Manual_database access – mediawiki

We use the convention $dbr for read and $dbw for write to help you keep track of whether the database object is a slave (read-only) or a master (read/write). Database xls If you write to a slave, the world will explode. C database library Or to be precise, a subsequent write query which succeeded on the master may fail when replicated to the slave due to a unique key collision. 510 k database Replication on the slave will stop and it may take hours to repair the database and get it back online.

How to become a database administrator Setting read_only in my.cnf on the slave will avoid this scenario, but given the dire consequences, we prefer to have as many checks as possible. Database xml Wrapper functions [ edit ]

We provide a query() function for raw SQL, but the wrapper functions like select() and insert() are usually more convenient. Database terminology They can take care of things like table prefixes and escaping for you under some circumstances. Database theory If you really need to make your own SQL, please read the documentation for tableName() and addQuotes(). Database 1 to 1 relationship You will need both of them.

Another important reason to use the high level methods rather than constructing your own queries is to ensure that your code will run properly regardless of the database type. Database testing Currently there is MySQL and reasonable support for SQLite and PostgreSQL, also somewhat limited for Oracle and DB2, but there could be other databases in the future such as MSSQL or Firebird.

In the following, the available wrapper functions are listed. Database graph For a detailed description of the parameters of the wrapper functions, please refer to class Database’s docs. Database naming conventions Particularly see Database::select for an explanation of the $table, $vars, $conds, $fname, $options, and $join_conds parameters that are used by many of the other wrapper functions.

MediaWiki developers who need to write DB queries should have some understanding of databases and the performance issues associated with them. Database entity Patches containing unacceptably slow features will not be accepted. Database developer Unindexed queries are generally not welcome in MediaWiki, except in special pages derived from QueryPage. Data recovery plan It’s a common pitfall for new developers to submit code containing SQL queries which examine huge numbers of rows. Data recovery kansas city Remember that COUNT(*) is O(N), counting rows in a table is like counting beans in a bucket. N k database Replication [ edit ]

The largest installation of MediaWiki, Wikipedia, uses a large set of slave MySQL servers replicating writes made to a master MySQL server. Data recovery 2016 It is important to understand the issues associated with this setup if you want to write code destined for Wikipedia.

It’s often the case that the best algorithm to use for a given task depends on whether or not replication is in use. 510 k database fda Due to our unabashed Wikipedia-centrism, we often just use the replication-friendly version, but if you like, you can use wfGetLB()->getServerCount() > 1 to check to see if replication is in use. Database programmer Lag [ edit ]

Lag primarily occurs when large write queries are sent to the master. Data recovery osx Writes on the master are executed in parallel, but they are executed in serial when they are replicated to the slaves. Database integrity The master writes the query to the binlog when the transaction is committed. Database backup The slaves poll the binlog and start executing the query as soon as it appears. Hollywood u database They can service reads while they are performing a write query, but will not read anything more from the binlog and thus will perform no more writes. Data recovery ipad This means that if the write query runs for a long time, the slaves will lag behind the master for the time it takes for the write query to complete.

Lag can be exacerbated by high read load. Database vs server MediaWiki’s load balancer will stop sending reads to a slave when it is lagged by more than 30 seconds. Database is in transition If the load ratios are set incorrectly, or if there is too much load generally, this may lead to a slave permanently hovering around 30 seconds lag.

If all slaves are lagged by more than 30 seconds (according to $wgDBservers Manual:$wgDBservers), MediaWiki will stop writing to the database. Data recovery ios All edits and other write operations will be refused, with an error returned to the user. Database data types This gives the slaves a chance to catch up. Data recovery johannesburg Before we had this mechanism, the slaves would regularly lag by several minutes, making review of recent edits difficult.

In addition to this, MediaWiki attempts to ensure that the user sees events occurring on the wiki in chronological order. Iphone 5 data recovery software A few seconds of lag can be tolerated, as long as the user sees a consistent picture from subsequent requests. Database operations This is done by saving the master binlog position in the session, and then at the start of each request, waiting for the slave to catch up to that position before doing any reads from it. Database index If this wait times out, reads are allowed anyway, but the request is considered to be in “lagged slave mode”. Database crud Lagged slave mode can be checked by calling wfGetLB()->getLaggedSlaveMode(). Drupal 8 database The only practical consequence at present is a warning displayed in the page footer.

Shell users can check replication lag with getLagTimes.php Manual:getLagTimes.php; the other users with the siteinfo API:Siteinfo#dbrepllag API.

Databases often have their own monitoring systems in place as well, see for instance wikitech:MariaDB#Replication lag (Wikimedia) and wikitech:Help:Tool Labs/Database#Identifying lag (Wikimedia Labs). Data recovery disk Lag avoidance [ edit ]

To avoid excessive lag, queries that write large numbers of rows should be split up, generally to write one row at a time. Database 3 tier architecture Multi-row INSERT … Data recovery orlando SELECT queries are the worst offenders and should be avoided altogether. Database cardinality Instead do the select first and then the insert. Database unit testing Working with lag [ edit ]

Despite our best efforts, it’s not practical to guarantee a low-lag environment. I data recovery software free download Replication lag will usually be less than one second, but may occasionally be up to 30 seconds. O review database For scalability, it’s very important to keep load on the master low, so simply sending all your queries to the master is not the answer. Database in recovery So when you have a genuine need for up-to-date data, the following approach is advised:

To avoid swamping the master every time the slaves lag, use of this approach should be kept to a minimum. Data recovery wizard professional In most cases you should just read from the slave and let the user deal with the delay. Data recovery open source Lock contention [ edit ]

Due to the high write rate on Wikipedia (and some other wikis), MediaWiki developers need to be very careful to structure their writes to avoid long-lasting locks. Gif database By default, MediaWiki opens a transaction at the first query, and commits it before the output is sent. Data recovery lifehacker Locks will be held from the time when the query is done until the commit. Top 10 data recovery software 2014 So you can reduce lock time by doing as much processing as possible before you do your write queries. Database gale Update operations which do not require database access can be delayed until after the commit by adding an object to $wgPostCommitUpdateList.

Often this approach is not good enough, and it becomes necessary to enclose small groups of queries in their own transaction. Database life cycle Use the following syntax:

Use of locking reads (e.g. Data recovery dallas the FOR UPDATE clause) is not advised. Data recovery usb They are poorly implemented in InnoDB and will cause regular deadlock errors. Database 4th normal form It’s also surprisingly easy to cripple the wiki with lock contention.

Instead of locking reads, combine your existence checks into your write queries, by using an appropriate condition in the WHERE clause of an UPDATE, or by using unique indexes in combination with INSERT IGNORE. V database in oracle Then use the affected row count to see if the query succeeded. Data recovery tampa Database schema [ edit ]

Don’t forget about indexes when designing databases, things may work smoothly on your test wiki with a dozen of pages, but will bring a real wiki to a halt. R studio data recovery with crack See above for details.

When writing MySQL table definitions or upgrade patches, it is important to remember that SQLite shares MySQL’s schema, but that works only if definitions are written in a specific way: