add plugin documentation

docs/INPUTS.md

@@ -39,6 +39,7 @@ import (
 
 type Simple struct {
     Ok  bool            `toml:"ok"`
+    Log telegraf.Logger `toml:"-"`
 }
 
 func (s *Simple) Description() string {
@@ -52,6 +53,7 @@ func (s *Simple) SampleConfig() string {
 `
 }
 
+// Init is for setup, and validating config.
 func (s *Simple) Init() error {
 	return nil
 }

docs/OUTPUTS.md

@@ -31,6 +31,7 @@ import (
 
 type Simple struct {
     Ok  bool            `toml:"ok"`
+    Log telegraf.Logger `toml:"-"`
 }
 
 func (s *Simple) Description() string {
@@ -43,20 +44,25 @@ func (s *Simple) SampleConfig() string {
 `
 }
 
+// Init is for setup, and validating config.
 func (s *Simple) Init() error {
 	return nil
 }
 
 func (s *Simple) Connect() error {
-    // Make a connection to the URL here
+    // Make any connection required here
     return nil
 }
 
 func (s *Simple) Close() error {
-    // Close connection to the URL here
+    // Close any connections here.
+    // Write will not be called once Close is called, so there is no need to synchronize.
     return nil
 }
 
+// Write should write immediately to the output, and not buffer writes
+// (Telegraf manages the buffer for you). Returning an error will fail this
+// batch of writes and the entire batch will be retried automatically.
 func (s *Simple) Write(metrics []telegraf.Metric) error {
     for _, metric := range metrics {
         // write `metric` to the output sink here

docs/PROCESSORS.md

@@ -33,6 +33,7 @@ import (
 )
 
 type Printer struct {
+	Log telegraf.Logger `toml:"-"`
 }
 
 var sampleConfig = `
@@ -46,6 +47,7 @@ func (p *Printer) Description() string {
 	return "Print all metrics that pass through this filter."
 }
 
+// Init is for setup, and validating config.
 func (p *Printer) Init() error {
 	return nil
 }
@@ -97,6 +99,7 @@ import (
 )
 
 type Printer struct {
+	Log telegraf.Logger `toml:"-"`
 }
 
 var sampleConfig = `
@@ -110,13 +113,27 @@ func (p *Printer) Description() string {
 	return "Print all metrics that pass through this filter."
 }
 
+// Init is for setup, and validating config.
 func (p *Printer) Init() error {
 	return nil
 }
 
+// Start is called once when the plugin starts; it is only called once per
+// plugin instance, and never in parallel.
+// Start should return once it is ready to receive metrics.
+// The passed in accumulator is the same as the one passed to Add(), so you
+// can choose to save it in the plugin, or use the one received from Add().
 func (p *Printer) Start(acc telegraf.Accumulator) error {
 }
 
+// Add is called for each metric to be processed. The Add() function does not
+// need to wait for the metric to be processed before returning, and it may
+// be acceptable to let background goroutine(s) handle the processing if you
+// have slow processing you need to do in parallel.
+// Keep in mind Add() should not spawn unbounded goroutines, so you may need
+// to use a semaphore or pool of workers (eg: reverse_dns plugin does this).
+// Metrics you don't want to pass downstream should have metric.Drop() called,
+// rather than simply omitting the acc.AddMetric() call
 func (p *Printer) Add(metric telegraf.Metric, acc telegraf.Accumulator) error {
 	// print!
 	fmt.Println(metric.String())
@@ -127,6 +144,12 @@ func (p *Printer) Add(metric telegraf.Metric, acc telegraf.Accumulator) error {
 	return nil
 }
 
+// Stop gives you an opportunity to gracefully shut down the processor.
+// Once Stop() is called, Add() will not be called any more. If you are using
+// goroutines, you should wait for any in-progress metrics to be processed
+// before returning from Stop().
+// When stop returns, you should no longer be writing metrics to the
+// accumulator.
 func (p *Printer) Stop() error {
 }
 

input.go

@@ -4,7 +4,7 @@ type Input interface {
 	PluginDescriber
 
 	// Gather takes in an accumulator and adds the metrics that the Input
-	// gathers. This is called every "interval"
+	// gathers. This is called every agent.interval
 	Gather(Accumulator) error
 }
 
