5 ms·

Right. The flavors of Lamport clocks I stated in the article are used in CRDTs designs I studied. While CRDTs are eventually consistent, I wouldn't dismiss the

Right. The flavors of Lamport clocks I stated in the article are used in CRDTs designs I studied. While CRDTs are eventually consistent, I wouldn't dismiss them as such without qualification. They are causal-consistent when offline and sequential-consistent when online. (This duality is why CRDTs have been hard for me to wrap my head around them).

preseinger 4y ago
the properties of CRDTs are invariant to the concept of "online" or "offline" i think you may be making things harder for yourself by thinking in these terms
- thoughtlede 4y ago
  Indeed, certain properties of CRDT are invariable to network state. However, it is worth pointing out that in ops-based CRDT “implementations”, you deal with local ops case differently from remote ops case. That is, while the properties are invariant, how you produce them are different. So I was on a my quest to understand the “essence” of CRDTs, not just understand them to be able to practically use them. Atomic broadcast and Raft were easy enough for me to wrap my head around (although quite challenging to implement). But not CRDTs. I found common statements about CRDTs that they ensure ops to be commutative to be superficial. A slightly deeper characteristic was that ops-based CRDTs are just causally-linked ops (aka causal tree). But what about state-based? Finally, when I realized CRDTs are dual-consistent and that’s what makes any data structure a CRDT, that was a moment of epiphany for me.
  - preseinger 4y ago
    so an important "eureka" observation about CRDTs is that the op-based model is theoretical, useful for proofs insofar as any op-based CRDT can be translated to a state-based CRDT, but not something that can actually exist in practice all practical CRDTs are state-based CRDTs that's because it's not possible to assert a causal order for arbitrary operations on arbitrary data structures (in useful contexts) CRDTs don't _ensure_ ops are commutative (and associative, and idempotent) but rather they _require_ that ops are commutative (and associative, and idempotent) and it's definitely not the case that any data structure is a CRDT, it's possible to translate many data structures to CRDTs, but that translation rarely preserves the operations in full fidelity
    - thoughtlede 4y ago
      > CRDTs don't _ensure_ ops are commutative (and associative, and idempotent) but rather they _require_ that ops are commutative (and associative, and idempotent) I disagree. You can create a CRDT flavor of data structure whose ops are not commutative. For example, a Set's add and delete operations. These are not commutative. You cannot switch the order of the ops for meaningfully processing them. However, you can create a CRDT Set. You do that by adding metadata to the ops, and having the instances always process them in the only order that makes sense even if such instances receive the ops in a different order. In that sense, you are "ensuring" ops are behaving like they are commutative and not "requiring" them to be so. > it's definitely not the case that any data structure is a CRDT I could have worded my statement better. I meant any data structure that has the aforementioned duality property is a CRDT. Not that any data structure unconditionally can be translated into a CRDT. > an important "eureka" observation about CRDTs is that the op-based model is theoretical, I do not understand your statement. Perhaps you could elaborate. My understanding is that a CRDT is op-based or state-based depending on what is "communicated" between the instances. If ops are communicated, then it is op-based CRDT, whereas if states (or delta-states) are communicated, then it is state-based. At least in that sense, op-based model is NOT theoretical. Perhaps you have a different point in mind that I fail to observe.
      - preseinger 4y ago
        > I disagree. You can create a CRDT flavor of data structure whose ops are not commutative. For example, a Set's add and delete operations. These are not commutative. You cannot switch the order of the ops for meaningfully processing them. However, you can create a CRDT Set. set add (union) is commutative, set delete is not. so a set with only the add (union) operation is a CRDT, but a set that supports both add and delete is not a CRDT, at least not without very specific caveats. you can create an add-only CRDT set, or an add-remove CRDT set, or a variety of other CRDT sets that support specific operations with specific caveats. but you can't create a CRDT set that is a fully-fledged set, with all the operations that sets generally provide. > You do that by adding metadata to the ops, and having the instances always process them in the only order that makes sense even if such instances receive the ops in a different order. so this is the crux of the issue, I think -- "having the instances always process [ops] in the same order" is basically not possible in any real-world network first, because it's not possible to decide what the "correct" set of ops actually is, nodes are always subject to partitions, faults, etc. which prevent reliable dissemination of knowledge, and plus all the stuff about light cones and etc. second, because "the same order" implies a specific total ordering of events is unknowable (see prior comments) > you are "ensuring" ops are behaving like they are commutative and not "requiring" them to be so converting a non-commutative operation to a commutative operation in a distributed system requires reliable delivery, which no network provides the whole point of CRDTs is that they give you a formally strong version of eventual consistency that holds even in the face of (unavoidably) unreliable delivery > My understanding is that a CRDT is op-based or state-based depending on what is "communicated" between the instances. If ops are communicated, then it is op-based CRDT, whereas if states (or delta-states) are communicated, then it is state-based. this is true in the abstract, but the issue is that "what is communicated" is not a given, it's subject to the choices you make when you encounter a network fault -- or in the CAP model, a partition CRDTs are tools for eventually consistent (AP) systems, which means that you have to keep making forward progress if there are partitions, which means that message delivery between nodes is not reliable, it can always fail for state-based CRDTs if you fail to deliver a message it's fine, the information in that message is not lost forever, it will be included in the next message, and (if the partition is eventually healed) the ultimate state will converge. this is also true for delta states but for op-based CRDTs if you fail to deliver a message it's not fine, the information in that message is lost forever, it won't be included in the next message, and (even if the partition is eventually healed) the ultimate state will not converge

