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Alessandro Pisa: PAS adventures

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Talk by Alessandro Pisa at the Plone Conference 2019 in Ferrara.

What is PAS? It is the Pluggable Authentication Service from Zope and Plone. It manages everything related to users and groups, like authentication, permissions, searching. It can get or set information from your site or the request or an external service like LDAP.

It uses plugins, so you can register your own. You can see a lot of those in the ZMI at the Zope root, at http://localhost:8080/acl_users/plugins/manage_active.

There are about twenty different plugin types, and they interact with each other.

In Plone we have Products.PlonePAS. It adds Plone-specific plugins and types in PAS, for example the local roles plugins. Local roles are regular parts of Zope, but not pluggable there. So this is at http://localhost:8080/Plone/acl_users/plugins/manage_active

So this is the basis to manage security, users and groups, using plugins that can be activated, deactivated, and ordered, and these plugins interact with each other.

Now let's go to the kitchen and make a plugin. We want our Plone Intranet to use an LDAP plugin. At the time, we started using Products.PloneLDAP, which builds on a few other Zope packages. (Currently pas.plugins.ldap is the more modern choice.)

We also wanted Products.membrane: this allows to create content that works as user or group. It uses a separate membrane_catalog, a sort-of copy of the portal_catalog, for better or worse.

We wanted collective.workspace to manage roles through groups instead of sharing.

And Plone Intranet itself had some extra sauce (plugins) on top. And important: caching, for performance.

It worked fine! But after a while it slowed down. We had many calls to LDAP, in the wrong way, even with the cache in place. We were using the membrane catalog too much, causing the catalog queue to flush, causing a slowdown in the standard portal_catalog search. We were also writing on SOLR, again external, which did not help for performance.

So what was the reason of the bottleneck? The big deal is when PAS does not find what it is looking for. It then tries all relevant plugins, which are intertwined. For example it finds a workspace group, and needlessly goes to LDAP to search for this group there, where it will not be found, etcetera.

So maybe we could store the LDAP information in Plone. We decided to sync LDAP in the Plone objects.

And then we also patched PAS. Finding a group results in trying to get the properties, parent groups, roles. Our patched PAS skipped plugins that looked unrelated.

There was another problem: a custom local role manager. There were users that belonged to lots of workspaces. So also to lots of groups. So role assignment was slow: you need to get all roles of all groups. We replaced Plone's local role manager with a custom one.

Another plugin is the recursive groups plugin. Standard in Plone. This can be really expensive. For each of those thousands of groups you would check if they have a parent group. We replaced this with a utility that could handle this fast, using graphs with networkx. With this, we could replace all our group plugins, especially the recursive groups plugin.

Possible problem: in the graph we had 20.000 nodes, 25.000 edges, 25 relations. But creating the graph took 50 milliseconds, so that was okay. The nodes are strings like principal_type:UID, for example collective.workspaces:Admins. We stored the information in OOBTrees: uid2path, path2name, name2uid, and an OOTreeSet called edges. And then nx.DiGraph(uid2path) to create the graph. This helped speed things up a lot.

To keep the data structure up to date, we use events.

To recap:

  • remove external connections when possible
  • know the limitations of your plugins
  • you can patch PAS to avoid some plugins
  • you can use a custom group plugin

Further ideas:

  • lazily create collective.workspace groups
  • lazily fetch group and user properties and roles

Takeaways:

  • PAS is great, but mixing and abusing plugins can be deadly
  • Sometimes replace plugins with a new one that does exactly what you need.