In this post we'd like to share an interesting way of dealing with configuration of a distributed system.
The configuration is represented directly in Scala language in a type safe manner. An example implementation is described in details. Various aspects of the proposal are discussed, including influence on the overall development process.

Overall configuration management process

(на русском)

Introduction

Building robust distributed systems requires the use of correct and coherent configuration on all nodes. A typical solution is to use a textual deployment description (terraform, ansible or something alike) and automatically generated configuration files (often — dedicated for each node/role). We would also want to use the same protocols of the same versions on each communicating nodes (otherwise we would experience incompatibility issues). In JVM world this means that at least the messaging library should be of the same version on all communicating nodes.

What about testing the system? Of course, we should have unit tests for all components before coming to integration tests. To be able to extrapolate test results on runtime, we should make sure that the versions of all libraries are kept identical in both runtime and testing environments.

When running integration tests, it's often much easier to have the same classpath on all nodes. We just need to make sure that the same classpath is used on deployment. (It is possible to use different classpaths on different nodes, but it's more difficult to represent this configuration and correctly deploy it.) So in order to keep things simple we will only consider identical classpaths on all nodes.

Configuration tends to evolve together with the software. We usually use versions to identify various
stages of software evolution. It seems reasonable to cover configuration under version management and identify different configurations with some labels. If there is only one configuration in production, we may use single version as an identifier. Sometimes we may have multiple production environments. And for each environment we might need a separate branch of configuration. So configurations might be labeled with branch and version to uniquely identify different configurations. Each branch label and version corresponds to a single combination of distributed nodes, ports, external resources, classpath library versions on each node. Here we'll only cover the single branch and identify configurations by a three component decimal version (1.2.3), in the same way as other artifacts.

In modern environments configuration files are not modified manually anymore. Typically we generate
config files at deployment time and never touch them afterwards. So one could ask why do we still use text format for configuration files? A viable option is to place the configuration inside a compilation unit and benefit from compile-time configuration validation.

In this post we will examine the idea of keeping the configuration in the compiled artifact.

Compilable configuration

In this section we will discuss an example of static configuration. Two simple services — echo service and the client of the echo service are being configured and implemented. Then two different distributed systems with both services are instantiated. One is for a single node configuration and another one for two nodes configuration.

A typical distributed system consists of a few nodes. The nodes could be identified using some type:

sealed trait NodeId
case object Backend extends NodeId
case object Frontend extends NodeId

or just

case class NodeId(hostName: String)

or even

object Singleton
type NodeId = Singleton.type

These nodes perform various roles, run some services and should be able to communicate with the other nodes by means of TCP/HTTP connections.

For TCP connection at least a port number is required. We also want to make sure that client and server are talking the same protocol. In order to model a connection between nodes let's declare the following class:

case class TcpEndPoint[Protocol](node: NodeId, port: Port[Protocol])

where Port is just an Int within the allowed range:

type PortNumber = Refined[Int, Closed[_0, W.`65535`.T]]

Refined types

See refined library. In short, it allows to add compile time constraints to other types. In this case Int is only allowed to have 16-bit values that can represent port number. There is no requirement to use this library for this configuration approach. It just seems to fit very well.

For HTTP (REST) we might also need a path of the service:

type UrlPathPrefix = Refined[String, MatchesRegex[W.`"[a-zA-Z_0-9/]*"`.T]]
case class PortWithPrefix[Protocol](portNumber: PortNumber, pathPrefix: UrlPathPrefix)

Phantom type

In order to identify protocol during compilation we are using the Scala feature of declaring type argument Protocol that is not used in the class. It's a so called phantom type. At runtime we rarely need an instance of protocol identifier, that's why we don't store it. During compilation this phantom type gives additional type safety. We cannot pass port with incorrect protocol.

One of the most widely used protocols is REST API with Json serialization:

sealed trait JsonHttpRestProtocol[RequestMessage, ResponseMessage]

where RequestMessage is the base type of messages that client can send to server and ResponseMessage is the response message from server. Of course, we may create other protocol descriptions that specify the communication protocol with the desired precision.

For the purposes of this post we'll use a simpler version of the protocol:

sealed trait SimpleHttpGetRest[RequestMessage, ResponseMessage]

In this protocol request message is appended to url and response message is returned as plain string.

