
Base resilient stack
====================

This stack is meant to be extended by SR profiles, or other stacks, that need to provide
automated backup/restore, election of backup candidates, and instance failover.

As reference implementations, both stack/lamp and stack/lapp define resilient behavior for
MySQL and Postgres respectively.

This involves three different software_types:

 * pull-backup
 * {mysoftware}_export
 * {mysoftware}_import

where 'mysoftware' is the component that needs resiliency (can be postgres, mysql, erp5, and so on).


pull-backup
-----------

This software type is defined in

    http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/stack/resilient/instance-pull-backup.cfg.in?js=1

and there should be no reason to modify or extend it.

An instance of type 'pull-backup' will receive data from an 'export' instance and immediately populate an 'import' instance.
The backup data is automatically used to build an historical, incremental archive in srv/backup/pbs.


export
------

example:
    http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/stack/lapp/postgres/instance-postgres-export.cfg.in?js=1

This is the *active* instance - the one providing live data to the application.

A backup is run via the bin/exporter script: it will
     1) run bin/{mysoftware}-backup
 and 2) notify the pull-backup instance that data is ready.

The pull-backup, upon receiving the notification, will make a copy of the data and transmit it to the 'import' instances.

You should provide the bin/{mysoftware}-exporter script, see for instance
  http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/slapos/recipe/postgres/__init__.py?js=1#l207
  http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/slapos/recipe/mydumper.py?js=1#l71

By default, as defined in
  http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/stack/resilient/pbsready-export.cfg.in?js=1#l27
the bin/exporter script is run every 60 minutes.



import
------

example:
    http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/stack/lapp/postgres/instance-postgres-import.cfg.in?js=1

This is the *fallback* instance - the one that can be activated and thus become active.
Any number of import instances can be used. Deciding which one should take over can be done manually
or through a monitoring + election script.


You should provide the bin/{mysoftware}-importer script, see for instance

  http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/slapos/recipe/postgres/__init__.py?js=1#l233
  http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/slapos/recipe/mydumper.py?js=1#l71




In practice
-----------

Add resilience to your software

Let's say you already have a file instance-mysoftware.cfg.in that instantiates your
software. In which there is a part [mysoftware] where there is the main recipe
that instantiates the program.

You need to create two new files, instance-mysoftware-import.cfg.in and
instance-mysoftware-export.cfg.in, following this layout:


IMPORT:

[buildout]
extends = ${instance-mysoftware:output}
          ${pbsready-import:output}

parts +=
    mysoftware
    import-on-notification

[importer]
recipe = YourImportRecipe
wrapper = $${rootdirectory:bin}/$${slap-parameter:namebase}-importer
backup-directory = $${directory:backup}
...



EXPORT:

[buildout]
extends = ${instance-mysoftware:output}
          ${pbsready-export:output}

parts +=
    mysoftware
    cron-entry-backup

[exporter]
recipe = YourExportRecipe
wrapper = $${rootdirectory:bin}/$${slap-parameter:namebase}-exporter
backup-directory = $${directory:backup}
...


In the [exporter] / [importer] part, you are free to do whatever you want, but
you need to dump / import your data from $${directory:backup} and specify a
wrapper. I suggest you only add options and specify your export/import recipe.



-----------------------------------------------------------------------------------------

Finally, instance-mysoftware-import.cfg.in and
instance-mysoftware-export.cfg.in need to be downloaded and accessible by
switch_softwaretype, and you need to extend stack/resilient/buildout.cfg and
stack/resilient/switchsoftware.cfg to download the whole resiliency bundle.

Here is how it's done in the mariadb case for the lamp stack:



 ** buildout.cfg **

extends =
   ../resilient/buildout.cfg

[instance-mariadb-import]
recipe = slapos.recipe.template
url = ${:_profile_base_location_}/mariadb/instance-mariadb-import.cfg.in
output = ${buildout:directory}/instance-mariadb-import.cfg
md5sum = ...
mode = 0644

[instance-mariadb-export]
recipe = slapos.recipe.template
url = ${:_profile_base_location_}/mariadb/instance-mariadb-export.cfg.in
output = ${buildout:directory}/instance-mariadb-export.cfg
md5sum = ...
mode = 0644



 ** instance.cfg.in **

extends =
  ../resilient/switchsoftware.cfg

[switch-softwaretype]
...
mariadb = ${instance-mariadb:output}
mariadb-import = ${instance-mariadb-import:output}
mariadb-export = ${instance-mariadb-export:output}
...



Then, in the .cfg file where you want to instantiate your software, you can do, instead of requesting your software

 * template-resilient.cfg.in *

[buildout]
...
parts +=
  {{ parts.replicate("Name","3") }}
  ...

[...]
...
[ArgLeader]
...

[ArgBackup]
...

{{ replicated.replicate("Name", "3",
                        "mysoftware-export", "mysoftware-import",
                        "ArgLeader","ArgBackup") }}

and it'll expend into the sections require to request Name0, Name1 and Name2,
backuped and resilient. The leader will expend the section [ArgLeader], backups
will expend [ArgBackup]. If you don't need to specify any options, you can
omit the last two arguments in replicate().

Since you will compile your template with jinja2, there should be no $${},
because it is not yet possible to use jinja2 -> buildout template.

To compile with jinja2, see jinja2's recipe.


