Difference between revisions of "Lazy Module"
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| '''Summary''' | | '''Summary''' | ||
|Returns a materialized instance of the specified {{Code|$value}}:<br /> | |Returns a materialized instance of the specified {{Code|$value}}:<br /> | ||
− | * if an item is streamable, its value will be retrieved | + | * if an item is streamable, its value will be retrieved and cached. |
* other, non-streamable items will simply be passed through. | * other, non-streamable items will simply be passed through. | ||
− | Materialization is advisable if | + | Materialization of a streamable value is advisable if it may not be available anymore when it will eventually be required (see the example below). |
|- | |- | ||
| '''Example''' | | '''Example''' | ||
− | |In the following example, a file will be deleted before its content is returned. To avoid a | + | |In the following example, a file will be deleted before its content is returned. To avoid a “file not found” error, the content will be materialized: |
<pre class="brush:xquery"> | <pre class="brush:xquery"> | ||
let $file := 'data.txt' | let $file := 'data.txt' |
Revision as of 21:38, 26 October 2017
This XQuery Module contains functions for handling streamable items.
In contrast to standard XQuery items, a streamable item contains only a reference to the actual data. The data itself will be retrieved if it is requested by an expression, or if the item is to be serialized. Hence, a streamable item only uses a few bytes, and no additional memory is occupied during serialization.
The following BaseX functions return streamable items:
- Streamable Base64 binaries:
- Streamable strings:
Some functions are capable of consuming items in a streamable fashion: data will never be cached, but instead passed on to another target (file, the calling expression, etc.). The following streaming functions are currently available:
The XQuery expression below serves as an example on how large files can be downloaded and written to a file with constant memory consumption:
file:write-binary('output.data', fetch:binary('http://files.basex.org/xml/xmark111mb.zip'))
Conventions
All functions in this module are assigned to the http://basex.org/modules/stream
namespace, which is statically bound to the stream
prefix.
All errors are assigned to the http://basex.org/errors
namespace, which is statically bound to the bxerr
prefix.
Functions
stream:materialize
Signatures | stream:materialize($value as item()*) as item()*
|
Summary | Returns a materialized instance of the specified $value :
Materialization of a streamable value is advisable if it may not be available anymore when it will eventually be required (see the example below). |
Example | In the following example, a file will be deleted before its content is returned. To avoid a “file not found” error, the content will be materialized:
let $file := 'data.txt' let $data := stream:materialize(file:read-text($file)) return (file:delete($file), $data) |
stream:is-streamable
Signatures | stream:is-streamable($item as item()) as item()
|
Summary | Checks whether the specified $item is streamable.
|
Changelog
- Version 8.0
- Update: stream:materialize extended to sequences.
This module was introduced with Version 7.7.