A service configuration could be described by the service name, a collection of ports and some dependencies. There are a few possible ways of how to represent all these elements in Scala (for instance, HList, algebraic data types). For the purposes of this post we'll use Cake Pattern and represent combinable pieces (modules) as traits. (Cake Pattern is not a requirement for this compilable configuration approach. It just one possible implementation of the idea.)

Dependencies could be represented using the Cake Pattern as endpoints of other nodes:

  type EchoProtocol[A] = SimpleHttpGetRest[A, A]

  trait EchoConfig[A] extends ServiceConfig {
    def portNumber: PortNumber = 8081
    def echoPort: PortWithPrefix[EchoProtocol[A]] = PortWithPrefix[EchoProtocol[A]](portNumber, "echo")
    def echoService: HttpSimpleGetEndPoint[NodeId, EchoProtocol[A]] = providedSimpleService(echoPort)
  }

Echo service only needs a port configured. And we declare that this port supports echo protocol. Note that we do not need to specify a particular port at this moment, because trait's allows abstract methods declarations. If we use abstract methods, compiler will require an implementation in a configuration instance. Here we have provided the implementation (8081) and it will be used as the default value if we skip it in a concrete configuration.

We can declare a dependency in the configuration of the echo service client:

  trait EchoClientConfig[A] {
    def testMessage: String = "test"
    def pollInterval: FiniteDuration
    def echoServiceDependency: HttpSimpleGetEndPoint[_, EchoProtocol[A]]
  }

Dependency has the same type as the echoService. In particular, it demands the same protocol. Hence, we can be sure that if we connect these two dependencies they will work correctly.

Services implementation

A service needs a function to start and gracefully shutdown. (Ability to shutdown a service is critical for testing.) Again there are a few options of specifying such a function for a given config (for instance, we could use type classes). For this post we'll again use Cake Pattern. We can represent a service using cats.Resource which already provides bracketing and resource release. In order to acquire a resource we should provide a configuration and some runtime context. So the service starting function might look like:

  type ResourceReader[F[_], Config, A] = Reader[Config, Resource[F, A]]

  trait ServiceImpl[F[_]] {
    type Config
    def resource(
      implicit
      resolver: AddressResolver[F],
      timer: Timer[F],
      contextShift: ContextShift[F],
      ec: ExecutionContext,
      applicative: Applicative[F]
    ): ResourceReader[F, Config, Unit]
  }

where

Config — type of configuration that is required by this service starter
AddressResolver — a runtime object that has the ability to obtain real addresses of other nodes (keep reading for details).

the other types comes from cats:

F[_] — effect type (In the simplest case F[A] could be just () => A. In this post we'll use cats.IO.)
Reader[A,B] — is more or less a synonym for a function A => B
cats.Resource — has ways to acquire and release
Timer — allows to sleep/measure time
ContextShift — analog of ExecutionContext
Applicative — wrapper of functions in effect (almost a monad) (we might eventually replace it with something else)

Using this interface we can implement a few services. For instance, a service that does nothing:

  trait ZeroServiceImpl[F[_]] extends ServiceImpl[F] {
    type Config <: Any
    def resource(...): ResourceReader[F, Config, Unit] =
      Reader(_ => Resource.pure[F, Unit](()))
  }

(See Source code for other services implementations — echo service,
echo client and lifetime controllers.)

A node is a single object that runs a few services (starting a chain of resources is enabled by Cake Pattern):

object SingleNodeImpl extends ZeroServiceImpl[IO]
  with EchoServiceService
  with EchoClientService
  with FiniteDurationLifecycleServiceImpl
{
  type Config = EchoConfig[String] with EchoClientConfig[String] with FiniteDurationLifecycleConfig
}

Note that in the node we specify the exact type of configuration that is needed by this node. Compiler won't let us to build the object (Cake) with insufficient type, because each service trait declares a constraint on the Config type. Also we won't be able to start node without providing complete configuration.

Node address resolution

In order to establish a connection we need a real host address for each node. It might be known later than other parts of the configuration. Hence, we need a way to supply a mapping between node id and it's actual address. This mapping is a function:

case class NodeAddress[NodeId](host: Uri.Host)
trait AddressResolver[F[_]] {
  def resolve[NodeId](nodeId: NodeId): F[NodeAddress[NodeId]]
}

There are a few possible ways to implement such a function.

