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simple config and logging setup for scala

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Configgy is a library for handling config files and logging for a scala daemon. The idea is that it should be simple and straightforward, allowing you to plug it in and get started quickly.


You would think config files would be a solved problem by now, but for some reason they're not. Java is still trying to shoehorn "property files" for the purpose (keys and values are strings, no blocks). Python is still trying to use INI files (!), and Ruby invented a new text marshalling format (YAML) and decided to abuse that for config files too. It's all kind of a sad mess.

Configuration doesn't have to be a hard problem. My experience shows that only a few features are needed:

  • key/value pairs can be set
  • values can be bools, ints, strings, or lists of strings
  • sets of key/value pairs can be grouped into blocks and nested inside each other
  • some blocks can inherit default values from other blocks
  • values can be based (in some way) on previous values
  • files can be "imported" into each other

From the API side, it should be easy to find and manage key/value pairs and blocks. A library should also be able to "subscribe" to a block and be notified when values have been changed from somewhere else.

None of property files, INI files, or YAML files can do this. So here we are.

At Danger, after several false starts, we ended up with a server config file format very much like what I've implemented here (but it was more complex and had a few major flaws). Other projects have created similar formats. So I think this is an idea whose time has come.

Logging is tightly integrated with configuration, since config files are usually used to setup logging, and some sort of logging is usually required at the same time as a config file. So this library puts them together, using the simple config format to setup logging, too -- allowing most of the flexibility of java logging, but making it easy to do the common case of creating a single logfile with one debug level.

Quick Start

This is all you need to know to use the library:

import net.lag.configgy.Configgy
import net.lag.logging.Logger

// load our config file and configure logfiles:

// read hostname and port:
val config = Configgy.config
val hostname = config.get("hostname", "localhost")
val port = config.getInt("port", 3000)

// log an error:
val log = Logger.get
log.error("Unable to listen on %s:%d!", hostname, port)

// or an exception:
try {
} catch {
    case e: IOException => log.error(e, "IOException while doodling")

The following config file will setup a logfile at debug level, that rolls every night, and also sets a few simple config values:

    filename = "/var/log/pingd.log"
    roll = "daily"
    level = "debug"

hostname = ""
port = 3000

The rest of this README just describes the config file format, the logging options, and how to use the library in more detail.

Basic Use

Config Files

The config file format is simple and clean. It borrows from XML, but isn't XML. It has a few advanced features, but limits itself to representing a nested structure of key/value pairs. It is always in UTF-8. Key names are always case-insensitive.

Values are set like so:

server_name = "pingd"

Key names follow the same rules as C (or Java or Python) symbols, except they may also contain dashes ("-") if you like.

String values are in quotes, and have the usual escape codes, like \r. \xHH and \uHHHH also work.

Int values don't need quotes, but are treated like strings and converted only at the last minute.

Boolean values are turned "on" or "off" (or "true" or "false", with an optional equal sign -- whatever makes you happy):

use_root_privs off
drink_milk = false

String lists are just strings enclosed in brackets, separated by commas:

states = ["California", "Tennessee", "Idaho"]

Groups of keys may be enclosed in an XML-like tag:

    # set a high timeout for now.
    timeout = 30

Nested values can be accessed from the API and from within the config file as if they were in C structs, using dotted-name notation. So the above config is the same as:

pingd.timeout = 30


Logging is configured in a special <log> block. The main logging options are described below.

  • filename - the file to write log entries into (optional)
  • level - the lowest severity log entry that should be written to the logfile (defaults to INFO) (described below)
  • console - true (on) if logs should be written to the stderr console
  • syslog_host - hostname (or hostname:port) to send syslog formatted log data to (optional)
  • syslog_server_name - server name to attach to log messages when sending to a syslog (optional)
  • roll - when the logfile should be rolled (described below)

Logging severities are:

Severity   Description
=========  ================
FATAL      the server is about to exit
CRITICAL   something happened that is so bad that someone should probably
             be paged
ERROR      an error occurred that may be limited in scope, but was
WARNING    a coder may want to be notified, but the error was probably not
INFO       normal informational logging
DEBUG      coder-level debugging information
TRACE      intensive debugging information

Logfile rolling policies are:

Name     Description
=======  =================
never    always use the same logfile
hourly   roll to a new logfile at the top of every hour
daily    roll to a new logfile at midnight every night
sunday   roll to a new logfile at midnight between saturday and sunday,
           once a week

You can omit a rolling policy, or use policy "never", to avoid rolling the logfiles. For weekly logfile rolling, you may use any day of the week ("monday", "tuesday", etc), not just "sunday".

When a logfile is rolled, the current logfile is renamed to have the date (and hour, if rolling hourly) attached, and a new one is started. So, for example, test.log may become test-20080425.log, and test.log will be reopened as a new file.

So, for example:

    filename = "test.log"
    level = "warning"
    roll = "tuesday"

creates a logfile test.log that captures log entries only at warning, error, critical, or fatal levels. It's rolled once a week, at midnight on Tuesday morning.

None of filename, console, or syslog_host are mutually exclusive, so you can define any or all of those targets, to have log messages sent to any possible combination of places.

Advanced Features

There are a few features you may not use right away, but you'll usually start wanting after the code matures a bit.