@@ -15,6 +15,9 @@ type ServiceInput interface {
 	// Stop returns.
 	Start(Accumulator) error
 
-	// Stop stops the services and closes any necessary channels and connections
+	// Stop stops the services and closes any necessary channels and connections.
+	// Metrics should not be written out to the accumulator once stop returns, so
+	// Stop() should stop reading and wait for any in-flight metrics to write out
+	// to the accumulator before returning.
 	Stop()
 }

output.go

@@ -3,9 +3,12 @@ package telegraf
 type Output interface {
 	PluginDescriber
 
-	// Connect to the Output
+	// Connect to the Output; connect is only called once when the plugin starts
 	Connect() error
-	// Close any connections to the Output
+	// Close any connections to the Output. Close is called once when the output
+	// is shutting down. Close will not be called until all writes have finished,
+	// and Write() will not be called once Close() has been, so locking is not
+	// necessary.
 	Close() error
 	// Write takes in group of points to be written to the Output
 	Write(metrics []Metric) error

plugin.go

@@ -10,7 +10,9 @@ type Initializer interface {
 }
 
 // PluginDescriber contains the functions all plugins must implement to describe
-// themselves to Telegraf
+// themselves to Telegraf. Note that all plugins may define a logger that is
+// not part of the interface, but will receive an injected logger if it's set.
+// eg: Log telegraf.Logger `toml:"-"`
 type PluginDescriber interface {
 	// SampleConfig returns the default configuration of the Processor
 	SampleConfig() string

plugins/inputs/http_response/README.md

@@ -7,9 +7,7 @@ This input plugin checks HTTP/HTTPS connections.
 ```toml
 # HTTP/HTTPS request given an address a method and a timeout
 [[inputs.http_response]]
-  ## Deprecated in 1.12, use 'urls'
-  ## Server address (default http://localhost)
-  # address = "http://localhost"
+  ## address is Deprecated in 1.12, use 'urls'
 
   ## List of urls to query.
   # urls = ["http://localhost"]

processor.go

@@ -14,17 +14,27 @@ type Processor interface {
 type StreamingProcessor interface {
 	PluginDescriber
 
-	// Start is the initializer for the processor
-	// Start is only called once per plugin instance, and never in parallel.
-	// Start should exit immediately after setup
+	// Start is called once when the plugin starts; it is only called once per
+	// plugin instance, and never in parallel.
+	// Start should return once it is ready to receive metrics.
+	// The passed in accumulator is the same as the one passed to Add(), so you
+	// can choose to save it in the plugin, or use the one received from Add().
 	Start(acc Accumulator) error
 
-	// Add is called for each metric to be processed.
+	// Add is called for each metric to be processed. The Add() function does not
+	// need to wait for the metric to be processed before returning, and it may
+	// be acceptable to let background goroutine(s) handle the processing if you
+	// have slow processing you need to do in parallel.
+	// Keep in mind Add() should not spawn unbounded goroutines, so you may need
+	// to use a semaphore or pool of workers (eg: reverse_dns plugin does this)
+	// Metrics you don't want to pass downstream should have metric.Drop() called,
+	// rather than simply omitting the acc.AddMetric() call
 	Add(metric Metric, acc Accumulator) error
 
-	// Stop gives you a callback to free resources.
-	// by the time Stop is called, the input stream will have already been closed
-	// and Add will not be called anymore.
+	// Stop gives you an opportunity to gracefully shut down the processor.
+	// Once Stop() is called, Add() will not be called any more. If you are using
+	// goroutines, you should wait for any in-progress metrics to be processed
+	// before returning from Stop().
 	// When stop returns, you should no longer be writing metrics to the
 	// accumulator.
 	Stop() error