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The BaseX client-server architecture offers ACID -safe transactions,

=TransactionIntroduction=

Incoming requests are parsed and checked for errors on the server. If the command or query is not correct, the request will not be executed,and the user will receive an error message. Otherwise the request becomes a transaction and gets into the transaction monitor.

NotePlease note that:An unexpected abort of the server during a transaction, caused by a hardwarefailure or power cut, may lead to an inconsistent database state if a transaction was active at the shutdown time. So we advise to usethe [[Commands#CREATE BACKUP|BACKUP]] command to backup your database regularly. If the worst case occurs, you can try the [[Commands#INSPECT|INSPECT]] command to check if your database has obvious inconsistencies, and [[Commands#RESTORE|RESTORE]] to restore a previous version of the database.

* Locks ''cannot be synchronized'' 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 is advisable to use the [[Commands#CREATE BACKUP|BACKUP]] command to regularly backup your database. If the worst case occurs, you can try the [[Commands#INSPECT|INSPECT]] command to check if your database has obvious inconsistencies, and use [[Commands#RESTORE|RESTORE]] to restore the last backed up version of the database. ==XQuery Update Transactions==

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 (read or write) will be queued until the updating transaction has completed. Each database has its own queue: An update on database level. Writing transactions do A will not necessarily block all other transactions any moreoperations 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 limited by setting adjusted with the [[Options#PARALLEL|PARALLEL]] option. With {{Version|8.6}}, locking has been improved:

Deadlocks are prevented by using preclaiming two phase lockingBy default, access to external resources (files on hard disk, HTTP requests, . Execution is starvation-free as lock acquisition is queued per database. Due to the specifics of XQuery Update, all updates are written at the end of the query. Locking ) is strict with not controlled by the exception that databases for which transaction monitor of BaseX recognizes it will not write to are downgraded . You can use custom XQuery locks to read locks.do so:

Access In the following two example modules, locks have been added to external resources (files prevent concurrent write operations on hard disk, HTTP requests, ...) is not controlled by BaseX' transaction monitor unless specified by the user.same file:

<pre class="brush:xquery">module namespace read ==XQuery Locking Options==='read';

Custom locks can be acquired by setting the BaseX-specific XQuery options {{Code|query(:read-~ Read lock}} and {{Code|queryon CONFIG key. :write-lock}}. Multiple )declare option declarations may occur in the prolog of a query, but multiple values can also be separated with commas in a single declaration. These locks are in another namespace than the database names: the read-lock value {{Code|factbook}} will not lock a database named factbook.'CONFIG';

These option declarations will put declare function read locks on ''foo'', ''bar':config() { file:read-text(' and config.txt''batz'' and a write lock on ''quix'':)};</pre>

declare option querymodule namespace write = 'write'; (:read-~ Write lock "foo,bar";on CONFIG key. :)declare option query:readwrite-lock "batz"'CONFIG'; declare option queryfunction write:file($data) { file:write-lock "quix"text('config.txt', $data)};

As XQuery is a very powerful language, deciding Deciding which databases will be accessed by a query complex XQuery expression is a non-trivialtask. Optimization is work in progress.The current identification Database detection works for the following types 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. Some examples on database-locking enabled queries, all of these can be executed in parallel:

Some examples on All databases will be locked by queries that are not supported by database-locking yet: * <code>let $db := 'factbook' return doc($db)</code>, will read-lock: referencing database names isn’t supported yet* {{Code|for $db in ('factbook') return doc($db)}}, will read-lock globally* {{Code|doc(doc('test')/reference/text())}}, will read-lock globally* <code>let $db := 'test' return insert nodes <test/> into doc($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 GUI's [[GUI#Visualizations|Info View]] --> If you think that too much is locked, please give us a note on our [httpfollowing kind://basex.org/open-source/ mailing list] with some example code. ===GUI=== Database locking is currently disabled if the BaseX GUI is used.

* {{Code|for $db in ('db1', 'db2') return doc($db)}}* {{Code|doc(doc('test')/reference/text())}}* <code>let $db :==Process Locking=='test' return insert nodes <test/> into doc($db)</code>

In order to enable locking on global You can consult the query info output (process) level, which you find in the [[GUI#Visualizations|Info View]] of the option GUI or which you can turn on by setting <code>[[Options#GLOBALLOCKQUERYINFO|GLOBALLOCKQUERYINFO]]</code> can be set to {{Code|true}}. This can e.g. be done ) to find out which databases have been locked by editing your {{Code|.basex}} file (see [[Options]] for more details). If process locking is active, a process that performs write operations will queue all other operationsquery.

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.