9 ms·

This is an open problem. It might be surprising to find out that it's quite difficult. CRDTs usually work as last-write-wins, meaning that if you have a key-va

This is an open problem. It might be surprising to find out that it's quite difficult. CRDTs usually work as last-write-wins, meaning that if you have a key-value store, the last update to update a key 'wins' the value via the way oplog reduction works. If you reverse that to a FIRST-write-wins log, you can grant permissions and ownership on a first-come, first-serve basis. Revocation, then, becomes the issue. What do you do with the records they already have? Questions like that are plentiful. The approach most people take is to find workarounds or "good enough" solutions here, either by using encryption and allowing the encrypted data to be public, or by using some sort of other OrbitDB store as their ACL and management, and only giving select keys access to write to said ACL store in the first place. Adding encryption into the mix though, particularly multi-writer, becomes exponentially harder.

sagichmal 6y ago
> CRDTs usually work as last-write-wins Um.
- markhenderson 6y ago
  Ah, sorry, yes I stepped on a rake and whacked myself in the face here. What I meant, since LWW is nomenclature for an alternative to CRDTs, is that the last writer by _logical clock_ in a CRDT, not by _wall clock_ time, will "win" the key.
  - sagichmal 6y ago
    > LWW is nomenclature for an alternative to CRDTs I think you're still stepping on rakes...
- markhenderson 6y ago
  Can you elaborate?
  - sagichmal 6y ago
    Last-Writer-Wins is a conflict resolution strategy that can be used by any kind of data type that needs conflicts resolved, CRDTs included. Unfortunately it's not a very good one: even if you use vector clocks instead of wall clocks, it doesn't give you much stronger guarantees than determinism. That is, given two concurrent writes, the winner is essentially arbitrary. LWW is a merge strategy of last resort; if that's the only thing your CRDT system offers, I'm not sure it's really fair to call it a CRDT system.
  - haadcode 6y ago
    > Last-Writer-Wins is a conflict resolution strategy that can be used by any kind of data type that needs conflicts resolved, CRDTs included. Unfortunately it's not a very good one: even if you use vector clocks instead of wall clocks, it doesn't give you much stronger guarantees than determinism. That is, given two concurrent writes, the winner is essentially arbitrary. LWW is a merge strategy of last resort; if that's the only thing your CRDT system offers, I'm not sure it's really fair to call it a CRDT system. Can't reply to the comment below, so replying here. I believe what markhenderson was trying to say is that in OrbitDB, the default merge strategy for concurrent operations is LWW. The comment above is conflating a lot of things here. 1) determinism is exactly the guarantee one needs for CRDTs, and I'd argue generally is a good thing in distributed system but 2) adding vector clocks (OrbitDB uses Lamport clocks, or Merkle Clocks [1], by default), nor wall clocks, have nothing to do with determinism and in fact there's a good reason to not use vector clocks by default: they grow unbounded in a system where users (=IDs) are not known. In my experience, LWW is a good baseline merge strategy. I don't think it's at all correct to say that "the winner is essentially arbitrary" because it's not. The "last" in LWW can be determined based on any number of facts. For example "in case of concurrent operations, always take the one that is written by the ID of the user's mobile device", or "in case of concurrent operations, always take the one that <your preferred time/ordering service> says should come first". It'd be more correct say "the winner is based on the logical time ordering function, which may not be chronological, real world time order". As for the last comment, I'm pretty sure it's a CRDT system :) Want to elaborate your reasoning why you think it's not a CRDT? [1] "Merkle-CRDTs: Merkle-DAGs meet CRDTs" - https://arxiv.org/abs/2004.00107 https://arxiv.org/abs/2004.00107

sagichmal 6y ago

> CRDTs usually work as last-write-wins Um.

markhenderson 6y ago
Ah, sorry, yes I stepped on a rake and whacked myself in the face here. What I meant, since LWW is nomenclature for an alternative to CRDTs, is that the last writer by _logical clock_ in a CRDT, not by _wall clock_ time, will "win" the key.
- sagichmal 6y ago
  > LWW is nomenclature for an alternative to CRDTs I think you're still stepping on rakes...
markhenderson 6y ago
Can you elaborate?
- sagichmal 6y ago
  Last-Writer-Wins is a conflict resolution strategy that can be used by any kind of data type that needs conflicts resolved, CRDTs included. Unfortunately it's not a very good one: even if you use vector clocks instead of wall clocks, it doesn't give you much stronger guarantees than determinism. That is, given two concurrent writes, the winner is essentially arbitrary. LWW is a merge strategy of last resort; if that's the only thing your CRDT system offers, I'm not sure it's really fair to call it a CRDT system.
- haadcode 6y ago
  > Last-Writer-Wins is a conflict resolution strategy that can be used by any kind of data type that needs conflicts resolved, CRDTs included. Unfortunately it's not a very good one: even if you use vector clocks instead of wall clocks, it doesn't give you much stronger guarantees than determinism. That is, given two concurrent writes, the winner is essentially arbitrary. LWW is a merge strategy of last resort; if that's the only thing your CRDT system offers, I'm not sure it's really fair to call it a CRDT system. Can't reply to the comment below, so replying here. I believe what markhenderson was trying to say is that in OrbitDB, the default merge strategy for concurrent operations is LWW. The comment above is conflating a lot of things here. 1) determinism is exactly the guarantee one needs for CRDTs, and I'd argue generally is a good thing in distributed system but 2) adding vector clocks (OrbitDB uses Lamport clocks, or Merkle Clocks [1], by default), nor wall clocks, have nothing to do with determinism and in fact there's a good reason to not use vector clocks by default: they grow unbounded in a system where users (=IDs) are not known. In my experience, LWW is a good baseline merge strategy. I don't think it's at all correct to say that "the winner is essentially arbitrary" because it's not. The "last" in LWW can be determined based on any number of facts. For example "in case of concurrent operations, always take the one that is written by the ID of the user's mobile device", or "in case of concurrent operations, always take the one that <your preferred time/ordering service> says should come first". It'd be more correct say "the winner is based on the logical time ordering function, which may not be chronological, real world time order". As for the last comment, I'm pretty sure it's a CRDT system :) Want to elaborate your reasoning why you think it's not a CRDT? [1] "Merkle-CRDTs: Merkle-DAGs meet CRDTs" - https://arxiv.org/abs/2004.00107 https://arxiv.org/abs/2004.00107