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By default, access to external resources (files on hard disk, HTTP requests, ...) is not controlled by the transaction monitor of BaseX. You can use custom XQuery locks to do so:===Commands===
===Option DeclarationsDatabase locking works with all commands unless the glob syntax is used, Pragmas, Function Annotations===such as in the following command call:
* You can declare custom locks via options, pragmas or function annotations.* The value of the lock may contain one or multiple lock keys (separated with commas).* Similar to the internal database locks, write locks block all other operations while read locks allow parallel access.* The internal locks and ===XQuery locks can co-exist (there will be no conflicts, even if your lock string equals the name of a database that will be locked by the transaction manager).===
Before {{Version|9Deciding which databases will be accessed by a complex XQuery expression is a non-trivial task.1}}, locks were declared in Database detection works for the {{Code|query}} namespace.following types of queries:
Local locks can also be declared via pragmas:==Pragmas==
Locks for The write locks is enforced via the functions of a module can be assigned via option declarations{{Code|Update|update:output}}.
<pre class="brush:xquery">declare option basex:write-lock 'HTTPLOCK';file:write('config.xml', <config/>)</pre>=Options==
===Java Modules===Locks for the functions of a module can also be assigned via option declarations:
Locks can also be acquired on [[Java Bindings#Locking|Java functions]] which are imported and invoked from an XQuery expression. It is advisable to explicitly <syntaxhighlight lang="xquery">declare option basex:lock Java code whenever it performs sensitive read and write operations.'CONFIG';
==Limitations==update:output(file:write('config.xml', <config/>))</syntaxhighlight>
===Commands===Once again, a write lock is enforced.
Database locking works with all commands unless the glob syntax is used, such as in the following command call:==Java Modules==
* {{Code|DROP DB new*}}: drop all databases starting with "new" ===XQuery=== Deciding which databases will Locks can also be accessed by a complex XQuery expression is a non-trivial task. Database detection works for the following types of queries: * {{Code|//item}}, read-locking of the database opened by a client* {{Code|doc('factbook')}}, read-locking of "factbook"* {{Code|collection('db/path/to/docs')}}, read-locking of "db"* {{Code|fn:sum(1 to 100)}}, locking nothing at all* {{Code|delete nodes doc('test')//*[string-length(local-name(.)) > 5]}}, write-locking of "test" All databases will be locked by queries of the following kind: * {{Code|for $db in ('db1', 'db2') return doc($db)}}* {{Code|doc(doc('test')/reference/text())}}* <code>let $db := 'test' return insert nodes <test/> into doc($db)</code> You can consult the query info output (which you find in the acquired on [[GUIJava Bindings#VisualizationsLocking|Info ViewJava functions]] of the GUI or which you can turn on by setting <are imported and invoked from an XQuery expression. It is advisable to explicitly lock Java code>[[Options#QUERYINFO|QUERYINFO]]</code> to {{Code|true}}) to find out which databases have been locked by a querywhenever it performs sensitive read and write operations.
→XQuery Locks
* By default, read transactions are favored, and transactions that access no databases can be evaluated even if the transactions limit has been reached. This behavior can be changed via the {{Option|FAIRLOCK}} option.
==XQuery LocksLimitations==
* {{MarkCode|Introduced with Version 9.1DROP DB new*}}: locks via pragmas and function annotations.drop all databases starting with "new"
* {{Code|basex://item}}, read-locking of the database opened by a client* {{Code|doc('factbook')}}, read-locking of "factbook"* {{Code|collection('db/path/to/docs')}}, read-locklocking of "db"* {{Code|fn:sum(1 to 100)}} and , locking nothing at all* {{Code|basexdelete nodes db:open('test')//*[string-length(local-name(.)) > 5]}}, write-locking of "test" A global lockwill be assigned if the name of the database is not a static string: * {{Code|for $db in ('db1', 'db2') return db:open($db)}} * {{Code|doc(doc('test')/reference/text())}}* <code>let $db := 'test' return insert nodes <test/> into db:open($db)</code> The functions [[Databases#XML Documents|fn:doc]] and [[Databases#XML Documents|fn:collection]] can also be used to address that are not stored in a database. However, this may lead to unwanted locks, and you have two options to reduce the number of locks: No database lookups will take place if {{Option|WITHDB}} option is disabled, or if {{Function|Fetch|fetch:xml}} is used instead of [[Databases#XML Documents|fn:doc]]. You can consult the query info output (which you find in the [[GUI#Visualizations|Info View]] of the GUI or which you can turn on by setting {{Option|QUERYINFO}} to {{Code|true}}) to find out which databases have been locked by a query prolog . =XQuery Locks= {{Mark|Updated with Version 9.4:}} Single lock option for reads and writes. By default, access to external resources (files on hard disk, HTTP requests, ...) is not controlled by the transaction monitor of BaseX. Custom locks can be assigned via annotations, pragmas or options: * A lock string may consist of a single key or multiple keys separated with commas.* Internal locks and XQuery locks can co-exist. No conflicts arise, even if a lock string equals the name of a database that is locked by the transaction manager.* The lock is transformed into a write lock by making the corresponding expression updating. ==Annotations==
In the following module, lock annotations are used to prevent concurrent write operations on the same file:
<pre classsyntaxhighlight lang="brush:xquery">
module namespace config = 'config';
declare %basex:read-lock('CONFIG') function config:read() as xs:string {
file:read-text('config.txt')
};
declare %updating %basex:write-lock('CONFIG') function config:write($dataas xs:string) {
file:write-text('config.txt', $data)
};
</presyntaxhighlight>
Some explanations:
* If a query calls the <code>config:read</code> function, a read lock will be acquired for the user-defined {{Code|CONFIG}} lock string before query evaluation.
* If <code>config:write</code> is called by a query, a write lock on this lock string will be set for this queryapplied.* If a another query calls <code>config:write</code>, it will be queued until there is no running query left that has {{Code|CONFIG}} locked.* If the writing first query will be is evaluated, all other queries that will set a {{Code|CONFIG}} lock (reading or writing) will have to wait.
Locks can also be declared via pragmas: <pre classsyntaxhighlight lang="brush:xquery">update:output((# basex:write-lock CONFIGLOCK CONFIG #) {
file:write('config.xml', <config/>)
})</presyntaxhighlight>
=File-System Locks=
=Changelog=
;Version 9.4
* Updated: Single lock option for reads and writes.
;Version 9.1
