9 Jan
2012
9 Jan
'12
11:36 a.m.
On Mon, Jan 9, 2012 at 11:23, Vincent Massol <vincent(a)massol.net> wrote:
I am myself on MyISAM since long, since there is other drawback using
InnoDB.
I do not experience much issue with corruption up to now. So you could
expect other to have similar setup.
On Jan 9, 2012, at 11:09 AM, Denis Gervalle wrote:
I think we should have one transaction per document update instead. We've
had this problem in the past when upgrading very large systems. The
migration was never going through in one go for some reason which I have
forgotten so we had needed to use several tx so that the migrations could
be restarted when it failed and so that it could complete.
This could be done easily if you want it so. Just note that all other
migration are single transaction based AFAICS.
On Mon, Jan 9, 2012 at 10:07, Vincent Massol
<vincent(a)massol.net> wrote:
since
+1 with the following caveats:
* We need to guarantee that a migration cannot corrupt the DB.
The evolution of the migration was the first steps in that procedure, accessing a DB with an inappropriate XWiki core
could have corrupt it.
> For example imagine that we change a document id but this id is also
used
in some
other tables and the migration stops before it's changed in the
other tables. The change needs to be done in transactions for each doc
being changed across all tables.
That would be nice, but MySQL does not support transaction on ISAM table.
I use a single transaction for the whole migration process, so on systems
that support it (Oracle ?), there will be migration or not. But I could
not
secure MySQL better that it is possible to.
It should work fine on MySQL with InnoDB which recommend (see
http://platform.xwiki.org/xwiki/bin/view/AdminGuide/InstallationMySQL).
Thanks
-Vincent
each
you
like
--
Denis Gervalle
SOFTEC sa - CEO
eGuilde sarl - CTO
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Denis Gervalle
SOFTEC sa - CEO
eGuilde sarl - CTO
> Said differently the migrator should be
allowed to be ctrl-c-ed at any
> time and you safely restart xwiki and the migrator will just carry on
from
where it
was.
The migrator will restart were it left-off, but the granularity is the
document. I proceed the updates by documents, updating all tables for one. If there is some issue during the migration
let say on MySQL, and it
is restarted, it will start again skipping documents that have been
converted previously. So the corruption could be limited to a single
document.
> * OR we need to have a configuration parameter for deciding to run this
> migration or not so that users run it only when they decide thus
ensuring
that
they've done the proper backups and saving of DBs.
This is true using the new migration procedure, but not as flexible as seems to expect. Supporting two hashing algorithm
is not a feature, but
an
augmented risk of causing corruption for me.
Now, if you use a recent core, that use new id, and on the other side,
you
have not activated migrations and access an old
db, you will simply be
unable to access the database. You will receive a "db require migration"
exception.
Anyway, migration are disable by default, and should be enabled by an
administrator in xwiki.cfg. The release notes will mention the needs to
proceed to it, and of course, to make a backup before. And you are always
supposed to have backup when you upgrade, or you are not a system admin
;)
I prefer the first option but we need to
guarantee it.
We will never be able to guarantee it, but I have done my best to have it
the most secure.
>
> Thanks
> -Vincent
>
> On Jan 7, 2012, at 10:39 PM, Denis Gervalle wrote:
>
>> Now that the database migration mechanism has been improved, I would >> 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,
--
Denis Gervalle
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