# Introduction

mtail is intended to run one per machine, and serve as monitoring glue for multiple applications running on that machine.  It runs one or more programs in a 1:1 mapping to those client applications.

## Configuration Overview

mtail is configured through commandline flags.

The `--help` flag will print a list of flags for configuring `mtail`.

(Flags may be prefixed with either `-` or `--`)

Basic flags necessary to start `mtail`:

  * `--logs` is a comma separated list of filenames to extract from, but can also be used multiple times, and each filename can be a [glob pattern](http://godoc.org/path/filepath#Match).  Named pipes can be read from when passed as a filename to this flag.
  * `--progs` is a directory path containing [mtail programs](Language.md). Programs must have the `.mtail` suffix.

mtail runs an HTTP server on port 3903, which can be changed with the `--port` flag.

# Details

## Launching mtail

```
mtail --progs /etc/mtail --logs /var/log/syslog --logs /var/log/ntp/peerstats
```

`mtail` will start to read the specified logs from their current end-of-file,
and read new updates appended to these logs as they arrive.  It will attempt to
correctly handle log files that have been rotated by renaming or symlink
changes.

### Getting the logs in

Use `--logs` multiple times to pass in glob patterns that match the logs you
want to tail.  This includes named pipes.

### Polling the file system

`mtail` polls every `--poll_interval`, or 250ms by default, the supplied `--logs` patterns for newly created or deleted log pathnames.

Known and active logs are read until EOF every 250ms by default.

Example:
```
mtail --progs /etc/mtail --logs /var/log/syslog --poll_interval 250ms
```


### Setting garbage collection intervals

`mtail` accumulates metrics and log files during its operation.  By default, *every hour* both a garbage collection pass occurs looking for expired metrics, and stale log files.

An expired metric is any metric that hasn't been updated in a time specified by a `del after` form in a program.

A stale log file is any log being watched that hasn't been read from in 24 hours.

The interval between garbage collection runs can be changed on the commandline with the `--expired_metrics_gc_interval` and `--stale_log_gc_interval` flags, which accept a time duration string compatible with the Go [time.ParseDuration](https://golang.org/pkg/time/#ParseDuration) function.


### Runtime error log rate

If your programs deliberately fail to parse some log lines then you may end up generating lots of runtime errors which are normally logged at the standard INFO level, which can fill your disk.

You can disable this with `--novm_logs_runtime_errors` or `--vm_logs_runtime_errors=false` on the commandline, and then you will only be able to see the most recent runtime error in the HTTP status console.

### Launching under Docker

`mtail` can be run as a sidecar process if you expose an application container's logs with a volume.

`docker run -d --name myapp -v /var/log/myapp myapp`

for example exports a volume called `/var/log/myapp` (named the same as the
hypothetical path where `myapp`s logs are written.

Then launch the `mtail` docker image and pass in the volume:

    docker run -dP \
       --name myapp-mtail \
       --volumes-from myapp \
       -v examples:/etc/mtail \
       mtail --logs /var/log/myapp --progs /etc/mtail

This example fetches the volumes from the `myapp` container, and mounts them in
the mtail container (which we've called `myapp-mtail`).  We also mount the
`examples` directory as `/etc/mtail` in the container.  We launch `mtail` with
the `logs` and `progs` flags to point to our two mounted volumes.

The `-P` flag ensures `mtail-myapp`'s port 3903 is exposed for collection,
refer to `docker ps` to find out where it's mapped to on the host.

## Writing the programme

Read the [Programming Guide](Programming-Guide.md) for instructions on how to write an `mtail` program.

### Reloading programmes

`mtail` does not automatically reload programmes after it starts up.  To ask `mtail` to scan for and reload programmes from the supplied `--progs` directory, send it a `SIGHUP` signal on UNIX-like systems.

## Getting the Metrics Out

### Pull based collection

Point your collection tool at `localhost:3903/json` for JSON format metrics.

Prometheus can be directed to the /metrics endpoint for Prometheus text-based format.

### Push based collection

Use the `collectd_socketpath` or `graphite_host_port` flags to enable pushing to a collectd or graphite instance.

Configure collectd on the same machine to use the unixsock plugin, and set `collectd_socketpath` to that unix socket.

```
mtail --progs /etc/mtail --logs /var/log/syslog,/var/log/rsyncd.log --collectd_socketpath=/var/run/collectd-unixsock
```

Set `graphite_host_port` to be the host:port of the carbon server.

```
mtail --progs /etc/mtail --logs /var/log/syslog,/var/log/rsyncd.log --graphite_host_port=localhost:9999
```

Likewise, set `statsd_hostport` to the host:port of the statsd server.

Additionally, the flag `metric_push_interval_seconds` can be used to configure the push frequency.  It defaults to 60, i.e. a push every minute.

## Setting a default timezone

The `--override_timezone` flag sets the timezone that `mtail` uses for timestamp conversion.  By default, `mtail` assumes timestamps are in UTC.

To use the machine's local timezone, `--override_timezone=Local` can be used.

## Troubleshooting

Lots of state is logged to the log file, by default in `/tmp/mtail.INFO`.  See [Troubleshooting](Troubleshooting.md) for more information.

N.B. Oneshot mode (the `one_shot` flag on the commandline) can be used to check
that a program is correctly reading metrics from a log, but with the following
caveats:

* Unlike normal operations, oneshot mode will read the logs from the start of
  the file to the end, then close them -- it does not continuously tail the
  file
* The metrics will be printed to standard out when the logs are finished being
  read from.
* mtail will exit after the metrics are printed out.

This mode is useful for debugging the behaviour of `mtail` programs and
possibly for permissions checking.
