Difference between revisions of "Lazy Module"

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This [[Module Library|XQuery Module]] contains functions for handling ''streamable'' items.
+
This [[Module Library|XQuery Module]] contains functions for handling ''lazy'' items.
  
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In contrast to standard XQuery items, a streamable item contains only a reference to the actual data. The data itself will be retrieved if it is requested by an expression, or if the item is to be serialized. Hence, a streamable item only uses a few bytes, and no additional memory is occupied during serialization.
+
In contrast to standard XQuery items, a lazy item contains a reference to the actual data, and the data itself will only be retrieved if it is processed. Hence, possible errors will be postponed, and no memory will be occupied by a lazy item as long as its content has not been requested yet.
  
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The following BaseX functions return streamable items:
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The following BaseX functions return lazy items:
  
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* Streamable Base64 binaries:
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* Lazy Base64 binaries:
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** <code>[[Database Module#db:retrieve|db:retrieve]]</code>
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** {{Function|Fetch|fetch:binary}}
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** <code>[[Fetch Module#fetch:binary|fetch:binary]]</code>
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** {{Function|File|file:read-binary}}
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** <code>[[File Module#file:read-binary|file:read-binary]]</code>
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** {{Function|Database|db:get-binary}}
  
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* Streamable strings:
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* Lazy strings:
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** <code>[[Fetch Module#fetch:text|fetch:text]]</code>
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** {{Function|Fetch|fetch:text}}
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** <code>[[File Module#file:read-text|file:read-text]]</code>
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** {{Function|File|file:read-text}}
  
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Some functions are capable of consuming items in a ''streamable'' fashion: data will never be cached, but instead passed on to another target (file, the calling expression, etc.). The following streaming functions are currently available:
+
Some functions are capable of consuming the contents of lazy items in a ''streamable'' fashion: data will not be cached, but instead passed on to another target (file, the calling expression, etc.). The following streaming functions are currently available:
  
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* <code>[[Conversion Module#convert:binary-to-bytes|convert:binary-to-bytes]]</code>
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* [[Archive Module]] (most functions)
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* <code>[[Database Module#db:store|db:store]]</code>
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* Conversion Module: {{Function|Conversion|convert:binary-to-string}}
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* <code>[[File Module#file:write-binary|file:write-binary]]</code>
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* File Module: {{Function|File|file:write-binary-text}}, {{Function|File|file:write-text}} (if no encoding is specified)
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* <code>[[Fetch Module#file:write-text|file:write-text]]</code>
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* Database Module: {{Function|Database|db:put-binary}}
 +
* [[Hashing Module]] (all functions)
  
 
The XQuery expression below serves as an example on how large files can be downloaded and written to a file with constant memory consumption:
 
The XQuery expression below serves as an example on how large files can be downloaded and written to a file with constant memory consumption:
  
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<pre class="brush:xquery">
+
<pre lang='xquery'>
 
file:write-binary('output.data', fetch:binary('http://files.basex.org/xml/xmark111mb.zip'))
 
file:write-binary('output.data', fetch:binary('http://files.basex.org/xml/xmark111mb.zip'))
 
</pre>
 
</pre>
 +
 +
If lazy items are serialized, they will be streamed as well.
  
 
=Conventions=
 
=Conventions=
  
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All functions in this module are assigned to the {{Code|http://basex.org/modules/stream}} namespace, which is statically bound to the {{Code|stream}} prefix.<br/>
+
All functions and errors in this module are assigned to the <code><nowiki>http://basex.org/modules/lazy</nowiki></code> namespace, which is statically bound to the {{Code|lazy}} prefix.<br/>
−
All errors are assigned to the {{Code|http://basex.org/errors}} namespace, which is statically bound to the {{Code|bxerr}} prefix.
 
  
 
=Functions=
 
=Functions=
  
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==stream:materialize==
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==lazy:cache==
 +
 
 
{| width='100%'
 
{| width='100%'
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|-
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|- valign="top"
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| width='120' | '''Signatures'''
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| width='120' | '''Signature'''
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|{{Func|stream:materialize|$item as item()|item()}}
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|<pre>lazy:cache(
−
|-
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  $input  as item()*,
 +
  $lazy  as xs:boolean?  := false()
 +
) as item()*</pre>
 +
|- valign="top"
 
| '''Summary'''
 
| '''Summary'''
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|Returns a materialized instance of the specified {{Code|$item}}:<br />
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|Caches the data of lazy {{Code|$input}} items:<br/>
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* if an item is streamable, its value will be retrieved, and a new item containing the value will be returned.
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* data of lazy items are retrieved and cached inside the item.
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* other, non-streamable items will simply be passed through.
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* non-lazy items, or lazy items with cached data, are simply passed through.
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Materialization is advisable if a value is to be processed more than once, and is expensive to retrieve. It is get mandatory whenever a value is invalidated before it is requested (see the example below).
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* If {{Code|$lazy}} is set to {{Code|true()}}, caching is deferred until the data is eventually requested. Streaming will be disabled: Data will be cached before a stream is returned.
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|-
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Caching is advisable if an item is processed more than once, or if the data may not be available anymore at a later stage.
 +
|- valign="top"
 
| '''Example'''
 
| '''Example'''
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|In the following example, a file will be deleted before its content is returned. To avoid a "file not found" error, the content will first be materialized:
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|In the following example, a file is deleted before its content is returned. To avoid a “file not found” error when serializing the result, the content must be cached:
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<pre class="brush:xquery">
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<pre lang='xquery'>
 
let $file := 'data.txt'
 
let $file := 'data.txt'
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let $data := stream:materialize(file:read-text($file))
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let $text := lazy:cache(file:read-text($file))
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return (file:delete($file), $data)
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return (file:delete($file), $text)
 
