Project meeting T0+24, 22-23 November 2012, Nantes, France
This meeting is joint with the ANR Streams M24 meeting.
More information is also available on STREAMS website.
- Carlos Baquero, U. Minho (invited);
- Valter Balegas, UNL, Concordant;
- Annette Bieniusa, Uni Kaiserslautern (invited);
- Russell Brown, Basho (invited speaker);
- Alexey Gotsman, IMDEA (invited speaker);
- Luis Ibáñez, U. Nantes, Streams;
- Stéphane Martin, U. Rennes, Streams;
- Pascal Molli, U. Nantes, Streams (organiser);
- Achour Mostefaoui, U. Nantes, Streams;
- Mahsa Najafzadeh, INRIA, Concordant;
- David Navalho, UNL, Concordant;
- Brice Nedelec, U. Nantes, Streams;
- Gérald Oster, LORIA, Streams (co-chair);
- Nuno Preguiça, UNL, Concordant (by video);
- Rodrigo Rodrigues, UNL, Concordant (by video);
- Hala Skaf-Molli, U. Nantes, Streams;
- Marc Shapiro, INRIA, Concordant and Streams (co-chair);
- Pascal Urso, LORIA, Concordant and Streams;
- Marek Zawirski, INRIA, Concordant and Streams;
Talks and discussion sessions
Semantics of eventual consistency (Alexey Gotsman)
- Interesting formalisation. Clean separation of concerns.
- Concordant authors wish to revisit the "Semantics of concurrent sets" paper with Alexey.
- Liveness: no modal logic; consider infinite histories. Expressed in the same logic, but proof techniques will differ.
- You don't mention semi-lattice properties, why? Hidden in the data type semantics.
SwiftCloud: It's Time to Move the Data to the Edge (Annette Bieniusa)
- Contrast to Riak? Riak state-based, oriented towards replication within data centre and DC geo-replication.
CRDT transactions in a scalable way with SwiftCloud (Marek Zawirski)
- How does SwiftCloud scale to large numbers of DCs, clients? Epidemic propagation + quorum reads in DCs. No vector clocks entriesfor clients.
- Why is it hard? Partial replication, dynamic topology, pruning.
SwiftFS: A CRDT Filesystem (Annette Bieniusa)
- What are the correctness conditions for composing CRDTs? See Carlos' presentation later in the workshop.
- Red-Blue distinction not sufficient: operations that commute with one another but not with strong ones.
- Total order of red ops? Implemented with ~atomic broadcast.
- How does escrow fit in?
Limitations of SwiftCloud approach (Valter Balegas)
- Referential integrity: is it an issue of CRDTs or just a DB issue? Both: DB, but harder with CRDTs and SI.
- Do you leverage commutativity? Yes (and associativity and compensation).
Benchmarking collaborative editing CRDTs (Pascal Urso)
Support for CRDTs on riak_core (Russell Brown)
- Do Riak customers require transactions? No, just basic CRDTs are a massive improvement for them. SwiftCloud is way ahead.
- Small replica count (3--5) but vector clocks grow. Consensus required to clean vectors.
- Read-repair only so far.
Quasi-Causal Delivery (Achour Mostefaoui)
- Collision detection: no false negatives, but false positives possible.
- How to recover from collisions? Dependent on data semantics.
Composition and Fragmenting of CRDTs (Carlos Baquero)
- Composition is not just juxtaposition: need more rules. For instance: graph = composition of sets + rule about add-edge(v,v') || remove-vertex(v). Extra layer of abstraction map that hides some internal states that do not make sense.
- Decomposition: Consistency between fragments? Need more mechanism???
- Hand-off counters: monotonic protocol for moving private state from secondary replica to primary replica.
Live Linked Data (Luis Ibáñez)
- Implemented in Corese, W3C reference implementation.
- To be submitted to W3C as a standard.
Programming CRDTs on SwiftCloud (Marek Zawirski)
- Why must CRDTs be both state- and op-based? This is an optimization. State-based allows more efficient representation after pruning. We are not sure if the state-based part needs to mergeable (something to discuss).
- Why two different abstractions (versioned + transactional) for a single object type? The transactional abstraction is simpler, isolated.
Designing new CRDT types using layers Δ (Stéphane Martin)
- Link with Carlos' talk on composition + extra abstraction map.
- D3.3 replaced by SwiftCloud paper + tech report on 1-phase handoff (to
- D5.2: delayed to M36
- D6.2+6.3: delayed to M36
- M30 Concordant workshop: Porto or Braga. Organisers: Nuno and Carlos.
- Final workshop:
- Organise as a follow-up to Dagstuhl workshop on consistency in distributed systems (Feb. 2013).
- Target date: between November 2013 and April 2014.
- Find a convenient location. Co-locate with some conference?
- Set up a PC: Marc (chair), Alexey, Ahmed, DB and systems people, mostly from Europe.
New ANR proposals:
- ANR Blanc International with Germany: Kaiserslautern, Rostock, INRIA, LORIA. In charge: Annette. Deadline: 17 January.
- ANR Infra: In charge: Pascal U. Deadline: 7 February.
- Interested parties include INRIA, LORIA, Nantes, Grenoble, Eurecom, LIAFA, PlayAdz, Technicolor.
- Use cases to be defined by industrial partners.
- Possible application area: games at the edge (geographical locality of players at a same server; disconnected operation; latency and QoS constraints).
- Stream computation using CRDTs.
- Placement of data and computation.
- Byzantine fault tolerance, security.
- Tools for analysis, test, debugging of conflict-free programs.