(This gets a little bit off the topic, but it should still be helpful for the current discussion. But if we want to discuss a more general data management architecture for MW, then it might be sensible to make a new thread ;-)
On Freitag, 28. Mai 2010, Michael Dale wrote:
More important than file_metadata and page asset metadata working with the same db table backed, its important that you can query export all the properties in the same way.
Within SMW you already have some "special" properties like pagelinks, langlinks, category properties etc, that are not stored the same as the other SMW page properties ... The SMW system should name-space all these file_metadata properties along with all the other structured data available and enable universal querying / RDF exporting all the structured wiki data. This way file_metadata would just be one more special data type with its own independent tables. ...
SMW should abstract the data store so it works with the existing structured tables. I know this was already done for categories correct?
More recent versions of SMW actually do no longer use MW's category table for this, mostly to improve query performance. [In a nutshell: SMW properties can refer to non-existing pages, and the full version of SMW therefore has its own independent page id management (because we want to use numerical IDs for all pages that are used as property values, whether or not they exist). Using IDs everywhere improves our query performance and reduces SQL query size, but it creates a barrier for including MW table data since more joins would be needed to translate between IDs. This is one reason SMW Light will not support queries: it uses a much simpler DB layout and less code, but the resulting DB is not as suitable for querying.]
Was enabling this for all the other links and usage tables explored?
Having a unified view on the variety of MediaWiki data (page metadata, user- edited content data, file metadata, ...) would of course be great. But accomplishing this would require a more extensive effort than our little SMW extension. What SMW tries to provide for now is just a way of storing user- edited data in a wiki (and also for displaying/exporting it).
Of course SMW already has a PHP abstraction for handling the property-value pairs that were added to some page, and this abstraction layer completely hides the underlying DB tables. This allows us to make more data accessible even if it is in other tables, and even to change the DB layout of our custom tables if required. You are right that such an abstraction could be extended to cover more of the native data of MediaWiki, so that data dumps can include it as well.
I think this idea is realistic, and I hope that SMW helps to accomplish this in some future. Yet, this is not a small endeavour given that not even most basic data management features are deployed on the big Wikimedia projects today. To get there, we first need a code review regarding security and performance, and so for the moment we are pressed to reduce features and to shrink your code base. This is why we are currently building the "Light" version that only covers data input (without link syntax extensions), storage, look-up, and basic export/dump. For this step, I really think that sharing a data table with the EXIF extension would make sense, since the data looks very similar and a more complex DB layout is not necessary for the initial goals. We can always consider using more tables if the need arises.
But I would be very happy if there were more people who want to make concrete progress toward the goal you describe. Meanwhile, we are planning in smaller steps ;-)
This also make sense from an architecture perspective, where file_metadata is tied to the file asset and SMW properties are tied to the asset wiki description page. This way you know you don't have to think about that subset of metadata properties on page updates since they are tied to the file asset not the wiki page propriety driven from structured user input. Likewise uploading a new version of the file would not touch the page data tables.
Right, it might be useful to distinguish the internal handles (and external URIs) of the Image page and of the image file. But having a dedicated meta_schema value for user-edited properties of the page might suffice to accomplish this on the DB level. I am fairly agnostic about the details, but I have a tendency to wait with developing a more sophisticated DB layout until we have some usage statistics from the initial deployment to guide us.
-- Markus
Markus Krötzsch wrote:
Hi Bawolff,
interesting project! I am currently preparing a "light" version of SMW that does something very similar, but using wiki-defined properties for adding metadata to normal pages (in essence, SMW is an extension to store and retrieve page metadata for properties defined in the wiki -- like XMP for MW pages; though our data model is not quite as sophisticated ;-).
The use cases for this light version are just what you describe: simple retrieval (select) and basic inverse searches. The idea is to thus have a solid foundation for editing and viewing data, so that more complex functions like category intersections or arbitrary metadata conjunctive queries would be done on external servers based on some data dump.
It would be great if the table you design could be used for such metadata as well. As you say, XMP already requires extensibility by design, so it might not be too much work to achieve this. SMW properties are usually identified by pages in the wiki (like categories), so page titles can be used to refer to them. This just requires that the meta_name field is long enough to hold MW page title names. Your meta_schema could be used to separate wiki properties from other XMP properties. SMW Light does not require nested structures, but they could be interesting for possible extensions (the full SMW does support one-level of nesting for making compound values).
Two things about your design I did not completely understand (maybe just because I don't know much about XMP):
(1) You use mediumblob for values. This excludes range searches for numerical image properties ("Show all images of height 1000px or more") which do not seem to be overly costly if a suitable schema were used. If XMP has a typing scheme for property values anyway, then I guess one could find the numbers and simply put them in a table where the value field is a number. Is this use case out of scope for you, or do you think the cost of reading from two tables too high? One could also have an optional helper field "meta_numvalue" used for sorting/range-SELECT when it is known from the input that the values that are searched for are numbers.
