" : { "count" : 99 } }
+    { "_id" : "PH207x", "value" : { "count" : 25 } }
+    { "_id" : "edx4edx", "value" : { "count" : 50 } }
+    { "_id" : "templates", "value" : { "count" : 11 } }