identity propagation and failing nodes

[Topper Bowers]

Hello!

I'm new to the cothority codebase and I was looking through the identity code. I see where identity is propagated down: https://github.com/dedis/cothority/blob/master/identity/service.go#L302 . However, if a node is offline during that time it seems like it will never get that identity? Is the "catchup when back online" in there somewhere or handled by onet and I don't see it?

Thanks for any info!

Topper

[kelon...@epfl.ch]

Hello again,

We do not have a "catchup when back online" functionality for identity. However, we are starting to improve the fault-tolerance aspect of cothority. Currently, skipchain is fault tolerant and a node is able to catch up after going offline.

Thanks,
Kelong

[Gasser Linus]

Hi Topper,

Thanks for your question. The identity-service depends on the skipchain-service. As such all nodes will fetch necessary blocks if they need to:

Having 4 nodes: A, B, C, D

Node D is down

Node A gets request for identity change and gets B, C to sign, all three store the new identity

Node D gets up

Node A gets new request for identity change and gets B, C, D to sing. D will ask any of the three other nodes for the missing block, verify it, and sign the request

All four nodes have the same state

There is a big caveat: for the moment we didn’t implement a view-change of the leader! So as long as A is the leader and A is up, everything will work fine. If A is down, the service will stop working.

We’re working on having a correct change-of-leader, but that won’t be ready before mid-March.

If you have other questions, don’t hesitate to contact us,

Linus

PS: forgot to include the cothority-discussion list, sorry.

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[jeff....@epfl.ch]

Hello Topper,

Sorry to pile on with even more answers to your simple question, but my two colleagues didn't point you to a document I wrote that mentions this behaviour:

  https://github.com/dedis/cothority/wiki/Operating-a-Conode#catch-up-behavior

This was only recently implemented, so you'd only see this "catching up" happening if you are using cothority compiled from the tip.

A quick note on versions: My current development focus is to bring onet and cothority to a stable point that we call "v2". At that point (which will be announced on this mailing list) you can move off of tip and use "gopkg.in/dedis/cothority.v2".

  -jeff