Lazy Module
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
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
lazy:cache
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
Signature | lazy:is-lazy( $item as item() ) as xs:boolean |
Summary | Checks whether the specified $item is lazy.
|
lazy:is-cached
Signature | lazy:is-cached( $item as item() ) as xs:boolean |
Summary | Checks whether the contents of the specified $item are cached. The function will always return true for non-lazy items.
|
Changelog
- Version 9.1
- Updated:
lazy:cache
:$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.