Debugging Designs Alongside Tla+
This postal service talks close why you lot should model your systems in addition to exhaustively seek these models/designs amongst the TLA+ framework. In the showtime part, I volition hash out why modeling your designs is of import in addition to beneficial, in addition to inwards the minute purpose I volition explicate why TLA+ is a real suitable framework for modeling, specially for distributed in addition to concurrent systems.
There are many corner cases because at that topographic point are many parameters, in addition to these arrive at interfere inwards unanticipated ways amongst each other. The corner cases violate your seemingly reasonable implicit assumptions close the organization components in addition to environment, e.g.,"1-hop is faster than 2-hops", "0-hop is faster than 1-hop", in addition to "processes locomote amongst the same rate". There are abundant race weather because today (with the ascension of SOA, cloud, in addition to microservices) all systems are distributed systems. Code that is supposedly "atomic block of execution" fails due to other processes executing concurrently. Finally, faults occur in addition to their effects are almost ever underestimated pre-deployment. Faults guide hold your organization to unanticipated states, in addition to from at that topographic point on amongst the interleaving of recovery actions amongst normal organization actions, the organization may survive thrown to fifty-fifty to a greater extent than unanticipated states.
In large software systems, which are inevitably distributed systems, at that topographic point are many unknown-unknowns in addition to an interplanetary space number of highly-improbable ways things tin perish wrong. Human encephalon in addition to reasoning cannot scale to grip all these possibilities. To alleviate these problems, the manufacture developed tools for amend observability in addition to fifty-fifty testing inwards production for improving availability. These tools are real of import in addition to indispensable. But past times the fourth dimension you lot figure out simply about inherent problems amongst your pattern it may survive likewise difficult in addition to expensive to prepare things. What you lot thought would survive the final 10% of the projection ends upwards taking 90% of your fourth dimension at production in addition to operations.
If you lot model your designs showtime in addition to exhaustively seek in addition to debug these models for correctness against corner cases, failed assumptions, concurrency, in addition to failures, you lot tin grab errors at the pattern fourth dimension in addition to prepare them earlier they prepare into problems in addition to perish costly to fix.
TLA+ uses basic math to model in addition to argue close algorithms: practical logic, laid theory, in addition to temporal logic are used for specifying systems. Best of all, the framework integrates a model checker that exhaustively tests your models to the confront of corner cases, failed assumptions, concurrency, in addition to failures. The model checker tries all executions possible for your model in addition to tells you lot for which executions, your invariants and system guarantees break.
Invariant-based reasoning
TLA+ framework promotes invariant-based reasoning to preclude the problems that arise from operational reasoning. In operational reasoning, you lot start amongst a "happy path", in addition to and hence you lot seek to figure out "what tin perish wrong?" in addition to how to preclude them. Of course, you lot ever autumn brusk inwards that enumeration of occupation scenarios in addition to overlook corner cases, race conditions, in addition to cascading failures. In contrast, invariant-based reasoning focuses on "what needs to perish right?" in addition to how to ensure this properties equally invariants of your organization at all times. Invariant-based reasoning takes a principled state-based rather than operation/execution-based persuasion of your system.
To attain invariant-based reasoning, nosotros specify security in addition to liveness properties for our models. Safety properties specify "what the organization is allowed to do". For example, at all times, all committed information is acquaint in addition to correct. Liveness properties specify "what the organization should eventually do". For example, whenever the organization receives a request, it must eventually reply to that request. In other words, security properties are concerned amongst "nothing bad happens", in addition to liveness properties amongst "something skilful eventually happens".
Modeling amongst TLA+
The TLA+ framework supports you lot inwards edifice a model in addition to figuring out its invariant properties inwards 2 major ways. Firstly, the math-based formal linguistic communication helps you lot compass precision spell all the same working amongst high-level declarative statements. Secondly, the integrated model checker exhaustively debugs your model to the confront of concurrency in addition to failures, in addition to produces counterexamples for which your candidate invariants fail. (After years of working amongst TLA+, I am all the same surprised close the counterexamples the model checkers spit out for my models: It is real tardily to overlook simply about scenarios, but the model checker sets you lot straight.) You address these problems past times improving your model or sometimes past times relaxing your candidate invariants, in addition to subsequently many iterations converge to an exhaustively debugged model which guarantees the invariants.
