Support of the session engine with Redis

[khaledk2]

This PR adds the support of the session engine with Redis for the Omero web role
It includes an example of deploying the web client with the Redis session engine in the readme file.

[sbesson]

In general big 👍 for adding Redis to the example. See also #43 for a possible alternative approach usingomero-web[redis]

What puzzles me though is that the IDR deployment playbook seems to install django-redis without any extra task. So far I cannot find where the installation happens

[pwalczysko]

where the installation happens

We have a specific redis role, so maybe that is how ? At least this is how (via the role) I install redis in the learning playbook.

[joshmoore]

So far I cannot find where the installation happens

https://github.com/IDR/deployment/blob/825c70b6b778e76d9efbee0aea492d967c0f3753/ansible/idr-omero-web.yml#L7 ?

[sbesson]

We have a specific redis role, so maybe that is how ? At least this is how (via the role) I install redis in the learning playbook.

The ome.redis role installs the Redis server but that's not sufficient to install the Redis backend for Django sessions.

After a bit of searching, I think I identified how the installation of django-redis happens: omero-mapr depends on omero-web[redis]>=5.14.0 which will install django-redis - https://github.com/ome/omero-mapr/blob/9631b155c44fcec9986b03315dd9f5c5c64aa7eb/requirements.txt#L2.
That's very fragile at best, I completely support bringing this installation at the level of the role, possibly using an variable if we want to make optional.

[pwalczysko]

The ome.redis role installs the Redis server but that's not sufficient to install the Redis backend for Django sessions.

After a bit of searching, I think I identified how the installation of django-redis happens: omero-mapr depends on omero-web[redis]>=5.14.0 which will install django-redis - https://github.com/ome/omero-mapr/blob/9631b155c44fcec9986b03315dd9f5c5c64aa7eb/requirements.txt#L2. That's very fragile at best, I completely support bringing this installation at the level of the role, possibly using an variable if we want to make optional.

Understood and makes sense, thanks for explanation.

[jburel]

@pwalczysko that will make sense especially if we want to decommission mapr

[khaledk2]

I have added a variable to control adding the support of the Redis session engine, i.e. omero_web_setup_redis_session. The variable default is false, so you may not change any of the current installation ansible-playbooks which install the package.
What do you think?

[sbesson]

I can definitely see how to consume these changes in the IDR deployment playbooks. I'll leave @pwalczysko and @jburel to comment on the UoD deployment side.

[jburel]

Proposing to merge and tag as 5.1.0
Any objections @pwalczysko?