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From Jure Zitnik <>
Subject Re: [BEP-0003] Misc questions
Date Fri, 14 Dec 2012 11:25:42 GMT
On 12/13/12 4:17 PM, Gary Martin wrote:
> On 13/12/12 13:41, Jure Zitnik wrote:
>> Hi,
>> On 12/12/12 5:31 PM, Gary Martin wrote:
>>> On 12/12/12 15:18, Jure Zitnik wrote:
>>>>>> 2. Something that we might forgot: What about 3rd party plugin 
>>>>>> tables that
>>>>>> reference multiproductized Trac tables?
>>>>>>      Will probably need to proclaim these incompatible when more

>>>>>> than one
>>>>>> product is in effect?
>>>>> Good point. To keep track of records from tables from third party 
>>>>> plugins, this approach doesn't quite work. I would have thought 
>>>>> that we would be better off using a separate table to keep track 
>>>>> of the resources that belong to a product. Is this another area 
>>>>> that has not been updated based on discussions?
>>>> The current SQL translator implementation would show 3rd party 
>>>> plugins a view of translated tables that would only include 
>>>> resources from the currently selected product scope. If the plugin 
>>>> makes a reference to a resource by it's name everything should work 
>>>> fine as the reference would be consistent each time when in that 
>>>> specific product environment (as the plugin would always get the 
>>>> same view of the database).
>>>> Things start breaking if there's a resource with the same name in 
>>>> multiple products, unless the translator is changed to return names 
>>>> with product namespace being prefixed to the actual resource name 
>>>> for example. The plugins would get version name 'BH:1.0' instead of 
>>>> '1.0' for example. Still, this doesn't solve the problem entirely 
>>>> as the plugin (that's not aware of products) would end up (in it's 
>>>> own tables) with references to different resources from different 
>>>> products and maybe that's not exactly what's expected to happen...
>>>> Keeping track of resource belonging to a product using a separate 
>>>> resource mapping table also unfortunately doesn't solve the issue. 
>>>> We'd need to change the schema anyway as in the current database 
>>>> model, all tables have 'name' column as their key. We could of 
>>>> course reference the same resource from different products using 
>>>> the separate mapping table but we'd be referencing the same record 
>>>> and changing the name of that record would change the resource in 
>>>> all products which is, at least imo, not what we want.
>>> Ah yes, I forgot about my ideas for that. For the purposes of unique 
>>> keys I was thinking of including some kind of prefix as part of the 
>>> name - not necessarily the product namespace as we could consider it 
>>> better to leave this as a constant with a means to link the prefix 
>>> to the namespace.
>> As I wrote above, only prefixing names doesn't solve the issue. If we 
>> have a product unaware plugin, that plugin should only see resources 
>> from the current product scope and this (for trac/bh resources) is 
>> currently accomplished by translating SQLs in such a way that the 
>> plugin only sees a product specific view of resources. The problem 
>> arises when the plugin stores references to that resources in their 
>> own tables as it might end up (unless we do something about it) with 
>> resource mixed from different products.
>> Let's say we have a custom table named 'sorted_milestones' with 
>> columns 'milestone_name' and 'sequence' and database constraint that 
>> 'sequence' is unique. If plugins sees only resources (in this case 
>> milestones) from one product at a time, constraint will fail as logic 
>> sees only product scope specific part of milestones. When product 
>> scope would change, the plugin sees completely different set of 
>> milestones and has no way of knowing that certain sequences are 
>> already 'taken'.
>> On the other hand, assuming we only prefix resources without 
>> filtering them for multi-product unaware plugins would cause the user 
>> interface for those plugins to show everything (every resource 
>> defined regardless of product scope). In addition to that, it'd be 
>> hard (if not impossible) to remove the prefix before showing that to 
>> the user. Also, it's not exactly obvious that we can remove the 
>> prefix before passing that to plugins/UI as we never know how those 
>> will be used further down the line...
>>> The problem with just adding fields to each model is not so much a 
>>> problem from the point of view of 3rd party plugins accessing those 
>>> models that are modified in such a way, but rather with those 
>>> resource tables that are added by the third party plugins. 
>> Completely agree.
>>> These would have to be modified to add the product to their tables 
>>> too. Is the suggestion that we do that modification for externally 
>>> defined resources or only provide the ability for specific plugins?
>> The idea we are playing with is that in addition to translating 3rd 
>> party plugin DMLs (SELECT/INSERT/UPDATE/DELETE) (to present plugins 
