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Three stages of logging

It explains how a log message passed to the output. It assumed that logger instance initialized before alongside with a number of outputs.

Stage 1: forming a record

Firstly you add key/value pairs to a log record. There are a lot methods for it around Logger struct. They stored in Logger but not passed further to output until you call Logger.Log(). This method flushes current record.

Stage 2: filtering

Each flushed record passed to all outputs. The each output can has arbitrary number of filters. After using a new output it has no defined filters. You add them with methods of Output struct. There are two groups of filtering methods:

• first group allows drop out unwanted records
• second group allows drop out unwanted pairs for more compact record output

By default no restrictions so a new output will pass all records logged by application. Combination of filters defines which records will pass to each output. It is normal when the same record passed to several outputs.

Stage 3: formatting & writing

After record passed for writing it should be appropriately formatted. Each output declares which format it wants. Currently there are two predefined formats: Logfmt and JSON. You can pass a custom format by implementing Formatter interface.

Formatter interface has three methods:

• Begin() for prefixing each record
• Pair(key, val string, quoted bool) called for each pair inside a record
• Finish() []byte should return formatter record as a byte array

With Begin and Finish you can define prefix and suffix for each record. It is their main purpose. See the sample realization in formatter.go for Logfmt and JSON formatters.

After formatting bytes passed to io.Writer of the output.