RE: [Exist-open] The case of the disappearing node

[Gil T. <Gil...@we...>]

I suspected as much. But if the references are pointing to real-life
persistent nodes, then shouldn't those nodes be
referenced-counted/garbage-collected, i.e. live as long as somebody is
referencing them?

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________________________________

From: Wolfgang Meier [mailto:wol...@ex...]=20
Sent: Wednesday, 05 January, 2005 16:56
To: exi...@li...
Subject: Re: [Exist-open] The case of the disappearing node

=20

This isn't a bug: eXist is based on a persistent DOM, so the processed=20
nodes are life, i.e. they reference the real nodes stored in the db (the

XQuery engine internally represents persistent nodes as instances of=20
class NodeProxy, which is a pointer to the real node). The node=20
reference in $client becomes invalid after the node has been removed=20
from the database, and $client/title finds nothing.=20

To preserve the contents of a node, you have to copy it to an in-memory=20
representation as you did by casting it to string (this reminds me that=20
we need a kind of deep-copy() function for cases where you need to have=20
a real copy of a document fragment).=20

Wolfgang=20

=20

> I believe I found a bug when keeping references to deleted nodes. Let
me=20
> explain. I have this XQuery:=20
>=20
> =20
>=20
> let=20
>=20
> *  **$**client **:=3D** **//**client[**@**id **=3D**
**$**clientIdParameter]***=20
>=20
> return=20
>=20
>   if ($client) then=20
>=20
>     let $remove :=3D=20
>=20
> *      **xmldb:remove**(**concat**(**'clients/client-**',=20
> **$**client**/@**id))***=20
>=20
>     return=20
>=20
> *      **<**success title**=3D**"{$client/title}**"**/>***=20
>=20
>   else=20
>=20
>     <error>=20
>=20
>       <not-exist>=20
>=20
>         <id>{$clientIdParameter}</id>=20
>=20
>         <type>client</type>=20
>=20
>       </not-exist>=20
>=20
>     </error>=20
>=20
> =20
>=20
> This query will find a node ($client), delete the document containing=20
> it, and return a <success> with a subnode (title) of the $client node.

> Running this query returned <success title=3D"" />.=20
>=20
> =20
>=20
> I suspected that the node was deleted and somehow the reference I have

> to that node ($client) was "nulled". So I tried modifying it:=20
>=20
> =20
>=20
> let=20
>=20
>   $client :=3D //client[@id =3D $clientIdParameter],=20
>=20
>   $clientTitle :=3D $client/title=20
>=20
> return=20
>=20
>   if ($client) then=20
>=20
>     let $remove :=3D=20
>=20
>       xmldb:remove(concat('clients/client-', $client/@id))=20
>=20
>     return=20
>=20
>       <success title=3D"{$clientTitle}"/>=20
>=20
>   else=20
>=20
>     <error>=20
>=20
>       <not-exist>=20
>=20
>         <id>{$clientIdParameter}</id>=20
>=20
>         <type>client</type>=20
>=20
>       </not-exist>=20
>=20
>     </error>=20
>=20
> =20
>=20
> I added the temporary variable $clientTitle, but got the same result.=20
> So, last chance - get a /copy/ of the title:=20
>=20
> =20
>=20
> let=20
>=20
>   $client :=3D //client[@id =3D $clientIdParameter],=20
>=20
>   $clientTitle :=3D string($client/title)=20
>=20
> return=20
>=20
>   if ($client) then=20
>=20
>     let $remove :=3D=20
>=20
>       xmldb:remove(concat('clients/client-', $client/@id))=20
>=20
>     return=20
>=20
>       <success title=3D"{$clientTitle}"/>=20
>=20
>   else=20
>=20
>     <error>=20
>=20
>       <not-exist>=20
>=20
>         <id>{$clientIdParameter}</id>=20
>=20
>         <type>client</type>=20
>=20
>       </not-exist>=20
>=20
>     </error>=20
>=20
> =20
>=20
> This time it worked! By using string(...) I copied the string and now
got=20
> the result I wanted.=20
>=20
> =20
>=20
> Looks like a bug...=20
>=20
> =20
>=20
> =20
>=20
> Yours,=20
>=20
> Gil Tayar=20
>=20
> WebCollage=20
>=20
> Chief Technology Officer=20
>=20
> Business: +972 (3) 766 1806=20
>=20
> Mobile:    +972 (54) 634 4457=20
>=20
> =20
>=20

=20

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