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.

Database All commands come with a detector for local locks. Global locking works with all commands unless is applied if the glob syntax is used, such as in the following command call:

* {{Code|DROP DB new*}}: drop Drop all databases starting with "the prefix string {{Code|new"}}.

Deciding which databases will be accessed Since BaseX 10, the [[BaseX 10#Compilation|lock detection has been fundamentally improved]], by a complex XQuery expression is a non-trivial tasksplitting compilation into multiple steps. 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 Local locks can be applied if it is possible after compile time to 100)}}, locking nothing at associate all* {{Code|delete nodes dbdatabase operations with static databases names:open('test')//*[string-length(local-name(.)) > 5]}}, write-locking of "test"

A global {| class="wikitable"|- valign="top"! Query! Description|- valign="top"| <code>//item</code>| Read lock will be assigned if 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 name {{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 not a static string[[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 Optimizations#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.|}

* {{Code|for $db in ('db1', 'db2') return dbA global lock will be assigned if the static detection fails:open($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{| class="wikitable"|- valign="top"! Query! Description|- valign="top"|fn<code>db:get(doc]] and [[Databases#XML Documents('test')/reference/text())</code>|fn:collection]] can also The name of the database to be opened will only be used to address that are not stored in a databaseknown at evaluation time. However, this may lead |- valign="top"| <code>(1 to unwanted locks100) ! db:get(concat('db', and you have two options to reduce the number of locks: No database lookups will take place if .))</code>| The {{Option|WITHDBUNROLLLIMIT}} option is disabled, or if can be increased to generate 100 {{Function|FetchCode|fetchdb:xmlget}} is used instead of [[Databases#XML Documentsfunction calls and corresponding locks.|fn:doc]].}

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 (which you find in via the [[GUI#Visualizations|Info View]] of the GUI or which you can turn on by setting , via {{OptionCode|QUERYINFO-V}} to on [[Command-Line]] or via turning on the {{CodeOption|trueQUERYINFO}}option) to find out which databases have been 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 Custom locks can use custom XQuery locks to do sobe assigned via annotations, pragmas or options:

==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, Pragmas, Annotations==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.

<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 #) {

})</syntaxhighlightpre>

Locks for The write locks is enforced via the functions of a module can be assigned via option declarations{{Code|Update|update:output}}.

==Options== Locks for the functions of a module can also be assigned via option declarations: <syntaxhighlight pre lang="'xquery"'>declare option basex:write-lock 'CONFIG'; update:output(file:write('config.xml', <config/>))</syntaxhighlightpre> Once again, a write lock is enforced.