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DistributedTransactions.Mechanism.md

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Distributed Transactions as a Mechanism Overview

Sources:

There are some cases, where we need to implement distributed transactions because of multiple databases and etc. There are a couple of approaches to take:

Two-Phase Commit

Two Phase Commit (2PC) consists of 2 stages:

  • PREPARE
  • COMMIT

Here is an example of create customer order two phase commit

Pros:

  • well planned strategy
  • easy to understand because of statefullness of that approach.

Cons:

  • hard to implement
  • no mechanism to rollback the other transaction is available (if one microservice goes down in the commit phase)
  • others are waiting for the slowest microservice to respond with successful result of confirmation.

SAGA Pattern

There are two ways to implement SAGA pattern:

  • Choreography: each local transaction publishes domain events that trigger local transactions in other services
  • Orchestraction: an orchestrator (object) tells the participants what local transactions to execute.

Orchestration-based SAGA

We need to define a Saga Execution Service (SEC) with the sole responsibility to tell each service what operation to perform and when.

Let's look at specific stages of transaction:

  1. Order service saves a pending order and asks SEC to start a create order transaction
  2. SEC sends an Execute Payment command to Payment service and it replies with a Payment executed message
  3. SEC sends a Prepare Order command to Stock service and it replies with an Order prepared message
  4. SEC send a Deliver Order command to Delivery service and it replies with an Order Delivered message

Pros:

  • no cyclic dependencies between services. All interaction is done using SEC
  • easy to maintain unique ID for single distributed transaction
  • avoids synchronous operations

Cons:

  • complex approach
  • we need to implement compensating transactions to have an availability to rollback transactions.