Recently at iHeartRadio we decided to migrate our one monolithic Java backend service into multiple microservices, more specifically we decided to implement these microservices as Akka apps and use Akka cluster to weave them into our microservice cluster. This post is about how we reached this decision. There are three main sections in this post: first a brief discussion of the goals we wanted to achieve with microservices, second, the specific reasons why we think Akka apps without Http interface makes the most sense for our goals, and third, a brief look at the initial architecture design.
Goals we want to achieve
Goal 1. Easier and faster release cycle.
Monolithic code base for the Java backend is one of the major factors preventing us from being able to release services at a more granular pace. Code changes to multiple services have to be tested/QA as a whole, which means small changes have to wait for all other changes (relevant or irrelevant), before they can be released. We want to address this problem with microservices that can be released on a microservice by microservice basis - each microservice can have its own release schedule. Thus we can deliver new features/fixes to clients at a faster pace (with smaller steps).
Goal 2. Improve development productivity with looser and clearer inter-module dependencies
In our monolithic Java backend code, different functional areas depend on each other tightly. The dependencies are also hard to track without carefully inspecting the code, thus making them harder to manage. These over-dependencies make the whole code cumbersome to change - to change code at one place you may have to change code at multiple places accordingly. The implication of such changes are hard to understand. By dividing code into clearly separated microservices, the dependencies are much looser through messages and easier to inspect in simple configuration files.
Goal 3. Better reusability/composability
Classes in a monolithic backend tend to grow larger and larger congregating logic from different functional areas, which makes reusing more and more difficult. We want to take this opportunity to redesign the modules so that each microservice will have a smaller interface and clearly defined responsibilities. This will make it easier to reuse them as modules and compose higher level microservices with lower level ones.
Goal 4. Easier team integration
The monolithic backend codebase is huge in size and very complex to understand. It creates a higher barrier for developers from outside the dedicated backend team to contribute to the backend. Code size within each microservice, on the other hand, is much more modest and easier to learn. This opens the doors to different development organizations, such as having client developers contributing directly to the code base or a more vertically oriented team structure.
Why we picked Akka cluster as the core architecture for our microservice
Now let me go through a few reasons why we made this pick:
- Out-of-the-box clustering infrastructure
- Loose coupling without the cost of JSON parsing.
- Transparent programming model within and across microservices.
- Strong community and commercial support
- High resiliency, performance and scalability.
Out-of-the-box clustering infrastructure
One of the costs of microservices is the clustering infrastructure you need to build - that includes but is not limited to discovery, load balancing, monitoring, failover and partitioning the microservices. There are 3rd party tools that can help with these clustering functionalities, but they require a strenuous integration effort and introduce significant complexity to the stack. Akka cluster provides these clustering infrastructure components out of the box. We had the cluster up and running with only a couple lines of configuration changes. Actually, these clustering functionalities are so ready that Typesafe implemented their general distributed system management tool Conductr using Akka cluster.
Loose(relatively) coupling without the cost of JSON parsing
One of the common protocols used by microservices is HTTP with JSON body. This setup has a performance cost of text (JSON) parsing and a development cost of writing JSON parsers. Akka’s message communication over binary protocol is more optimized for performance and there is no extra parsing code to write. We value both benefits over the looser coupling provided by HTTP with JSON. We also separate our message API with the service implementation to provide enough decoupling between the clients and services.
Transparent programming model within and across microservices
The Akka programming model, actor model, is transparent within and across microservices. All the calling is done through asynchronous message passing regardless of whether the caller and responder are on the same microservice or different ones. This single programming model within and across microservices has two major benefits:1) it makes development experience consistent - when writing clients, you don’t need to remember where the service actor is, local or remote. 2) it makes it easier to move logic around, you can easily merge or split microservices with little-to-no code change.
High resiliency, performance and scalability
Apache Spark uses Akka under the hood for driver-worker communication. Their team is building masterless standalone Spark mode using Akka cluster. Not much more was needed to convince us that it will comfortably satisfy our performance requirements.
Strong community and commercial support
The Akka community is unquestionably substantial and vigorous, but what sets Akka different from other candidates in our list is the option of having commercial support from Typesafe, which is provided by the core developers in the Akka teams. It’s precious for us to have the confidence that you will always be able to get the “most correct” answer to related problems and you never get blocked by a technical problem for more than 24 hours.
With these features of an Akka cluster tallying well with our goals, it was an easy decision to make.
Microservices with Akka Cluster
As a final note, here is a brief review our initial architecture design of the microservice platform.
All microservices are implemented in Akka apps and running in a single Akka cluster. Above this Akka cluster layer, a REST layer is composed of one or more Play! web applications serving as Http (mostly JSON) public interfaces for the microservices. They communicate with the microservices by having router actors deployed inside the akka cluster - we call these actors agents. The web apps also handle some cross cutting functionalities such as security and caching. Below the Akka cluster is the lower data storage layer, which represent the root sources of data/information for our backend.
Instances of microservices can join and leave Akka cluster according to demand. When a redundant instance of a microservice joins the cluster, all members get notified, and will automatically start to include that instance when load balancing requests to the service. Same is true when an instance leaves the cluster, members will get notified that it’s leaving the cluster. This way we can scale up and down at a service by service level.
There you go. As of now we have several microservices live in production and the cluster has served us very well. I will post further updates, hopefully good ones, later.Continue