If we know actual addresses before deployment, during node hosts instantiation, then we can generate Scala code with the actual addresses and run the build afterwards (which performs compile time checks and then runs integration test suite). In this case our mapping function is known statically and can be simplified to something like a Map[NodeId, NodeAddress].
Sometimes we obtain actual addresses only at a later point when the node is actually started, or we don't have addresses of nodes that haven't been started yet. In this case we might have a discovery service that is started before all other nodes and each node might advertise it's address in that service and subscribe to dependencies.
If we can modify /etc/hosts, we can use predefined host names (like my-project-main-node and echo-backend) and just associate this name with ip address at deployment time.

In this post we don't cover these cases in more details. In fact in our toy example all nodes will have the same IP address — 127.0.0.1.

In this post we'll consider two distributed system layouts:

Single node layout, where all services are placed on the single node.
Two node layout, where service and client are on different nodes.

The configuration for a single node layout is as follows:

Single node configuration

object SingleNodeConfig extends EchoConfig[String] 
  with EchoClientConfig[String] with FiniteDurationLifecycleConfig
{
  case object Singleton // identifier of the single node 
  // configuration of server
  type NodeId = Singleton.type
  def nodeId = Singleton

  /** Type safe service port specification. */
  override def portNumber: PortNumber = 8088

  // configuration of client

  /** We'll use the service provided by the same host. */
  def echoServiceDependency = echoService

  override def testMessage: UrlPathElement = "hello"

  def pollInterval: FiniteDuration = 1.second

  // lifecycle controller configuration
  def lifetime: FiniteDuration = 10500.milliseconds // additional 0.5 seconds so that there are 10 requests, not 9.
}

Here we create a single configuration that extends both server and client configuration. Also we configure a lifecycle controller that will normally terminate client and server after lifetime interval passes.

The same set of service implementations and configurations can be used to create a system's layout with two separate nodes. We just need to create two separate node configs with the appropriate services:

Two nodes configuration

  object NodeServerConfig extends EchoConfig[String] with SigTermLifecycleConfig
  {
    type NodeId = NodeIdImpl

    def nodeId = NodeServer

    override def portNumber: PortNumber = 8080
  }

  object NodeClientConfig extends EchoClientConfig[String] with FiniteDurationLifecycleConfig
  {
    // NB! dependency specification
    def echoServiceDependency = NodeServerConfig.echoService

    def pollInterval: FiniteDuration = 1.second

    def lifetime: FiniteDuration = 10500.milliseconds // additional 0.5 seconds so that there are 10 request, not 9.

    def testMessage: String = "dolly"
  }

See how we specify the dependency. We mention the other node's provided service as a dependency of the current node. The type of dependency is checked because it contains phantom type that describes protocol. And at runtime we'll have the correct node id. This is one of the important aspects of the proposed configuration approach. It provides us with the ability to set port only once and make sure that we are referencing the correct port.

Two nodes implementation

For this configuration we use exactly the same services implementations. No changes at all. However, we create two different node implementations that contain different set of services:

  object TwoJvmNodeServerImpl extends ZeroServiceImpl[IO] with EchoServiceService with SigIntLifecycleServiceImpl {
    type Config = EchoConfig[String] with SigTermLifecycleConfig
  }

  object TwoJvmNodeClientImpl extends ZeroServiceImpl[IO] with EchoClientService with FiniteDurationLifecycleServiceImpl {
    type Config = EchoClientConfig[String] with FiniteDurationLifecycleConfig
  }

The first node implements server and it only needs server side config. The second node implements client and needs another part of config. Both nodes require some lifetime specification. For the purposes of this post service node will have infinite lifetime that could be terminated using SIGTERM, while echo client will terminate after the configured finite duration. See the starter application for details.

Overall development process

Let's see how this approach changes the way we work with configuration.

The configuration as code will be compiled and produces an artifact. It seems reasonable to separate configuration artifact from other code artifacts. Often we can have multitude of configurations on the same code base. And of course, we can have multiple versions of various configuration branches. In a configuration we can select particular versions of libraries and this will remain constant whenever we deploy this configuration.

A configuration change becomes code change. So it should be covered by the same quality assurance process:

Ticket -> PR -> review -> merge -> continuous integration -> continuous deployment

There are the following consequences of the approach:

The configuration is coherent for a particular system's instance. It seems that there is no way to have incorrect connection between nodes.
It's not easy to change configuration just in one node. It seems unreasonable to log in and change some text files. So configuration drift becomes less possible.
Small configuration changes are not easy to make.
Most of the configuration changes will follow the same development process, and it will pass some review.

