Client Module

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This XQuery Module contains functions to access BaseX server instances from XQuery. With this module, you can execute database commands and evaluate XQuery expressions.

Please note that the client module should always be used to address independent BaseX server instances. You can create deadlocks if you evaluate a query with a server instance, and if you are addressing the same server instance in your query. See the following example:

(: Retrieve documents from database :)
let $client-id := client:connect('localhost', 1984, 'admin', 'admin')
let $docs := client:query($client-id, 'db:open("conflict")')
(: Create database with same name :)
return db:create('conflict', $docs, $docs ! db:path(.)) 

The read-only query cannot be processed, because the conflict database is currently write-locked by the main query. See Transaction Management for more background information.

Contents

[edit] Conventions

Updated with Version 9.0:

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

[edit] Functions

[edit] client:connect

Signatures client:connect($host as xs:string, $port as xs:integer, $user as xs:string, $password as xs:string) as xs:anyURI
Summary This function establishes a connection to a remote BaseX server, creates a new client session, and returns a session id. The parameter $host is the name of the database server, $port specifies the server port, and $user and $password represent the login data.
Errors connect: an error occurs while creating the session (possible reasons: server not available, access denied).

[edit] client:execute

Signatures client:execute($id as xs:anyURI, $command as xs:string) as xs:string
Summary This function executes a command and returns the result as string. The parameter $id contains the session id returned by client:connect. The $command argument represents a single command, which will be executed by the server.
Errors error: an I/O error occurs while transferring data from or to the server.
command: an error occurs while executing a command.
Examples The following query creates a new database TEST on a remote BaseX server:
client:connect('basex.server.org', 8080, 'admin', 'admin') !
  client:execute(., 'create database TEST')

[edit] client:info

Signatures client:info($id as xs:anyURI) as xs:string
Summary This function returns an information string, created by the last call of client:execute. $id specifies the session id.

[edit] client:query

Signatures client:query($id as xs:anyURI, $query as xs:string) as item()*
client:query($id as xs:anyURI, $query as xs:string, $bindings as map(*)?) as item()*
Summary Evaluates a query and returns the result as sequence. The parameter $id contains the session id returned by client:connect, and $query represents the query string, which will be evaluated by the server.
Variables and the context item can be declared via $bindings. The specified keys must be QNames or strings:
  • If a key is a QName, it will be directly adopted as variable name.
  • If a key is a string, it may be prefixed with a dollar sign. A namespace can be specified using the Clark Notation. If the specified string is empty, the value will be bound to the context item.
Errors error: an I/O error occurs while transferring data from or to the server.
query: an error occurs while evaluating a query, and if the original error cannot be extracted from the returned error string.
function: function items (including maps and arrays) cannot be returned.
Examples The following query sends a query on a local server instance, binds the integer 123 to the variable $n and returns 246:
let $c := client:connect('localhost', 1984, 'admin', 'admin')
return client:query($c, "declare variable $n external; $n * 2", map { 'n': 123 })

The following query performs a query on a first server, the results of which are passed on to a second server:

let $c1 := client:connect('basex1.server.org', 8080, 'jack', 'C0S19tt2X')
let $c2 := client:connect('basex2.server.org', 8080, 'john', '465wFHe26')
for $it in client:query($c1, '1 to 10')
return client:query($c2, $it || '* 2')

[edit] client:close

Signatures client:close($id as xs:anyURI) as empty-sequence()
Summary This function closes a client session. $id specifies the session id.
Opened connections will automatically be closed after the XQuery expression has been evaluated, but it is recommendable to explicitly close them with this function if you open many connections.
Errors error: an I/O error occurs while transferring data from or to the server.

[edit] Errors

Updated with Version 9.0:

Code Description
command An error occurred while executing a command.
connect An error occurred while creating a new session (possible reasons: server not available, access denied).
error An I/O error occurred while transferring data from or to the server.
function Function items (including maps and arrays) cannot be returned.
id The id with the specified session is unknown, or has already been closed.
query An error occurred while evaluating a query. Will only be raised if the XQuery error cannot be extracted from the returned error string.

[edit] Changelog

Version 9.0
Version 8.0
Version 7.5

The module was introduced with Version 7.3.

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