statsd_exporter/pkg/mapper/fsm/README.md

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# FSM Mapping
## Overview
This package implements a fast and efficient algorithm for generic glob style
string matching using a finite state machine (FSM).
### Source Hierachy
```
'-- fsm
'-- dump.go // functionality to dump the FSM to Dot file
'-- formatter.go // format glob templates using captured * groups
'-- fsm.go // manipulating and searching of FSM
'-- minmax.go // min() max() function for interger
```
## FSM Explained
Per [Wikipedia](https://en.wikipedia.org/wiki/Finite-state_machine):
> A finite-state machine (FSM) or finite-state automaton (FSA, plural: automata),
> finite automaton, or simply a state machine, is a mathematical model of
> computation. It is an abstract machine that can be in exactly one of a finite
> number of states at any given time. The FSM can change from one state to
> another in response to some external inputs; the change from one state to
> another is called a transition. An FSM is defined by a list of its states, its
> initial state, and the conditions for each transition.
In our use case, each *state* is a substring after the input StatsD metric name is splitted by `.`.
### Add state to FSM
`func (f *FSM) AddState(match string, matchMetricType string,
maxPossibleTransitions int, result interface{}) int`
At first, the FSM only contains three states, representing three possible metric types:
____ [gauge]
/
(start)---- [counter]
\
'--- [observer]
Adding a rule `client.*.request.count` with type `counter` will make the FSM to be:
____ [gauge]
/
(start)---- [counter] -- [client] -- [*] -- [request] -- [count] -- {R1}
\
'--- [observer]
`{R1}` is short for result 1, which is the match result for `client.*.request.count`.
Adding a rule `client.*.*.size` with type `counter` will make the FSM to be:
____ [gauge] __ [request] -- [count] -- {R1}
/ /
(start)---- [counter] -- [client] -- [*]
\ \__ [*] -- [size] -- {R2}
'--- [observer]
### Finding a result state in FSM
`func (f *FSM) GetMapping(statsdMetric string, statsdMetricType string)
(*mappingState, []string)`
For example, when mapping `client.aaa.request.count` with `counter` type in the
FSM, the `^1` to `^7` symbols indicate how FSM will traversal in its tree:
____ [gauge] __ [request] -- [count] -- {R1}
/ / ^5 ^6 ^7
(start)---- [counter] -- [client] -- [*]
^1 \ ^2 ^3 \__ [*] -- [size] -- {R2}
'--- [observer] ^4
To map `client.bbb.request.size`, FSM will do a backtracking:
____ [gauge] __ [request] -- [count] -- {R1}
/ / ^5 ^6
(start)---- [counter] -- [client] -- [*]
^1 \ ^2 ^3 \__ [*] -- [size] -- {R2}
'--- [observer] ^4
^7 ^8 ^9
## Debugging
To see all the states of the current FSM, use `func (f *FSM) DumpFSM(w io.Writer)`
to dump into a Dot file. The Dot file can be further renderer into image using:
```shell
$ dot -Tpng dump.dot > dump.png
```
In StatsD exporter, one could use the following:
```shell
$ statsd_exporter --statsd.mapping-config=statsd.rules --debug.dump-fsm=dump.dot
$ dot -Tpng dump.dot > dump.png
```
For example, the following rules:
```yaml
mappings:
- match: client.*.request.count
name: request_count
match_metric_type: counter
labels:
client: $1
- match: client.*.*.size
name: sizes
match_metric_type: counter
labels:
client: $1
direction: $2
```
will be rendered as:
![FSM](fsm.png)
The `dot` program is part of [Graphviz](https://www.graphviz.org/) and is
available in most of popular operating systems.