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+
+High-level design document
+==========================
+
+
+Data model
+----------
+
+  - Users: Django built in user model
+  - Session: Django session is used to store information under the following keys:
+      - `experiments_enrollments`  **WHAT SHOULD THIS CONTAIN?**
+      - `experiments_goals`
+      - `experiments_session_key`  **WHEN IS IT WRITTEN?**
+      - `experiments_verified_human` 
+  - Cookies:
+      - `sessionid` used to associate with Django session stored on server side
+  - Experiments:
+    - Experiments and experiment alternatives are automatically created when first
+      occurrence in a template or view code appears. They are stored in table `experiments_experiment`
+      
+          +-----------+-------------+---------------+---------------------+--------------------+-------+------------+----------+
+          | name (PK) | description | alternatives" | relevant_chi2_goals | relevant_mwu_goals | state | start_date | end_date |
+          
+  - Enrolments
+    - Bots are ignored    
+    - Session-based users are enroled based on session id **STORED WHERE?**
+    - Logged in users are enroled based on their user id using table `experiments_enrollment`
+      
+          +-----+-----------------+-----------+-------------+---------------+---------+
+          | id  | enrollment_date | last_seen | alternative | experiment_id | user_id |
+
+  - Goals:
+    - Definitions, goals must be created in advance and described in `settings.EXPERIMENTS_GOALS`.
+    - Counters are stored in Redis using the following key names:
+
+          COUNTER_CACHE_KEY = 'experiments:participants:'
+          PARTICIPANT_KEY = '%s:%s:participant' % (experiment.name, alternative_name)
+          COUNTER_FREQ_CACHE_KEY = 'experiments:freq:%s'
+          GOAL_KEY = '%s:%s:%s:goal' % (experiment.name, alternative_name, goal_name)
+
+
+
+Request-response cycle
+----------------------
+The workflow is different for logged in users and session-based auth.
+
+### Session users
+
+  - `experiments.utils.SessionUser`
+  - **WHAT KEYS ARE USED ON SESSION?**
+  - **WHEN ARE THESE STORED?**
+
+
+### Logged in users
+
+   - `experiments.utils.AuthenticatedUser`
+
+
+
+Goal tracking
+-------------
+
+  - Check if user is enrolled 
+  - Increment appropriate counter in Redis
+
+
+
+
+
+
+Analytics
+---------
+
+  - Read goal counters and enrolment denominator to compute rates
+  - Compute stats
+
+
+