# System Monitoring
_This functionality should be considered experimental and is subject to change._

_System Monitoring_ means all statistical and diagnostic information made availabe to the user of InfluxDB system, about the system itself. Its purpose is to assist with troubleshooting and performance analysis of the database itself.

## Statistics vs. Diagnostics
A distinction is made between _statistics_ and _diagnostics_ for the purposes of monitoring. Generally a statistical quality is something that is being counted, and for which it makes sense to store persistently for historical analysis. Diagnostic information is not necessarily numerical, and may not make sense to store.

An example of statistical information would be the number of points received over UDP, or the number of queries executed. Examples of diagnostic information would be a list of current Graphite TCP connections, the version of InfluxDB, or the uptime of the process.

## System Statistics
`SHOW STATS [FOR <module>]` displays statisics about subsystems within the running `influxd` process. Statistics include points received, points indexed, bytes written to disk, TCP connections handled etc. These statistics are all zero when the InfluxDB process starts. If _module_ is specified, it must be single-quoted. For example `SHOW STATS FOR 'httpd'`.

All statistics are written, by default, by each node to a "monitor" database within the InfluxDB system, allowing analysis of aggregated statistical data using the standard InfluxQL language. This allows users to track the performance of their system. Importantly, this allows cluster-level statistics to be viewed, since by querying the monitor database, statistics from all nodes may be queried. This can be a very powerful approach for troubleshooting your InfluxDB system and understanding its behaviour.

## System Diagnostics
`SHOW DIAGNOSTICS [FOR <module>]` displays various diagnostic information about the `influxd` process. This information is not stored persistently within the InfluxDB system. If _module_ is specified, it must be single-quoted. For example `SHOW STATS FOR 'build'`.

## Standard expvar support
All statistical information is available at HTTP API endpoint `/debug/vars`, in [expvar](https://golang.org/pkg/expvar/) format, allowing external systems to monitor an InfluxDB node. By default, the full path to this endpoint is `http://localhost:8086/debug/vars`.

## Configuration
The `monitor` module allows the following configuration:

 * Whether to write statistical and diagnostic information to an InfluxDB system. This is enabled by default.
 * The name of the database to where this information should be written. Defaults to `_internal`. The information is written to the default retention policy for the given database.
 * The name of the retention policy, along with full configuration control of the retention policy, if the default retention policy is not suitable.
 * The rate at which this information should be written. The default rate is once every 10 seconds.

# Design and Implementation

A new module named `monitor` supports all basic statistics and diagnostic functionality. This includes:

 * Allowing other modules to register statistics and diagnostics information, allowing it to be accessed on demand by the `monitor` module.
 * Serving the statistics and diagnostic information to the user, in response to commands such as `SHOW DIAGNOSTICS`.
 * Expose standard Go runtime information such as garbage collection statistics.
 * Make all collected expvar data via HTTP, for collection by 3rd-party tools.
 * Writing the statistical information to the "monitor" database, for query purposes.

## Registering statistics and diagnostics

To export statistical information with the `monitor` system, code simply calls `influxdb.NewStatistics()` and receives an `expvar.Map` instance in response. This object can then be used to store statistics.

For example, if you have a component called `Service`, you can statistics like so:

```
import (
     "expvar"
     "github.com/influxdb/influxdb"
)
.
.
.
.
type Service struct {
  ....some other fields....
  statMap *expvar.Map         /// Add a map of type *expvar.Map. Check GoDocs for how to use this.
}


func NewService() *Service {
  s = &NewService{}
  s.statMap = NewStatistics(key, name, tags)
}
```
When calling `NewStatistics` `key` should be unique for the Service instance (if a network service, the protocol and binding port are good to include in the key). `name` will be the name of the Measurement used to store these statistics. Finally, when these statistics are written to the `monitor` database, all points will be tagged with `tags`. A value of nil for `tags` is legal. 

To register diagnostic information, `monitor.RegisterDiagnosticsClient` is called, passing a `influxdb.monitor.DiagsClient` object to `monitor`. Implementing the `influxdb.monitor.DiagsClient` interface requires that your component have function returning diagnostic information in specific form, so that it can be displayed by the `monitor` system.

## expvar
Statistical information is gathered by each package using [expvar](https://golang.org/pkg/expvar). Each package registers a map using its package name.

Due to the nature of `expvar`, statistical information is reset to its initial state when a server is restarted.