Building a TLA+ model is beneficial fifty-fifty for systems that are already implemented in addition to running. Through edifice the model, you lot acquire close your organization better, in addition to figure out simply about latent failure modes and right them earlier they occur inwards production.
Finally, maintaining a TLA+ model of your organization provides of import benefits for continuous development. While software systems ask to survive extended amongst novel features frequently, these extensions may interfere inwards unanticipated agency amongst the organization in addition to Pb to downtimes. With the TLA+ model at hand, you lot tin showtime add together these features to your model, in addition to catch/debug the problems at the design-level using the model-checker. This agency you lot resolve potential issues earlier they fifty-fifty perish problems.
TLA+ is practical
Since using TLA+ genuinely saves fourth dimension for edifice large software systems, TLA+ modeling is adopted equally a practise past times many software companies.
I am on sabbatical at Cosmos DB, Microsoft globally distributed cloud-native database. The squad has been using TLA+ to model the replication in addition to global distribution protocols in addition to exhaustively tests the designs for correctness against failures. We guide hold of late published the customer-facing purpose of the model which exactly defines the five consistency levels offered past times Cosmos DB.
Amazon has also used TLA+ modeling for simply about of their AWS offerings in addition to has written a squeamish sense written report on this. There are also reports of using TLA+ for modeling hardware systems equally well.
For the final iv years, I guide hold been incorporating TLA+ inwards my distributed systems classes. TLA+ enables students to acquire close concurrency in addition to invariant-based reasoning in addition to it provides them hands-on sense amongst distributed protocols. I also usage TLA+ exhaustively inwards my interrogation on novel distributed algorithms.
In my experience, it is possible to pick upwards TLA+ upwards inwards a twosome weeks. This is firstly because TLA+ adopts a real uncomplicated state-machine approach to model systems. A organization consists of: (1) A laid of variables which define the nation of the system, in addition to (2) A finite laid of assignments/actions that serves to transition the organization from 1 nation to another.
Furthermore, PlusCal provides syntactic a carbohydrate for the TLA+, which has a vogue to grow long (due to its low-level state-transition centric syntax) in addition to await cryptic for simply about people. PlusCal is a pseudocode for writing algorithms at a higher-level of abstraction, in addition to it is translated to the underlying TLA+ specifications for model checking. To give you lot simply about thought close the PlusCal, hither is an illustration of a PlusCal code for a database replica process. While this is a straightforward code, you lot tin run into a nondeterministic choice build "either or" inwards action. The model checker volition exhaustively seek all possible combinations of these "either or" actions in addition to banking concern check if a sure enough sequence would interruption 1 of your security in addition to liveness specifications.
My weblog includes many examples of TLA+/PlusCal modeling of distributed algorithms/systems.
LearnTLA provides a user-friendly introduction to TLA+/PlusCal.
Lamport's site includes TLA+/PlusCal resources (videos/books/examples) in addition to links to download the toolkit.
Modeling is important
If you lot guide hold worked on a large software system, you lot know that they are prone to corner cases, failed assumptions, race conditions, in addition to cascading faults.There are many corner cases because at that topographic point are many parameters, in addition to these arrive at interfere inwards unanticipated ways amongst each other. The corner cases violate your seemingly reasonable implicit assumptions close the organization components in addition to environment, e.g.,"1-hop is faster than 2-hops", "0-hop is faster than 1-hop", in addition to "processes locomote amongst the same rate". There are abundant race weather because today (with the ascension of SOA, cloud, in addition to microservices) all systems are distributed systems. Code that is supposedly "atomic block of execution" fails due to other processes executing concurrently. Finally, faults occur in addition to their effects are almost ever underestimated pre-deployment. Faults guide hold your organization to unanticipated states, in addition to from at that topographic point on amongst the interleaving of recovery actions amongst normal organization actions, the organization may survive thrown to fifty-fifty to a greater extent than unanticipated states.
