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158 lines
8.0 KiB
Go
158 lines
8.0 KiB
Go
// Package flume is a logging package, build on top of zap. It's structured and leveled logs, like zap/logrus/etc.
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// It adds global, runtime re-configuration of all loggers, via an internal logger registry.
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//
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// There are two interaction points with flume: code that generates logs, and code that configures logging output.
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// Code which generates logs needs to create named logger instances, and call log functions on it, like Info()
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// and Debug(). But by default, all these logs will be silently discarded. Flume does not output
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// log entries unless explicitly told to do so. This ensures libraries can freely use flume internally, without
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// polluting the stdout of the programs importing the library.
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//
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// The Logger type is a small interface. Libraries should allow replacement of their Logger instances so
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// importers can entirely replace flume if they wish. Alternately, importers can use flume to configure
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// the library's log output, and/or redirect it into the overall program's log stream.
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//
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// Logging
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//
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// This package does not offer package level log functions, so you need to create a logger instance first:
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// A common pattern is to create a single, package-wide logger, named after the package:
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//
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// var log = flume.New("mypkg")
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//
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// Then, write some logs:
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//
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// log.Debug("created user", "username", "frank", "role", "admin")
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//
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// Logs have a message, then matched pairs of key/value properties. Child loggers can be created
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// and pre-seeded with a set of properties:
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//
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// reqLogger := log.With("remoteAddr", req.RemoteAddr)
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//
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// Expensive log events can be avoid by explicitly checking level:
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//
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// if log.IsDebug() {
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// log.Debug("created resource", "resource", resource.ExpensiveToString())
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// }
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//
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// Loggers can be bound to context.Context, which is convenient for carrying
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// per-transaction loggers (pre-seeded with transaction specific context) through layers of request
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// processing code:
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//
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// ctx = flume.WithLogger(ctx, log.With("transactionID", tid))
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// // ...later...
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// flume.FromContext(ctx).Info("Request handled.")
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//
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// The standard Logger interface only supports 3 levels of log, DBG, INF, and ERR. This is inspired by
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// this article: https://dave.cheney.net/2015/11/05/lets-talk-about-logging. However, you can create
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// instances of DeprecatedLogger instead, which support more levels.
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//
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// Configuration
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//
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// There are several package level functions which reconfigure logging output. They control which
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// levels are discarded, which fields are included in each log entry, and how those fields are rendered,
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// and how the overall log entry is rendered (JSON, LTSV, colorized, etc).
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//
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// To configure logging settings from environment variables, call the configuration function from main():
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//
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// flume.ConfigFromEnv()
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//
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// This reads the log configuration from the environment variable "FLUME" (the default, which can be
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// overridden). The value is JSON, e.g.:
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//
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// {"level":"INF","levels":"http=DBG","development"="true"}
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//
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// The properties of the config string:
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//
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// - "level": ERR, INF, or DBG. The default level for all loggers.
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// - "levels": A string configuring log levels for specific loggers, overriding the default level.
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// See note below for syntax.
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// - "development": true or false. In development mode, the defaults for the other
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// settings change to be more suitable for developers at a terminal (colorized, multiline, human
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// readable, etc). See note below for exact defaults.
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// - "addCaller": true or false. Adds call site information to log entries (file and line).
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// - "encoding": json, ltsv, term, or term-color. Configures how log entries are encoded in the output.
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// "term" and "term-color" are multi-line, human-friendly
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// formats, intended for terminal output.
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// - "encoderConfig": a JSON object which configures advanced encoding settings, like how timestamps
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// are formatted. See docs for go.uber.org/zap/zapcore/EncoderConfig
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//
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// - "messageKey": the label of the message property of the log entry. If empty, message is omitted.
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// - "levelKey": the label of the level property of the log entry. If empty, level is omitted.
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// - "timeKey": the label of the timestamp of the log entry. If empty, timestamp is omitted.
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// - "nameKey": the label of the logger name in the log entry. If empty, logger name is omitted.
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// - "callerKey": the label of the logger name in the log entry. If empty, logger name is omitted.
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// - "lineEnding": the end of each log output line.
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// - "levelEncoder": capital, capitalColor, color, lower, or abbr. Controls how the log entry level
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// is rendered. "abbr" renders 3-letter abbreviations, like ERR and INF.
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// - "timeEncoder": iso8601, millis, nanos, unix, or justtime. Controls how timestamps are rendered.
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// "millis", "nanos", and "unix" are since UNIX epoch. "unix" is in floating point seconds.
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// "justtime" omits the date, and just prints the time in the format "15:04:05.000".
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// - "durationEncoder": string, nanos, or seconds. Controls how time.Duration values are rendered.
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// - "callerEncoder": full or short. Controls how the call site is rendered.
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// "full" includes the entire package path, "short" only includes the last folder of the package.
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//
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// Defaults:
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//
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// {
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// "level":"INF",
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// "levels":"",
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// "development":false,
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// "addCaller":false,
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// "encoding":"term-color",
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// "encoderConfig":{
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// "messageKey":"msg",
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// "levelKey":"level",
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// "timeKey":"time",
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// "nameKey":"name",
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// "callerKey":"caller",
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// "lineEnding":"\n",
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// "levelEncoder":"abbr",
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// "timeEncoder":"iso8601",
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// "durationEncoder":"seconds",
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// "callerEncoder":"short",
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// }
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// }
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//
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// These defaults are only applied if one of the configuration functions is called, like ConfigFromEnv(), ConfigString(),
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// Configure(), or LevelsString(). Initially, all loggers are configured to discard everything, following
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// flume's opinion that log packages should be silent unless spoken too. Ancillary to this: library packages
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// should *not* call these functions, or configure logging levels or output in anyway. Only program entry points,
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// like main() or test code, should configure logging. Libraries should just create loggers and log to them.
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//
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// Development mode: if "development"=true, the defaults for the rest of the settings change, equivalent to:
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//
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// {
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// "addCaller":true,
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// "encoding":"term-color",
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// "encodingConfig": {
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// "timeEncoder":"justtime",
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// "durationEncoder":"string",
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// }
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// }
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//
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// The "levels" value is a list of key=value pairs, configuring the level of individual named loggers.
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// If the key is "*", it sets the default level. If "level" and "levels" both configure the default
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// level, "levels" wins.
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// Examples:
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//
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// * // set the default level to ALL, equivalent to {"level"="ALL"}
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// *=INF // same, but set default level to INF
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// *,sql=WRN // set default to ALL, set "sql" logger to WRN
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// *=INF,http=ALL // set default to INF, set "http" to ALL
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// *=INF,http // same as above. If name has no level, level is set to ALL
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// *=INF,-http // set default to INF, set "http" to OFF
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// http=INF // leave default setting unchanged.
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//
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// Factories
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//
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// Most usages of flume will use its package functions. The package functions delegate to an internal
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// instance of Factory, which a the logger registry. You can create and manage your own instance of
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// Factory, which will be an isolated set of Loggers.
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//
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// tl;dr
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//
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// The implementation is a wrapper around zap. zap does levels, structured logs, and is very fast.
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// zap doesn't do centralized, global configuration, so this package
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// adds that by maintaining an internal registry of all loggers, and using the sync.atomic stuff to swap out
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// levels and writers in a thread safe way.
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package flume
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