Transaction Management

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

Introduction

In a nutshell, a transaction is equal to a command or query. So each command or query sent to the server becomes a transaction.

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.

Please note that:

• 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. It is advisable to use the CREATE BACKUP command to regularly back up your database. If the worst case occurs, you can try the INSPECT command to check if your database has obvious inconsistencies, and use RESTORE to restore the last backed up version of the database.

XQuery Update

Many update operations are triggered by XQuery Update expressions. When executing an updating query, all update operations of the query are stored in a pending update list. They will be executed all at once, so the database is updated atomically. If any of the update sub-operations is erroneous, the overall transaction will be aborted.

Concurrency Control

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.
• 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 below).
• The number of maximum parallel transactions can be adjusted with the 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 FAIRLOCK option.

Limitations

Commands

All commands come with a detector for local locks. Global locking is applied if the glob syntax is used. The following example drops all databases starting with the prefix string new; it results in a global lock:

DROP DB new*

XQuery

Since BaseX 10, the 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:

//item

doc('factbook')

collection('documents/path/to/docs')

delete nodes db:get('test')//*[@type = 'misc']

declare variables $db external;
db:get($db)

for $db in ('db1', 'db2')
return db:get($db)

let $db := 'test'
return insert nodes <test/> into db:get($db)

sum(1 to 100)

declare variable $SIMULATE := true();
if ($SIMULATE) then <doc/> else db:get('doc')

A global lock will be assigned if the static detection fails:

db:get(doc('test')/reference/text())

for $n in (1 to 100)
return db:get('db' || $n)

The functions fn:doc and fn:collection can be used for both accessing databases resources and fetching resources at the specified URI (see Access Resources for more details). There are two ways to reduce the number of locks:

1. Turn off the WITHDB option to prevent the functions from accessing databases; or
2. use fetch:doc for fetching resources from URIs, and use db:get for accessing databases.

You can consult the query info output (via the Info View of the GUI, via -V on Command-Line or via turning on the QUERYINFO option) to find out which databases are locked by a query, and if local 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. Custom locks can be assigned via annotations, pragmas or 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, 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.

Annotations

In the following module, lock annotations are used to prevent concurrent write operations on the same file:

module namespace config = 'config';

declare %basex:lock('CONFIG') function config:read() as xs:string {
  file:read-text('config.txt')
};

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

Some explanations:

• If a query calls a reading basex:lock function, a read lock will be acquired for the user-defined CONFIG lock string before query evaluation.
• If an updating basex:lock is called by a query, a write lock will be applied.
• If another query calls a basex:lock function, it will be queued until the first query is evaluated.

Pragmas

Locks can also be declared via pragmas:

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

The write locks is enforced via the Update|update:output.

Options

Locks for the functions of a module can also be assigned via option declarations:

declare option basex:lock 'CONFIG';

update:output(file:write('config.xml', <config/>))

Once again, a write lock is enforced.

Java Modules

Locks can also be acquired on 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.

File-System Locks

Update Operations

During a database update, a locking file 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 will not be deleted. In this case, the database cannot be opened anymore, 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.

Database Locks

To avoid database corruptions that are caused by accidental write operations from different JVMs, a shared lock is requested on the database table file (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.”.

Please note that you cannot 100% rely on this mechanism, as it is not possible to synchronize operations across different JVMs. You will be safe when using the client/server or HTTP architecture.

Changelog

• Updated: Lock detection was improved by splitting compilation into multiple steps.

• Updated: Single lock option for reads and writes.

• Updated: Query lock options were moved from query to basex namespace.

• Updated: New FAIRLOCK option, improved detection of lock patterns.

• Added: Locks can also be acquired on Java functions.

• 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.