ContainerSSH Logging Library

This library provides internal logging for ContainerSSH. Its functionality is very similar to how syslog is structured.

⚠⚠⚠ Warning: This is a developer documentation. ⚠⚠⚠
The user documentation for ContainerSSH is located at containerssh.io.

Basic concept

This is not the logger you would expect from the go-log library. This library combines the Go errors and the log messages into one. In other words, the Message object can be used both as an error and a log message.

The main Message structure has several properties: a unique error code, a user-safe error message and a detailed error message for the system administrator. Additionally, it also may contain several key-value pairs called labels to convey additional information.

Creating a message

If you want to create a message you can use the following methods:

log.UserMessage()

The log.UserMessage method creates a new Message with a user-facing message structure as follows:

msg := log.UserMessage(
    "E_SOME_ERROR_CODE",
    "Dear user, an internal error happened.",
    "Details about the error (%s)",
    "Some string that will end up instead of %s."
)

• The first parameter is an error code that is unique so people can identify the error. This error code should be documented so users can find it easily.
• The second parameter is a string printed to the user. It does not support formatting characters.
• The third parameter is a string that can be logged for the system administrator. It can contain fmt.Sprintf-style formatting characters.
• All subsequent parameters are used to replace the formatting characters.

log.NewMessage()

The log.NewMessage() method is a simplified version of log.UserMessage() without the user-facing message. The user-facing message will always be Internal Error.. The method signature is the following:

msg := log.NewMessage(
    "E_SOME_ERROR_CODE",
    "Details about the error (%s)",
    "Some string that will end up instead of %s."
)

log.WrapUser()

The log.WrapUser() method can be used to create a wrapped error with a user-facing message. It automatically appends the original error message to the administrator-facing message. The function signature is the following:

msg := log.WrapUser(
    originalErr,
    "E_SOME_CODE",
    "Dear user, some error happened."
    "Dear admin, an error happened. %s" + 
        "The error message will be appended to this message."
    "This string will appear instead of %s in the admin-message."
)

log.Wrap()

Like the log.WrapUser() method the log.Wrap() will skip the user-visible error message and otherwise be identical to log.Wrap().

msg := log.Wrap(
    originalErr,
    "E_SOME_CODE",
    "Dear admin, an error happened. %s" + 
        "The error message will be appended to this message."
    "This string will appear instead of %s in the admin-message."
)

Adding labels to messages

Labels are useful for recording extra information with messages that can be indexed by the logging system. These labels may or may not be recorded by the logging backend. For example, the syslog output doesn't support recording labels due to size constraints. In other words, the message itself should contain enough information for an administrator to interpret the error.

You can add labels to messages like this:

msg.Label("labelName", "labelValue")

Hint: Label() calls can be chained.

Using messages

As mentioned before, the Message interface implements the error interface, so these messages can simply be returned like a normal error would.

Logging

This library also provides a Logger interface that can log all kinds of messages and errors, including the Message interface. It provides the following methods for logging:

• logger.Debug(message ...interface{})
• logger.Info(message ...interface{})
• logger.Notice(message ...interface{})
• logger.Warning(message ...interface{})
• logger.Error(message ...interface{})
• logger.Critical(message ...interface{})
• logger.Alert(message ...interface{})
• logger.Emergency(message ...interface{})

You are encouraged to pass a Message implementation to it.

We also provide the following compatibility methods which log at the info level.

• logger.Log(v ...interface{})
• logger.Logf(format string, v ...interface{})

We provide a method to create a child logger that has a different minimum log level. Messages below this level will be discarded:

newLogger := logger.WithLevel(log.LevelInfo)

We can also create a new logger copy with default labels added:

newLogger := logger.WithLabel("label name", "label value")

Finally, the logger also supports calling the Rotate() and Close() methods. Rotate() instructs the output to close all handles and reopen them to facilitate rotating logs. Close() permanently closes the writer.

Creating a logger

The Logger interface is intended for generic implementations. The default implementation can be created as follows:

logger, err := log.NewLogger(config)

Alternatively, you can also use the log.MustNewLogger method to skip having to deal with the error. (It will panic if an error happens.)

If you need a factory you can use the log.LoggerFactory interface and the log.NewLoggerFactory to create a factory you can pass around. The Make(config) method will make a new logger when needed.

