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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.
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)
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).
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 #6543 and TracDev/Proposals/TracRelations.
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_customtable (#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
intfields, this could also be useful to implementtextfields for Oracle (working around the limits on varchars and not having to use CLOBs)
- (+) 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
- (-) slower, less memory efficient (?)
- (-) more complex code (?)
(?) means yet to be verified
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. Note also that by an appropriate use of indexes, we might eventually get better performance compared to what we have today.
So the new model could be simply:
ticket
| id | name | value | seq |
|---|
seq is the sequence number in case of multiple entries with the same name.
or even:
resource_prop
| realm | id | name | value | seq |
|---|
(if we use one mega table for all resources)
Note by the way that newer "resources", like repository in MultiRepos already have this (id,name/value) form.
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.
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 |
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.
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 | seq |
|---|
and the realm information could simply be stored 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 | seq |
|---|
milestone_change
| id | changeid | name | value | seq |
|---|
or:
resource_change
| id | changeid | name | value | seq |
|---|
(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.
In the following:
- int stands for the integer column type
- string stands for a short text column type (e.g. varchar(65))
- text stands for a long text column type, of unbounded size
Minimal Model
- surrogate keys for all resources
- only text fields
Schema
resource_schema
| realm | prop | name | value |
|---|---|---|---|
| string | string | string | text |
resource_prop
| id | name | value | seq |
|---|---|---|---|
| int | string | text | int |
resource_revprop
| changeid | name | value | seq |
|---|---|---|---|
| int | string | text | int |
resource_change
| id | changeid | name | value | seq |
|---|---|---|---|---|
| int | int | string | text | int |
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 | seq |
|---|---|---|---|
| 0 | id | 130 | 0 |
| 0 | summary | Multiple Project Support | 0 |
| 0 | description | One day… | 0 |
| 0 | reporter | joe | 0 |
resource_revprop
| changeid | name | value | seq |
|---|---|---|---|
| 1 | author | joe | 0 |
| 1 | date | 5 years ago | 0 |
| 2 | author | joe | 0 |
| 2 | date | 2 years ago | 0 |
| 2 | comment | come on… | 0 |
| 3 | author | cboos | 0 |
| 3 | date | 1 year ago | 0 |
| 3 | comment | sure… | 0 |
resource_change
| id | changeid | name | value | seq |
|---|---|---|---|---|
| 0 | 1 | id | 130 | 0 |
| 0 | 1 | summary | Multiple Project Support | 0 |
| 0 | 1 | description | Should be easy… | 0 |
| 0 | 1 | reporter | joe | 0 |
| 0 | 2 | description | Should be easy… Redmine has it! | 0 |
| 0 | 3 | description | One day… | 0 |
Intermediate Model
- surrogate keys for all resources
- text and int fields (same as Complete Model, without the
*_stringtables)
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 ids will be integers).
Complete Model
- surrogate keys for all resources
- int, bigint, 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.
bigint is needed since we use this to store microsecond precision timestamps (#6466).
Schema
FIXME need to add the seq columns here as well
resource_schema
| realm | prop | name | value |
| string | string | string | text |
resource_prop
| id | name | value |
| int | string | text |
resource_revprop
| changeid | name | value |
| int | string | text |
resource_change
| id | changeid | name | value |
| int | int | string | text |
resource_prop_string
| id | name | value |
| int | string | string |
resource_revprop_string
| changeid | name | value |
| int | string | string |
resource_change_string
| id | changeid | name | value |
| int | int | string | string |
resource_prop_int
| id | name | value |
| int | string | int |
resource_revprop_int
| changeid | name | value |
| int | string | int |
resource_change_int
| id | changeid | name | value |
| int | int | string | int |
Example Dataset
FIXME add example of multivalued property
resource_schema
| realm | prop | name | value |
| ticket | summary | type | text |
| ticket | description | type | wiki |
| ticket | reporter | type | string |
resource_prop
| id | name | value |
| 0 | summary | Multiple Project Support |
| 0 | description | One day… |
resource_revprop
| changeid | name | value |
| 2 | comment | come on… |
| 3 | comment | sure… |
resource_change
| 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_string
| id | name | value |
| 0 | reporter | joe |
resource_revprop_string
| changeid | name | value |
| 1 | author | joe |
| 2 | author | joe |
| 3 | author | cboos |
resource_change_string
| id | changeid | name | value |
| 0 | 1 | reporter | joe |
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 |
- Comment Why not implement all of the different resource_prop* tables into a single table, where each tupel has multiple attributes, see for example the JBPM datamodel for a working and presumably also fast approach. Here, there exists a process_variable or some similar table that stores all the different value types in single table. And please rename the name field to prop so that it matches the one in the resource_schema table.The schema would be like so:
table resource_prop { id prop type int_val string_val datetime_val ... } table resource_revprop { change_id prop type int_val string_val ... } table resource_change { id change_id prop type int_val string_val ... }
- Comment Also I would like to have the resource_schema table extended so that it will support different schemas for, say, different ticket types. That way, users can define their personal ticket type schemas. Of course, derivation would also be nice, but that could be implemented at a later point in time, requiring yet another table. That way we could have both inheritance at the schema level and also multiple different models per realm ;)
table resource_schema
{
realm
urn -- the urn of the ticket schema or wiki page schema to which this belongs to, defaults are for example trac::ticket or trac::milestone or trac::wiki-page and so on
prop
name
type
}
and the inheritance table for the schemas would then be
table resource_schema_inheritance
{
realm
urn
base -- the urn of the base schema
}
- inheritance would then provide for also multiple inheritance
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, …)
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
