Changes

The BaseX client-server architecture offers ACID -safe transactions,with multiple readers and writers. Here are is some moreinformations information about the transaction management.

=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, will probably lead to an inconsistent database state if a transaction was active at the shutdown time. So we advise to usethe [[Commands#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==

{{Mark|Version 7.6}} of BaseX introduces 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 lockingAccess to external resources (files on hard disk, HTTP requests, . Execution is starvation-free as lock aquiration 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 to read locksunless specified by the user.

Locks are not synchronized between multiple ===XQuery Locking Options=== Custom locks can be acquired by setting the BaseX instances-specific XQuery options {{Code|query:read-lock}} and {{Code|query:write-lock}}. We generally recommend working Multiple 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 clientdatabase names: the lock value {{Code|factbook}} will not lock a database named factbook. These option declarations will put read locks on ''foo'', ''bar'' and ''batz'' and a write lock on ''quix'': <pre class="brush:xquery">declare option query:read-lock "foo,bar";declare option query:read-lock "batz";declare option query:write-lock "quix";</server architecture if concurrent pre> ===Java Modules=== Locks can also be acquired on [[Java Bindings#Locking|Java functions]] which are imported and invoked from an XQuery expression. It is advisable to explicitly lock Java code whenever it performs sensitive read and write operations are to be performed.

Database locking works with all commands unless no the glob syntax is used, such as in the following command call:

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 queries that are not supported by database-locking yet: * <code>let $db := 'factbook' return doc($db)</code>, All databases will read lock globally: 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>doc(doc('test')[.]</code>, will read lock globally, as the context in the predicate cannot be associated yet with the correct database reference* <code>let $db := 'test' return insert nodes <test/> into doc($db)</code>, will write lock globally A list locked by queries 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 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 $db :==How to disable=='test' return insert nodes <test/> into doc($db)</code>

In order to disable traditional process locking, You can consult the query info output (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|falsetrue}}. This can e.g. be done by editing your {{Code|.basex}} file (see [[Options]] for more details). To enable it again, set it to {{Code|true}}find out which databases have been locked by a query.

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.

 * Added: database locking introduced, replacing process locking.

 * Updated: pin files replaced with shared/exclusive filesystem locking.

 * Added: pin files to mark open databases.

 * Added: update lock files [[Category:Server]][[Category:Internals]].