Re: [xwiki-devs] [VOTE] Change document id stored in the database to reduce the likelihood of duplicate id

Now that the database migration mechanism has been improved, I would like to go ahead with my patch to improve document ids. Currently, ids are simple string hashcode of a locally serialized document reference, including the language for translated documents. The likelihood of having duplicates with the string hashing algorithm of java is really high. What I propose is:  1) use an MD5 hashing which is particularly good at distributing.  2) truncate the hash to the first 64bits, since the XWD_ID column is a 64bit long.  3) use a better string representation as the source of hashing Based on previous discussion, point 1) and 2) has already been agreed, and this vote is in particular about the string used for 3). I propose it in 2 steps:  1) before locale are fully supported in document reference, use this format:  <lengthOfLastSpaceName>:<lastSpaceName><lengthOfDocumentName>:<documentName><lengthOfLanguage>:<language>     where language would be an empty string for the default document, so it would look like 7:mySpace5:myDoc0: and its french translation could be 7:mySpace5:myDoc2:fr  2) when locale are included in reference, we will replace the implementation by a reference serializer that would produce the same kind of representation, but that will include all spaces (not only the last one), to be prepared for the future. While doing so, I also propose to fix the cache key issue by using the same reference, but prefixed by <lengthOfWikiName>:<wikiName>, so the previous examples will have the following key in the document cache: 5:xwiki7:mySpace5:myDoc0: and 5:xwiki7:mySpace5:myDoc2:fr Using such a key (compared to the usual serialization) has the following advantages:  - ensure uniqueness of the reference without requiring a complex escaping algorithm, which is unneeded here.  - potentially reversible  - faster than the usual serialization  - support language  - independent of the current serialization that may evolved independently, so it will be stable over time which is really important when it is used as a base for the hashing algorithm used for document ids stored in the database. I would like to introduce this as early as possible, which means has soon has we are confident with the migration mechanism recently introduced. Since the migration of ids will convert 32bits hashes into 64bits ones, the risk of collision is really low, and to be careful, I have written a migration algorithm that would support such collision (unless it cause a circular reference collision, but this is really unexpected). However, changing ids again later, if we change our mind, will be really more risky and the migration difficult to implements, so it is really important that we agree on the way we compute these ids, once for all. Here is my +1,