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Excellent paper. As someone who has worked with filesystems and ACLs, but never touched Spanner before, I have some questions for any Googler who has played with Zanzibar. (in part because full-on client systems examples are limited)

A check my understanding: Zanzibar is being optimized to handle zookies that are a bit stale (say 10s) old. In this case, the indexing systems (such as Leapord) can be used to vastly accelerate query evaluation.

Questions I have (possibly missed explanations in the paper):

1. If I understand the zookie time (call it T) evaluation correctly, access questions for a given user are effectively "did a user have access to a document at or after T"? How in practice is this done with a check() API? The client/Zanzibar can certainly use the snapshots/indexes to give a True answer, but if the snapshot evaluation is false, is live data used (and if so by the client or Zanzibar itself?)? (e.g. how is the case handled of a user U just gaining access to a group G that is a member of some resource R?)

2. Related to #1, when is a user actually guaranteed to lose access to a document (at a version they previously had access to?) E.g. if a user has access to document D via group G and user is evicted from G, the protocol seems to inherently allow user to forever access D unless D is updated. In practice, is there some system (or application control) that will eventually block U from accessing D?

3. Is check latency going to be very high for documents that are being modified in real time (so zookie time is approximately now or close to now) that have complex group structures? (e.g. a document nested 6 levels deep in a folder where I have access to the folder via a group)? That is, there's nothing Zanzibar can do but "pointer chase", resulting in a large number of serial ACL checks?

4. How do clients consistently update ACLs alongside their "reverse edges"? For instance, the Zanzibar API allows me to view the members of a group (READ), but how do I consistently view which groups a user is a member of? (Leapord can cache this, but I'm not sure if this is available to clients and regardless it doesn't seem to be able to answer the question for "now" - only for a time earlier than indexed time).

Or for a more simple example, if I drag a document into a folder, how is the Zanzibar entry that D is a child of F made consistent with F's views of its children?

E.g. can you do a distributed transaction with ACL changes and client data stored in spanner?

5. It looks like the Watch API is effectively pushing updates whenever the READ(obj) would change, not the EXPAND(object). Is this correct? How are EXPAND() changes tracked by clients? Is this even possible? (e.g. if G is a member of some resource R and U is added to G, how can a client determine U now has access to R?)

Excellent questions. We have encountered all of them in practice and have solutions for most of them, e.g., (4) requires an ACL-aware search index.

Unfortunately we don't have enough space to explain them in the paper. Please consider coming to Usenix. :-)

Used to be a Googler and worked on an ACL model built on top of Zanzibar. I didn't work directly on Zanzibar so listen to ruomingpang over me.

> 3. There's nothing Zanzibar can do but "pointer chase", resulting in a large number of serial ACL checks?

Zanzibar enforced a max depth and would fail if the pointer-chasing traversed too deeply. Zanzibar would also fail if it traversed too many nodes.

> 4. How do clients consistently update ACLs alongside their "reverse edges"?

One of the recommended solutions was to store your full ACL (which includes a Zookie) in the same Spanner row of whatever it protected. So, if your ACL is for books, you might have:

    CREATE TABLE books (
      acl ZanzibarAcl
Alternately, you could opt to only store the current zookie instead of the full ACL. Then the check becomes:

1. Fetch Zookie from Spanner

2. Call Zanzibar.Check with the zookie

> but how do I consistently view which groups a user is a member of?

I remember this as a large source of pain to implement. Zanzibar didn't support this use-case directly. As rpang mentioned in a sibling comment, you need an ACL-aware index. Essentially, the algorithm is:

1. Call Zanzibar.Check on all groups the user might be a part of.

There's a bunch of clever tricks you can use to prune the search space that I don't know the details of.

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