Configuration

The configuration structure for the default logger implementation is contained in the log.Config structure.

Configuring the output format

The most important configuration is where your logs will end up:

log.Config{
    Output: log.OutputStdout,
}

The following options are possible:

• log.OutputStdout logs to the standard output or any other io.Writer
• log.OutputFile logs to a file on the local filesystem.
• log.OutputSyslog logs to a syslog server using a UNIX or UDP socket.

Logging to stdout

If you set the Output option to log.OutputStdout the output will be written to the standard output in the format specified below (see "Changing the log format"). The destination can be overridden:

log.Config {
    Output: log.OutputStdout,
    Stdout: someOtherWriter,
}

Logging to a file

The file logger is configured as follows:

log.Config {
    Output: log.OutputFile,
    File: "/var/log/containerssh.log",
}

You can call the Rotate() method on the logger to close and reopen the file. This allows for log rotation.

Logging to syslog

The syslog logger writes to a syslog daemon. Typically, this is located on the /dev/log UNIX socket, but sending logs over UDP is also supported. TCP, encryption, and other advanced Syslog features are not supported, so adding a Syslog daemon on the local node is strongly recommended.

The configuration is the following:

log.Config{
    Output: log.OutputSyslog,
    Facility: log.FacilityStringAuth, // See facilities list below
    Tag: "ProgramName", // Add program name here
    Pid: false, // Change to true to add the PID to the Tag
    Hostname: "" // Change to override host name 
}

The following facilities are supported:

• log.FacilityStringKern
• log.FacilityStringUser
• log.FacilityStringMail
• log.FacilityStringDaemon
• log.FacilityStringAuth
• log.FacilityStringSyslog
• log.FacilityStringLPR
• log.FacilityStringNews
• log.FacilityStringUUCP
• log.FacilityStringCron
• log.FacilityStringAuthPriv
• log.FacilityStringFTP
• log.FacilityStringNTP
• log.FacilityStringLogAudit
• log.FacilityStringLogAlert
• log.FacilityStringClock
• log.FacilityStringLocal0
• log.FacilityStringLocal1
• log.FacilityStringLocal2
• log.FacilityStringLocal3
• log.FacilityStringLocal4
• log.FacilityStringLocal5
• log.FacilityStringLocal6
• log.FacilityStringLocal7

Changing the log format

We currently support two log formats: text and ljson. The format is applied for the stdout and file outputs and can be configured as follows:

log.Config {
    Format: log.FormatText|log.FormatLJSON,
}

The text format

The text format is structured as follows:

TIMESTAMP[TAB]LEVEL[TAB]MODULE[TAB]MESSAGE[NEWLINE]

• TIMESTAMP is the timestamp of the message in RFC3339 format.
• LEVEL is the level of the message (debug, info, notice, warning, error, critical, alert, emergency)
• MODULE is the name of the module logged. May be empty.
• MESSAGE is the text message or structured data logged.

This format is recommended for human consumption only.

The ljson format

This format logs in a newline-delimited JSON format. Each message has the following format:

{"timestamp": "TIMESTAMP", "level": "LEVEL", "module": "MODULE", "message": "MESSAGE", "details": "DETAILS"}

• TIMESTAMP is the timestamp of the message in RFC3339 format.
• LEVEL is the level of the message (debug, info, notice, warning, error, critical, alert, emergency)
• MODULE is the name of the module logged. May be absent if not sent.
• MESSAGE is the text message. May be absent if not set.
• DETAILS is a structured log message. May be absent if not set.

Creating a logger for testing

You can create a logger for testing purposes that logs using the t *testing.T log facility:

logger := log.NewTestLogger(t)

Generating message code files

This package also includes a utility to generate and update a CODES.md from a codes.go file in your repository to create a documentation about message codes.

To use this utility your codes.go must contain message codes in individual const declarations with documentation, not as a const block. You can install this package as follows:

go get -u github.com/containerssh/log/cmd/containerssh-generate-codes

You can then execute containerssh-generate-codes to generate CODES.md. Optionally, you can pass the filenames as parameters:

containerssh-generate-codes source.go DESTINATION.md

We recommend creating a gencodes.md file with the following content:

//go:generate go run containerssh-generate-codes

This lets you generate CODES.md using go generate.