Choosing a mean to connect microservices is never an easy task, many factors are taken into account before resorting to an option. If you are building a production-ready system, I guess the principle of weighing all factors hold true. Yes, I know this doesn’t apply to visionaries :)
In this article, I will run through some common communication means, briefly describe the background of our project, and my arguments on choosing RPC over the remaining options.
Before deciding on how we should wire our microservices, we have to understand two concepts:
1. Architectural Style
2. Transport Protocol
Think about: How the payload is formed when consuming a service? Is it stateless or stateful? Should we use REST, SOAP, JSON, XML, or some other messaging formats?
Think about: Which transport protocol should we use? Should we call a remote service over HTTP, HTTP2, a message bus, TCP socket or even UDP?
Popular Communication Means
Let us look at some relatively popular options available:
1. REST over HTTP(S)
2. Messaging over Message Broker
3. RPC (cross-language or single-language)
REST over HTTP(S)
Even since RESTful architecture style was proposed by Roy Fielding, we’ve been seeing a huge wave of adoption especially in web application development. The constraints proposed by Fielding despite not being a standard, shall always be adhered before declaring our API as RESTful.
There are variety of REST over HTTP(S), since there is no standard to be enforced. Developers are free to choose forming a request payload in JSON, XML or some self-defined format.
REST over HTTP(S) simply means using REST architectural style and send requests over HTTP(S).
Messaging over Message Broker
This basically works by connecting microservices to a centralize message bus and all communications between services are done by sending messages through the backbone.
Eg: Nameko in Python
RPC (cross-language or single-language)
Remote Procedural Call is not a new thing in distributive systems, it works by executing functions/methods/procedures on another device over the network.
According to the standard of RPC, RPC 5531:
1. RPC should be transport protocol agnostic: TCP, UDP, egal! Thus, reliability is not guatanteed.
2. Transaction ID is used to insure execute-at-most-once semactics and to allow client application to match replies to calls.
3. Time-outs and reconnection required to handle server crash, even if a connection-oriented protocol (TCP) is used.
4. Does not specify binding of services and clients, up to implementer to decide.
5. Mandatory requirements for RPC implementation:
(1) Unique specification of a procedure to be called.
(2) Provisions for matching response messages to request messages.
(3) Provisions for authenticating the caller to service and vice-versa
Background of The Project
In the organization we work for, we have a monolithic web application (written in Django) with acceptable performance. There are some services can be decoupled as separate services. I was taking the initiative of transforming our system architecture into microservice architecture in a gradual approach. One of the important aspects is to decide communication mean(s).
Why I chose RPC over the other two popular ones?
Take a look at RPC before ruling them out. I’ve read articles and comments advocating a replacement of RPC with REST. Some argued RPC is a stone-age technology, some said RPC is simple not easy to use. My stance is neutral, as the choice depends on individual use-case.
These are our main requirements:
1. No single-point-of-failure -> This ruled messaging queue out
2. Errors are propagated back to caller/client/consumer
3. Service interface which provides native experience
Since errors propagation to callers is important to us, RPC is a good candidate as many RPC frameworks return any exception raised in server function back to RPC function caller.
Most RPC frameworks eliminate the need of message broker, thus, single-point-of-failure is avoided.
Most RPC frameworks allow remote procedural call like:
except ValueError as e:
Given my scenario, what’s the better option than RPC?
Categories of RPC Frameworks
Within my course of exploring different RPC frameworks. I roughly categorize them into:
1. Monolingual RPC frameworks
2. Cross-language RPC frameworks
Monolingual framework, well, supports only a single programming language. A good candidate of such category in Python is RPyC. RPyC comes with easy-to-use pretty standard RPC features, and uses TCP as its transport protocol.
The pros of using RPyC (monolingual framework) is the absence of need to write a separate service interface. The downside is insufficient support of different Python version, of course, missing support of cross-language as its name suggest.
On the other hand, cross-language RPC frameworks supports multiple programming languages with a great cost. gRPC is one of the frameworks I uses. gRPC supported by Google, comes with wide coverage of programming languages from C++, Ruby, Python to Dart. In order to support multiple programming languages, a common service contract has to be defined. It is usually a protocol buffer (.proto) file. A service contract defines functions with arguments provided by the servers and to be consumed by clients, as well as the message format to be transported. In the case of gRPC, a protocol buffer file is then compiled into language-specific file (Eg: .py file in Python), this creates a problem when you started to have multiple versions of service contracts. It makes us difficult to keep track of different versions of client stubs and service functions.
To wrap up, communication mediums are chosen from use-case to use-case, these are basically my humble considerations on selecting a communication medium between microservices. Feel free to suggest any improvement of my choice.
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