Preemptive cache

1.php

<?php

$data = [
	'element_data'            =>
		[
			7   =>
				[
					'translation_id' => '18',
					'trid'           => '12',
					'lang'           => 'en',
					'source_lang'    => null,
					'type'           => 'page',
				],
			11  =>
				[
					'translation_id' => '23',
					'trid'           => '12',
					'lang'           => 'ru',
					'source_lang'    => 'en',
					'type'           => 'page',
				],
			391 =>
				[
					'translation_id' => '165',
					'trid'           => '12',
					'lang'           => 'uk',
					'source_lang'    => 'en',
					'type'           => 'page',
				],
		],
	'translations'            =>
		[
			7   =>
				[
					'en' => '7',
					'ru' => '11',
					'uk' => '391',
				],
			11  =>
				[
					'en' => '7',
					'ru' => '11',
					'uk' => '391',
				],
			391 =>
				[
					'en' => '7',
					'ru' => '11',
					'uk' => '391',
				],
		],
	'trid_groups'             =>
		[
			12 =>
				[
					'en' => '7',
					'ru' => '11',
					'uk' => '391',
				],
		],
	'translation_ids_element' =>
		[
			18  => '7',
			23  => '11',
			165 => '391',
		],
];

/**
 * @return array
 */
protected function get_data_keys() {
	return [
		'element_data',
		'translations',
		'trid_groups',
		'translation_ids_element',
	];
}

public function init() {
	$this->data = get_transient( $this->get_transient_name() );
	delete_transient( $this->get_transient_name() );
	if ( $this->data ) {
		$this->data = json_decode( $this->data, true );
		$keys       = $this->get_data_keys();
		foreach ( $keys as $key ) {
			$this->{$key} = $this->data[ $key ];
		}
	}
}

public function shutdown_action() {
	$keys = $this->get_data_keys();
	$data = [];

	foreach ( $keys as $key ) {
		$this->data[ $key ] = isset( $this->data[ $key ] ) ? $this->data[ $key ] : [];
		$data[ $key ]       = array_replace( $this->{$key}, $this->data[ $key ] );
	}

	set_transient( $this->get_transient_name(), $this->maybe_reduce_transient_size( $data ) );
}

/**
 * @param array $data
 *
 * @return string
 */
private function maybe_reduce_transient_size( $data ) {
	$json = wp_json_encode( $data );
	$size = strlen( $json );

	while ( $size > self::MAX_TRANSIENT_SIZE ) {
		$count        = count( $data['element_data'] );
		$keep_count   = intval( $count / ( $size / self::MAX_TRANSIENT_SIZE ) );
		$keep_count   = $keep_count * 2; // 'element_data' takes approximately one half of the total $data size.
		$remove_count = $count - $keep_count;

		$i = 0;
		foreach ( $data['element_data'] as $key => $element_datum ) {
			if ( $i > $remove_count ) {
				break;
			}
			// Remove 'element_data' and relevant items on FIFO principle.
			$translation_id = $data['element_data'][ $key ]['translation_id'];
			$trid           = $data['element_data'][ $key ]['trid'];
			unset( $data['element_data'][ $key ] );
			unset( $data['translations'][ $key ] );
			unset( $data['trid_groups'][ $trid ] );
			unset( $data['translation_ids_element'][ $translation_id ] );
			$i ++;
		}

		$json     = wp_json_encode( $data );
		$new_size = strlen( $json );
		if ( $new_size === $size ) {
			break; // Prevent infinite loop if remove_count was estimated improperly.
		}
		$size = $new_size;
	}

	return $json;
}