preseinger 4y ago

the properties of CRDTs are invariant to the concept of "online" or "offline" i think you may be making things harder for yourself by thinking in these terms

thoughtlede 4y ago
Indeed, certain properties of CRDT are invariable to network state. However, it is worth pointing out that in ops-based CRDT “implementations”, you deal with local ops case differently from remote ops case. That is, while the properties are invariant, how you produce them are different. So I was on a my quest to understand the “essence” of CRDTs, not just understand them to be able to practically use them. Atomic broadcast and Raft were easy enough for me to wrap my head around (although quite challenging to implement). But not CRDTs. I found common statements about CRDTs that they ensure ops to be commutative to be superficial. A slightly deeper characteristic was that ops-based CRDTs are just causally-linked ops (aka causal tree). But what about state-based? Finally, when I realized CRDTs are dual-consistent and that’s what makes any data structure a CRDT, that was a moment of epiphany for me.
- preseinger 4y ago
  so an important "eureka" observation about CRDTs is that the op-based model is theoretical, useful for proofs insofar as any op-based CRDT can be translated to a state-based CRDT, but not something that can actually exist in practice all practical CRDTs are state-based CRDTs that's because it's not possible to assert a causal order for arbitrary operations on arbitrary data structures (in useful contexts) CRDTs don't _ensure_ ops are commutative (and associative, and idempotent) but rather they _require_ that ops are commutative (and associative, and idempotent) and it's definitely not the case that any data structure is a CRDT, it's possible to translate many data structures to CRDTs, but that translation rarely preserves the operations in full fidelity
  - thoughtlede 4y ago
    > CRDTs don't _ensure_ ops are commutative (and associative, and idempotent) but rather they _require_ that ops are commutative (and associative, and idempotent) I disagree. You can create a CRDT flavor of data structure whose ops are not commutative. For example, a Set's add and delete operations. These are not commutative. You cannot switch the order of the ops for meaningfully processing them. However, you can create a CRDT Set. You do that by adding metadata to the ops, and having the instances always process them in the only order that makes sense even if such instances receive the ops in a different order. In that sense, you are "ensuring" ops are behaving like they are commutative and not "requiring" them to be so. > it's definitely not the case that any data structure is a CRDT I could have worded my statement better. I meant any data structure that has the aforementioned duality property is a CRDT. Not that any data structure unconditionally can be translated into a CRDT. > an important "eureka" observation about CRDTs is that the op-based model is theoretical, I do not understand your statement. Perhaps you could elaborate. My understanding is that a CRDT is op-based or state-based depending on what is "communicated" between the instances. If ops are communicated, then it is op-based CRDT, whereas if states (or delta-states) are communicated, then it is state-based. At least in that sense, op-based model is NOT theoretical. Perhaps you have a different point in mind that I fail to observe.
    - preseinger 4y ago
      > I disagree. You can create a CRDT flavor of data structure whose ops are not commutative. For example, a Set's add and delete operations. These are not commutative. You cannot switch the order of the ops for meaningfully processing them. However, you can create a CRDT Set. set add (union) is commutative, set delete is not. so a set with only the add (union) operation is a CRDT, but a set that supports both add and delete is not a CRDT, at least not without very specific caveats. you can create an add-only CRDT set, or an add-remove CRDT set, or a variety of other CRDT sets that support specific operations with specific caveats. but you can't create a CRDT set that is a fully-fledged set, with all the operations that sets generally provide. > You do that by adding metadata to the ops, and having the instances always process them in the only order that makes sense even if such instances receive the ops in a different order. so this is the crux of the issue, I think -- "having the instances always process [ops] in the same order" is basically not possible in any real-world network first, because it's not possible to decide what the "correct" set of ops actually is, nodes are always subject to partitions, faults, etc. which prevent reliable dissemination of knowledge, and plus all the stuff about light cones and etc. second, because "the same order" implies a specific total ordering of events is unknowable (see prior comments) > you are "ensuring" ops are behaving like they are commutative and not "requiring" them to be so converting a non-commutative operation to a commutative operation in a distributed system requires reliable delivery, which no network provides the whole point of CRDTs is that they give you a formally strong version of eventual consistency that holds even in the face of (unavoidably) unreliable delivery > My understanding is that a CRDT is op-based or state-based depending on what is "communicated" between the instances. If ops are communicated, then it is op-based CRDT, whereas if states (or delta-states) are communicated, then it is state-based. this is true in the abstract, but the issue is that "what is communicated" is not a given, it's subject to the choices you make when you encounter a network fault -- or in the CAP model, a partition CRDTs are tools for eventually consistent (AP) systems, which means that you have to keep making forward progress if there are partitions, which means that message delivery between nodes is not reliable, it can always fail for state-based CRDTs if you fail to deliver a message it's fine, the information in that message is not lost forever, it will be included in the next message, and (if the partition is eventually healed) the ultimate state will converge. this is also true for delta states but for op-based CRDTs if you fail to deliver a message it's not fine, the information in that message is lost forever, it won't be included in the next message, and (even if the partition is eventually healed) the ultimate state will not converge