Re: Alias handling issues

[Emmanuel Lecharny]

On 6/15/11 6:17 PM, Pierre-Arnaud Marcelot wrote:
On 15 juin 2011, at 18:01, Emmanuel Lecharny wrote:
On 6/15/11 5:19 PM, Emmanuel Lecharny wrote:
Hi,

sadly, things are not as rosy as I exepected...

I have written a few tests to check if we correctly detect cycles while 
processing a search, and at some point, I found that cycle are not an issue at 
all. But wait, it's not a good news :/

Let's say you have the following entries :

cn=test
  cn=foo,cn=test
    cn=barAlias,cn=foo,cn=test ->  cn=bar,cn=test
  cn=bar,cn=test
    cn=dohAlias,cn=foo,cn=test ->  cn=doh,cn=test
  cn=doh,cn=test

Logically, doing a SUBTREE search on cn=foo,cn=test, you should get the 
following entries :
cn=foo,cn=test
cn=bar,cn=test
cn=doh,cn=test

This is *not* what we get. Currently, you'll have :
cn=foo,cn=test
cn=bar,cn=test

The second alias dereferencing is never done.

Obviously, it helps when it come sto avoid cycle, but this is certainly not the 
expected behavior.

Now, if we want to do alias chasing on the server, we will have to modify the 
way we handle alias : each one of them will issue a new search, wth a new 
cursor.

Hopefully, stacking the aliases to be processed works well with the fact that 
we have to stack the aliases for cycle detection. One possible solution would 
be to process this stack alias after alias, and if we get back an alias, we add 
it in the stack if it's not already present (otherwise, that means we have a 
cycle).

In our sample, we will have the following stack :
() : empty stack
(cn=barAlias,cn=foo,cn=test) : first alias met
->  here, we have processed all the entries for the initial search
  [cn=foo,cn=test]
  [cn=bar,cn=test] (the dereferenced alias)
->  now, we get the leftmost element in the stack, and launch a new search
(<cn=barAlias,cn=foo,cn=test>) : this alias is being processed, thus the<>
(<cn=barAlias,cn=foo,cn=test>, cn=dohAlias,cn=foo,cn=test) : second alias met
->  again, all the entries have been processed, we take the next alias in the 
stack
  [cn=doh,cn=test]
(<cn=barAlias,cn=foo,cn=test>,<cn=dohAlias,cn=foo,cn=test>) : second alias met
->  there are no further entries. We are done

Seems to work... Did I miss something ?
Yes, I missed something (thanks Pierre-Arnaud for pointing this out) :

if the alias get dereferenced to an entry below one of the DN already stored in 
the stack, or to the descendant of one of the stored DNs, then we don't need to 
proceed the search for this alias, as the entries have already been provided. 
That will avoid sending duplicate entries to the user.
Yeah, a basic example would be to execute the same search as above but one level higher, 
from "cn=test".

In that case, the first search will already give us the three resulting entries:
- cn=foo,cn=test
- cn=bar,cn=test
- cn=doh,cn=test

And then, in the other searches, triggered when following aliases, some of 
these entries would match again. For instance:
- cn=bar,cn=test
- cn=doh,cn=test

We have to make sure that we don't send them multiple times to the user.
Maybe by storing a list of all the DNs we already returned to the user... But 
that might be a little bit overkill and suck too much memory.
By storing the root DN of each consecutive search, we will avoid such 
problem.

In this case, the initial search is done with cn=foo,cn=test, and this 
is the stored DN.
The first search will just returns the cn=foo,cn=test entry and the 
cn=barAlias,cn=foo,cn=test entry, which will be dereferenced.

This dereferemced operation consists on doing a search on the 
aliasedObject stored in the alias. As it's a new search, we store the 
base DN (cn=bar,cn=test), and we launch the new search. Etc...

Should work pretty well without having to store all the entries' DNs


--
Regards,
Cordialement,
Emmanuel Lécharny
www.iktek.com