Saga pattern for distributed transactions in HLD - Architecture Diagram

System Overview - Saga pattern for distributed transactions

The Saga pattern manages distributed transactions by breaking them into smaller steps, each handled by a separate service. It ensures data consistency across services by executing compensating actions if any step fails, avoiding the need for a global lock.

This system supports long-running transactions that span multiple microservices, providing reliability and fault tolerance.

Architecture Diagram

User
  |
  v
API Gateway
  |
  v
+-------------------+
|   Order Service    |
+-------------------+
  |          |
  v          v
+---------+  +----------------+
| Payment |  | Inventory      |
| Service |  | Service        |
+---------+  +----------------+
  |             |
  v             v
+-------------------+
|  Compensation     |
|  Coordinator      |
+-------------------+

Components

User

actor

Initiates the distributed transaction request

API Gateway

api_gateway

Receives user requests and routes them to the Order Service

Order Service

service

Starts the saga by creating an order and coordinating subsequent steps

Payment Service

service

Processes payment as part of the transaction

Inventory Service

service

Reserves inventory for the order

Compensation Coordinator

service

Manages compensating actions to undo previous steps if a failure occurs

Request Flow - 10 Hops

User → API Gateway

API Gateway → Order Service

Order Service → Payment Service

Payment Service → Order Service

Order Service → Inventory Service

Inventory Service → Order Service

Order Service → User

Order Service → Compensation Coordinator

Compensation Coordinator → Payment Service

Compensation Coordinator → Inventory Service

Failure Scenario

Component Fails:Inventory Service

Impact:Inventory reservation fails causing the overall transaction to fail after payment is processed

Mitigation:Compensation Coordinator triggers rollback on Payment Service to refund payment and informs Order Service to notify the user of failure

Architecture Quiz - 3 Questions

Which component initiates the rollback process when a transaction step fails?

A API Gateway

B Compensation Coordinator

C User

D Order Service

Design Principle

This architecture demonstrates the Saga pattern, which manages distributed transactions by coordinating local transactions and compensations across microservices. It avoids global locks and ensures eventual consistency through compensating actions, improving system scalability and fault tolerance.

Practice

1. What is the main purpose of the Saga pattern in distributed systems?

easy

A. To replicate data across multiple servers for backup

B. To lock all resources until the transaction completes

C. To speed up database queries by caching results

D. To manage long transactions by splitting them into smaller steps with compensations

Solution

1. Step 1: Understand the problem Saga solves

   The Saga pattern handles distributed transactions by breaking them into smaller steps that can be undone if needed.

2. Step 2: Compare options with Saga's goal

   Locking resources or caching are unrelated to Saga's main goal of managing distributed transactions with compensations.

3. Final Answer:

   To manage long transactions by splitting them into smaller steps with compensations -> Option D

4. Quick Check:

   Saga pattern purpose = Manage transactions with compensations [OK]

Hint: Saga splits big tasks into steps with undo actions [OK]

Common Mistakes:

• Thinking Saga locks resources like traditional transactions
• Confusing Saga with caching or replication techniques
• Assuming Saga only works with single database systems

2. Which of the following is the correct sequence in a Saga transaction?

easy

A. Execute steps sequentially, running compensations for previous steps if any step fails

B. Run compensations first, then execute all steps

C. Execute steps and compensations simultaneously

D. Execute steps without any compensations

Solution

1. Step 1: Recall Saga transaction flow

   Saga executes steps one by one. If a step fails, compensations undo previous steps.

2. Step 2: Eliminate incorrect sequences

   Running compensations before steps or simultaneously is incorrect. Skipping compensations breaks consistency.

3. Final Answer:

   Execute steps sequentially, running compensations for previous steps if any step fails -> Option A

4. Quick Check:

   Saga sequence = Steps then compensations on failure [OK]

Hint: Steps run first; compensations only if failure occurs [OK]

Common Mistakes:

• Running compensations before any step executes
• Assuming compensations run regardless of success
• Thinking steps and compensations run at the same time

3. Consider a Saga with three steps: A, B, and C. Step B fails after A succeeds. What happens next?

medium

A. Compensate step A, then abort the Saga

B. Retry step B indefinitely

C. Proceed to step C despite failure

D. Ignore failure and commit all steps

Solution

1. Step 1: Identify failure handling in Saga

   If step B fails, Saga triggers compensations for all previous successful steps, here step A.

2. Step 2: Understand why other options fail

   Retrying indefinitely can cause blocking; proceeding ignores failure; ignoring failure breaks consistency.

3. Final Answer:

   Compensate step A, then abort the Saga -> Option A

4. Quick Check:

   Failure in step B triggers compensation of A [OK]

Hint: Failure triggers undo of prior successful steps [OK]

Common Mistakes:

• Assuming Saga retries failed steps endlessly
• Skipping compensations and continuing steps
• Ignoring failure and committing partial results

4. A developer implemented a Saga but noticed data inconsistencies after failures. What is a likely cause?

medium

A. Saga uses asynchronous messaging

B. Compensation actions are missing or incomplete

C. Steps are idempotent and retry safe

D. All steps are executed sequentially

Solution

1. Step 1: Analyze cause of inconsistencies

   Missing or incomplete compensation means failed steps do not undo prior changes, causing inconsistency.

2. Step 2: Evaluate other options

   Sequential execution, idempotency, and async messaging are good practices and do not cause inconsistencies alone.

3. Final Answer:

   Compensation actions are missing or incomplete -> Option B

4. Quick Check:

   Missing compensations cause inconsistencies [OK]

Hint: Check if compensations are properly implemented [OK]

Common Mistakes:

• Blaming sequential execution for inconsistency
• Ignoring importance of compensation actions
• Assuming async messaging causes inconsistency

5. You design a Saga for an e-commerce order process with payment, inventory, and shipping services. Which approach best ensures data consistency across these services?

hard

A. Use a global database lock across all services during the order process

B. Allow each service to commit independently without rollback

C. Implement compensating transactions for each service step to rollback on failure

D. Retry failed steps indefinitely without compensation

Solution

1. Step 1: Understand distributed transaction challenges

   Locking globally is impractical; independent commits without rollback cause inconsistency.

2. Step 2: Apply Saga pattern best practice

   Compensating transactions allow rollback of previous steps if any step fails, ensuring consistency.

3. Final Answer:

   Implement compensating transactions for each service step to rollback on failure -> Option C

4. Quick Check:

   Compensations ensure consistency in distributed Saga [OK]

Hint: Use compensations, not global locks, for distributed consistency [OK]

Common Mistakes:

• Trying to lock all services globally
• Ignoring rollback on failure
• Relying on infinite retries without undo