Teaching Paxi
Paxos household unit of measurement of protocols (which I refer to equally Paxi) is immensely useful for edifice distributed systems due to their fantabulous fault-tolerance properties. Many cloud computing services together with distributed databases employ Paxi for the world motorcar replication (SMR). Paxi save the security of consensus work (no 2 nodes commit dissimilar values for the same slot) fifty-fifty to the human face upwards of a fully asynchronous execution, crash faults, message losses, together with network partitions. Paxi satisfy liveness of consensus work (some value is eventually committed for the slot) when the organization moves exterior the realm of the coordinated laid on together with FLP impossibility results.
Paxi are perennially misunderstood together with their sophistication underrated. While at that spot has been a lot of run on Paxi, nosotros accept been able to explore only a fraction of the algorithm blueprint space. A hitting evidence of this arrived inward 2016, where nosotros had a flexible quorum breakthrough later xxx years, which no i had anticipated.
There is a demand to unpack Paxi together with explicate them ameliorate for both students together with practitioners alike. Paxi supply a bang-up chance for instruction the principles of distributed systems together with nosotros should seize on this opportunity.
Raft has been proposed equally a unproblematic explanation of consensus together with Paxos. While many developers honey the operationalized explanation vogue of Raft together with the implementation that accompany it, tying the explanation to a constrained implementation is unnecessarily restrictive. The generality of Paxos household unit of measurement of protocols are lost, together with the context together with principles of distributed consensus is non communicated satisfactorily.
We demand ameliorate explanations of non simply Paxi but the context together with derivation of these protocols, explaining why each activity is needed together with why this is a difficult problem. However, explaining the protocol entirely inward a declarative agency using derivation is also difficult to follow for students, together with to a greater extent than or less intuition should locomote provided equally well. The students should also locomote provided amongst ample chance to acquire a lot of hands-on exercise together with sense amongst the protocols, their implementation, together with their integration into practical applications/systems.
Every twelvemonth my favorite role of the distributed systems shape is when I acquire to learn Paxos for 2 weeks. Every year, I am able to do a ameliorate task of it past times gradual/gradient-descent improvement. But these days, I am planning an overhaul of how I learn Paxos, together with set to a greater extent than or less existent endeavour behind this to realize my ideal setup for instruction Paxi. Here is my explanation of this setup inward damage of the content of the module together with supporting tools for it.
To learn nearly the context, the module volition embrace questions such as: What makes the distributed consensus work thus hard? How has our agreement of distributed consensus modify later "Attacking Generals" together with FLP impossibility results? What are the cornercases that haunt correctness together with liveness? Covering these volition tending the students appreciate the depth together with ingenuity of Paxi.
To learn nearly derivation together with principles of the protocol, the module volition employ a stepwise refinement from the high-level consensus specification to an intermediate round-based abstraction (for which Heidi's generalized consensus framework is a practiced candidate), together with thus to the Paxos algorithm. The module volition explore both leader-based (as inward Paxi) together with non-leader-based (as inward Ben-Or together with Texel) refinements of this round-base intermediate specification, together with volition hash out the advantages together with disadvantages of each approach.
The module volition also relate distributed consensus to decentralized protocols inward the blockchain domain. By showing a Texel to Avalanche transition, the module volition necktie consensus (where Texel shows an option solution to leader based consensus equally inward Paxos) to blockchains (where Avalanche shows how to scale together with operationalize Texel to large-scale decentralized environments).
To enable the students to experiment at the implementation level, nosotros volition utilisation our Paxi framework implemented inward Go (available equally opensource at https://github.com/ailidani/paxi). Our Go Paxi framework provides a leveled playground for protocol evaluation together with comparison. The protocols are implemented using mutual edifice blocks for networking, message handling, quorums, etc., together with the developer needs to only create sum inward 2 modules for describing the distributed coordination protocol. Paxi includes benchmarking back upwards to evaluate the protocols inward damage of their performance, availability, scalability, together with a linearizability checker to banking concern check the protocol for consistency. We accept a dozen Paxos variants already implemented inward Paxi. We volition invite students to implement more, peculiarly Byzantine versions of Paxos protocols, together with reckon tie-ins to permissioned blockchain protocols. In guild to link the high-level blueprint to the implementation, nosotros volition supply a mapping from TLA+ model to the Paxi implementation of distributed consensus.
In guild to showcase the integration of Paxi protocols to distributed systems applications, nosotros volition utilisation the globally distributed database FleetDB (https://github.com/acharapko/fleetdb) equally the hands-on application. We volition starting fourth dimension extend FleetDB to locomote instantiable amongst dissimilar Paxi protocols equally plugins, together with brand the Paxi protocols exchangeable based on workload together with environment. FleetDB tin atomic number 82 the agency together with tending distributed databases for integrating Paxi protocols inward their replication operation. Currently only a handful databases (including Spanner together with CockroachDB) utilisation Paxos equally role of their replication. Although Paxos provides fantabulous fault-tolerance properties together with forestall whatever loss of consistency, the vanilla Paxos protocol is non a practiced jibe for WAN deployments, together with performs poorly nether for certain topologies together with workloads.
Couple other tools are worth mentioning. One is the DSLabs tool from UW for model checking distributed systems projects implementations. Another is the DistAlgo tool from SUNY Stonybrook.
While instruction Paxos, I convey v students to the board to perform alive reenactments the Paxos consensus algorithm (each i simulating a Paxos node) nether several mistake scenarios. This role is the most fun i together with most beneficial inward my distributed systems course. I do this twice inward 2 dissimilar classes. Through this exercises the students acquire to larn how the protocol works, together with encounter how it deals amongst the cornercases.
Another favorite minute for me is to lookout adult man the students intend they empathize the protocol, together with thus forget nearly it together with acquire confused again. This is inevitable, together with a large role of how learning works. Learning needs self evaluation together with self correction. Without doing run yourself, y'all can't genuinely larn anything. Easy come, slowly go. I also similar watching the students larn the application of the single-instance Paxos inward the MultiPaxos protocol, together with encounter them larn nearly to a greater extent than or less Paxos variants. The students thus realize that what they learned was only the tip of the iceberg.
2. Are at that spot other algorithms/systems y'all suggest tin serve equally capstone projects inward a distributed systems class?
Paxi are perennially misunderstood together with their sophistication underrated. While at that spot has been a lot of run on Paxi, nosotros accept been able to explore only a fraction of the algorithm blueprint space. A hitting evidence of this arrived inward 2016, where nosotros had a flexible quorum breakthrough later xxx years, which no i had anticipated.
There is a demand to unpack Paxi together with explicate them ameliorate for both students together with practitioners alike. Paxi supply a bang-up chance for instruction the principles of distributed systems together with nosotros should seize on this opportunity.
Problems amongst instruction Paxi
Most coverage of Paxos inward courses is dry out together with superficial: the Paxos protocol is described together with the students memorize the protocol. While the Paxos protocol looks simple, it has a lot depth together with subtleties. It is non possible to appreciate these together with genuinely empathize distributed consensus past times simply memorizing the Paxos protocol. To empathize Paxos, y'all should non only empathize how it works, but also why it works, together with what cornercases it prevents, together with how else it could locomote realized.Raft has been proposed equally a unproblematic explanation of consensus together with Paxos. While many developers honey the operationalized explanation vogue of Raft together with the implementation that accompany it, tying the explanation to a constrained implementation is unnecessarily restrictive. The generality of Paxos household unit of measurement of protocols are lost, together with the context together with principles of distributed consensus is non communicated satisfactorily.
We demand ameliorate explanations of non simply Paxi but the context together with derivation of these protocols, explaining why each activity is needed together with why this is a difficult problem. However, explaining the protocol entirely inward a declarative agency using derivation is also difficult to follow for students, together with to a greater extent than or less intuition should locomote provided equally well. The students should also locomote provided amongst ample chance to acquire a lot of hands-on exercise together with sense amongst the protocols, their implementation, together with their integration into practical applications/systems.
Every twelvemonth my favorite role of the distributed systems shape is when I acquire to learn Paxos for 2 weeks. Every year, I am able to do a ameliorate task of it past times gradual/gradient-descent improvement. But these days, I am planning an overhaul of how I learn Paxos, together with set to a greater extent than or less existent endeavour behind this to realize my ideal setup for instruction Paxi. Here is my explanation of this setup inward damage of the content of the module together with supporting tools for it.
