Difference between revisions of "Databases"

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<syntaxhighlight lang="xquery">
 
<syntaxhighlight lang="xquery">
db:add('database', '/path/to/xml/documents/')
+
db:add('database', '/path/to/xml/resources/')
 
</syntaxhighlight>
 
</syntaxhighlight>
  
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<syntaxhighlight lang="xquery">
 
<syntaxhighlight lang="xquery">
 
let $db := 'documents'
 
let $db := 'documents'
let $root := '/path/to/documents/'
+
let $root := '/path/to/resources/'
 
for $path in file:list($root)
 
for $path in file:list($root)
 
where matches($path, '\d+')
 
where matches($path, '\d+')
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<syntaxhighlight lang="xquery">
 
<syntaxhighlight lang="xquery">
let $failures := db:get-value('misc')
+
let $failures := db:get-value('resources')
 
return map:for-each($failures, function($filename, $error) {
 
return map:for-each($failures, function($filename, $error) {
 
   $filename || ': ' || $error
 
   $filename || ': ' || $error

Revision as of 13:39, 1 August 2022

This page is part of the Getting Started Section.

In BaseX, a database is a pretty light-weight concept. It may contain one or more resources, which are addressed by a unique database path. There is no explicit layer for collections: Instead, collections are implicitly created and deleted, and collections result from the existence of documents in specific paths.

As a single database is restricted to 2 billion XML nodes (see Statistics), but resources can easily be distributed across multiple database instances. Multiple databases can be addressed (queried, updated) by a single XQuery expression.

Three different resource types exist:

Resource Type Description
XML Documents The default resource type. The storage and index features are optimized for XML contents, or any other contents stored in an XML representation.
Binary Data Binary data: Raw data of any type, stored in its binary representation. See Binary Data for more information.
XQuery Values Introduced with Version 10: Results of XQuery expressions, stored in a binary representation for fast retrieval. All value types are supported, including maps and arrays, but excluding any other function items.

Create Databases

Databases can be created via Commands, via XQuery, in the GUI, and with various APIs. If an initial input is specified with a create operation, some time can be saved, as the specified resources will be added to the database in a bulk operation:

  • Command-Line: CREATE DB documents /path/to/resources: Add resources in the specified path to a database named documents.
  • GUI: Go to DatabaseNew, press Browse… to choose an initial file or directory, and press OK.

The database name is composed of a restricted set of characters (see Valid Names). Various Parsers can be selected to control the import process, or to convert data of different input type to XML.

Access Resources

Stored resources and external documents can be accessed in different ways:

XML Documents

Various XQuery functions exist to access XML documents in databases:

Function Example Description
db:get db:get("db", "path/to/docs") Returns all documents that are found in the database db at the (optional) path path/to/docs.
fn:collection collection("db/path/to/docs") Returns all documents at the location path/to/docs in the database db.
If no path is specified after the database, all documents in the database will be returned.
If no argument is specified, all documents of the database will be returned that has been opened in the global context.
fn:doc doc("db/path/to/doc.xml") Returns the document at the location path/to/docs in the database db.
An error is raised if the specified yields zero or more than one document.

You can access multiple databases in a single query:

for $i in 1 to 100
return db:get('books' || $i)//book/title

If the DEFAULTDB option is turned on, the path argument of the fn:doc or fn:collection functions will first be resolved against the globally opened database.

Two more functions are available for retrieving information on database nodes:

Function Example Description
db:name db:name($node) Returns the name of the database in which the specified $node is stored.
db:path db:path($node) Returns the path of the database document in which the specified $node is stored.

