Backup 2.0 - Improve XML parsing

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Warning: This page is no longer in use. The information contained on the page should NOT be seen as relevant or reliable.

Backup 2.0 -> Improve XML parsing

Summary

Current (Moodle 1.x) restore uses too much memory parsing some "parts" of the XML information. We need to change current approach to one providing optimal memory usage and acceptable throughput. In this page you'll find the different alternatives researched, with their strengths and weaknesses. Finally one solution will be provided in order to be implemented in new Moodle 2.0 backup & restore. Also, if possible, some changes will be performed in Moodle 1.9 in order to achieve better results and solve bugs like MDL-14302, MDL-15489, MDL-9838...).

Research

Below, you will find some information about different methods used to perform the XML parsing of one 12.5MB file, corresponding to one real quiz module with only 788 attempts and 20115 states (answers). It has been proved to be the problematic "part" in one production server using current Moodle 1.9.x (the file isn't available here for privacy matters, obviously).

Each method, in order to be considered valid must fulfil these basic objectives:

• Parse the XML file.
• Provide one in-memory object with all the needed info in order to be processed later by the corresponding restore plugin. Discussion of this point moved to the talk page.

For each method, some common information bits are provided (to allow comparisons later).

• name: mnemonic to easily reference each method later in comments.
• file size: the size of the original XML file being parsed.
• memory: max memory used by PHP in the execution (provided by memory_get_peak_usage()).
• time: time required to perform the parsing to memory (measured in seconds).
• data size: size of the in-memory generated object (final result of the execution).
• data format: specifications of the in-memory generated object (xmlize compatible, custom...).

Table of Results

This table summarises the raw results obtained running each method, providing links to details about each of them.

Method	File size	Memory	Time	Data size	Format
Method 0: Current behaviour (xmlize)	12.5MB	311.3MB	4.8 seconds	14MB	xmlize
Method 1: SimpleXML parsing + conversion to xmlize format	12.5MB	165.8MB	5.2 seconds	14MB	xmlize
Method 2: Method 2: SimpleXML parsing, no conversion	12.5MB	36.5MB	0.5 seconds	8.5MB	simplexml
Method 3: Custom SAX parser + conversion to xmlize format	12.5MB	158.3MB	47.5 seconds	14MB	xmlize
Method 4: Custom SAX parser + conversion to xmlize-reduced format	12.5MB	72MB	51.9 seconds	7.7MB	xmlize-reduced
Method 5: Custom SAX parser + conversion to custom (simple) format	12.5MB	64.5MB	15.1 seconds	5.4MB	custom-simple

• Temp Note: To avoid forgetting to talk about :
  • Improve4memory and improve4speed
  • SimpleXML in core (possibility to provide transformation to xmlize for easier contrib code upgrade of backup/restore.
  • Split moodle.xml into multiple files (one for each current "TODO" - part in Moodle 1.9 + 1 per module). Some of this has been worked out/tested in MDL-18468 showing important speed benefits.
  • With that split performed it will be easier to support different "envelopes" for the same modules (section backup/course backup/1 activity backup...).
  • Analyse the possibility to, in a declarative way, be able to define the whole backup & restore process. One XML to specify the whole thing would be amazing... prospect possibilities. Different from install.xml for sure (just in case somebody is thinking about that) (see talk page).

Eloy, declarative is good, but please not XML. We need an machine-readable format that developers understand. Let's use a PHP API. Defining databases with XML that can only be edited with the built in editor sucks. We should not repeat that choice.--Tim Hunt 04:53, 8 March 2009 (CDT)

Tim, I don't get the point. What exactly sucks? What is a PHP API? I think XML is the way, 100% --Eloy Lafuente (stronk7) 07:23, 8 March 2009 (CDT)

OK, easy example, when you have to edit an XMLDB file by hand becuase the stupid XMLDB editor is buggy, you find yourself editing stuff like

<FIELD NAME="questiondecimalpoints" TYPE="int" LENGTH="4" NOTNULL="true" UNSIGNED="false" DEFAULT="-2" SEQUENCE="false" ENUM="false" COMMENT="The number of decimal digits to use when displaying question grades. -1 = use decimalpoints, otherwise a separate setting." PREVIOUS="decimalpoints" NEXT="review"/>

In particular, when you are trying to edit the previous and next links (which are completely unnecessary, separate rant) your editor is scrolled so far over to the right that you cannot actually see which field you are editing the prev and next attributes of. That is my point. XML is a horrible format to edit by hand.

Instead, I would rather work with a PHP API, for example the admin settings API. That is also more flexible. With XML, you can only do the things that the XML lets you do. With PHP, where necessary, you can write PHP code to do anything necessary, in the admin settings case, that may mean making a new settings subclass, or writing some code to compute a default value to feed to a setting constructor. For backup and restore, we will need that sort of flexibility I think.--Tim Hunt 01:30, 10 March 2009 (UTC)

• • Allow modules to request "sub-parts" of the whole module.xml file. While we can accept each module as a whole (an load it straight to memory) some modules (quiz) are big enough to make this approach to cause problems (see MDL-15489). Should be able to request "different sub-parts" using some simple mini-API. (quiz example: "giveme the quiz module data", or "iterate over the attempts in a nice loop"...). (see talk page).
  • Modules implementing own parser?
  • Backport or analyse some interim solution for 1.9 quizzes.

The alternatives analysed

Here each of the alternatives is analysed and compared

Method 0: Current behaviour (xmlize)

Summary: file size: 12.5MB, memory: 311.3MB, time: 4.8 seconds, data size: 14MB, data format: xmlize format

Method 1: SimpleXML parsing + conversion to xmlize format

Summary: file size: 12.5MB, memory: 165.8MB, time: 5.2 seconds, ata size: 14MB, data format: xmlize format

Method 2: SimpleXML parsing, no conversion (use simplexml as final object)

Summary: file size: 12.5MB, memory: 36.5MB, time: 0.5 seconds, data size: 8.5MB, data format: simplexml format

Method 3: Custom SAX parser + conversion to xmlize format

Summary: file size: 12.5MB, memory: 158.3MB, time: 47.5 seconds, data size: 14MB, data format: xmlize format

Method 4: Custom SAX parser + conversion to xmlize-reduced format

Summary: file size: 12.5MB, memory: 72MB, time: 51.9 seconds, data size: 7.7MB, data format: xmlize-reduced format

Method 5: Custom SAX parser + conversion to custom (simple) format

Summary: file size: 12.5MB, memory: 64.5MB, time: 15.1 seconds, data size: 5.4MB, data format: custom simple format

Formats

In this section there are some explanations about the different object formats generated by the different methods:

Code

Here its' the code used for the different parsing methods commented above:

Method 0: Current behaviour

$contents = file_get_contents('moodle.xml');
$data = xmlize($contents);