Changes

* Locks *''cannot be synchronized* between '' across BaseX instances that run in different JVMs. If concurrent write operations are to be performed, we generally recommend working with the client/server or the HTTP architecture .* 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.

BaseX provides support for multiple read and single write operations (using preclaiming and starvation-free two phase locking). This means that read transactions are executed in parallel. If an updating transaction comes in, it will be queued and executed after all previous read transaction have been executed. Subsequent operations will also be queued until the updating transaction has completed.:

* Read transactions are executed in parallel.* If an updating transaction comes in, it will be queued and executed after all previous read transaction have been executed.* Subsequent operations (read or write) will be queued until the updating transaction has completed.* Jobs without database access will never be locked. Globally locking jobs can now be executed in parallel with non-locking jobs.* 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 can be statically determined (, i.e., before the transaction is evaluated) , which databases will be accessed by a transaction:(see [[#Limitations|below]]).* The number of maximum parallel transactions can be adjusted with the {{Option|PARALLEL}} option.* 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.

==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"| <pre classcode>//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="brushtop"| <code>delete nodes db:javaget('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}}.OPEN |- valign="top"| <code>for $db; ADD factbookin ('db1', 'db2')<br/>return db:get($db)</code>| Read lock of {{Code|db1}} and {{Code|db2}}, as the query is [[XQuery Optimizations#Loop Unrolling|unrolled at compile time]].xml; CLOSEXQUERY |- valign="top"| <code>let $db := 'test'<br/>return insert node nodes <atest/> into db:openget($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('dbdoc')</*code>| No lock required, as the query is simplified to {{Code|<doc/pre>}} at compile time.|}

In the following example, all databases will be blocked, because the name of the second database, which will be opened in the query, A global lock will only be known after having opened assigned if the first databasestatic detection fails:

<pre {| class="brush:xquerywikitable"|- valign="top">let $db1 :! Query! Description|- valign= "top"| <code>db:openget(doc('catalogtest')/reference/dbtext())</code>| The name of the database to be opened will only be known at evaluation time.|-name[@id valign= '123']"top"let $db2 := | <code>(1 to 100) ! db:openget(concat($'db', .)return delete node $db2//text()</precode>| The {{Option|UNROLLLIMIT}} can be increased to generate 100 {{Code|db:get}} function calls and corresponding locks.|}

The number of maximum parallel transactions functions {{Code|fn:doc}} and {{Code|fn:collection}} can be adjusted with used for both accessing databases resources and fetching resources at the specified URI (see [[OptionsDatabases#PARALLELAccess Resources|PARALLELAccess Resources]] optionfor more details).There are two ways to reduce the number of locks:

Access to external resources You can consult the query info output (files via the [[GUI#Visualizations|Info View]] of the GUI, via {{Code|-V}} on [[Command-Line]] or via turning on hard diskthe {{Option|QUERYINFO}} option) to find out which databases are locked by a query, HTTP requests, ...) and if local locks or a global lock is not controlled by the transaction monitor of BaseX unless specified by the userapplied.

===XQuery Locking Options==Locks=

Custom locks can be acquired By default, access to external resources (files on hard disk, HTTP requests, …) is not controlled by setting the transaction monitor of BaseX-specific XQuery options {{Code|query:read-lock}} and {{Code|query:write-lock}}. Multiple option declarations may occur in the prolog of a query, but multiple values Custom locks can also be separated with commas in a single declaration. These locks are in another namespace than the database namesassigned via annotations, pragmas or options: the lock value {{Code|factbook}} will not lock a database named factbook.

These option declarations will put read * A lock string may consist of a single key or multiple keys separated with commas.* Internal locks on ''foo''and XQuery locks can co-exist. No conflicts arise, ''bar'' and ''batz'' and 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 on ''quix'':by making the corresponding expression updating.

<pre class="brush:xquery">declare option query:read-lock "foo,bar";declare option query:read-lock "batz";declare option query:write-lock "quix";</pre>=Annotations==

Database locking works with all commands unless the glob syntax is used, such as in the following command callSome explanations:

* If a query calls a reading <code>basex:lock</code> function, a read lock will be acquired for the user-defined {{Code|DROP DB new*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: drop all databases starting with "new"lock</code> function, it will be queued until the first query is evaluated.

===XQuery=Pragmas==

As XQuery is a very powerful language, deciding which databases will Locks can also be accessed by a query is non-trivial. Optimization is work in progress.The current identification of which databases to lock is limited to queries that access the currently opened database, XQuery functions that explicitly specify a database, and expressions that address no database at all.declared via pragmas:

Some examples on database-locking enabled queries<pre lang='xquery'>update:output((# basex:lock CONFIG #) { file:write('config.xml', all of these can be executed in parallel:<config/>)})</pre>

* {{Code|//item}}, read-locking of The write locks is enforced via the database opened by a client* {{Code|doc('factbook')}}, read-locking of "factbook"* {{CodeUpdate|collection('db/path/to/docs')}}, read-locking of "db"* {{Code|fnupdate:sum(1 to 100)output}}, locking nothing at all* {{Code|delete nodes doc('test')//*[string-length(local-name(.)) > 5]}}, write-locking of "test"

* <code>let $db := 'factbook' return doc($db)</code>, will read-lock: referencing database names isn’t supported yet* {{Code|Locks for $db in ('factbook') return doc($db)}}, will read-lock globally* {{Code|doc(doc('test')/reference/text())}}, will read-lock globally* <code>let $db the functions of a module can also be assigned via option declarations:= 'test' return insert nodes <test/> into doc($db)</code>, will write-lock globally

A list of all locked databases is output if <code>[[Options#QUERYINFO|QUERYINFO]]</code> is set to {{Code|true}}. <!-- and in the GUIpre lang='xquery's [[GUI#Visualizations|Info View]] --> If you think that too much is locked, please give us a note on our [httpdeclare option basex://basex.org/open-source/ mailing list] with some example code.lock 'CONFIG';

Database locking Once again, a write lock is currently disabled if the BaseX GUI is usedenforced.

==Process LockingJava Modules==

In order to enable locking Locks can also be acquired on global (process) level, the option <code>[[OptionsJava Bindings#GLOBALLOCK|GLOBALLOCK]]</code> can be set to {{Code|true}}. This can e.g. be done by editing your {{CodeLocking|.basex}} file (see [[OptionsJava functions]] for more details)which are imported and invoked from an XQuery expression. If process locking It is active, a process that advisable to explicitly lock Java code whenever it performs sensitive read and write operations will queue all other operations.

During the term of a database update, a locking file {{Code|upd.basex}} will reside in that database directory. If the update fails for some unexpected reason, or if the process is killed ungracefully, this file may will not be deleted. In this case, the database cannot be opened anymore using the default commands, and the message "Database ... is being updated, or update was not completed" will be shown instead.  If the locking file is manually removed, you may be able to reopen the database, but you should be aware that database may have got corrupt due to the interrupted update process, and you should revert to the most recent database backup.

To avoid database corruptions that are caused by accidental write operations running in from different JVMs, a shared lock is requested on the database table file ({{Code|tbl.basex}}) whenever a database is opened. If an update operation is triggered, and if no exclusive lock can be acquired, it will be rejected with the message "Database ... is currently opened by another process." if no exclusive lock can be acquired.

As the standalone versions of BaseX (command-linePlease note that you cannot 100% rely on this mechanism, GUI) cannot as it is not possible to synchronize operations across different JVMs. You will be synchronized with other BaseX instances, we generally recommend working with safe when using the client/server or HTTP architecture if concurrent write operations are to be performed.