Difference between revisions of "Databases"

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This page is part of the [[Getting Started]] Section.
 
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. Resources can either be '''XML documents''' or '''raw files''' (binaries). Some information on [[Binary Data|binary data]] can be found on an extra page.
+
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. The resource types exist: '''XML documents''', '''Binary data''', and '''XQuery Values'''. Additional information on [[Binary Data|binary data]] can be found on an extra page.
  
Multiple databases can be addressed (queries, updated) with a single XQuery expression. As a single database is restricted to 2 billion nodes (see [[Statistics]]), resources can be distributed across multiple database instances.
+
Multiple databases can be addressed (queried, updated) with a single XQuery expression. As a single database is restricted to 2 billion XML nodes (see [[Statistics]]), resources can be distributed across multiple database instances.
  
 
=Create Databases=
 
=Create Databases=
  
Databases can be created via commands, via XQuery, in the GUI, or with any of our [[Developing|APIs]]. If an initial input is specified with create, some time can be saved, as the specified resources will be added to the database in a bulk operation:
+
Databases can be created via [[Commands]], via [[XQuery]], in the [[GUI]], and with various [[Developing|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:
  
* [[Startup#BaseX Standalone|Console]]: <code>CREATE DB db /path/to/resources</code> will add initial documents to a database
+
* [[Startup#BaseX Standalone|Console]]: <code>CREATE DB db /path/to/resources</code>: Add documents in the specified path to a database.
* [[Startup#BaseX GUI|GUI]]: Go to ''Database'' → ''New'', press ''Browse'' to choose an initial file or directory, and press ''OK''
+
* [[Startup#BaseX GUI|GUI]]: Go to ''Database'' → ''New'', press ''Browse…'' to choose an initial file or directory, and press ''OK''.
  
The name of a database is restricted to a restricted set of characters (see [[Valid Names]]). Various [[parsers]] can be chosen to control the import process, or to convert different formats to XML.
+
The name of a database is composed of a restricted set of characters (see [[Valid Names]]). Various [[parsers]] can be selected to control the import process, or to convert different formats to XML.
  
'''Note:''' A main-memory database will be created if the {{Option|MAINMEM}} option is enabled ([[Databases#In Memory Database|see below]] for more).
+
'''Note:''' A database can be created in main-memory by enabling the {{Option|MAINMEM}} option ([[Databases#In Memory Database|see below]] for more details).
  
 
=Access Resources=
 
=Access Resources=
Line 82: Line 82:
 
* {{Code|collection("/path/to/docs")}}: returns a main-memory collection with all XML documents found at the addressed file path.
 
* {{Code|collection("/path/to/docs")}}: returns a main-memory collection with all XML documents found at the addressed file path.
  
==Binary Files==
+
==Binary Data==
  
The {{Command|RETRIEVE}} command and the {{Function|Database|db:get-binary}} function can be used to return files in their native byte representation.
+
The {{Command|BINARY GET}} command and the {{Function|Database|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 (this is e.g. the case for various [[Clients|Client]] language bindings), you need to convert your binary data to its string representation before returning it to the client:
+
If the API you use does not support binary output (which is e.g. the case for various [[Clients|Client]] language bindings), you can convert your binary data to its string representation before returning it to the client:
  
 
<syntaxhighlight lang="xquery">
 
<syntaxhighlight lang="xquery">
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</syntaxhighlight>
 
</syntaxhighlight>
  
==HTTP Services==
+
==XQuery Values==
  
* With [[REST]] and [[WebDAV]], all database resources can be requested in a uniform way, no matter if they are well-formed XML documents or binary files.
+
With {{Function|Database|db:get-value}}, XQuery values can be retrieved. In the following example, we assume that an XQuery map {{Code|cities}} was stored in an {{Code|indexes}} database:
 +
 
 +
<syntaxhighlight lang="xquery">
 +
let $city-map := db:get-value('indexes', 'cities')
 +
return $city-map?Chile
 +
</syntaxhighlight>
  
 
=Update Resources=
 
=Update Resources=
 +
 +
==Commands==
  
 
Once you have created a database, additional commands exist to modify its contents:
 
Once you have created a database, additional commands exist to modify its contents:
  
* XML documents can be added with the {{Command|ADD}} command.
+
* XML documents can be added with the {{Command|PUT}} and {{Command|ADD}} commands.
* Raw files are added with {{Command|STORE}}.
+
* Binary data is stored with {{Command|BINARY PUT}}.
* Existing resources can be replaced with the {{Command|PUT}} command.
+
* Resources of all types can be deleted via {{Command|DELETE}}.
* Resources can be deleted via {{Command|DELETE}}.
 
  
The {{Option|AUTOFLUSH}} option can be turned off before ''bulk operations'' (i.e. before a large number of new resources is added to the database).
+
{{Option|AUTOFLUSH}} can be turned off before ''bulk operations'' (i.e., before numerous new resources are added to the database).
  
If {{Option|ADDCACHE}} is enabled, the input will be cached before it is added to the database. This is helpful when the input documents to be added are expected to consume too much main memory.
+
If {{Option|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.
  
