Merge pull request #19 from heetch/bootstap-documentation

Bootstrap documentation

Author: tealeg

README.md

@@ -10,3 +10,10 @@ Felice is very much a work in progress.  We expect to move forward quickly, but
 ## Why "Felice"?
 Felice Bauer was, at one time, Franz Kafka's fiance.  He wrote her many messages, which she faithfuly kept, and later published.
 
+## Where should I start?
+If you wish to send messages via Kafka, you should start by reading
+the documentation for the `producer` package.  If you wish to consume
+messages from Kafka, you should start by reading the documentation for
+the `consumer` package.  The `message` package contains the `Message` type that is
+shared by both `consumer` and `producer` code.
+

consumer/consumer.go

@@ -22,7 +22,10 @@ type clusterConsumer interface {
 	Close() error
 }
 
-// Consumer is a Kafka consumer.
+// Consumer is the structure used to consume messages from Kafka.
+// Having constructed a Conusmer you should use its Handle method to
+// register per-topic handlers and, finally, call it's Serve method to
+// begin consuming messages.
 type Consumer struct {
 	RetryInterval time.Duration
 	Metrics       MetricsReporter
@@ -81,7 +84,12 @@ func newClusterConfig(clientID string) *cluster.Config {
 	return c
 }
 
-// Serve runs the consumer and listens for new messages on the given topics.
+// Serve runs the consumer and listens for new messages on the given
+// topics.  You must provide it with unique clientID and the address
+// of one or more Kafka brokers.  Serve will block until it is
+// instructed to stop, which you can achieve by calling Consumer.Stop.
+// When Serve terminates it will return an Error or nil to indicate
+// that it excited without error.
 func (c *Consumer) Serve(clientID string, addrs ...string) error {
 	c.setup()
 

consumer/consumer_internal_test.go

@@ -344,7 +344,7 @@ type testHandler struct {
 
 // HandleMessage will keep a count of how many times it is called and,
 // if a testCase is set on the testHandler, it will run it with the
-// message that HandleMessage recieved, allowing us to make assertions
+// message that HandleMessage received, allowing us to make assertions
 // about the nature of that message.
 func (h *testHandler) HandleMessage(m *message.Message) error {
 	h.CallCount++

consumer/doc.go

@@ -0,0 +1,55 @@
+// Consumer is Felice's primary entrance point for receiving messages
+// from a Kafka cluster.
+//
+// There is no special construction function for the Consumer
+// structure as all of its public members are optional, and we shall
+// discuss them below.  Thus you construct a Consumer by the normal Go
+// means:
+//
+//    c := felice.Consumer{}
+//
+// Once you've constructed a consumer you must add message handlers to
+// it.  This is done by calling the Consumer.Handle method.  Each time
+// you call Handle you'll pass a topic name and a type that implements
+// the handler.Handler interface.  Their can only ever be one handler
+// associated with a topic so, if you call Handle multiple times with
+// the same topic, they will update the handler registered for the
+// topic, and only the final one will count.  A typical call to Handle
+// looks like this:
+//
+//    c.Handle("testmsg", handler.HandlerFunc(func(m *message.Message) error {
+//        // Do something of your choice here!
+//        return nil // .. or return an actual error.
+//    }))
+//
+// Once you've registered all your handlers you may call
+// Consumer.Serve. Serve requires a client ID and a slice of strings,
+// each of which is the address of a Kafka broker to attempt to
+// communicate with. Serve will start a go routine for each partition
+// to consume messages and pass them to their per-topic
+// handlers. Serve itself will block until Consumer.Stop is called.
+// When Serve terminates it will return an error, which will be nil
+// under normal circumstances.
+// 
+// Note that any calls to Consumer.Handle after
+// Consumer.Serve has been called will have no effect.
+//
+// Tweaking the consumer
+// --------------------------
+// The first public member is the RetryInterval, a time.Duration that
+// controls how long the Felice consumer will wait before trying to
+// consume a message from Kafka that failed the first time around.
+// The default value if 1 second.
+//
+// The second public member is Metrics.  Metrics stores a type that
+// implements the felice.MetricsReporter interface.  If you provide an
+// implementation, then it's Report function will be called every time
+// a message is successfully handled.  The Report function will
+// receive a copy of the message.Message that was handled, along with
+// a map[string]string containing metrics about the handling of the
+// message.  Currently we pass the following metrics: "attempts" (the
+// number of attempts it took before the message was handled);
+// "msgOffset" (the marked offset for the message on the topic); and
+// "remainingOffset" (the difference between the high water mark and
+// the current offset).
+package consumer

consumer/handler/collection.go

@@ -45,8 +45,8 @@ func (h *Collection) Topics() []string {
 }
 
 // Set associates the given Handler to the given Topic within the
-// collection.  If a Handler was already associated with the Topic,
-// then that association will be lost and replaced by the new one.  It
+// collection. If a Handler was already associated with the Topic,
+// then that association will be lost and replaced by the new one. It
 // is safe to use Set from concurrent code.
 func (h *Collection) Set(topic string, handler Handler) {
 	h.Lock()

