Fault Tolerance Via Idempotence (Paper Summary)

This newspaper (by Ramalingam in addition to Vaswani) proposes a generic/automatic way to utilisation idempotence---which requires the organisation to tolerate duplicate requests--- for treatment communication in addition to procedure failures inwards a software organisation efficiently.

Using idempotence for dealing alongside procedure in addition to communication failures has been investigated likewise inwards the "Idempotence is non a medical condition", but at that spot no (generic) solution for achieving idempotence was provided.

Automating idempotence

For automatically ensuring idempotence for a system, the newspaper makes utilisation of the monad idea, in addition to designs & implements the idempotence monad.

The regard underlying the idempotence monad is simple: "Given a unique identifier associated alongside a computation, the monad adds logging in addition to checking to each effectful measurement inwards the workflow to ensure idempotance". Armed alongside this idempotence monad, the newspaper shows that idempotence, when coupled alongside a unproblematic retry mechanism, provides a solution to the procedure in addition to communication failures.

Consider a banking concern example, where the parameter requestId serves to distinguish betwixt dissimilar transfer requests in addition to position duplicate requests. This service tin live on made idempotent in addition to failfree past times (1) using this identifier to log debit in addition to credit operations, in addition to (2) modifying the debit in addition to credit steps to check--using the log-- if the steps bring already been performed. This strategy ensures that multiple (potentially partial in addition to concurrent) invocations of transfer alongside the same identifier bring the same number every bit a unmarried invocation.

But, manually ensuring idempotence is tedious, error-prone in addition to makes implementation less comprehensible. So the newspaper describes a monad-based library that realizes idempotence in addition to failure-freedom inwards a generic way. "A type alongside a monad structure defines what it way to chain operations, or nest functions of that type together. This allows the programmer to create pipelines that procedure information inwards steps, inwards which each activeness is decorated alongside additional processing rules provided past times the monad. As such, monads bring been described every bit programmable semicolons." This is likewise the aspect-oriented programming way.

The Idempotence Monad:

• Associate every computation instance (that nosotros wishing to brand idempotent) alongside a unique identifier.
• Associate every measurement inwards an idempotent computation alongside a unique number.
• Whenever an effectful measurement is executed, persistently tape the fact that this measurement has executed in addition to salve the value produced past times this step.
• Every effectful measurement is modified to kickoff banking concern gibe if this measurement has already been    executed. If it has, in addition to thus the previously saved value (for this step) is used instead of executing the measurement again.

Decentralized Idempotence

The implementation of the idempotence monad is designed to piece of employment alongside key-value information stores, in addition to does non assume a dedicated storage for logs that tin live on accessed atomically alongside each transaction. The implementation adopts the key-value datastore to copy a distinct address infinite for logging.

This leads to a decentralized implementation of idempotence that does non require whatever centralized storage or whatever (distributed) coordination betwixt dissimilar stores. Thus, this implementation of idempotance preserves the decentralized nature of the underlying computation.

Evaluation in addition to critique

The idempotence monad has been implemented inwards C# in addition to F# targeting Windows Azure, which provides a key-value store. The evaluations inwards the newspaper shows that the performance overheads of using the idempotence monad over hand-coded implementations of generic idempotence are non significant.

But, this is an incomplete evaluation: The compared baseline of hand-coded implementations is constrained to add together logging in addition to checking to the functioning ---as it was the instance for the monad. So, the alone overheads inwards monad solution over hand-coded are that the compiler generated monad code "tend to capture a lot to a greater extent than nation than hand-coded implementations, in addition to may likewise add together unnecessary logging to transactions that are already idempotent". It could live on interesting to repeat the evaluations alongside a non-constrained developer that tin implement custom solution to fault-tolerance leveraging to a greater extent than on the application logic in addition to implementation information.

Another affair to compare inwards time to come evaluations could live on to that of option generic fault-tolerance approaches: e.g., using Replicated State Machines (e.g., via Paxos, or primary/secondary replication), or using the CRDT approach (for replicating the node, when it is applicable).

Actually, taking a measurement dorsum to re-examine the problem, nosotros tin let out that leveraging on TCP gets us through almost of the way: We won't bring message losses if nosotros utilisation TCP, in addition to would become message losses alone because of procedure crashes. So, nosotros alone would demand a way to tolerate procedure crashes, tell via replication, checkpointing, or a fast-restartable system.

Finally, the trends toward providing amend plumbing at datacenters likewise alleviates the communication/process failures problems. (For example, Amazon Simple Queue Service (Amazon SQS) offers a reliable, highly scalable, hosted queue for storing messages every bit they go betwixt computers. By using Amazon SQS, developers tin exactly motion information betwixt distributed components of their applications that perform dissimilar tasks, without losing messages or requiring each factor to live on ever available.)

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