Lazy Module
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 the value needs to be retrieved before other actions take place that invalidate the resource, such as the deletion of an addressed file. |
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.