Config Files

Previously-set config values can be included in new ones by using variable substitution, with shell syntax. This also pulls in values from the environment:

server_home = "$(HOME)/servers/pingd"
pid_file = "$(server_home)/"

Files can be included/imported into each other:

include "$(server_home)/config/local.conf"

(The context is preserved, so if you are inside a block, the imported file is injected inside that block too.) The default importer uses the local filesystem. You can set your own importer to use other sources.

To set a value only if it hasn't been set earlier (possibly by an included file), use shell syntax:

server_home ?= "/opt/pingd"

Common config blocks can be inherited by later blocks:

    timeout = 15
    chroot = "/opt/magic"

<pingd inherit="daemon-base">
    timeout = 30

The pingd block will use its own value of "timeout" (30), but will inherit "chroot" from daemon-base. That is, "pingd.chroot" will be "/opt/magic".


There are a handful of options to tune logging more directly:

  • utc - on to log in UTC (previously known as GMT) time instead of local time (default: off)
  • truncate - number of characters to allow in a single log line before eliding with "..." (default: 0 = never truncate)
  • truncate_stack_traces - number of lines of a stack trace to show before eliding (default: 30)
  • syslog_use_iso_date_format - set off to use old-style BSD date format in syslog messages (default: on)
  • use_full_package_names - set on to use full package names in log lines ("net.lag.configgy") instead of the toplevel node ("configgy") (default: off)
  • append - set off to create a new logfile each time the app starts (default: on, meaning to append to any existing logfile)

The logging options are usually set on the root node of java's "logging tree", at "". You can set options or logging handlers at other nodes by putting them in config blocks inside <log>. For example:

    filename = "test.log"
    level = "warning"
    utc = true

        node = "com.example.libnoise"
        level = "critical"

(You don't have to name the block "squelch_noisy"; they can have any name.)

The "com.example.libnoise" node will log at "critical" level (presumably to silence a noisy library), while everything else will log at "warning" level. You can put any of the logging options inside these blocks, including those for logging to files or syslog nodes, so in this way you can create multiple logfiles.

The extra options you can use in these inner blocks are:

  • node - define the log node name (as a string)
  • use_parents - whether to fall back to parent log-node configuration (java's setUseParentHandlers) (default: on)

Usage from within scala

You can build the scaladocs with the ant rule:

$ ant docs

The main interface is Configgy, which can be used to load a config file and configure logging. The loaded config object is of type Config which is just a ConfigMap with methods added for handling subscriptions.

Basic Config

Usually you will just want to set or get config values.

Getting values can return an Option, or (if you provide a default value), a direct String (or Int, or Boolean, etc):

scala> config.getString("name")
res1: Option[String] = Some(Bender)

scala> config("name")
res2: String = Bender

scala> config("name", "Frank")
res3: String = Bender

scala> config.getInt("age", 16)   
res4: Int = 23

Setting values is similar:

scala> config("name") = "Bender"
scala> config("age") = 23

To access a nested attribute, you can either use a dotted key like:

scala> config.getInt("pingd.port")
res10: Option[Int] = Some(3000)

Or you can access the intermediate ConfigMap object:

scala> config.getConfigMap("pingd")
res1: Option[net.lag.configgy.ConfigMap] = Some({pingd: port="3000" })

scala> config.getConfigMap("pingd").get.getInt("port")
res2: Option[Int] = Some(3000)

Subscription API

Subscribing to an AttributesMap causes the subscriber to get called on every change to that node. For monitored config nodes, changes happen in two phases: validate and commit.

In validation, you can verify that the new settings for this config node are reasonable and consistent, and if not, you can throw an exception to reject the change.

If all subscribers successfully validate a change, the commit method is called to notify subscribers that the change was successful. If any subscriber refuses to validate a change, the code that called set should be prepared to catch the validation exception.


To access a logger from within a class or object, you can usually just use:

import net.lag.logging.Logger
private val log = Logger.get

This creates a Logger object that uses the current class or object's package name as the logging node, so class "" will log to node "" (generally showing "foo" as the name in the logfile). You can also get a logger explicitly by name:

private val log = Logger.get("")

Logger objects wrap everything useful from "java.util.logging.Logger", as well as adding some convenience methods:

// log a string with sprintf conversion:"Starting compaction on level %d...", level)

try {
} catch {
    // log an exception backtrace with the message:
    case x: IOException =>, "I/O exception: %s", x.getMessage)

Each of the log levels (from "fatal" to "trace") has these two convenience methods. You may also use log directly:

log(Logger.DEBUG, "Logging %s at debug level.", name)

An advantage to using sprintf ("%s", etc) conversion, as opposed to:

log(Logger.DEBUG, "Logging " + name + " at debug level.")

is that java & scala perform string concatenation at runtime, even if nothing will be logged because the logfile isn't writing debug messages right now. With sprintf parameters, the arguments are just bundled up and passed directly to the logging level before formatting. If no log message would be written to any file or device, then no formatting is done and the arguments are thrown away. That makes it very inexpensive to include excessive debug logging which can be turned off without recompiling and re-deploying.

The logging classes are done as an extension to the java.util.logging API, and so if you want to use the java interface directly, you can. Each of the java classes (Logger, Handler, Formatter) is just wrapped by a scala class with a cleaner interface.


Configgy was written by Robey Pointer and is licensed under the Apache 2 license (included). Please write me with questions or comments, but the software is provided as-is.

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