</pre>
 
</pre>
 
|}
 
|}
  
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==stream:is-streamable==
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==lazy:is-lazy==
 +
 
 +
{| width='100%'
 +
|- valign="top"
 +
| width='120' | '''Signature'''
 +
|<pre>lazy:is-lazy(
 +
  $item  as item()
 +
) as xs:boolean</pre>
 +
|- valign="top"
 +
| '''Summary'''
 +
|Checks whether the specified {{Code|$item}} is lazy.
 +
|}
 +
 
 +
==lazy:is-cached==
 +
 
 
{| width='100%'
 
{| width='100%'
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|-
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|- valign="top"
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| width='120' | '''Signatures'''
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| width='120' | '''Signature'''
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|{{Func|stream:is-streamable|$item as item()|item()}}
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|<pre>lazy:is-cached(
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|-
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  $item as item()
 +
) as xs:boolean</pre>
 +
|- valign="top"
 
| '''Summary'''
 
| '''Summary'''
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|Checks whether the specified {{Code|$item}} is streamable.  
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|Checks whether the contents of the specified {{Code|$item}} are cached. The function will always return {{Code|true}} for non-lazy items.
 
|}
 
|}
  
 
=Changelog=
 
=Changelog=
 +
 +
;Version 9.1
 +
 +
* Updated: {{Function||lazy:cache}}: {{Code|$lazy}} argument added; support for sequences.
 +
 +
;Version 9.0
 +
 +
* Updated: Renamed from Streaming Module to Lazy Module.
 +
* Added: {{Function||lazy:is-cached}}
 +
 +
;Version 8.0
 +
 +
* Updated: {{Function||stream:materialize}} extended to sequences.
  
 
This module was introduced with Version 7.7.
 
This module was introduced with Version 7.7.
−
 
−
[[Category:XQuery]]
 

Latest revision as of 18:33, 1 December 2023

This XQuery Module contains functions for handling lazy items.

In contrast to standard XQuery items, a lazy item contains a reference to the actual data, and the data itself will only be retrieved if it is processed. Hence, possible errors will be postponed, and no memory will be occupied by a lazy item as long as its content has not been requested yet.

The following BaseX functions return lazy items:

Some functions are capable of consuming the contents of lazy items in a streamable fashion: data will not be cached, but instead passed on to another target (file, the calling expression, etc.). The following streaming functions are currently available:

The XQuery expression below serves as an example on how large files can be downloaded and written to a file with constant memory consumption: 

file:write-binary('output.data', fetch:binary('http://files.basex.org/xml/xmark111mb.zip'))

If lazy items are serialized, they will be streamed as well.

Conventions[edit]

All functions and errors in this module are assigned to the http://basex.org/modules/lazy namespace, which is statically bound to the lazy prefix.

Functions[edit]

lazy:cache[edit]

Signature 
lazy:cache(
  $input  as item()*,
  $lazy   as xs:boolean?  := false()
) as item()*
Summary Caches the data of lazy $input items:
  • data of lazy items are retrieved and cached inside the item.
  • non-lazy items, or lazy items with cached data, are simply passed through.
  • If $lazy is set to true(), caching is deferred until the data is eventually requested. Streaming will be disabled: Data will be cached before a stream is returned.

Caching is advisable if an item is processed more than once, or if the data may not be available anymore at a later stage.

Example In the following example, a file is deleted before its content is returned. To avoid a “file not found” error when serializing the result, the content must be cached: 
let $file := 'data.txt'
let $text := lazy:cache(file:read-text($file))
return (file:delete($file), $text)

lazy:is-lazy[edit]

Signature 
lazy:is-lazy(
  $item  as item()
) as xs:boolean
Summary Checks whether the specified $item is lazy.

lazy:is-cached[edit]

Signature 
lazy:is-cached(
  $item  as item()
) as xs:boolean
Summary Checks whether the contents of the specified $item are cached. The function will always return true for non-lazy items.

Changelog[edit]

Version 9.1
  • Updated: lazy:cache: $lazy argument added; support for sequences.
Version 9.0
  • Updated: Renamed from Streaming Module to Lazy Module.
  • Added: lazy:is-cached
Version 8.0

This module was introduced with Version 7.7.

Retrieved from "http://docs.basex.org/index.php?title=Lazy_Module&oldid=16809"