Lazy Module

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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:
  • 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.

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.