>> with a view of trac/bh resources as seen from the current product 
>> scope), the DDLs (CREATE/ALTER/DROP) for the custom tables should 
>> also be translated to support product scope(s) - this would be 
>> accomplished by creating per product custom table(s), prefixing the 
>> table name with product prefix. This (in combination with the 
>> currently implemented SQL translation) would present product unaware 
>> 3rd party plugins with product scoped tables for both, trac/bh tables 
>> and any custom table the plugin would create. References between the 
>> tables would also work and the content of the tables would always 
>> represent data based on the resources in the product scope.
>> Naive approach would be to solve 3rd party custom table the same way 
>> as trac/bh tables (by adding product column). This does not work for 
>> two reasons:
>> - schema upgrades - if the plugin chooses to upgrade it's custom 
>> table schema the usual way of doing this is to copy data to temp 
>> table, drop original table (this would effectively drop data for all 
>> products), recreate new table with changed schema and fill it from 
>> temp table
>> - table schema changes - not really sure how to implement ALTER TABLE 
>> if we modify the original schema (connected with the first reason)
>> To summarize the idea:
>> 1. for 3rd party plugins that are product unaware, any custom table 
>> being created is namespaced to the product by prefixing the table 
>> name with the product name (in a similar way as discussed resource 
>> name columns). SQL translator functionality will need to be extended 
>> to support DDL.
>> 2. the SQL translator will be changed in such a way that it will 
>> support the following table 'types':
>>    - non-translated tables - tables that need no translation 
>> (session, cache, attachments,...)
>>    - trac/bh tables with product scope - these are tables with 
>> product specific resource (enum, component, ticket, milestone, 
>> version,...) - the product scope for these tables is implemented 
>> using 'product' column
>>    - 3rd party, product-unaware plugin custom tables - product scope 
>> for these tables is implemented by prefixing the table name with 
>> product prefix
>> 3. changes to product unaware plugin install/upgrade process - 
>> install/upgrade (IEnvironmentSetupParticipant) would need to be 
>> invoked for all currently defined products (within that product scope 
>> of course)
>> 4. adding a new product would need to invoke 3rd party (product 
>> unaware) plugin installation
>> How does that sound to everyone?
> Thanks.. we definitely needed this to evaluate the solution properly!
> I don't strictly mind what solution is implemented as long as we are 
> not going too far down a dead end. I would also generally prefer 
> whichever solution turned out to be simplest but that will be far 
> harder to judge.
> Interestingly, if you are also looking at managing table names, it is 
> not inconceivable that you could add the product fields to the 3rd 
> party resource tables after all, as long as you were also prepared to 
> discover which fields should be unique together with product. One of 
> the possible dangers with this might be that a plugin is at some point 
> allowed to adjust the schema (obviously we wouldn't mean to allow it - 
> it would still be possible though) so it still might be wise to change 
> the table name anyway. There is the possible advantage that it limits 
> the differences in processing a little.

That (adding product column to 3rd party resources) was one of the ideas 
that were evaluated. The problem with that approach is the schema 
upgrade process that usually goes like this (even in trac itself):
1. copy data to temp table
2. drop original
3. create new table with changed schema
4. fill new table from temp table & drop temp table

The problem is step 2 (table drop) as there is no way of preventing 
plugin from dropping the table regardless of the fact that the table 
holds data for all product scopes and not just the current one.

> There is actually one final thing that seems to have been left without 
> any consideration, and that is whether we can get away with allowing 
> third party resources to be available across all scopes. I assume that 
> the initial implementations will start off this way anyway!

If we take the 'table name prefix' approach, 3rd party resource will 
only be available in current product scope. Only product aware plugins 
and bloodhound code will be able to access resources through all scopes 
using the global database connection that will have the SQL translator 
turned off. The interesting issue here is how to present that 3rd party 
resource (scattered across multiple tables) through global scope without 
requiring them to do UNION SELECTs on all tables. We might provide 
helper functions for that though ... something to think about but 
definitely not critical at the moment.


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