= GenericTrac Data Model = 

This page defines a new data model for Trac that should be suitable
for storing most of the data from resources, along with their change history.

In the longer term, it would be more appropriate to keep the change history in a dedicated system (a version control backend, even a [DistributedTrac distributed] one).
In that picture, we could even provide a simple "dbvcs" backend, which would use the database for storing the change history much like we do now.

The main benefits expected from the new model are:
 - simplification of the internals of Trac, especially for the ticket model,
   in which the storage of changes is quite cumbersome (see #454, #6466)
 - solve a few design problems with the current data model (like #1890, #4582)
 - allow better code reuse and share of the base features 
   among different kinds of resources (like #695, #1113, etc.
   see [#RelatedTickets] for more)

== Related Proposals ==

The idea is not new, it has its root in the TracObjectModelProposal, and was refined several times (TracDev/Proposals/DataModel, TracDev/Proposals/Journaling, see also [googlegroups:trac-dev:8cf3f5fe0e476ce5 this mail).

As this will be a major redesign of the data model, 
it could also be a good opportunity to take the 
**[TracMultipleProjects multiple project]** considerations into account (#130).

Somehow related to the generic data model, but not strictly depending on it,
Trac should also make it possible to implement new kinds of plugins that would
perform ''generic'' operations on Trac resources.
This could allow the (re-)implementation of the TracCrossReferences idea
as a plugin, for example.
See #31, #886, #6543 and TracDev/Proposals/TracRelations.


== Design Discussion ==

Requirements for the new model:
 1. it has to be ''simple'';
 2. it must be ''flexible'', in order to accommodate different kinds of resources and   
    allow for dynamic evolution, like addition or removal of fields by plugin or via the web admin;
 3. it should remain ''fast'', if not become faster than what we currently have;
 4. it should lead to a more ''compact'' representation of data
 5. all the existing constraints about maintaining resource history and associated metadata should be taken into account

Note that the persistence constraints imposed by the Trac data model are not necessarily only (or even best) approached using the RelationalModel, one could imagine that a future version could use more document-oriented persistence layers (e.g. [http://www.mongodb.org/ MongoDB]), or object-oriented databases (e.g. [http://www.zodb.org/ ZODB]). Also, as said above, the versioning of resources should be delegated to a version control backend, with a default, simple, in-database VCS backend.

=== Resource Content ===

The ticket model is by far richer data model we would have to support, 
so we could use it as a basis to lay out the foundations of the new model.

For ticket, we currently have a fixed set of properties
(as columns in the `ticket` table)
and a flexible set of properties
(as prop/value columns in a `ticket_custom` table).

Note that we might soon get ''relations'' (or ''links'') as well, see #31.

==== Properties
Let's examine the advantages and disadvantages of each different kind of storage, for the properties:
 1. properties stored in columns of a main table:
   - (-) only flexibility is to not use some fields (e.g. 'severity', 'url') 
   - (-) no multiple values per field possible  
   - (+) faster especially for multicriteria queries (?) 
   - (+) straightforward code (`for field1,field2, ... in cursor: ...`) (?)
 2. properties stored in prop/value columns of a generic table
   - (+) highest flexibility, add or remove fields at will
   - (+) allow for multiple values per property, provided we don't use a primary key
     as we currently do for the `ticket_custom` table (#918)
     - (+) a slight extension would allow ordered multiple values (sequences), otherwise we have no control over the order in which those multiple values are retrieved, which might introduce confusion as this order will vary when new values are added;
       while primarily useful for `int` fields, this could also be useful to implement `text` fields for Oracle (working around the limits on varchars and not having to use CLOBs)
   - (-) slower, less memory efficient, especially for multicriteria queries(?)
   - (-) more complex code (?)
(?) means ''yet to be verified''

In order to reduce the overall complexity, the idea is to pick only one approach, instead of having to support both.
By using the second style, we could also have our "fixed" set of properties, 
while obviously the first style can't support the second.

It remains to be seen whether the second approach is really less efficient than the first, but this doesn't really matter as we anyway have already to pay the 
price for that flexibility.
Note also that by an appropriate use of indexes, we might eventually get 
''better'' performance compared to what we have today.

The second style (prop/value) can be implemented in several ways: 
 - a single `properties` table with `prop` and `value` string columns (this is what our current `ticket_custom`, `system` and `repository` tables do)
 - a single `properties` table with `prop` and several typed columns (`int_value`, `string_value`, `date_value`, etc.), only the appropriate column being used - see discussion ([[./Brainstorm#JBPM-approach|JBPM's approach]])
 - several `properties` tables with each `prop` and `value` columns, each table dedicated to a given type (`int_properties`, `string_properties`, etc.)
 - ... more ...


So for example the new model could be simply:

 ||||||||=     '''ticket_prop''    ||
 ||= id ||= prop ||= value ||= seq ||

//seq// is the sequence number, which is used to support multiple values for the same property.

