Difference between revisions of "Lazy Module"
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* if an item is streamable, its value will be retrieved, and a new item containing the value will be returned. | * if an item is streamable, its value will be retrieved, and a new item containing the value will be returned. | ||
* other, non-streamable items will simply be passed through. | * other, non-streamable items will simply be passed through. | ||
− | Materialization is helpful if a value is to be processed more than once. It is even mandatory if | + | Materialization is helpful if a value is to be processed more than once. It is even mandatory if a value will be invalidated before it is processed (see the example below). |
|- | |- | ||
| '''Example''' | | '''Example''' |
Revision as of 22:45, 12 March 2013
This XQuery Module contains functions for handling streamable items.
In contrast to standard XQuery items, a streamable item only contains 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:
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 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($item as item()) as item()
|
Summary | Returns a materialized instance of the specified $item :
Materialization is helpful if a value is to be processed more than once. It is even mandatory if a value will be invalidated before it is processed (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 first 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 if the specified $item is streamable.
|
Changelog
This module was introduced with Version 7.7.