Idempotent Transactional Workflow

(POPL 2013)

G. RamalingamKapil Vaswani

Microsoft Research India

Application

The Problem

Partitioned Data

scale-out

Can we simplify

writing suchapplications?

Transfer (amt, acct1, acct2) {Debit amt from acct1;Credit amt to acct2;

}

Transfer (amt, acct1, acct2) atomic {

Debit amt from acct1; Credit amt to acct2;

}

ACID Transaction+ Strong consistency− Distributed

transaction

Transfer (amt, acct1, acct2) atomic {Debit …}; atomic {Credit …};

Workflow− Weaker consistency

− No isolation+ No distributed transaction

What about process failure? Claim: Workflows are common in applications over partitioned data

Storage Layer

Application Logic

Stopping (non-byzantine) failure

The Problem

(failures handled by storage layer)

Goal• Fault-tolerance in application• A transactional workflow

engine• decentralized!

Modern Cloud Platforms

request response

Making Workflows Fault-Tolerant

Request or response

may be lost!

Taking a step back …

Resending messages

is a critical elementof fault-tolerance

Must be Idempotent!

(tolerate duplicatemessages)

Transfer (amt, acct1, acct2) {Debit amt from acct1;Credit amt to acct2;

}

Goal:Idempotent Fault-Tolerance

• (Idempotent Workflow)• A program is said to be idempotent & fault-

tolerant iff– its behavior is unaffected by process failures– its behavior is unaffected by duplicate input

requests• Behavioral equivalence:– duplicate output responses allowed– progress (liveness) conditions

• slightly weakened

request response

Making WorkflowsIdempotent & Fault-Tolerant

request response

Making Computations Idempotent

Make every effectful step idempotent:1. Associate unique id with every step2. Modify step to log execution of step3. Modify step to check if it has already

executedAll must be done atomically !

AutomatedIdempotent Fault-Tolerance

• As a library– In C# & F#– Technically, a monad

• As a compiler

• As a programming-language construct

Formal ResultsTheorem. A well-typed monadic program isidempotent and fault-tolerant.

Theorem. compile[e] is an idempotent and fault-tolerant realization of e.

Any (well-typed) program e can be automatically translated (compiled) into a program compile[e]

Idempotence: A Language Construct

• “idworkflow uid e’’transfer (uid, amt, acct1, acct2) { idworkflow uid {

atomic T1 Debit amt from acct1 atomic T2 Credit amt to acct2}}

}

Extensions• Compensating actions– Undo earlier actions when later actions

encounter logical failure

• Automatic retry– Detect process failures & restart

• Checkpointing– Restart at most recent checkpoint

Questions?

Fault-Tolerance & Idempotence: Simpler Together

Storage Layer

Application Logic

client

service

partitioneddata

Problem Setting