Do we need a separate repository for production configuration? The production configuration might contain sensitive information that we would like to keep out of reach of many people. So it might worth keeping a separate repository with restricted access that will contain the production configuration. We may split the configuration into two parts — one that contains most open parameters of production and one that contains the secret part of configuration. This would enable access to most of the developers to the vast majority of parameters while restricting access to really sensitive things. It's easy to accomplish this using intermediate traits with default parameter values.

Variations

Let's see pros and cons of the proposed approach compared to the other configuration management techniques.

First of all, we'll list a few alternatives to the different aspects of the proposed way of dealing with configuration:

Text file on the target machine.
Centralized key-value storage (like etcd/zookeeper).
Subprocess components that could be reconfigured/restarted without restarting process.
Configuration outside artifact and version control.

Text file gives some flexibility in terms of ad-hoc fixes. A system's administrator can login to the target node, make a change and simply restart the service. This might not be as good for bigger systems. No traces are left behind the change. The change is not reviewed by another pair of eyes. It might be difficult to find out what have caused the change. It has not been tested. From distributed system's perspective an administrator can simply forget to update the configuration in one of the other nodes.

(Btw, if eventually there will be a need to start using text config files, we'll only have to add parser + validator that could produce the same Config type and that would be enough to start using text configs. This also shows that the complexity of compile-time configuration is a little smaller that the complexity of text-based configs, because in text-based version we need some additional code.)

Centralized key-value storage is a good mechanism for distributing application meta parameters. Here we need to think about what we consider to be configuration values and what is just data. Given a function C => A => B we usually call rarely changing values C "configuration", while frequently changed data A — just input data. Configuration should be provided to the function earlier than the data A. Given this idea we can say that it's expected frequency of changes what could be used to distinguish configuration data from just data. Also data typically comes from one source (user) and configuration comes from a different source (admin). Dealing with parameters that can be changed after process initialization leads to an increase of application complexity. For such parameters we'll have to handle their delivery mechanism, parsing and validation, handling incorrect values. Hence, in order to reduce program complexity, we'd better reduce the number of parameters that can change at runtime (or even eliminate them altogether).

From the perspective of this post we should make a distinction between static and dynamic parameters. If service logic requires rare change of some parameters at runtime, then we may call them dynamic parameters. Otherwise they are static and could be configured using the proposed approach. For dynamic reconfiguration other approaches might be needed. For example, parts of the system might be restarted with the new configuration parameters in a similar way to restarting separate processes of a distributed system.
(My humble opinion is to avoid runtime reconfiguration because it increases complexity of the system.
It' might be more straightforward to just rely on OS support of restarting processes. Though, it might not always be possible.)

One important aspect of using static configuration that sometimes makes people consider dynamic configuration (without other reasons) is service downtime during configuration update. Indeed, if we have to make changes to static configuration, we have to restart the system so that new values become effective. The requirements for downtime vary for different systems, so it might not be that critical. If it is critical, then we have to plan ahead for any system restarts. For instance, we could implement AWS ELB connection draining. In this scenario whenever we need to restart the system, we start a new instance of the system in parallel, then switch ELB to it, while letting the old system to complete servicing existing connections.

What about keeping configuration inside versioned artifact or outside? Keeping configuration inside an artifact means in most of the cases that this configuration has passed the same quality assurance process as other artifacts. So one might be sure that the configuration is of good quality and trustworthy. On the contrary configuration in a separate file means that there are no traces of who and why made changes to that file. Is this important? We believe that for most production systems it's better to have stable and high quality configuration.

Version of the artifact allows to find out when it was created, what values it contains, what features are enabled/disabled, who was responsible for making each change in the configuration. It might require some effort to keep configuration inside an artifact and it's a design choice to make.

Pros & cons

Here we would like to highlight some advantages and to discuss some disadvantages of the proposed approach.

