Lazy Module

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:
  • db:retrieve
  • fetch:binary
  • file:read-binary

• Streamable strings:
  • fetch:text
  • file:read-text

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:

• convert:binary-to-bytes
• db:store
• file:write-binary
• file:write-text

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

Template:Mark: Extended to sequences.

Signatures	stream:materialize($value as item()*) as item()*
Summary	Returns a materialized instance of the specified $value: 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 advisable if a value is to be processed more than once, and is expensive to retrieve. It is get mandatory whenever a value is invalidated before it is requested (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 whether the specified $item is streamable.

Changelog

Version 8.0

• Update: stream:materialize extended to sequences.

This module was introduced with Version 7.7.