Session handling

The state of a survey needs to be remembered. The state consists of a lot of data: the profile, evaluation, inventory, action plan answers and company details. This is too much information to manage via form variables or HTTP cookies so we need to use a server-side storage mechanism. The Zope session mechanism is known to quickly generate a lot of conflicts, which makes it unsuitable (the faster product improves things a lot though). An extra requirement to consider is that we expect to need to handle persistent sessions in the future. In light of these considerations the session system is based on a SQL database.

Survey tree

A survey starts with all profile questions. Once these have been answered a tree is generated for all items that are exposed to the user via the survey user interface. This also takes care of profile questions: modules are skipped or repeated as necessary based on the profile question answers.

This tree is stored in a mixed path enumeration (also called materialized tree) and adjancency list model. This combination of models allows for fast location of tree nodes during traversal as well as an efficient way to walk up the tree to find parent nodes.

As path a simple numbering scheme is used, which makes it possible to conver the path into a dotted numbering scheme in the user interface. For example this survey:

1. Do you have a mobile shop?
1.1 Is it serviced annually?
1.2 Is the tire pressured checked every week?
2. Are you open nights?

will have ‘‘001’‘, ‘‘001001’‘, ‘‘001002’’ and ‘‘002’’ as paths in the database.

Each survey has a session record in the database which holds data for the session ans is located using a HTTP cookie. Each session has its own tree with module and question information.

Schema structure

Session information is stored in a series of SQL tables.

Table Description
account Registered users.
tree Tree node data.
session User sessions. Each session is a tree root.
profile_answer An answer to a profile question.
module A module.
risk Answeres for a risk (for all phases).
action_plan Action plan items for questions.

The session table has a modification timestamp which is used to remove stale sessions. This timestamp is also updated when the session is resumed. This guarantees that a session does not expire if a user looks at an earlier session but does not make any changes.

For performance reasons every row in the tree table lists the title for the item and a pointer to the root session. This makes it possible to build a tree without having to load any objects from the ZODB.

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