Advantages

Features of the compilable configuration of a complete distributed system:

Static check of configuration. This gives a high level of confidence, that the configuration is correct given type constraints.
Rich language of configuration. Typically other configuration approaches are limited to at most variable substitution.
Using Scala one can use wide range of language features to make configuration better. For instance, we can use traits to provide default values, objects to set different scope, we can refer to vals defined only once in the outer scope (DRY). It's possible to use literal sequences, or instances of certain classes (Seq, Map, etc.).
DSL. Scala has decent support for DSL writers. One can use these features to establish a configuration language that is more convenient and end-user friendly, so that the final configuration is at least readable by domain users.
Integrity and coherence across nodes. One of the benefits of having configuration for the whole distributed system in one place is that all values are defined strictly once and then reused in all places where we need them. Also type safe port declarations ensures that in all possible correct configurations the system's nodes will speak the same language. There are explicit dependencies between nodes which makes it hard to forget to provide some services.
High quality of changes. The overall approach of passing configuration changes through normal PR process establishes high standards of quality also in configuration.
Simultaneous configuration changes. Whenever we make any changes in the configuration automatic deployment ensures that all nodes are being updated.
Application simplification. The application doesn't need to parse and validate configuration and handle incorrect configuration values. This simplifies the overall application. (Some complexity increase is in the configuration itself, but it's a conscious trade-off towards safety.) It's pretty straightforward to return to ordinary configuration — just add the missing pieces. It's easier to get started with compiled configuration and postpone implementation of additional pieces to some later times.
Versioned configuration. Due to the fact that configuration changes follow the same development process, as a result we get an artifact with unique version. It allows us to switch configuration back if needed. We can even deploy a configuration that was used a year ago and it will work exactly the same way. Stable configuration improves predictability and reliability of the distributed system. The configuration is fixed at compile time and cannot be easily tampered on a production system.
Modularity. The proposed framework is modular and modules could be combined in various ways to
support different configurations (setups/layouts). In particular, it's possible to have a small scale single node layout and a large scale multi node setting. It's reasonable to have multiple production layouts.
Testing. For testing purposes one might implement a mock service and use it as a dependency in a type safe way. A few different testing layouts with various parts replaced by mocks could be maintained simultaneously.
Integration testing. Sometimes in distributed systems it's difficult to run integration tests. Using the described approach to type safe configuration of the complete distributed system, we can run all distributed parts on a single server in a controllable way. It's easy to emulate the situation
when one of the services becomes unavailable.

Disadvantages

The compiled configuration approach is different from "normal" configuration and it might not suit all needs. Here are some of the disadvantages of the compiled config:

Static configuration. It might not be suitable for all applications. In some cases there is a need of quickly fixing the configuration in production bypassing all safety measures. This approach makes it more difficult. The compilation and redeployment are required after making any change in configuration. This is both the feature and the burden.
Configuration generation. When config is generated by some automation tool this approach requires subsequent compilation (which might in turn fail). It might require additional effort to integrate this additional step into the build system.
Instruments. There are plenty of tools in use today that rely on text-based configs. Some of them
won't be applicable when configuration is compiled.
A shift in mindset is needed. Developers and DevOps are familiar with text configuration files. The idea of compiling configuration might appear strange to them.
Before introducing compilable configuration a high quality software development process is required.

There are some limitations of the implemented example:

If we provide extra config that is not demanded by the node implementation, compiler won't help us to detect the absent implementation. This could be addressed by using HList or ADTs (case classes) for node configuration instead of traits and Cake Pattern.
We have to provide some boilerplate in config file: (package, import, object declarations;
override def's for parameters that have default values). This might be partially addressed using a DSL.
In this post we do not cover dynamic reconfiguration of clusters of similar nodes.

Conclusion

In this post we have discussed the idea of representing configuration directly in the source code in a type safe way. The approach could be utilized in many applications as a replacement to xml- and other text- based configs. Despite that our example has been implemented in Scala, it could also be translated to other compilable languages (like Kotlin, C#, Swift, etc.). One could try this approach in a new project and, in case it doesn't fit well, switch to the old fashioned way.

Of course, compilable configuration requires high quality development process. In return it promises to provide equally high quality robust configuration.

This approach could be extended in various ways:

One could use macros to perform configuration validation and fail at compile time in case of any business-logic constraints failures.
A DSL could be implemented to represent configuration in a domain-user-friendly way.
Dynamic resource management with automatic configuration adjustments. For instance, when we adjust the number of cluster nodes we might want (1) the nodes to obtain slightly modified configuration; (2) cluster manager to receive new nodes info.

Thanks

I would like to say thank you to Andrey Saksonov, Pavel Popov, Anton Nehaev for giving inspirational feedback on the draft of this post that helped me make it clearer.

Compilable configuration of a distributed system