plugins-ordering.md

Plugins Ordering

Outdated

Warning Many of the things described below are outdated. Parts of this document are still valid for the backend plugin.

Introduction

You should first read elektra-plugins to get an idea about plugins.

This document describes how elektra-plugins are ordered with elektra-backends(7).

Multiple plugins open up many ways in which they can be arranged. A simple way is to have one array with pointers to plugins. To store a KeySet, the first plugin starts off with the KeySet passed to it. The resulting KeySet is given to the next plugin. This is repeated for every plugin in the array.

To obtain a KeySet, the array of plugins is processed the other way round. An empty KeySet is passed to the last plugin. The resulting KeySet is handed over to the previous one until the first plugin is executed.

This approach has shown not to be powerful enough to express all use cases by counter-evidence. Logging should take place after the storage plugin performs its actions in both directions. It is not possible to do this with a single array processed in a way as described above.

Separated lists

Two individual lists (get+set list) of plugins solve the described problem. Instead of bidirectional processing of a single list two separate arrays are used. It turns out that error scenarios also make this approach unsuccessful. When a plugin fails, no other plugin must be executed later on because it might depend on the previous plugin to work correctly. In kdbGet(), this works well -- the update process will be stopped. But during kdbSet(), changes to the file system must be reverted. Plugins can leave a lock, temporary file or journal. These resources need to be cleaned up properly.

Error List

So every backend additionally needs a third array that is executed in error scenarios during kdbSet(). Plugins responsible for the cleanup, rollback or error notification are inserted into it.

The resolver plugin requires this error list to do a proper rollback. Another use case is logging after a failure has happened.

Arrays of linked lists

A disadvantage of using arrays is that they contain a fixed amount of slots. This means that when a certain amount of slots is full, it is no longer possible to add more. Therefore, the plugin arrays are structured as arrays of linked lists.

Each plugin role is assigned its own slot in the array, which contains a linked list. Plugins fulfilling that role are added to the list. That way, much greater and more flexible plugin numbers can be added to a single role.

Placements

The ordering of plugins inside these three arrays is controlled by elektra-contracts(7). Each of the three arrays has one slot for each role. These slots have names to be referred to in the contract.

Here you see a table that contains all names:

Slot	Error	Get	Set
0	prerollback	getresolver	setresolver
1	rollback	pregetstorage	presetstorage
2	postrollback	getstorage	setstorage
3		postgetstorage	precommit
4			commit
5			postcommit

How the placement is influenced using infos/placement, infos/ordering and infos/stacking is described in CONTRACT.ini.