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.

Each database has its own queue: An update on database A will not block operations on database B. This is under the premise that it Local locks can be statically determined, i.e., before the transaction applied if it is evaluated, which possible after compile time to associate all database operations with static databases will be accessed by a transaction (see [[#Limitations|below]]). The number of maximum parallel transactions can be adjusted with the [[Options#PARALLEL|PARALLEL]] option.names:

With {| 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 {Version{Code|8factbook}} 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}}.6|- 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}}, locking has been improvedas 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.|}

* Jobs without 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 access to be opened will never only be known at evaluation time.|- valign="top"| <code>(1 to 100) ! db:get(concat('db', .))</code>| The {{Option|UNROLLLIMIT}} can be lockedincreased to generate 100 {{Code|db:get}} function calls and corresponding locks. Globally locking jobs |} The functions {{Code|fn:doc}} and {{Code|fn:collection}} can now be executed in parallel with non-locking jobsused 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: * A # Turn off the {{Option|FAIRLOCKWITHDB}} option has been addedto prevent the functions from accessing databases; or# use {{Function|Fetch|fetch: By default, read transactions will now be favoreddoc}} for fetching resources from URIs, and transactions that access no use {{Function|Database|db:get}} for accessing databases . You can be evaluated even 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 the transactions limit has been reachedlocal locks or a global lock is applied.

==XQuery Locks==

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:

In the following module, a write lock is declared annotations are used to avoid multiple clients writing to a config file at prevent concurrent write operations on the same timefile:

<pre classlang="brush:'xquery"'>

(:~ Write lock for all functions of the module. :)declare option query%basex:write-lock "files";(:~ Config file. :)declare variable $config:file := 'config.txtCONFIG'; declare ) function config:writeread($config) as xs:string { file:writeread-text($config:file, $'config.txt')

declare %updating %basex:lock('CONFIG') function config:readwrite($data as xs:string) { file:readwrite-text('config.txt', $config:filedata)

===Java Modules===Some explanations: * If a query calls a reading <code>basex:lock</code> function, a read lock will be acquired for the user-defined {{Code|CONFIG}} lock string before query evaluation.* If an updating <code>basex:lock</code> is called by a query, a write lock will be applied.* If another query calls a <code>basex:lock</code> function, it will be queued until the first query is evaluated.

<pre lang===Commands==='xquery'>update:output((# basex:lock CONFIG #) { file:write('config.xml', <config/>)})</pre>

Database locking works with all commands unless the glob syntax The write locks is used, such as in enforced via the following command call{{Code|Update|update:output}}.

Deciding which databases will be accessed by a complex XQuery expression is a non-trivial task. Database detection works for the following types of queries<pre lang='xquery'>declare option basex:lock 'CONFIG';

* {{Code|//item}}, read-locking of the database opened by a client* {{Code|docupdate:output(file:write('factbookconfig.xml')}}, read-locking of "factbook"* {{Code|collection('db<config/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(.)) pre> 5]}}, write-locking of "test"

* {{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>=Java Modules==

You Locks can consult the query info output (which you find in the also be acquired on [[GUIJava Bindings#VisualizationsLocking|Info ViewJava functions]] of the GUI or which you can turn on by setting <code>[[Options#QUERYINFO|QUERYINFO]]</are imported and invoked from an XQuery expression. It is advisable to explicitly lock Java code> to {{Code|true}}) to find out which databases have been locked by a querywhenever it performs sensitive read and write operations.