:compat-mode: legacy
= Evolution of the project =

anchor:ch-evolution[Chapter 5. Evolution]

[id="evolution-foreword"]
== Foreword ==

This chapter is currently not meant as a definite summary of how
Pacemaker got into where it stands now, but rather to provide few valuable
pointers an entusiasts (presumably software archeologist type of person)
may find useful.  Moreover, well-intentioned contributors to Pacemaker
project may want to review them occasionally since, as the famous quote
has it, "those who do not learn history are doomed to repeat it".

For anything more talkative with less emphasis on actual code, other
places will serve better for the time being (and if not, should not be
too hard to volunteer extensions to those writeups):

* https://wiki.clusterlabs.org/wiki/Pacemaker[main entry at ClusterLabs
  community wiki]
* https://en.wikipedia.org/wiki/Pacemaker_(software)[entry at wikipedia.org]
* https://www.alteeve.com/w/AN!Cluster_Tutorial_2#What_about_Pacemaker.3F[
  brief section dedicated to Pacemaker in Digimer's tutorial regarding
  setting up the cluster with the old Red Hat Cluster Suite like stack]

== Common ancestor: 'heartbeat' project ==

Pacemaker can be considered as a spin-off from `heartbeat', original
comprehensive HA suite started by Alan Robertson, and some portions of code
are shared, at least on the conceptual level if not verbatim, till today,
even if the effective percentage continually declines.  Note that till
Pacemaker 2.0, it also used to stand for one (and initially the only) of
supported messaging back-ends (removal of this support made for one such
notable drop of reused code), see also
https://github.com/ClusterLabs/pacemaker/commit/55ab749bf0f0143bd1cd050c1bbe302aecb3898e[
pre-2.0 commit 55ab749bf].

The codebase for heartbeat used to be hosted at http://hg.linux-ha.org,
but since that does not appear reliably available recently, an archive
checkout from 2016 is shared at https://gitlab.com/poki/archived-heartbeat[
as a dedicated read-only repository], and anchored there, the most
notable commits are:

* https://gitlab.com/poki/archived-heartbeat/commit/bb48551be418291c46980511aa31c7c2df3a85e4[
  initial check-in of what turned up to be the basis for Pacemaker later on]

* https://gitlab.com/poki/archived-heartbeat/commit/74573ac6182785820d765ec76c5d70086381931a[
  drop of now-detached Pacemaker code]

Regarding the Pacemaker's split from heartbeat, it evolved stepwise
(as opposed to one-off cut), and the last step of full dependency is depicted
in https://www.kernel.org/doc/ols/2008/ols2008v1-pages-85-100.pdf#page=14[
The Corosync Cluster Engine] paper, fig. 10.
This article also provides a good reference regarding wider historical
context of the tangentially (and deeper in some cases) meeting components
around that time.

=== Influence of 'heartbeat' on Pacemaker ===

On a closer look, we can identify these things in common:

* extensive use of data types and functions of
  https://wiki.gnome.org/Projects/GLib[GLib]

* Cluster Testing System (CTS) is inherited from initial implementation
  by Alan Robertson

* ...

== Notable Restructuring Steps in the Codebase ==

File renames may not appear as notable ... unless one runs into complicated
+git blame+ and +git log+ scenarios, so some more massive ones may be stated
as well.

* watchdog/'sbd' functionality spin-off:
** https://github.com/ClusterLabs/pacemaker/commit/eb7cce2a172a026336f4ba6c441dedce42f41092[
   start separating, eb7cce2a1]
** https://github.com/ClusterLabs/pacemaker/commit/5884db78080941cdc4e77499bc76677676729484[
   finish separating, 5884db780]

* daemons' rename for 2.0 (in chronological order)
** https://github.com/ClusterLabs/pacemaker/commit/318a2e003d2369caf10a450fe7a7616eb7ffb264[
   start of moving daemon sources from their top-level directories under
   new +/daemons+ hierarchy, 318a2e003]
** https://github.com/ClusterLabs/pacemaker/commit/01563cf2637040e9d725b777f0c42efa8ab075c7[
   +attrd+ -> +pacemaker-attrd+, 01563cf26]
** https://github.com/ClusterLabs/pacemaker/commit/36a00e2376fd50d52c2ccc49483e235a974b161c[
   +lrmd+ -> +pacemaker-execd+, 36a00e237]
** https://github.com/ClusterLabs/pacemaker/commit/e4f4a0d64c8b6bbc4961810f2a41383f52eaa116[
   +pacemaker_remoted+ -> +pacemaker-remoted+, e4f4a0d64]
** https://github.com/ClusterLabs/pacemaker/commit/db5536e40c77cdfdf1011b837f18e4ad9df45442[
   +crmd+ -> +pacemaker-controld+, db5536e40]
** https://github.com/ClusterLabs/pacemaker/commit/e2fdc2baccc3ae07652aac622a83f317597608cd[
   +pengine+ -> +pacemaker-schedulerd+, e2fdc2bac]
** https://github.com/ClusterLabs/pacemaker/commit/038c465e2380c5349fb30ea96c8a7eb6184452e0[
   +stonithd+ -> +pacemaker-fenced+, 038c465e2]
** https://github.com/ClusterLabs/pacemaker/commit/50584c234e48cd8b99d355ca9349b0dfb9503987[
   +cib daemon+ -> +pacemaker-based+, 50584c234]

////
TBD:
- standalone tengine -> part of crmd/pacemaker-controld
////