tesla-microservice

"If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found the object of his search." - Nikola Tesla

This is the common basis for some of otto.de's microservices. It is written in clojure using the component framework.

Breaking changes

tesla-microservice is used for a number of different services now. Still it is a work in progress. See CHANGES.md for instructions on breaking changes.

Features included

• Load configuration from filesystem.
• Aggregate a status.
• Execute functions with a scheduler
• Reply to a health check.
• Deliver a json status report.
• Report to graphite using the metrics library.
• Manage handlers using ring.
• Optional auto-hot-reloading of changed source files
• Shutdown gracefully. If necessary delayed, so load-balancers have time to notice.

Examples

• A growing set of example applications can be found at tesla-examples.
• David & Germán created an example application based, among other, on tesla-microservice. They wrote a very instructive blog post about it
• Moritz created tesla-pubsub-service. It showcases how to connect components via core.async channels. Also the embedded jetty was replaced by immutant.

Scheduler

The scheduler wraps a thread-pool which can be used for scheduling tasks. It is based on overtones at-at project. To actually use it you have to pass the :scheduler as a dependency to the component in which it should be used. Afterwards you can schedule tasks using the overtone api like this:

(overtone.at-at/every 100 #(println "Hello world") (de.otto.tesla.stateful.scheduler/pool scheduler) :desc "HelloWord Task")

The overtone-pool wrapped by the scheduler can be configured by the config-entry :scheduler. (See overtone.at-at/mk-pool) By default the pool holds no threads.

app-status

The app-status indicates the current status of your microservice. To use it you can register a status function to it.

Here is a simple example for a function that checks if an atom is empty or not.

(de.otto.tesla.stateful.app-status/register-status-fun app-status #(status atom))

The app-status is injected under the keyword :app-status from the base system.

(defn status [atom]
      (let [status (if @atom :error :ok)
            message (if @atom "Atom is empty" "Atom is not empty")]
           (de.otto.status/status-detail :status-id status message)))

For further information and usages take a look at the: status library

Choosing a server

As of version 0.1.15 there is no server included any more directly in tesla-microservice. This gives you the freedom to a) not use any server at all (e.g. for embedded use) b) choose another server e.g. a non-blocking one like httpkit or immutant. The available options are:

• tesla-jetty: The tried and tested embedded jetty.
• tesla-httpkit: The non-blocking httpkit.

Configuring

Applications build with tesla-microservices can be configured via edn-files, that have to be located in the class path.

For backwards compatibility, it is also possible to load config from properties-files. See below for noteworthy differences.

Order of loading and merging

1. A file named default.edn is loaded as a resource from classpath if present.
2. A file either named application.edn or overridden by the ENV-variable $CONFIG_FILE is loaded as a resource or, if that is not possible, from the filesystem.
3. A file name local.edn is loaded from classpath if present.

The configuration hash-map in those files is loaded and merged in the specified order. Which mean configurations for the same key is overridden by the latter occurrence.

ENV-variables

In contrast to former versions of tesla-microservice ENV-variables are not merged into the configuration.

But you can easily specify ENV-variables, that should be accessible in your configuration:

{
 :my-app-secret  #ts/env [:my-env-dep-app-secret "default"]
}

ENV-variables are read with environ. To see which keyword represents which ENV-var have a look in their docs.

Configuring via properties files

For backwards compatibility, it is also possible to load config from properties-files. You'll have to pass {:property-file-preferred true} as a runtime config to the base-system. It is not possible to load individual environment variables when using properties config. Adding :merge-env-to-properties-config true to the runtime config will add all system properties and environment variables, overiding any config from files.

Reporters

Applications utilizing Tesla-Microservice can use iapetos prometheus client for monitoring. Metrics are send by reporters which can be configured using the :metrics keyword. Each configured reporter will start at system startup automatically.

See example configuration below for all supported reporters.

:metrics {:graphite            {:host             "localhost"
                                :port             "2003"
                                :prefix           "my.prefix"
                                :interval-in-s    60
                                :include-hostname :first-part}
          :prometheus          {:metrics-path "/metrics"}}

Automatic hot-reloading of changed source files

Restarting the whole system after a small change can be cumbersome. A tesla-microservice can detect changes to your source files and load them into a running server. Add this to your config, to check for changes on each request to your system:

{:handler {:hot-reload? true}}

Note: This should only be enabled in development mode. Use your local.edn to enable this feature safely. You can add a private.edn as well for personal configurations. This file should be added to your .gitignore.

Securing internal info endpoints

The Tesla-Microservice comes with endpoints that hold information about the internal state of your application. Those endpoints can be the app-status or even metrics (Prometheus, see above). To secure those endpoints you can provide an authentication-middleware to the base-system.

E.g.:

(defn auth-middleware [config handler-fn]
  (fn [request] (if (authenticated? config request) 
                  (handler-fn request)
                  {:status 401 :body "access denied"})))

(defn example-system [runtime-config]
  (-> (de.otto.tesla.system/base-system runtime-config auth-middleware))) 

Addons

The basis included is stripped to the very minimum. Additional functionality is available as addons:

• tesla-zookeeper-observer: Read only access to zookeeper.
• tesla-mongo-connect: Read/write access to mongodb.
• tesla-cachefile: Read and write a cachefile. Locally or in hdfs.

More features will be released at a later time as separate addons.

FAQ

Q: Is it any good? A: Yes.

Q: Why tesla? A: It's a reference to the ingenious scientist and inventor.

Q: Are there alternatives? A: Yes. You might want to look at modularity.org, system and duct.

Initial Contributors

Christian Stamm, Felix Bechstein, Ralf Sigmund, Kai Brandes, Florian Weyandt

License

Released under Apache License 2.0 license.