Course module on Paxi
The Paxi class module volition supply context together with learn the principles together with derivation of the protocols.To learn nearly the context, the module volition embrace questions such as: What makes the distributed consensus work thus hard? How has our agreement of distributed consensus modify later "Attacking Generals" together with FLP impossibility results? What are the cornercases that haunt correctness together with liveness? Covering these volition tending the students appreciate the depth together with ingenuity of Paxi.
To learn nearly derivation together with principles of the protocol, the module volition employ a stepwise refinement from the high-level consensus specification to an intermediate round-based abstraction (for which Heidi's generalized consensus framework is a practiced candidate), together with thus to the Paxos algorithm. The module volition explore both leader-based (as inward Paxi) together with non-leader-based (as inward Ben-Or together with Texel) refinements of this round-base intermediate specification, together with volition hash out the advantages together with disadvantages of each approach.
The module volition also relate distributed consensus to decentralized protocols inward the blockchain domain. By showing a Texel to Avalanche transition, the module volition necktie consensus (where Texel shows an option solution to leader based consensus equally inward Paxos) to blockchains (where Avalanche shows how to scale together with operationalize Texel to large-scale decentralized environments).
Supporting tools
To allow the students to experiment amongst the protocols at the blueprint level, nosotros volition supply TLA+/Pluscal modeling of Paxos together with variants. With these, the students volition locomote able to model-check Paxi protocols together with experiment amongst modifications to encounter which security together with progress properties are satisfied nether dissimilar environments.To enable the students to experiment at the implementation level, nosotros volition utilisation our Paxi framework implemented inward Go (available equally opensource at https://github.com/ailidani/paxi). Our Go Paxi framework provides a leveled playground for protocol evaluation together with comparison. The protocols are implemented using mutual edifice blocks for networking, message handling, quorums, etc., together with the developer needs to only create sum inward 2 modules for describing the distributed coordination protocol. Paxi includes benchmarking back upwards to evaluate the protocols inward damage of their performance, availability, scalability, together with a linearizability checker to banking concern check the protocol for consistency. We accept a dozen Paxos variants already implemented inward Paxi. We volition invite students to implement more, peculiarly Byzantine versions of Paxos protocols, together with reckon tie-ins to permissioned blockchain protocols. In guild to link the high-level blueprint to the implementation, nosotros volition supply a mapping from TLA+ model to the Paxi implementation of distributed consensus.
In guild to showcase the integration of Paxi protocols to distributed systems applications, nosotros volition utilisation the globally distributed database FleetDB (https://github.com/acharapko/fleetdb) equally the hands-on application. We volition starting fourth dimension extend FleetDB to locomote instantiable amongst dissimilar Paxi protocols equally plugins, together with brand the Paxi protocols exchangeable based on workload together with environment. FleetDB tin atomic number 82 the agency together with tending distributed databases for integrating Paxi protocols inward their replication operation. Currently only a handful databases (including Spanner together with CockroachDB) utilisation Paxos equally role of their replication. Although Paxos provides fantabulous fault-tolerance properties together with forestall whatever loss of consistency, the vanilla Paxos protocol is non a practiced jibe for WAN deployments, together with performs poorly nether for certain topologies together with workloads.
Couple other tools are worth mentioning. One is the DSLabs tool from UW for model checking distributed systems projects implementations. Another is the DistAlgo tool from SUNY Stonybrook.
MAD questions
1. What is your favorite instruction technique for Paxi?While instruction Paxos, I convey v students to the board to perform alive reenactments the Paxos consensus algorithm (each i simulating a Paxos node) nether several mistake scenarios. This role is the most fun i together with most beneficial inward my distributed systems course. I do this twice inward 2 dissimilar classes. Through this exercises the students acquire to larn how the protocol works, together with encounter how it deals amongst the cornercases.
Another favorite minute for me is to lookout adult man the students intend they empathize the protocol, together with thus forget nearly it together with acquire confused again. This is inevitable, together with a large role of how learning works. Learning needs self evaluation together with self correction. Without doing run yourself, y'all can't genuinely larn anything. Easy come, slowly go. I also similar watching the students larn the application of the single-instance Paxos inward the MultiPaxos protocol, together with encounter them larn nearly to a greater extent than or less Paxos variants. The students thus realize that what they learned was only the tip of the iceberg.
2. Are at that spot other algorithms/systems y'all suggest tin serve equally capstone projects inward a distributed systems class?
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