The following commands create an empty database, add two resources, explicitly flush data structures to disk, and finally delete all inserted data:
+
With the following [[Commands#Command Scripts|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:
  
 
<pre>
 
<pre>
Line 117: Line 123:
 
SET ADDCACHE true
 
SET ADDCACHE true
 
ADD /path/to/xml/documents
 
ADD /path/to/xml/documents
STORE TO images/ 123.jpg
+
BINARY PUT TO images/ 123.jpg
FLUSH
+
EXPORT /path/to/file-system/
DELETE /
 
 
</pre>
 
</pre>
  
You may also use the BaseX-specific [[Database Module|XQuery Database Functions]] to create, add, replace, and delete XML documents:
+
==XQuery==
 +
 
 +
You can also use functions from the [[Database Module]] to add, replace, or delete XML documents:
  
 
<syntaxhighlight lang="xquery">
 
<syntaxhighlight lang="xquery">
let $root := "/path/to/xml/documents/"
+
db:add('database', '/path/to/xml/documents/')
for $file in file:list($root)
 
return db:add("database", $root || $file)
 
 
</syntaxhighlight>
 
</syntaxhighlight>
  
Last but not least, XML documents can also be added via the GUI and the ''Database'' menu.
+
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:
 +
 
 +
<syntaxhighlight lang="xquery">
 +
let $db := 'documents'
 +
let $root := '/path/to/documents/'
 +
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 || '.info')
 +
}
 +
</syntaxhighlight>
 +
 
 +
The error messages can e.g. be analyzed in a second step:
 +
 
 +
<syntaxhighlight lang="xquery">
 +
let $failures := db:get-value('misc')
 +
return map:for-each($failures, function($filename, $error) {
 +
  $filename || ': ' || $error
 +
})
 +
</syntaxhighlight>
  
 
=Export Data=
 
=Export Data=
  
All resources stored in a database can be ''exported'', i.e., written back to disk. This can be done in several ways:
+
All resources stored in a database can be ''exported'', i.e., written back to disk, e.g., as follows:
  
* Commands: {{Command|EXPORT}} writes all resources to the specified target directory
+
* Commands: {{Command|EXPORT}} writes all resources to the specified target directory.
* GUI: Go to ''Database'' → ''Export'', choose the target directory and press ''OK''
+
* GUI: Go to ''Database'' → ''Export'', choose the target directory and press ''OK''.
* WebDAV: Locate the database directory (or a subdirectory of it) and copy all contents to another location
+
* XQuery: Use {{Function|Database|db:export}}.
 +
* WebDAV: Locate the database directory (or a subdirectory of it) and copy all contents to another location.
  
 
=Main-Memory Database Instances=
 
=Main-Memory Database Instances=
Line 144: Line 172:
 
* In the standalone context, a main-memory database can be created (using <code>CREATE DB</code>), which can then be accessed by subsequent commands.
 
* In the standalone context, a main-memory database can be created (using <code>CREATE DB</code>), which can then be accessed by subsequent commands.
 
* If a BaseX server instance is started, and if a database is created in its context (using <code>CREATE DB</code>), other BaseX client instances can access (and update) this database (using OPEN, db:get, etc.) as long as no other database is opened/created by the server.
 
* If a BaseX server instance is started, and if a database is created in its context (using <code>CREATE DB</code>), other BaseX client instances can access (and update) this database (using OPEN, db:get, etc.) as long as no other database is opened/created by the server.
* You can force an ordinary database to being copied to memory by using <code>db:get('some-db') update {}</code>
+
* You can force an ordinary database to being copied to memory by applying an empty [[XQuery_Update#Main-Memory_Updates|main-memory update]] on a database node: <code>db:get('some-db') update { }</code>
  
'''Note:''' If you address a URI with <code>fn:doc</code> or <code>fn:collection</code> for which no database exists, the resulting internal representation is identical to those of main-memory database instances (no matter which value is set for {{Option|MAINMEM}}).
+
'''Note:''' If you open local or remote documents with <code>fn:doc</code> or <code>fn:collection</code>, the resulting internal representation is identical to those of main-memory database instances (regardless of which value is set for {{Option|MAINMEM}}).
  
 
=Changelog=
 
=Changelog=
 +
 +
;Version 10.0
 +
* Added: New resource type for XQuery values.
  
 
;Version 8.4
 
;Version 8.4
 
 
* Updated: [[#Raw Files|Raw Files]]: Items of binary type can be output without specifying the obsolete <code>raw</code> serialization method.
 
* Updated: [[#Raw Files|Raw Files]]: Items of binary type can be output without specifying the obsolete <code>raw</code> serialization method.
  
 
;Version 7.2.1
 
;Version 7.2.1
 
 
* Updated: {{Code|fn:document-uri}} and  {{Code|fn:base-uri}} now return strings that can be reused with {{Code|fn:doc}} or {{Code|fn:collection}} to reopen the original document.
 
* Updated: {{Code|fn:document-uri}} and  {{Code|fn:base-uri}} now return strings that can be reused with {{Code|fn:doc}} or {{Code|fn:collection}} to reopen the original document.

Revision as of 13:15, 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. The resource types exist: XML documents, Binary data, and XQuery Values. Additional information on binary data can be found on an extra page.

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

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:

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

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

Note: A database can be created in main-memory by enabling the MAINMEM option (see below for more details).

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/documents/')

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/documents/'
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 || '.info')
}

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

let $failures := db:get-value('misc')
return map:for-each($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

  • 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 (using CREATE DB), other BaseX client instances can access (and update) this database (using OPEN, db:get, etc.) as long as no other database is opened/created by the server.
  • You can force an ordinary database to being 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.