Upgrading plugin data

Every now and then there comes a time when a plugin needs to change the contents or the structure of the data it has stored either in the database or the dataroot.

The motivation for this may be that the data structure needs to be converted to more efficient or flexible structure. Or perhaps due to a bug the data items have been saved in an invalid way, and they needs to be converted to the correct format.

Migrations and convertions like this may take a long time if there is a lot of data to be processed. This is why Elgg provides the Elgg\Upgrade\Batch interface that can be used for implementing long-running upgrades.

Declaring a plugin upgrade

Plugin can communicate the need for an upgrade under the upgrades key in elgg-plugin.php file. Each value of the array must be the fully qualified name of an upgrade class that implements the Elgg\Upgrade\Batch interface.

Example from mod/blog/elgg-plugin.php file:

return [
        'upgrades' => [
                Blog\Upgrades\AccessLevelFix::class,
                Blog\Upgrades\DraftStatusUpgrade::class,
        ]
];
The class names in the example refer to the classes:
  • mod/blog/classes/Blog/Upgrades/AccessLevelFix
  • mod/blog/classes/Blog/Upgrades/DraftStatusUpgrade

Note

Elgg core upgrade classes can be declared in engine/lib/upgrades/async-upgrades.php.

The upgrade class

A class implementing the Elgg\Upgrade\Batch interface has a lot of freedom on how it wants to handle the actual processing of the data. It must however declare some constant variables and also take care of marking whether each processed item was upgraded successfully or not.

The basic structure of the class is the following:

<?php

namespace Blog\Upgrades;

use Elgg\Upgrade\Batch;
use Elgg\Upgrade\Result;

/**
 * Fixes invalid blog access values
 */
class AccessLevelFix implements Batch {

        /**
         * Version of the upgrade
         *
         * @return int
         */
        public function getVersion() {
                return 2016120300;
        }

        /**
         * Should the run() method receive an offset representing all processed items?
         *
         * @return bool
         */
        public function needsIncrementOffset() {
                return true;
        }

        /**
         * Should this upgrade be skipped?
         *
         * @return bool
         */
        public function shouldBeSkipped() {
                return false;
        }

        /**
         * The total number of items to process in the upgrade
         *
         * @return int
         */
        public function countItems() {
                // return count of all blogs
        }

        /**
         * Runs upgrade on a single batch of items
         *
         * @param Result $result Result of the batch (this must be returned)
         * @param int    $offset Number to skip when processing
         *
         * @return Result Instance of \Elgg\Upgrade\Result
         */
        public function run(Result $result, $offset) {
                // fix 50 blogs skipping the first $offset
        }
}

Warning

Do not assume when your class will be instantiated or when/how often its public methods will be called.

Class methods

getVersion()

This must return an integer representing the date the upgrade was added. It consists of eight digits and is in format yyyymmddnn where:

  • yyyy is the year
  • mm is the month (with leading zero)
  • dd is the day (with leading zero)
  • nn is an incrementing number (starting from 00) that is used in case two separate upgrades have been added during the same day

shouldBeSkipped()

This should return false unless the upgrade won’t be needed.

Warning

If true is returned the upgrade cannot be run later.

needsIncrementOffset()

If true, your run() method will receive as $offset the number of items aready processed. This is useful if you are only modifying data, and need to use the $offset in a function like elgg_get_entities*() to know how many you’ve already handled.

If false, your run() method will receive as $offset the total number of failures. false should be used if your process deletes or moves data out of the way of the process. E.g. if you delete 50 objects on each run(), you don’t really need the $offset.

countItems()

Get the total number of items to process during the upgrade. If unknown, Batch::UNKNOWN_COUNT can be returned, but run() must manually mark the upgrade complete.

run()

This must perform a portion of the actual upgrade. And depending on how long it takes, it may be called multiple times during a single request.

It receives two arguments:

  • $result: An instance of Elgg\Upgrade\Result object
  • $offset: The offset where the next upgrade portion should start (or total number of failures)

For each item the method processes, it must call either:

  • $result->addSuccesses(): If the item was upgraded successfully
  • $result->addFailures(): If it failed to upgrade the item

Both methods default to one item, but you can optionally pass in the number of items.

Additionally it can set as many error messages as it sees necessary in case something goes wrong:

  • $result->addError("Error message goes here")

If countItems() returned Batch::UNKNOWN_COUNT, then at some point run() must call $result->markComplete() to finish the upgrade.

In most cases your run() method will want to pass the $offset parameter to one of the elgg_get_entities*() functions:

/**
 * Process blog posts
 *
 * @param Result $result The batch result (will be modified and returned)
 * @param int    $offset Starting point of the batch
 * @return Result Instance of \Elgg\Upgrade\Result;
 */
public function run(Result $result, $offset) {
        $blogs = elgg_get_entitites([
                'type' => 'object'
                'subtype' => 'blog'
                'offset' => $offset,
        ]);

        foreach ($blogs as $blog) {
                if ($this->fixBlogPost($blog)) {
                        $result->addSuccesses();
                } else {
                        $result->addFailures();
                        $result->addError("Failed to fix the blog {$blog->guid}.");
                }
        }

        return $result;
}

Administration interface

Each upgrade implementing the Elgg\Upgrade\Batch interface gets listed in the admin panel after triggering the site upgrade from the Administration dashboard.

While running the upgrades Elgg provides:

  • Estimated duration of the upgrade
  • Count of processed items
  • Number of errors
  • Possible error messages