Ja-micro 
Ja-micro is a lightweight Java framework for building microservices.
Introduction
Ja-micro is a framework that allows developers to easily develop microservices in Java. It was developed at Sixt, primarily over the course of 2016, during a push to create a new platform. That platform was started with a goal of supporting two primary languages, being Golang and Java.
We use Go Micro as our framework for our Go services, so a primary concern for this framework is to be compatible with Micro. They diverge a bit in capabilities and methodologies, but they are indeed compatible.
The framework takes care of many of the concerns so that developers can simply focus on the functionality of their services instead.
See the wiki to get started.
Features
- Simply build service as docker container or fat jar.
- Configuration from environment, command-line and external configuration services.
- Standardized json logging.
- Standardized metrics reporting
- Simple interface for calling endpoints on other services and handling errors from them.
- Client-side load-balancer
- Simple interface for a service to support health checks.
- Database migrations built-in.
- Simplified event-handling using Kafka.
- Pluggable service registry to register and discover service instances.
- Compatible with Go Micro to allow choice of implementation language.
- Guice dependency injection for ease of implementation and testing scenarios.
- Components to create service integration testing scenarios.
Details
Service Registry
The service registry is pluggable with different backends (consul, etc.) The purpose to for each service instance (which may be coming up on a rather anonymous IP address and random port) to register so that other instances can locate it. It is also by extension, available for services to use to locate instances of other services. The service information contains specially-formatted tags which contain information about the format of the request and response messages for each endpoint, and other tags.
Client-side Load-balancer
There is a client-side load-balancer in place behind the facade of the RpcClient. This interacts with the service registry to track instances of services a service wants to interact with, tracks their state of health in order to know whether the instances should receive requests, and tracks the response results of each rpc call in order to maintain 'circuit breakers' to limit exposure to non-healthy service instances.
RpcClient and Error Handling
In order to allow a service to call other services, there is support to easily create RpcClients. An RpcClient is an object that abstracts an rpc endpoint of another service. One can create an RpcClient by using the RpcClientFactory. Once one has an RpcClient, calling another service endpoint is as simple as calling the callSynchronous method (a future version of the framework will also support asynchronous calls) with a protobuf request message, and it will return a protobuf response message.
The timeout policy, the retry policy, and the error-handling are all handled by the client. By default, we have said that the default retry policy should be to retry a failed request one time, if the response is retriable. In a future release, we will add support for time budgeting, where a client of a service can set how much time is allowed to service the whole request. For now, we have a static policy with a default timeout of 1000ms, and this can be customized per client.
The RpcCallException class defines all of our exception categories, their default retriable setting, and what the resulting HTTP status code is (when using an HTTP transport). When one service calls another (client calling server), if a server throws an exception during processing the response, the exception is transparently transported back to the client and can be rethrown on the client-side (ignoring the retry aspect here).
Default RpcClient retry policy (1) can be overridden with setting 'rpcClientRetries'. Default RpcClient timeout policy (1000ms) can be overridden with setting 'rpcClientTimeout'.
Health Checks
Every few seconds, the health state for a service instance is reported to the service registry plugin. Using annotations, there is support for a service developer to easily hook into health checking to mark a service instance as unhealhty. There is also an interface to immediately change the health state of an instance.
Configuration Handling
For any component in a service requiring configuration, it can simply get a ServiceProperties object injected into it, and request the configuration properties from it. Properties are located from three locations: command-line, environment variables, and configuration plugin. They are applied in that order. The same property with a different value from a later source overwrites the earlier source. The configuration plugin provides the ability to do long-polling so that each service can get real-time updates to changes of the configuration from arbitrary sources. There are hooks for service components to get real-time notifications of configuration changes.
Logging
There is standardized logging in place. The log records are json objects, and Markers can be used to dynamically create properties on those objects. The log format can be completely customized.
Metrics
There is standardized metric handling in place. This is heavily opinionated to Sixt's infrastructure, and uses metrics formatted in a specific format and sent to an influx agent to be reported back to a central influxdb cluster. The metrics reporting is pluggable to support reporting metrics in any desired format to any desired destination.
Kafka Events
There are factories/builders in place to easily create publishers and subscribers for topics in Kafka.
Output Artifacts
One can build a shadow jar (fat jar - all dependencies included), or a docker image. One might use the shadow jar for developer testing. The shadow jar is a dependency of the docker image tasks as well. To start a service in a debugger, use the JettyServiceBase as a main class.
Database Migrations
There is currently support for Flyway database migrations, which supports many different SQL databases. This support extends into carefully controlling the service lifecycle and health checks. A future version should support basic migration support for DynamoDB instances.
Dependency Injection
Dependency injection is heavily used in Ja-micro. It is strictly supporting Guice.
Service Integration Testing
We heavily use automation at Sixt in our microservice projects. To support this, the framework gives the ability to developers to automate service integration tests. What this means is that core infrastructure dependencies (for example, on one service, this is consul, postgres, zookeeper and kafka) and the service itself are started as containers under docker-compose. Additionally, to eliminate the problem that would arise of starting containers for every other service dependency (and their dependencies, etc.), there exists a class called ServiceImpersonator that can serve as a complete service mock, available in service registry and serving real rpc requests. However, the developer maps the requests and responses instead of a real instance serving those requests.
Compatibility
Ja-micro is meant to keep compatibility so that service developers can easily choose between developing a service in Java or Go. Other languages can also be supported by using the Go Micro sidecar.
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