In large software systems, which are inevitably distributed systems, at that topographic point are many unknown-unknowns in addition to an interplanetary space number of highly-improbable ways things tin perish wrong. Human encephalon in addition to reasoning cannot scale to grip all these possibilities. To alleviate these problems, the manufacture developed tools for amend observability in addition to fifty-fifty testing inwards production for improving availability. These tools are real of import in addition to indispensable. But past times the fourth dimension you lot figure out simply about inherent problems amongst your pattern it may survive likewise difficult in addition to expensive to prepare things. What you lot thought would survive the final 10% of the projection ends upwards taking 90% of your fourth dimension at production in addition to operations.
If you lot model your designs showtime in addition to exhaustively seek in addition to debug these models for correctness against corner cases, failed assumptions, concurrency, in addition to failures, you lot tin grab errors at the pattern fourth dimension in addition to prepare them earlier they prepare into problems in addition to perish costly to fix.
- Modeling showtime does non extend your evolution time, on the reverse it saves you lot fourth dimension past times reducing futile evolution attempts. Embarking on evolution amongst a flawed pattern almost ever ensures that the implementation is flawed. While having a precise in addition to right model at mitt does non guarantee that your implementation of the model is correct, it helps you lot avoid the big/intricate problems in addition to also provides a skilful reference for testing your implementation against.
- Constructing a precise model of your organization gives you lot clarity of thinking in addition to supports your evolution immensely. By modeling you lot detect close the inherent complexities of the problem; that helps you lot focus your attending in addition to ignore accidental/byproduct complexities.
- The model also helps you lot to communicate exactly amongst your squad in addition to others equally you lot avoid the ambiguity of natural linguistic communication in addition to the hand-waving in addition to generalizations involved.
- Finally amongst the model at hand, you lot also guide hold a run a jeopardy to gradually innovate pattern decisions, in addition to run into alternative ways to implement the design.
TLA+ is bully for modeling
TLA+ is a formal linguistic communication for describing in addition to reasoning close distributed in addition to concurrent systems. It is developed past times Dr. Leslie Lamport, Turing Award winner 2013. Lamport is a real of import figure inwards distributed systems due to his logical clocks work in addition to Paxos work amidst many others. For the final decade, he is real involved amongst improving the TLA+ framework to assist brand distributed systems to a greater extent than manageable.TLA+ uses basic math to model in addition to argue close algorithms: practical logic, laid theory, in addition to temporal logic are used for specifying systems. Best of all, the framework integrates a model checker that exhaustively tests your models to the confront of corner cases, failed assumptions, concurrency, in addition to failures. The model checker tries all executions possible for your model in addition to tells you lot for which executions, your invariants and system guarantees break.
Invariant-based reasoning
TLA+ framework promotes invariant-based reasoning to preclude the problems that arise from operational reasoning. In operational reasoning, you lot start amongst a "happy path", in addition to and hence you lot seek to figure out "what tin perish wrong?" in addition to how to preclude them. Of course, you lot ever autumn brusk inwards that enumeration of occupation scenarios in addition to overlook corner cases, race conditions, in addition to cascading failures. In contrast, invariant-based reasoning focuses on "what needs to perish right?" in addition to how to ensure this properties equally invariants of your organization at all times. Invariant-based reasoning takes a principled state-based rather than operation/execution-based persuasion of your system.
To attain invariant-based reasoning, nosotros specify security in addition to liveness properties for our models. Safety properties specify "what the organization is allowed to do". For example, at all times, all committed information is acquaint in addition to correct. Liveness properties specify "what the organization should eventually do". For example, whenever the organization receives a request, it must eventually reply to that request. In other words, security properties are concerned amongst "nothing bad happens", in addition to liveness properties amongst "something skilful eventually happens".
