Difference between revisions of "Lazy Module"
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This [[Module Library|XQuery Module]] contains functions for handling ''lazy'' items. | This [[Module Library|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 | + | 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: | The following BaseX functions return lazy items: | ||
Line 24: | Line 24: | ||
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: | ||
− | < | + | <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> |
If lazy items are serialized, they will be streamed as well. | If lazy items are serialized, they will be streamed as well. | ||
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{| width='100%' | {| width='100%' | ||
|- valign="top" | |- valign="top" | ||
− | | width='120' | ''' | + | | width='120' | '''Signature''' |
− | | | + | |<pre>lazy:cache( |
+ | $input as item()*, | ||
+ | $lazy as xs:boolean? := false() | ||
+ | ) as item()*</pre> | ||
|- valign="top" | |- valign="top" | ||
| '''Summary''' | | '''Summary''' | ||
− | |Caches the data of lazy {{Code|$ | + | |Caches the data of lazy {{Code|$input}} items:<br/> |
− | * data of lazy items | + | * data of lazy items are retrieved and cached inside the item. |
− | * non-lazy items, or lazy items with cached data, | + | * non-lazy items, or lazy items with cached data, are simply passed through. |
− | * If {{Code|$lazy}} is set to {{Code|true()}}, caching | + | * 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. |
− | Caching is advisable if an item | + | 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" | |- valign="top" | ||
| '''Example''' | | '''Example''' | ||
− | |In the following example, a file | + | |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: |
− | < | + | <pre lang='xquery'> |
let $file := 'data.txt' | let $file := 'data.txt' | ||
let $text := lazy:cache(file:read-text($file)) | let $text := lazy:cache(file:read-text($file)) | ||
return (file:delete($file), $text) | return (file:delete($file), $text) | ||
− | </ | + | </pre> |
|} | |} | ||
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{| width='100%' | {| width='100%' | ||
|- valign="top" | |- valign="top" | ||
− | | width='120' | ''' | + | | width='120' | '''Signature''' |
− | | | + | |<pre>lazy:is-lazy( |
+ | $item as item() | ||
+ | ) as xs:boolean</pre> | ||
|- valign="top" | |- valign="top" | ||
| '''Summary''' | | '''Summary''' | ||
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{| width='100%' | {| width='100%' | ||
|- valign="top" | |- valign="top" | ||
− | | width='120' | ''' | + | | width='120' | '''Signature''' |
− | | | + | |<pre>lazy:is-cached( |
+ | $item as item() | ||
+ | ) as xs:boolean</pre> | ||
|- valign="top" | |- valign="top" | ||
| '''Summary''' | | '''Summary''' |
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:
- Lazy Base64 binaries:
- Lazy strings:
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:
- Archive Module (most functions)
- Conversion Module:
convert:binary-to-string
- File Module:
file:write-binary-text
,file:write-text
(if no encoding is specified) - Database Module:
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:
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:
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
- Updated:
stream:materialize
extended to sequences.
This module was introduced with Version 7.7.