Usage - deps.edn coordinates

  com.github.tstout/sys-loader
    {:git/url "https://github.com/tstout/sys-loader"
     :git/tag "v1.1.7"
     :git/sha "4e1ce47"}

Overview

Sys-loader is a tool for spinning up a system of modules written in clojure. In this context the definition of a module is:

one of a set of parts that can be connected or combined to build or complete something.

Clojure’s cli-deps tools provide a compelling environment for combining code stored across multiple git or maven repositories. A sys-loader module is code stored in a maven or git repository.

Design

A module is defined by a module.edn file, typically made available on the classpath as a resource. The EDN file contains a namespace qualified init function and a vector of dependent modules, which are (required), then invoked. The cli-deps tooling makes building up the classpath and invoking configured functions simple. For example,

clojure -M:sys-loader -A:service-1:service-2:service-n

This composition of dependencies is a tenet of cli-deps. Simple, powerful, interesting. The term service here is abstract. It is not meant to imply a web service accepting http requests. It could be this, but not necessarily. Each dependency listed after the sys-loader alias given above, is assumed to contain a module.edn resource file containing the init function needed to initialize the module. The EDN file can also contain a list of dependencies. The sys-loader implementation will do a topological sort to invoke the init functions in the appropriate order. The cli-deps tooling can create uberjars for convenient deployment which does not require cli-deps at runtime.

An example module.edn file:

[{:sys/description "ring module"
  :sys/name        :ring-module
  :sys/deps        [:sys/logging]
  :sys/init        ring-module.core/init}]

Sys-loader provides

REPL

A prepl server is started on port 8000. nRepl is nice, but perhaps initially prepl (built-in) should be used to avoid dependencies. Look here for more info: https://oli.me.uk/clojure-socket-prepl-cookbook/

Logging

Storing logs in a database is useful. By default logs are written to an H2 database. tools.logging with log4j2 provides the logging implementation.

Database

An H2 server is provided. Most apps/services I have in mind will need several modules/plugins: DB, Logging, pub/sub, and scheduling. Consider supporting command line options to sys-loader to exclude baked-in modules/plugins from being loaded. The exclusion options are probably not needed. If the module is not listed in any dependency, then it won’t be started.

Database Migrations (forward only)

Building/Running

There is currently a java class that needs compiling. This is related to configuring log4j2 jdbc logging. To compile the class:

clojure -T:build compile

Note: there is a branch rm-java-code where I have attempted to configure log4j with clojure code to remove this java dependency. This currently does not work, and I'm putting it on hold for now. Apps having sys-loader as a dependency will need to execute

clojure -X:deps prep

to compile the class.

TODO

Monitoring

Is JMX sufficient for the beginning? Save this for nice to have, but not necessary.

Scheduling

Consider providing a standard scheduler perhaps based on chime. This has been working well for years now in fin-kratzen.