consumer/handler/doc.go

@@ -0,0 +1,56 @@
+// The handler package defines the mechanisms by which a received
+// message can be handled, and how handlers themselves can be managed.
+// The most obvious way to use the handler package is via the consumer
+// package, but this isn't an absolute requirement. We do however
+// require that the messages handled are felice's message.Message
+// type.
+//
+// The Handler interface defines the signature for all felice
+// Handlers. There are two common ways to comply with this interface.
+// The first is simply to create a type with the HandleMessage
+// function:
+//
+//    type MyFooHandler struct { }
+//
+//    func (mfh MyFooHandler) HandleMessage(msg *message.Message) error {
+//        fmt.Printf("%+v", *msg)
+//    }
+//
+// This approach has the advantage of not actually requiring you to
+// import the handler package when defining handlers for use with the
+// consumer.Consumer.
+//
+// The second approach is to cast a function to the HandlerFunc type
+// defined in this package:
+//
+//    h := handler.HandlerFunc(func(msg *message.Message) error {
+//        fmt.Printf("%+v", *msg)
+//    })
+//
+// Handlers can also be managed in Collections. The Collection struct
+// will allow exactly one handler to be associated with each topic,
+// and will, on demand return a list of all topics for which a handler
+// has been registered. For example:
+//
+//    col := handler.Collection{}
+//
+//    col.Set("my-topic", handler.HandlerFunc(func(msg *message.Message) error) {
+//        fmt.Printf("Got message on my-topic: %+v", *msg)
+//        return nil
+//    })
+//
+//    col.Set("your-topic", handler.HandlerFunc(func(msg *message.Message) error) {
+//        fmt.Printf("Got message on your-topic: %+v", msg)
+//        return nil
+//    })
+//
+//    yourHandler, ok := col.Get("your-topic")
+//    if !ok {
+//        fmt.Println("Couldn't find a handler for your-topic")
+//    }
+//
+//    for _, t := range col.Topics {
+//        fmt.Printf("We have a handler for: %s", t)
+//    }
+//
+package handler

consumer/handler/func.go

@@ -4,11 +4,13 @@ import (
 	"github.com/heetch/felice/message"
 )
 
-// A HandlerFunc is a function type that mimics the interface of the
-// HandleMessage function in the Handler interface.  If you cast a
-// function to this type, it will comply with the Handler interface.
+// A HandlerFunc is a function that supports the Handler interface.
+// Calls to Handler.HandleMessage made against any function cast to
+// this type will result in the function itself being called.
 type HandlerFunc func(*message.Message) error
 
+// HandleMessage implements the Handler interface by calling the
+// HandlerFunc to which it is associated.
 func (h HandlerFunc) HandleMessage(msg *message.Message) error {
 	return h(msg)
 }

consumer/handler/interface.go

@@ -2,7 +2,22 @@ package handler
 
 import "github.com/heetch/felice/message"
 
-// A Handler handles a Message.
+// Handler is the interface for handling consumed messages. You can
+// either add a compliant HandleMessage function to some type by hand,
+// or make use of the HandlerFunc. Note that felice Handlers receive
+// felice Message types.
 type Handler interface {
+	// HandleMessage functions will receive a *message.Message
+	// type, and may do with it what they wish.  However, some
+	// caution should be used when returning an error from a
+	// HandleMessage function.  Errors are considered an
+	// indication that the message has not been handled.  After
+	// waiting a respectable amount of time, the HandleMessage
+	// function will be called again with the same message.  This
+	// will continue until on of the following conditions is met:
+	//
+	// - 1. HandleMessage returns nil, instead of an error.
+	// - 2. A system administrator intervenes.
+	// - 3. The Earth is consumed by the sun.
 	HandleMessage(*message.Message) error
 }

message/doc.go

@@ -0,0 +1,27 @@
+// The message package contains the Message type. When using
+// the felice Producer, Message will be the type you send.
+// When using the felice Consumer, you will register handlers that
+// receive the Message type.
+//
+// You can create a new Message by calling New:
+//
+//    msg := New("my-topic", "simple string message")
+//
+// The value passed as the 2nd argument can be any Go type that can be
+// marshalled with encoding/json.  Two default headers are added to
+// all Messages:
+//
+// - Message-Id : a universally unique ID for the message
+// - Produced-At : the current time in the UTC timezone.
+//
+// New can also be passed zero, one or many additional Options. An
+// Option is a function that receives a pointer to the Message and can
+// modify it directly prior to it being returned by New. Two
+// predefined Options exists in felice: Header and Key.
+//
+// For example, if you want to create a Message with a custom header
+// field you could request it as follows:
+//
+//    msg := New("my-topic", "cold potatoes ain't hot!", Header("subject", "potatoes"))
+//
+package message

message/message.go

@@ -9,17 +9,23 @@ import (
 	uuid "github.com/satori/go.uuid"
 )
 
-// Message contains informations about the message to be sent to Kafka.
+// Message represents a message to be sent via Kafka, or received from
+// it. When using Felice's Consumer, any Handlers that you register
+// will receive Messages as they're arguments. When using the Felice
+// Producer, you will be sending Messages. When making a Message to
+// be sent it is essential that you use the New function to do so.
 type Message struct {
-	// Kafka topic.
+	// The Kafka topic this Message applies to.
 	Topic string
 
 	// If specified, messages with the same key will be sent to the same Kafka partition.
 	Key string
 
-	// Body of the Kafka message.
+	// Body of the Kafka message. For now this will always be a
+	// JSON marshaled form of whatever was passed to New.
 	Body []byte
 
+	// The time at which this Message was produced.
 	ProducedAt time.Time
 
 	// Partition where this publication was stored.
@@ -31,11 +37,20 @@ type Message struct {
 	// Headers of the message.
 	Headers map[string]string
 
-	// Unique id of the message.
+	// Unique ID of the message.
 	ID string
 }
 
-// New creates a new configured message.
+// New creates a new Message, correctly configured for use with the
+// felice Producer. You should not attempt to create Message types by
+// hand (unless you really know what you're doing! - I'm just some
+// documentation, not the police).
+//
+// Two headers will be added to all Messages:
+//
+// - Message-Id : a universally unique ID for the message
+// - Produced-At : the current time in the UTC timezone.
+//
 func New(topic string, value interface{}, opts ...Option) (*Message, error) {
 	if topic == "" {
 		return nil, fmt.Errorf("messages require a non-empty topic")