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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 serialized. 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:

* Lazy Base64 binaries:** <code>[[Database Module#db:retrieve|db:retrieve]]</code>** <code>[[Fetch Module#fetch:binary|fetch:binary]]</code>* <code>[[Fetch Module#fetch:text|fetch:text]]</code>* <code>[[File Module#file:read-binary|file:read-binary]]</code>* <code>[[File Module#file:read-text|file:read-text]]</code>

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 (** <code>[[Fetch Module#fetch:text|fetch:text]]</code>** <code>[[File Module#file:read-text|file, the calling expression, etc.). The following streaming functions are currently available:read-text]]</code>

* <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>

* [[Archive Module]] (most functions)* Conversion Module: <code>[[Conversion Module#convert:binary-to-bytes|convert:binary-to-bytes]]</code>, <code>[[Conversion Module#convert:binary-to-string|convert:binary-to-string]]</code>* Database Module: <code>[[Database Module#db:store|db:store]]</code>* File Module: <code>[[File Module#file:write-binary|file:write-binary]]</code>, <code>[[File Module#file:write-text|file:write-text]] </code> (if no encoding is specified)* [[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:

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== 

| width='90120' | '''Signatures'''|{{Func|streamlazy:materializecache|$items as item ()*|item()*}}<br/>{{Func|lazy:cache|$items as item()*, $lazy as xs:boolean|item()*}}

|Returns a materialized instance Caches the data of the specified lazy {{Code|$itemitems}}in a sequence:<br />* if an item is streamable, its value data of lazy items will be retrieved, and a new cached inside the item containing the value will be returned.* other, non-streamable lazy items , or lazy items with cached data, will simply be passed through.Materialization * If {{Code|$lazy}} is set to {{Code|true()}}, caching will be deferred until the data is helpful if eventually requested. Streaming will be disabled: Data will always be cached before a value stream is returned.Caching is to advisable if an item will 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.

|In the following example, a file will be 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:

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

==streamlazy:is-streamablelazy== {| width='100%'|-| width='120' | '''Signatures'''|{{Func|lazy:is-lazy|$item as item()|xs:boolean}}|-| '''Summary'''|Checks whether the specified {{Code|$item}} is lazy.|} ==lazy:is-cached== 

| width='90120' | '''Signatures'''|{{Func|streamlazy:is-streamablecached|$item as item()|item()xs:boolean}}

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