Modeling amongst TLA+
The TLA+ framework supports you lot inwards edifice a model in addition to figuring out its invariant properties inwards 2 major ways. Firstly, the math-based formal linguistic communication helps you lot compass precision spell all the same working amongst high-level declarative statements. Secondly, the integrated model checker exhaustively debugs your model to the confront of concurrency in addition to failures, in addition to produces counterexamples for which your candidate invariants fail. (After years of working amongst TLA+, I am all the same surprised close the counterexamples the model checkers spit out for my models: It is real tardily to overlook simply about scenarios, but the model checker sets you lot straight.) You address these problems past times improving your model or sometimes past times relaxing your candidate invariants, in addition to subsequently many iterations converge to an exhaustively debugged model which guarantees the invariants.
Building a TLA+ model is beneficial fifty-fifty for systems that are already implemented in addition to running. Through edifice the model, you lot acquire close your organization better, in addition to figure out simply about latent failure modes and right them earlier they occur inwards production.
Finally, maintaining a TLA+ model of your organization provides of import benefits for continuous development. While software systems ask to survive extended amongst novel features frequently, these extensions may interfere inwards unanticipated agency amongst the organization in addition to Pb to downtimes. With the TLA+ model at hand, you lot tin showtime add together these features to your model, in addition to catch/debug the problems at the design-level using the model-checker. This agency you lot resolve potential issues earlier they fifty-fifty perish problems.
TLA+ is practical
Since using TLA+ genuinely saves fourth dimension for edifice large software systems, TLA+ modeling is adopted equally a practise past times many software companies.
I am on sabbatical at Cosmos DB, Microsoft globally distributed cloud-native database. The squad has been using TLA+ to model the replication in addition to global distribution protocols in addition to exhaustively tests the designs for correctness against failures. We guide hold of late published the customer-facing purpose of the model which exactly defines the five consistency levels offered past times Cosmos DB.
Amazon has also used TLA+ modeling for simply about of their AWS offerings in addition to has written a squeamish sense written report on this. There are also reports of using TLA+ for modeling hardware systems equally well.
For the final iv years, I guide hold been incorporating TLA+ inwards my distributed systems classes. TLA+ enables students to acquire close concurrency in addition to invariant-based reasoning in addition to it provides them hands-on sense amongst distributed protocols. I also usage TLA+ exhaustively inwards my interrogation on novel distributed algorithms.
In my experience, it is possible to pick upwards TLA+ upwards inwards a twosome weeks. This is firstly because TLA+ adopts a real uncomplicated state-machine approach to model systems. A organization consists of: (1) A laid of variables which define the nation of the system, in addition to (2) A finite laid of assignments/actions that serves to transition the organization from 1 nation to another.
Furthermore, PlusCal provides syntactic a carbohydrate for the TLA+, which has a vogue to grow long (due to its low-level state-transition centric syntax) in addition to await cryptic for simply about people. PlusCal is a pseudocode for writing algorithms at a higher-level of abstraction, in addition to it is translated to the underlying TLA+ specifications for model checking. To give you lot simply about thought close the PlusCal, hither is an illustration of a PlusCal code for a database replica process. While this is a straightforward code, you lot tin run into a nondeterministic choice build "either or" inwards action. The model checker volition exhaustively seek all possible combinations of these "either or" actions in addition to banking concern check if a sure enough sequence would interruption 1 of your security in addition to liveness specifications.
To acquire more
There is a real active TLA+ forum at Google Groups. Leslie Lamport chimes inwards several threads.My weblog includes many examples of TLA+/PlusCal modeling of distributed algorithms/systems.
LearnTLA provides a user-friendly introduction to TLA+/PlusCal.
Lamport's site includes TLA+/PlusCal resources (videos/books/examples) in addition to links to download the toolkit.
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