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GenericTrac Data Model
This page attempts to define a new data model for Trac that could be suitable for most of its resources. 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 (numerous examples for that, see #RelatedTickets below)
This stems from the following former proposals:
See also this mail.
As this will be a major redesign of the data model,
it could also be a good opportunity to take the
multiple project considerations into account (#130).
Each resource related table could get a project
identifier field.
Working on the generic aspect of Trac should also make it possible to implement various generic operations on Trac resources as plugins, mainly being able to (re-)implement TracCrossReferences as a plugin (see also #6543).
Design Discussion
Requirements for the new model:
- it has to be simple;
- it must be flexible, in order to accommodate different kinds of resources and allow for dynamic evolution;
- it should remain fast, if not faster than what we currently have;
- it should lead to a more compact representation of data
Resource Content
The ticket model is by far richer data model we would have to support,
so we could take this 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 name/value columns in a ticket_custom
table).
Both styles have advantages and disadvantages:
- properties as columns:
- (-) only flexibility is to not use some fields (e.g. severity)
- (-) no multiple values per field possible
- (+) faster
- (+) straightforward code (
for field1,field2, ... in cursor: ...
)
- properties in name/value columns
- (+) highest flexibility, add or remove fields at will
- (+) allow for multiple values per name, provided we don't use a primary key
as we currently do for the
ticket_custom
table (#918) - (-) slower, less memory efficient (?)
- (-) more complex code (?)
In order to reduce the overall complexity, the idea would be 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.
So the new model could be simply:
ticket
id | name | value |
or even:
resource_prop
realm | id | name | value |
(if we use one mega table for all resources)
We could also keep the metadata associated to the properties in the database, instead of being hard-coded and present in the TracIni file.
resource_schema
realm | prop | name | value |
Here, possible values for name could be 'label', 'default', 'order', 'type', etc.
Example.
ticket | description | type | wiki |
ticket | priority | type | enum |
ticket | priority | enum | priority |
ticket | priority | default | normal |
ticket | need_review | type | checkbox |
ticket | need_review | default | 0 |
As a possible refining, it could be possible to have specialized tables, one for each different value column type we want to support:
- resource_prop for text values
- resource_prop_int for integer values
- (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).
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
- we have to support different type of keys (text for milestone, int for ticket).
- 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 (there would always be a name=id, value=the natural key entry
- renaming is easy (relations preserved)
This suggests that using surrogate keys would be preferable. Now if this is the case, the resource_prop table could as well become:
id | name | value |
and the realm information could simply be store as another name/value entry.
Resource 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 name/value pairs of "revision properties".
resource_revprop
changeid | name | value |
Typical example:
101001 | author | cboos |
101001 | auth | 1 |
101001 | date | 1231232114.12 |
101001 | comment | random change |
A given changeid 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)
- changes affecting changes (typically #454)
The property changes themselves are stored in other tables. We only need the changed properties here, no need to store the old/new values for each change, as this can be deduced from the past changes. Deletions of fields should be represented by setting a field to the NULL value.
Several possibilities here:
ticket_change
id | changeid | name | value |
milestone_change
id | changeid | name | value |
or:
resource_change
id | changeid | name | value |
(surrogate key approach)
The latter has the advantage that it would make easy to relate a given changeid
to the resource(s) that were affected by the change, without having to go through
each resource table.
We could also keep all property changes as text values
or have extra ..._int
(..._float
) tables for more compact
representation.
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.
The Model
To summarize the above discussion, here's what could be the new model.
Minimal Model
- surrogate keys for all resources
- only text fields
Schema
resource_schema
realm | prop | name | value |
resource_prop
id | name | value |
resource_revprop
changeid | name | value |
resource_change
id | changeid | name | value |
Example Dataset
resource_schema
realm | prop | name | value |
ticket | summary | type | text |
ticket | description | type | wiki |
ticket | reporter | type | text |
resource_prop
id | name | value |
0 | id | 130 |
0 | summary | Multiple Project Support |
0 | description | One day… |
0 | reporter | joe |
resource_revprop
changeid | name | value |
1 | author | joe |
1 | date | 5 years ago |
2 | author | joe |
2 | date | 2 years ago |
2 | comment | come on… |
3 | author | cboos |
3 | date | 1 year ago |
3 | comment | sure… |
resource_change
id | changeid | name | value |
0 | 1 | id | 130 |
0 | 1 | summary | Multiple Project Support |
0 | 1 | description | Should be easy… |
0 | 1 | reporter | joe |
0 | 2 | description | Should be easy… Redmine has it|| |
0 | 3 | description | One day… |
Intermediate Model
- surrogate keys for all resources
- text and int fields (same as Complete Model, without
*_text
tables)
The minimal model above is handy for showing the essence of the new model, but it's too simple in practice.
