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README.md

Apollo Core (a.k.a. Leto)

The apollo-core library manages the lifecycle (loading, starting, and stopping) of your service on production machines. You do not usually need to interact directly with apollo-core. Apollo Core doesn't enforce any service architecture, so it can be used standalone, within a Java Servlet, in a batch command, in a CLI tool, etc.

Simple usage

The simplest possible service that uses Apollo Core looks like:

package com.spotify.apollo.example;

import com.spotify.apollo.core.Service;
import com.spotify.apollo.core.Services;

import java.io.IOException;

public class App {

  public static void main(String... args) throws IOException {
    Service service = Services.usingName("test").build();
    Services.run(service, args);
  }
}

Life-cycle control

To gain control over the Apollo Core lifecycle, simply manage it yourself:

Service service = Services.usingName("test").build();

// Before service started
try (Service.Instance instance = service.start(args)) {
  // After service started

  instance.waitForShutdown();

  // Before service stopped
} catch (Exception e) {
  // Service crashed while running
} finally {
  // After service stopped
}
// After successful service run

Features

Apollo Core provides a standard configuration for Java services. It tries to be API-compatible with Apollo, and adds additional features that help solve some inconsistencies in Apollo, Helios, and other Spotify support libraries.

The API is summarized in the Apollo Core service interface.

Consistent configuration

A core principle of Apollo Core is that everything should be configured via the service configuration. No environment variables, weird command-line options or oracles. Apollo Core then defines a standard way of controlling this configuration (and mapping environment variables to configuration in the situations when it's necessary).

Service service = Services.usingName("test").build();

try (Service.Instance instance = service.start(args)) {
  instance.getConfig(); // typesafe.config
}

Apollo Core will look for a configuration file with the name <service-name>.conf on the classpath. test.conf in the example above. It is possible to overlay an explicit config file on top of the classpath resolved config by using the command line argument --config <config-file>.

Apollo Core adds support for overriding configuration keys using -D command-line options. For example, -Ddomain=example.org sets the domain config key to example.org.

Apollo Core also considers environment variables of the form APOLLO_X_Y=ab, where the APOLLO prefix is configurable with Service.Builder.withEnvVarPrefix(). These are translated into configuration keys of the form x.y=ab. The values of the environment variables are treated as strings and copied verbatim; no special syntax is supported.

The order of evaluating configuration sources (with later sources overriding earlier ones) is:

  1. The initial configuration, either:
  2. The file loaded by com.typesafe.config.ConfigFactory.load() invoked with the service name as the parameter. This happens if you start up your service with one of the Service.start methods that don't take a Config as an argument. The HttpService uses this way.
  3. The configuration specified as an argument to Service.start
  4. Values specified using -D as a JVM CLI argument.
  5. Values specified as an apollo-specific CLI argument such as --syslog
  6. Values specified using -D as a program CLI argument.
  7. Values specified in a file supplied via the --config command line argument
  8. Values specified by environment variables that are translated into configuration keys as described above.

Logging configuration

NOTE/TODO: a logging module is currently not included Apollo Core sets up logging for you. Some supported ways of doing that:

Argument Impact
-v, -vv, --verbose Increases amount of log output
-c, -cc, --concise Decreases amount of log output
-q, --quiet Outputs no log
--syslog[=true/false] Enables/disables logging to syslog

All of these options map directly to configuration keys.

Scoped executors

Many services are polluted by executors being allocated everywhere. It is not uncommon to have many hundred threads in an application. Most of the time, an application can instead use a limited set of threads that are managed by central executor services. This also makes it possible to avoid having to use daemon threads, which are known to have issues when shutting down the JVM. Apollo Core manages shared executors that are general purpose for just that reason.

Service service = Services.usingName("test").build();

try (Service.Instance instance = service.start(args)) {
  instance.getExecutorService(); // For long-running jobs
  instance.getScheduledExecutorService(); // For periodical, short-lived jobs
}

Managed application clean-up

Every service will have resources that need to be freed before the application exits. It is very hard to do this correctly if you manually have to control the clean-up. A common problem is for example that if two resources a and b need to be cleaned up, and a.close() throws an exception, the clean-up of b never happens.

Apollo Core exposes an API that lets you delegate clean-up to the life cycle manager, which will do the right thing:

Service service = Services.usingName("test").build();

try (Service.Instance instance = service.start(args)) {
  // Something convoluted that prevents you from doing try-with-resources
  Configuration c = loadConfigFromZK();

  Connection connection = connectionFactory.connect(c);

  // Guaranteed to call close() before application quits
  instance.getCloser().register(connection);

  instance.waitForShutdown();
}

Delegating command-line configuration

When implementing a command-line tool, that tool typically needs to do some command-line parsing on its own. Apollo Core conforms to the UNIX/POSIX tool behavior and preserves unprocessed command-line options similar to getopt(3).

Service service = Services.usingName("test").build();

try (Service.Instance instance = service.start(args)) {
  instance.getUnprocessedArgs(); // Unrecognized command-line args
}

Modules

Apollo Core has a module system that simplifies setting up common use-cases such as a HTTP server, Cassandra connections and so on.

The module system is built on top of Google Guice. This was deemed to be the most light-weight module system available for Java that still supports dynamic configuration (Dagger for example only supports static configuration). Apollo Core extends Guice by making modules auto-loadable, and configuration-driven.

Modules can be found under modules: