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This [[Module Library|XQuery Module]] contains functions for handling ''streamablelazy'' items.

In contrast to standard XQuery items, a streamable lazy item contains only a reference to the actual data. The , and the data itself will only be retrieved if it is requested by an expression, or if the item is to be serializedprocessed. Hence, a streamable item only uses a few bytespossible errors will be postponed, and no additional memory is will be occupied during serializationby a lazy item as long as its content has not been requested yet.

The following BaseX functions return streamable lazy items:

* <code>[[Database Module#dbLazy Base64 binaries:retrieve|db:retrieve]]</code>* <code>[[* {{Function|Fetch Module#fetch:binary|fetch:binary]]</code>}}* <code>[[Fetch Module#fetch:text* {{Function|fetch:text]]</code>* <code>[[File Module#file:read-binary|file:read-binary]]</code>}}* <code>[[File Module#file:read-text* {{Function|Database|filedb:readget-text]]</code>binary}}

Some functions are capable of consuming items in a ''streamable'' fashion* Lazy strings: data will never be cached, but instead passed on to another target (** {{Function|Fetch|fetch:text}}** {{Function|File|file, the calling expression, etc.). The following streaming functions are currently available:read-text}}

* <code>[[Conversion Module#convertSome functions are capable of consuming the contents of lazy items in a ''streamable'' fashion:binary-data will not be cached, but instead passed on to-bytes|convert:binary-to-bytes]]</code>* <code>[[Database Module#db:store|db:store]]</code>* <code>[[File Module#file:write-binary|file:write-binary]]</code>* <code>[[Fetch Module#file:write-text|another target (file, the calling expression, etc.). The following streaming functions are currently available:write-text]]</code>

The XQuery expression serves as an example on how large files can be downloaded and written * [[Archive Module]] (most functions)* Conversion Module: {{Function|Conversion|convert:binary-to a -string}}* File Module: {{Function|File|file with constant memory consumption:write-binary-text}}, {{Function|File|file:write-text}} (if no encoding is specified)* Database Module: {{Function|Database|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: <pre classlang="brush:'xquery"'>

All functions and errors in this module are assigned to the {{Code|<code><nowiki>http://basex.org/modules/stream}} lazy</nowiki></code> namespace, which is statically bound to the {{Code|streamlazy}} prefix.<br/>All errors are assigned to the {{Code|http://basex.org/errors}} namespace, which is statically bound to the {{Code|bxerr}} prefix.

==streamlazy:materializecache== 

|-valign="top"| width='90120' | '''SignaturesSignature'''|{{Func|stream<pre>lazy:materialize|cache( $input as item ()*, $lazy as itemxs:boolean? := false()|) as item()}}*</pre>|-valign="top"

|Returns a materialized instance Caches the data of the specified lazy {{Code|$iteminput}}items:<br />* if an item is streamable, its value will be data of lazy items are retrieved, and a new cached inside the item containing the value will be returned.* other, non-streamable lazy items will , or lazy items with cached data, are simply be passed through.Materialization * If {{Code|$lazy}} is helpful set to {{Code|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 a value an item is to be processed more than once. It is even mandatory , or if the data may not be available anymore at a value will be invalidated before it is processed (see the example below)later stage.|-valign="top"

|In the following example, a file will be is deleted before its content is returned. To avoid a "file “file not found" found” errorwhen serializing the result, the content will first must be materializedcached:<pre classlang="brush:'xquery"'>

let $data text := streamlazy:materializecache(file:read-text($file))return (file:delete($file), $datatext)

==streamlazy:is-streamablelazy== {| width='100%'|- valign="top"| width='120' | '''Signature'''|<pre>lazy:is-lazy( $item as item()) as xs:boolean</pre>|- valign="top"| '''Summary'''|Checks whether the specified {{Code|$item}} is lazy.|} ==lazy:is-cached== 

|-valign="top"| width='90120' | '''SignaturesSignature'''|{{Func|stream<pre>lazy:is-streamable|cached( $item as item()|item()}}as xs:boolean</pre>|-valign="top"

|Checks if whether the contents of the specified {{Code|$item}} is streamableare cached. The function will always return {{Code|true}} for non-lazy items.