Notes:
 - newer "resources", like `repository` in MultiRepos already have this `(id,prop,value)` form, we'd only have to add `seq`.
 - this sequence number could be a globally unique sequence number, which would make it possible to use it as a foreign key in specialized tables, in case properties are needed to describe the resource - property relation itself...

==== Schema
We could also keep the metadata associated to the properties in the database,
instead of being hard-coded and present in the TracIni file, permitting an
unification of the representation for fixed fields and custom fields.


Note: the existence of a schema describing the fields doesn't mean that modules
can't interpret fields as being special. Quite the opposite, as modules are what
provides the real "behavior" of resources.
Furthermore, properties not defined in the schema could simply be ignored, so this would allow a great deal of flexibility for plugins when they need to store "special" properties or revision properties.

==== The Generic Scheme #generic-scheme
Actually, for achieving requirement 3. & 4., we need specialized tables, 
one for each different value column type we want to support:
 - '''{resource}_prop''' for text values
 - '''{resource}_prop_int''' for integer values, identifiers and dates (using "bigint")
 - ('''{resource}_prop_float''' for float values, if really needed)
And we could even differentiate between short and long text values (requirement 4):
 - '''{resource}_prop''' for short text values 
 - '''{resource}_prop_text''' for long text values
(see #6986).

This set of tables is a "generic scheme" that can be easily created for any kind of "resource". There's also a trade-off here, between using one very generic set of "resource_..." table versus special instances like "ticket_...": having one set of tables presents the risk of introducing too much contention for backends using table locking, and leads to writing queries that will contain lots of difficult to joins. We will already have enough issues with this when writing multicriteria search queries...

For satisfying the requirement 3. (speed), we simply need to have enough indexes: 
 - on `id`, for retrieving all properties of an instance at once
 - on `(prop, value)`, for retrieving properties by values (searches and joins)

I don't see an use case yet for indexing on `seq`, or to use other combinations.


==== Identifier

Along the same lines there's also the question of what should be the ''id'':
a natural or a surrogate key?
 natural keys:: (''id'' would be 123 for ticket !#123, id would be 'milestone1' for milestone1, etc.)
   - we have to support different type of keys (text for milestone, int for ticket).
     - not a problem for separate tables
     - would require ''resource_int_prop'' style for resources having an ''int'' 
       id ... cumbersome
   - less compact but easier to "understand"
   - renaming is more difficult
 surrogate keys:: (''id'' would be a number in a sequence, never shown as such in the interface)
   - only one type of keys (int) - faster, simpler,
   - the unique ''resource_prop'' table approach is possible
   - more compact, not that difficult to read either as there would always be a prop/value pair (`'id'`,`<the natural key>`) 
   - renaming is easy (relations are preserved)

This all suggests that using surrogate keys would be preferable.

==== Relations

Storing the relations between resources is also an interesting topic.
We face several alternatives. Let's take the example of //ticket// and //project//. This is a simple example because we only have a "has_a / part_of" relation between ticket and project.
 1. in the `ticket_prop_int` table, via the `('project', <project_id>)` prop/value pair(s) (support of multiple projects per ticket is possible as any other multivalued field via multiple `seq`)
 2. in a dedicated `ticket_project` relation table `(<project_id>,  <ticket_id>)`
 3. in a global `project_relation` table `(<project_id>,  <resource_id>)`
 4. as part of the `project_prop_int` table, via `('ticket', <ticket_id>)` prop/value pair(s). This is symmetrical to 1., this time the relations are store on the project side.

We also have the more demanding example of generic //ticket// vs. //ticket// relations, as discussed in #31.
There we don't want to have to add a new table for each new type of relation, hence a solution of the type 1. / 4. would be more appropriate (3. + a "role" type would essentially be the same as 1. / 4.).

In order to decide between 1. or 4., we need to consider the nature of the relation, whether it's an essential or accidental property. 

A ticket doesn't really stand on its own, normally it's part of one project. Remember, in the context of MultiProject, even when there's no project defined, there's the implicit "whole" project. A ticket is eventually part of multiple projects but that's less common, if allowed at all. Even in that case, we can still consider that the //project// properties are essential properties of the ticket. 
On the opposite, a project can well exist without any tickets attached to it, so it's pretty clear that the tickets are not essential properties of a project. It's also clear that when we load a project instance, we don't necessarily want to load thousands of ticket properties.

