THEORY: Distributed Transactions and why you should avoid them (2 Phase Commit , Saga Pattern, TCC, Idempotency etc)

avoid-distributed-transactions.md

Distributed Transactions and why you should avoid them

1. Modern technologies won't support it (RabbitMQ, Kafka, etc.);
2. This is a form of using Inter-Process Communication in a synchronized way and this reduces availability;
3. All participants of the distributed transaction need to be avaiable for a distributed commit, again: reduces availability.

Implementing business transactions that span multiple services is not straightforward. Distributed transactions are best avoided because of the CAP theorem. Moreover, many modern (NoSQL) databases don’t support them. The best solution is to use the Saga Pattern.

[...]

One of the most well-known patterns for distributed transactions is called Saga. The first paper about it was published back in 1987 and has it been a popular solution since then.

There are a couple of different ways to implement a saga transaction, but the two most popular are:

• Events/Choreography: When there is no central coordination, each service produces and listen to other service’s events and decides if an action should be taken or not;
• Command/Orchestration: when a coordinator service is responsible for centralizing the saga’s decision making and sequencing business logic;

💡 Top insights (and one point I don't agree with) about the Gregor Hohpe's article "Event-driven = Loosely coupled? Not so fast!":

TLDR
This is fantastic post from Gregor for reading slowly and deep thinking. It gets to the core of buzzwords such as EDA and Coupling used daily without really analysing direct and second-level consequences.

https://www.linkedin.com/posts/bibryam_i-love-blog-posts-that-make-me-think-gregor-activity-7218910524107878400-ZMy_/

I love blog posts that make me think. Gregor Hohpe hits the nail on the head 100% with this one. Here are the top insights (and one point I don't agree with):

1. Messages vs. Events
   Messaging is an interaction style, whereas "Event" describes the semantics of a message.
   Commands, Events, Documents are all Messages.

2. Messages vs. Channels
   We must delineate which characteristics of an event-driven system derive from the properties of the channel rather than the message intent.
   Message + Channel = Some Decoupling

3. Events vs. Coupling
   This is the core of the article, introduces a unique perspective about coupling for the different interaction styles. We can now explore the Interaction Styles (RPC, P2P, Pub/Sub) with different Coupling types (change propagation).

4. Now vs Later
   I love the second-level effects analysis too

Folks pitching EDA as decoupled may be the ones early in the lifecycle (like vendors) and won't have to live with the consequences of their (coupling) decisions. The more you take advantage of your ability to add recipients, the harder it will become to make a change. Thus, taking advantage of one dimension (add recipients) of decoupling exposes you more to another dimension of coupling (ex: schema changes).

⭐️ 5. What I don't fully agree with Gregor

"Location Coupling propagates location changes of a recipient to a sender (or vice versa). Message channels decouple location changes because they are (mostly) logical constructs... RPC can achieve some decoupling through DNS or active load balancers, but it's much more likely that a change affects the sender."

⭐️ 5.1.
IMO, one of the main reasons for using RPC is the decoupling of service consumer and provider, so they can evolve in isolation but also start/stop, scale, move, independently. Otherwise, there’s no reason to split a monolith into services and in-memory calls should be preferred over RPC.

⭐️ 5.2.
Today, no caller has a hard-coded callee address; it’s almost always a logical name:

• Kubernetes service name, not Pod IP
• Dapr sidecar/Istio proxy, with the logical name of the target service
• AWS API Gateway address, rather than Lambda URL
• Client-side service discovery that looks up the location, still not affecting the client
• IMO, RPC today always involves some kind of proxy

⭐️ 5.3.
In all these cases, the provider can scale based on demand or move without affecting the client. You might say the client needs to know the seed location (K8s service, AWS gateway, sidecar, etc.), but that’s true for the location of the messaging channel too. Even if the topic is a logical name the broker URL is with fixed location.

How Complex Systems Fail

5. Complex systems run in degraded mode.
A corollary to the preceding point is that complex systems run as broken systems. The system continues to function because it contains so many redundancies and because people can make it function, despite the presence of many flaws. [...]

16. Safety is a characteristic of systems and not of their components
Safety is an emergent property of systems; it does not reside in a person, device or department of an organization or system. Safety cannot be purchased or manufactured; it is not a feature that is separate from the other components of the system.

Outbox Pattern - by Unico

• The interesting part is they use Protobuf as a content type when sending events to the broker. Still, for some reason that's unclear in the article, they serialize this Protobuf data into JSON format before persisting it in the outbox table. I guess they do so because they use Debezium under the hood.
• They also use the CloudEvents (v1.0.2) spec for defining the format of event data;

This is the Protobuf message using the CloudEvent spec:

syntax = "proto3";
import "google/protobuf/timestamp.proto";
import "google/protobuf/any.proto";  
message OutboxEvent {
  string specversion = 1;  
  string type = 2;  
  string source = 3;  
  string subject = 4;  
  string id = 5;  
  google.protobuf.Timestamp time = 6;  
  string datacontenttype = 7;  
  string dataschema = 8;  
  google.protobuf.Any data = 9;
}

And this is an example:

{
  "specversion": "1.0",
  "type": "someevent",
  "source": "integration",
  "subject": "1ec07712-79b7-485a-a0e2-0a1c33fd1016",
  "time": "2020-04-30T04:00:00Z",
  "datacontenttype": "application/json",
  "dataschema": "http://<schemapath>",
  "data": {
    "transactionId": "1ec07712-79b7-485a-a0e2-0a1c33fd1016",
    "doc": "123.123.123-00",
    "image_id": "ea02254f-28f4-4b31-99a5-957bb024f78d"
  }
}

Fidelis blog: System Design - Saga Pattern 🇧🇷 - artigo sobre Saga e Outbox Pattern escrito pelo Matheus Fidelis.

Do Caos a Consistência: A Ordem das Mensagens em Sistemas Distribuídos

River's blog: Building an idempotent email API with River unique jobs