Changes

* An ''unexpected abort'' of the server during a transaction, caused by a hardware failure or power cut, may lead to an inconsistent database state if a transaction was active at shutdown time. So it It is advisable to use the [[Commands#{{Command|CREATE BACKUP|BACKUP]] }} command to regularly backup back up your database. If the worst case occurs, you can try the [[Commands#INSPECT{{Command|INSPECT]] }} command to check if your database has obvious inconsistencies, and use [[Commands#RESTORE{{Command|RESTORE]] }} to restore the last backed up version of the database.

==Limitations== ===Commands=== All commands come with a detector for local locks. Global locking is applied if the glob syntax is used: * {{Code|DROP DB new*}}: Drop all databases starting with the prefix string {{Code|new}}. ===XQuery=== Since BaseX 10, the [[BaseX 10#Compilation|lock detection has been fundamentally improved]], by splitting compilation into multiple steps. Local locks can be applied if it is possible after compile time to associate all database operations with static databases names: {| class="wikitable"|- valign="top"! Query! Description|- valign="top"| <code>//item</code>| Read lock of the currently opened database|- valign="top"| <code>doc('factbook')</code>| Read lock of the {{Code|factbook}} database|- valign="top"| <code>collection('documents/path/to/docs')</code>| Read lock of the {{Code|documents}} database|- valign="top"| <code>delete nodes db:get('test')//*[@type = 'misc']</code>| Write lock of the {{Code|test}} database|- valign="top"| <code>declare variables $db external;<br/>db:get($db)</code>| Read lock of the database externally bound to {{Code|$db}}.|- valign="top"| <code>for $db in ('db1', 'db2')<br/>return db:get($db)</code>| Read lock of {{Code|db1}} and {{Code|db2}}, as the query is [[XQuery Optimizations#Loop Unrolling|unrolled at compile time]].|- valign="top"| <code>let $db := 'test'<br/>return insert nodes <test/> into db:get($db)</code>| Read lock of {{Code|test}}, as the [[XQuery LocksOptimizations#Variable_Inlining|variable is inlined]] at compile time.|- valign="top"| <code>sum(1 to 100)</code>| No lock required|- valign="top"| <code>declare variable $SIMULATE := true();<br/>if($SIMULATE) then <doc/> else db:get('doc')</code>| No lock required, as the query is simplified to {{Code|<doc/>}} at compile time.|} A global lock will be assigned if the static detection fails: {| class="wikitable"|- valign="top"! Query! Description|- valign="top"| <code>db:get(doc('test')/reference/text())</code>| The name of the database to be opened will only be known at evaluation time.|- valign="top"| <code>(1 to 100) ! db:get(concat('db', .))</code>| The {{Option|UNROLLLIMIT}} can be increased to generate 100 {{Code|db:get}} function calls and corresponding locks.|} The functions {{Code|fn:doc}} and {{Code|fn:collection}} can be used for both accessing databases resources and fetching resources at the specified URI (see [[Databases#Access Resources|Access Resources]] for more details). There are two ways to reduce the number of locks: # Turn off the {{Option|WITHDB}} option to prevent the functions from accessing databases; or# use {{Function|Fetch|fetch:doc}} for fetching resources from URIs, and use {{Function|Database|db:get}} for accessing databases. You can consult the query info output (via the [[GUI#Visualizations|Info View]] of the GUI, via {{Code|-V}} on [[Command-Line]] or via turning on the {{Option|QUERYINFO}} option) to find out which databases are locked by a query, and if local locks or a global lock is applied.

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:Locks=

===OptionsBy default, Pragmasaccess to external resources (files on hard disk, Annotations===HTTP requests, …) is not controlled by the transaction monitor of BaseX. Custom locks can be assigned via annotations, pragmas or options:

* You can declare custom read and write locks via options, pragmas or function annotations.* The value of the A lock string may contain one consist of a single key or multiple lock keys (separated with commas). The default value is an empty string.* Similar to the internal database locks, write locks block all other operations while read locks allow parallel access.* The internal Internal locks and XQuery locks can co-exist (there will be no . No conflictsarise, even if your a lock string equals the name of a database that will be is locked by the transaction manager).* The lock is transformed into a write lock by making the corresponding expression updating. ==Annotations==

<syntaxhighlight pre lang="'xquery"'>

declare %basex:read-lock('CONFIG') function config:read() as xs:string {

declare %updating %basex:write-lock('CONFIG') function config:write($dataas xs:string) {

</syntaxhighlightpre>

* If a query calls the a reading <code>configbasex:readlock</code> function, a read lock will be acquired for the user-defined {{Code|CONFIG}} lock string before query evaluation.* If an updating <code>configbasex:writelock</code> is called by a query, a write lock on this lock string will be set for this queryapplied.* If a another query calls a <code>configbasex:writelock</code>function, 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: <syntaxhighlight pre lang="'xquery"'>update:output((# basex:write-lock CONFIG #) {

}</syntaxhighlight> Locks for the functions of a module can be assigned via option declarations: <syntaxhighlight lang="xquery">declare option basex:write-lock 'CONFIG';file:write('config.xml', <config/>)</syntaxhighlightpre>

<pre lang===Commands==='xquery'>declare option basex:lock 'CONFIG';

Database locking works with all commands unless the glob syntax is usedupdate:output(file:write('config.xml', such as in the following command call:<config/>))</pre>

===XQuery=Java Modules==

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 db:open('test')//*[string-length(local-name(.)) > 5]}}, write-locking of "test" A global lock will 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 acquired on [[DatabasesJava Bindings#XML DocumentsLocking|fn:docJava functions]] which are imported and [[Databases#XML Documents|fn:collection]] can also be used to address that are not stored in a databaseinvoked from an XQuery expression. However, this may lead It is advisable to unwanted locks, explicitly lock Java code whenever it performs sensitive read 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 querywrite operations.