Lazy Module

This XQuery Module contains functions for handling lazy items.

In contrast to standard XQuery items, a lazy item contains a reference to the actual data, and the data itself will only be retrieved if it is processed. Hence, possible errors will be postponed, and no memory will be occupied by a lazy item as long as its content has not been requested yet.

The following BaseX functions return lazy items:

• Lazy Base64 binaries:
  • fetch:binary
  • file:read-binary
  • db:get-binary

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

Some functions are capable of consuming the contents of lazy items in a streamable fashion: data will not be cached, but instead passed on to another target (file, the calling expression, etc.). The following streaming functions are currently available:

• Archive Module (most functions)
• Conversion Module: convert:binary-to-string
• File Module: file:write-binary-text, file:write-text (if no encoding is specified)
• Database Module: db:put-binary
• Hashing Module (all functions)

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'))

If lazy items are serialized, they will be streamed as well.

Conventions

All functions and errors in this module are assigned to the http://basex.org/modules/lazy namespace, which is statically bound to the lazy prefix.

Functions

lazy:cache

Signature	lazy:cache( $input as item()*, $lazy as xs:boolean? := false() ) as item()*
Summary	Caches the data of lazy $input items: data of lazy items are retrieved and cached inside the item. non-lazy items, or lazy items with cached data, are simply passed through. If $lazy is set to true(), caching is deferred until the data is eventually requested. Streaming will be disabled: Data will be cached before a stream is returned. Caching is advisable if an item is processed more than once, or if the data may not be available anymore at a later stage.
Example	In the following example, a file is deleted before its content is returned. To avoid a “file not found” error when serializing the result, the content must be cached: let $file := 'data.txt' let $text := lazy:cache(file:read-text($file)) return (file:delete($file), $text)

lazy:is-lazy

Signature	lazy:is-lazy( $item as item() ) as xs:boolean
Summary	Checks whether the specified $item is lazy.

lazy:is-cached

Signature	lazy:is-cached( $item as item() ) as xs:boolean
Summary	Checks whether the contents of the specified $item are cached. The function will always return true for non-lazy items.

Changelog

Version 9.1

• Updated: lazy:cache: $lazy argument added; support for sequences.

Version 9.0

• Updated: Renamed from Streaming Module to Lazy Module.
• Added: lazy:is-cached

Version 8.0

• Updated: stream:materialize extended to sequences.

This module was introduced with Version 7.7.