(2) Each row in your table specifies property (name and schema), type, and the additional meta_qualifies. Does this mean that one XMP property can have values of many different types and with different flags for meta_qualifies? Otherwise it seems like a lot of redundant data. Also, one could put stuff like type and qualifies into the mediumblob value field if they are closely tied together (I guess, when searching for some value, you implicitly specify what type the data you search for has, so it is not problematic to search for the value + type data at once). Maybe such considerations could simplify the table layout, and also make it less specific to XMP.
But overall, I am quite excited to see this project progressing. Maybe we could have some more alignment between the projects later on (How about combining image metadata and custom wiki metadata about image pages in queries? :-) but for GSoC you should definitely focus on your core goals and solve this task as good as possible.
Best regards,
Markus
On Freitag, 28. Mai 2010, bawolff wrote:
Hi all,
For those who don't know me, I'm one of the GSOC students this year. My mentor is ^demon, and my project is to enhance support for metadata in uploaded files. Similar to the recent thread on interwiki transclusions, I'd thought I'd ask for comments about what I propose to do.
Currently metadata is stored in img_metadata field of the image table as a serialized php array. Well this works fine for the primary use case - listing the metadata in a little box on the image description page, its not very flexible. Its impossible to do queries like get a list of images with some specific metadata property equal to some specific value, or get a list of images ordered by what software edited them.
So as part of my project I would like to move the metadata to its own table. However I think the structure of the table will need to be a little more complicated then just <page id>, <name>, <value> triples, since ideally it would be able to store XMP metadata, which can contain nested structures. XMP metadata is pretty much the most complex metadata format currently popular (for metadata stored inside images anyways), and can store pretty much all other types of metadata. Its also the only format that can store multi-lingual content, which is a definite plus as those commons folks love their languages. Thus I think it would be wise to make the table store information in a manner that is rather close to the XMP data model.
So basically my proposed metadata table looks like:
*meta_id - primary key, auto-incrementing integer *meta_page - foreign key for page_id - what image is this for *meta_type - type of entry - simple value or some sort of compound structure. XMP supports ordered/unordered lists, associative array type structures, alternate array's (things like arrays listing the value of the property in different languages). *meta_schema - xmp uses different namespaces to prevent name collisions. exif properties have their own namespace, IPTC properties have their own namespace, etc *meta_name - The name of the property *meta_value - the value of the property (or null for some compound things, see below) *meta_ref - a reference to a meta_id of a different row for nested structures, or null if not applicable (or 0 perhaps) *meta_qualifies - boolean to denote if this property is a qualifier (in XMP there are normal properties and qualifiers)
(see http://www.mediawiki.org/wiki/User:Bawolff/metadata_table for a longer explanation of the table structure)
Now, before everyone says eww nested structures in a db are inefficient and what not, I don't think its that bad (however I'm new to the whole scalability thing, so hopefully someone more knowledgeable than me will confirm or deny that).
The XMP specification specifically says that there is no artificial limit on nesting depth, however in general practise its not nested very deeply. Furthermore in most cases the tree structure can be safely ignored. Consider: *Use-case 1 (primary usecase), displaying a metadata info box on an image page. Most of the time that'd be translating specific name and values into html table cells. The tree structure is totally unnecessary. for example the exif property DateTimeOriginal can only appear once per image (also it can only appear at the root of the tree structure but thats beside the point). There is no need to reconstruct the tree, just look through all the props for the one you need. If the tree structure is important it can be reconstructed on the php side, and would typically be only the part of the tree that is relevant, not the entire nested structure. *Use-case 2 (secondary usecase). Get list of images ordered by some property starting at foo. or get list of images where property bar = baz. In this case its a simple select. It does not matter where in the tree structure the property is.
Thus, all the nestedness of XMP is preserved (So we could re-output it into xmp form if we so desired), and there is no evil joining the metadata table with itself over and over again (or at all), which from what i understand, self-joining to reconstruct nested structures is what makes them inefficient in databases.
I also think this schema would be future proof because it can store pretty much all metadata we can think of. We can also extend it with custom properties we make up that are guaranteed to not conflict with anything (The X in xmp is for extensible).
As a side-note, based on my rather informal survey of commons (aka the couple people who happened to be on #wikimedia-commons at that moment) another use-case people think would be cool and useful is metadata intersections, and metadata-category intersections. I'm not planning to do this as part of my project, as I believe that would have performance issues. However doing a metadata table like this does leave the possibility open for people to do such intersection things on the toolserver or in a DPL-like extension.
I'd love to get some feedback on this. Is this a reasonable approach for me to take on this.
Thanks for reading.
-- -bawolff
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