For other relations, it can be less clear cut, we have to take sides. For example, concerning the ticket relations:
 - ''**parent**/child'': a child ticket necessarily has a ''parent'' ticket, otherwise it's not a child ticket ; the parent ticket may have one or more children tickets, but it can also exist on its own
 - ''**blocked-by**/is-blocking'': a ticket can be ''blocked-by'' one or more tickets ; the converse relation ''is-blocking'' is less strong
 - ''**duplicate-of**/duplicated-by'': a ticket may be qualified to be a ''duplicate-of'' of another ticket ; that other ticket is ''duplicated-by'' one or many tickets, but this is less strong 

==== History

We need to differentiate between the changes to the data, and the metadata about the change. The metadata is about who did the change, when, why the change was made, etc.
We can adopt the same flexible strategy as the one for resource properties and
store arbitrary key/value pairs of "revision properties".

For implementing a simple db backed VCS, we can even adopt the same [#generic-scheme generic scheme] as described above, but for two derived resource:
 - one for snapshotting the resource properties at a given point in time "{resource}_version", e.g. `ticket_version`
 - one representing the metadata corresponding to the change, describing the act of creating a new version, i.e. revisioning:  "{resource}_rev", e.g. a `ticket_rev` (or `ticket_revision`?)

||||||||=  '''ticket_version_prop'''  ||
||= id ||= prop ||= value ||= seq ||
||||||||=  '''ticket_rev_prop'''  ||
||= id ||= prop ||= value ||= seq ||

The main resource and the derived resources would be related together:
 - the main resource would point to the lastchange (shortcut, this could be deduced by inspecting the ''date'' stored in the _rev_prop table - so in a "normalized" schema we wouldn't need it)
 - a revision resource would point to the corresponding version which was created at the same time (again, matching with the ''date'' would be possible?)

Typical example:
||||||||=  '''ticket_prop'''  ||
|| 1001 || id || 1 || 2021010101 ||
|| 1001 || lastchange || 101001 ||
||||||||=  '''ticket_rev_prop'''  ||
|| 101001 || ticket  || 1001         ||  23232323333 ||
|| 101001 || auth    || 1            ||  23232323334 ||
|| 101001 || date    || 123123211422 ||  23232323335 ||
|| 101001 || version || 202002       ||  23232323336 ||
||||||||=  '''ticket_rev_prop_int''  ||
|| 202002 || author  || cboos        ||  23232323337 ||
|| 202002 || comment || random change ||  23232323338 ||

A given revision instance (here 101001) is usually related to a specific change in one resource,
but there could be other situations:
 - one change affecting lots of resources (typically #4582 and #5658, ticket batch changes #525)
 - changes affecting changes (typically #454); now that we have completed #454, it would be interesting to see what constraints this impose on the new model

Here we see that one change can easily cover multiple tickets, useful for storing metadata related to batch changes, via different sequence numbers for the ''ticket'' property, and a corresponding sequence of ''version'' property.

The property changes themselves are stored in other tables.
We only need the properties that have changed here, no need to store the old/new values for each change, as this can be deduced from the past changes. No need to store the properties which haven't changed either (requirement 4.).
Deletions of fields could be represented by setting a field to the NULL value.


See also ticket:6466#comment:10 and follow-ups for a discussion about how ticket changes and in particular ticket change edits, could be handled using this approach //(not sure if this still applies, though)//.




== Possible Implementation Plan ==

=== Milestone First ===
 - modify the Milestone module so that it uses the new proposed datamodel.
 - experiment new tabbed view for the milestone (''View'', ''Discussion'', ''History''). See TracProject/UiGuidelines.
 - milestone should be able to have attachments, too (#3068)
 - adapt the Roadmap module to the new model
 - adapt the Milestone admin component to the new model

Once this is complete, validate the genericity by promoting the components to be first class resources as well (#1233).

=== Ticket First ===

As the ticket module is by far the most complex, it might be worth to
try out the new model there first:
 - we could verify that we meet the expectations in terms of code simplification,
   solving open issues, etc.
 - we could detect early if there are no regressions or risk of losing current features
 - by redeploying the ticket infrastructure to the other components, 
   we could spread the most benefits of tickets (comments, 
   custom fields, queries, etc.) to other resources (milestone, wiki, component, ...)

=== Wiki First ===

Milestone and components are closer in spirit to a wiki page than to a ticket (we have the long standing #3776 ticket).

We could as well start simply on the wiki model and wiki module, to rework the basics there, and once this is done port the changes to the milestone module, then work on the components.

If we do this, we can also at the same time think about the storage model, in particular how past versions could be stored in a VCS (#1132). We could then think about a serialization model that would play nicely with the genericity of the Trac resources on one hand side, and with the external editors on the other hand.

=== Component First ===

An other possibility would be to start on fresh ground, implementing not yet existing resource typse in a generic way. That would be the less disrupting approach, as everything else should just continue to work unchanged (or mostly unchanged, except for things like #5211 and places where we deal with the old `component` table).


== Related Tickets ==

 - Data model issues: [[TicketQuery(status=!closed&keywords=~model)]]
 - Resource related: [[TicketQuery(status=!closed&keywords=~tracobject)]]