The fn:document-uri and fn:base-uri functions return URIs that can also be reused as arguments for the fn:doc and fn:collection functions. As a result, the following example query always returns true:

every $c in collection('anyDB')
satisfies doc-available(document-uri($c))

If the argument of fn:doc or fn:collection does not start with a valid database name, or if the addressed database does not exist, the string is interpreted as URI reference, and the documents found at this location will be returned. Examples:

  • doc("http://web.de"): retrieves the addressed URI and returns it as a main-memory document node.
  • doc("myfile.xml"): retrieves the given file from the file system and returns it as a main-memory document node. Note that updates to main-memory nodes are not automatically written back to disk unless the WRITEBACK option is set.
  • collection("/path/to/docs"): returns a main-memory collection with all XML documents found at the addressed file path.

Binary Data

The BINARY GET command and the db:get-binary function can be used to return files in their native byte representation.

If the API you use does not support binary output (which is e.g. the case for various Client language bindings), you can convert your binary data to its string representation before returning it to the client:

string(db:get-binary('multimedia', 'sample.avi'))

XQuery Values

With db:get-value, XQuery values can be retrieved. In the following example, we assume that an XQuery map cities was stored in an indexes database:

let $city-map := db:get-value('indexes', 'cities')
return $city-map?Chile

Update Resources

Commands

Once you have created a database, additional commands exist to modify its contents:

  • XML documents can be added with the PUT and ADD commands.
  • Binary data is stored with BINARY PUT.
  • Resources of all types can be deleted via DELETE.

AUTOFLUSH can be turned off before bulk operations (i.e., before numerous new resources are added to the database).

If ADDCACHE is enabled, the input will be cached before it is added to the database. This is helpful when the input documents are expected to consume too much main-memory.

With the following command script, an empty database is created, two resources are added (one directly, another one cached), and all data is exported to the file system:

CREATE DB example
SET AUTOFLUSH false
ADD example.xml
SET ADDCACHE true
ADD /path/to/xml/documents
BINARY PUT TO images/ 123.jpg
EXPORT /path/to/file-system/

XQuery

You can also use functions from the Database Module to add, replace, or delete XML documents:

db:add('database', '/path/to/xml/resources/')

Function from other modules, such as the File Module, can be utilized to filter the input. With the following code, all files that contain numbers in the filename are selected, and stored as XML. If an input file contains no well-formed XML, it is stored as binary resource, and the error message is stored as a string value:

let $db := 'documents'
let $root := '/path/to/resources/'
for $path in file:list($root)
where matches($path, '\d+')
return try {
  db:put($db, fetch:doc($root || $path), $path)
} catch * {
  db:put-binary($db, $root || $path, $path),
  db:put-value($db, $err:description, $path || '.error')
}

The error messages can e.g. be analyzed in a second step:

let $failures := db:get-value('resources')
return map:for-each($failures, function($filename, $error) {
  $filename || ': ' || $error
})

Export Data

All resources stored in a database can be exported, i.e., written back to disk, e.g., as follows:

  • Commands: EXPORT writes all resources to the specified target directory.
  • GUI: Go to DatabaseExport, choose the target directory and press OK.
  • XQuery: Use db:export.
  • WebDAV: Locate the database directory (or a subdirectory of it) and copy all contents to another location.

Main-Memory Database Instances

A database can be created in main-memory by enabling the MAINMEM option. Next,

  • in the standalone context, a main-memory database can be created (using CREATE DB), which can then be accessed by subsequent commands.
  • If a BaseX server instance is started, and if a database is created in its context at startup time (e.g., with the Template:Command-Line Options), other BaseX client instances can access and update this database.
  • You can force an ordinary database, or parts of it, to being temporarily copied to memory by applying an empty main-memory update on a database node: db:get('some-db') update { }

Note: If you open local or remote documents with fn:doc or fn:collection, the resulting internal representation is identical to those of main-memory database instances (regardless of which value is set for MAINMEM).

Changelog

Version 10.0
  • Added: New resource type for XQuery values.
Version 8.4
  • Updated: Raw Files: Items of binary type can be output without specifying the obsolete raw serialization method.
Version 7.2.1
  • Updated: fn:document-uri and fn:base-uri now return strings that can be reused with fn:doc or fn:collection to reopen the original document.