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Performance and scalability

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Performance is about allowing Moodle to support as many users as possible with a certain amount of hardware.

Of course you can always always buy a bigger server. Scalability means ensuring that if you buy a server that is about twice as powerful, it can then handle about twice as much load.

This page is part of the Moodle coding guidelines

Writing code that scales and performs

Every page should only use a fixed number of database queries

  • Be very suspicious if you ever see database code inside a loop.
    • This is sometimes hard to spot if the database access is hidden in a function.
  • Instead use JOINs and subqueries. (get_records_sql, get_recordset_sql, etc.).
    • Or look for a Moodle API function that gets the information you want as efficiently as possible. (For example, get_users_by_capability)
    • See the Database guidelines for how to write SQL that will work on all our supported databases.

Limit the amount of RAM each page requires to generate

  • Large reports should be broken into pages of a fixed size.
  • Processing large amounts of data from the database should be handled with a recordset (when you cannot do all the processing in the database with SQL) and using recordset_walk iterator, as there is no RAM benefit on using recordsets if you load all the results in a massive PHP array.

Be cautious about other external calls

Like a database query, there are other operations that are much slower than just executing PHP code. For example:

  • running a shell script
  • making a web-service call
  • (to a lesser extent) working with files.

Whenever you are doing these things, worry about performance.

Limit the scope of session locks

If you don't need a session lock, or only need it for part of your page, then unlock the session. Full details here: Session locks

How to improve the performance of your code

Measure during development

  • Use tools like JMeter to subject your new code to load.

Measure in production

  • If you're using postgres, there's a script that can parse the logs and output the top 10 slow queries, ready to be plugged into a cronjob to email you every day. It can be found here:;a=blob_plain;f=scripts/;hb=refs/heads/pg-log-process-multidb

You need to configure postgres a bit to make it log things in the correct format. Instructions are in the file.

How big can a Moodle site be?

Looking at the statistics, the largest sites in the world currently have

  • Up to 1 000 000 users
  • Up to 50 000 courses
  • Up to 5 000 users per course
  • Up to 50 roles
  • Up to 100 course categories nested up to about 10 levels deep.
  • Up to XXX activities in a course.
  • Please add more things here.

When planning and testing your code, these are the sorts of numbers you should be contemplating. However, do not assume that Moodle sites will never get bigger than this.

Even if you can't test sites this big on your development server, you should use the generator script so you can test your code in a Moodle site that is not tiny.

See also