I think we need at the very least to support 'integer' type columns, useful for storing dates efficiently, boolean values, and relations to other resources (as the surrogate id will be an integer).
Complete Model
- surrogate keys for all resources
- int, short and long text fields
Not absolutely necessary to go that far, this could nevertheless help a lot for the MySQL backend (#6986), possibly also for a future Oracle backend. Don't know about PostgreSQL, but for SQLite this should be indifferent.
Schema
resource_schema
realm | prop | name | value |
resource_prop
id | name | value |
resource_revprop
changeid | name | value |
resource_change
id | changeid | name | value |
resource_prop_text
id | name | value |
resource_revprop_text
changeid | name | value |
resource_change_text
id | changeid | name | value |
resource_prop_int
id | name | value |
resource_revprop_int
changeid | name | value |
resource_change_int
id | changeid | name | value |
Example Dataset
resource_schema
realm | prop | name | value |
ticket | summary | type | text |
ticket | description | type | wiki |
ticket | reporter | type | string |
resource_prop
id | name | value |
0 | reporter | joe |
resource_revprop
changeid | name | value |
1 | author | joe |
2 | author | joe |
3 | author | cboos |
resource_change
id | changeid | name | value |
0 | 1 | reporter | joe |
resource_prop_text
id | name | value |
0 | summary | Multiple Project Support |
0 | description | One day… |
resource_revprop_text
changeid | name | value |
2 | comment | come on… |
3 | comment | sure… |
resource_change_text
id | changeid | name | value |
0 | 1 | summary | Multiple Project Support |
0 | 1 | description | Should be easy… |
0 | 2 | description | Should be easy… Redmine has it|| |
0 | 3 | description | One day… |
resource_prop_int
id | name | value |
0 | id | 130 |
resource_revprop_int
changeid | name | value |
1 | date | 5 years ago |
2 | date | 2 years ago |
3 | date | 1 year ago |
resource_change_int
id | changeid | name | value |
0 | 1 | id | 130 |
Possible Implementation Plan
Milestone First
- modify the Milestone module so that it uses the new proposed datamodel. See #TheMilestoneExample.
- 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, …)
Related Tickets
- Data model issues:
- Resource related:
- #150
- User-centric storage. Last-viewed information and 'intelligent' followup
- #221
- Creating TR for multiple components
- #695
- Keywords for wiki pages
- #787
- Change attachment description
- #918
- [patch] Custom Ticket Fields should support the multiple selection type
- #1113
- Show milestone changes in timeline
- #1386
- Adding properties to Wiki Pages
- #1395
- Text box for duplicate when a bug is a duplicate
- #1678
- Show component view in timeline view for checkins
- #1835
- Add a discussion tab for Trac resources
- #2035
- Changeset commenting
- #2344
- sub milestone
- #2464
- Conditional fields in tickets
- #2465
- Add "custom query" functionality for changesets
- #2467
- Link user name in reports to custom query showing that user's open tickets
- #2662
- assign tickets to multiple users
- #2961
- custom comment fields
- #3003
- milestone could be a ticket
- #3080
- Custom field sorts only as text
- #3718
- Trac should use HTTP 301 Moved when milestones get renamed
- #3911
- implement an object system (xwiki, roundup)
- #4588
- User Page: trac links to these pages
- #5211
- Ticket - Wiki Integration enhancement/suggestion
- #7871
- Add start date to a milestone
- #8335
- Create People page with user profiles and status of who's doing what.
- #9263
- if SVN is used, milestone should manage/allowtoassign links for branch and tag