NATS Streaming is an extremely performant, lightweight reliable streaming platform powered by NATS.

NATS Streaming provides the following high-level feature set:

• Log based persistence
• At-Least-Once Delivery model, giving reliable message delivery
• Rate matched on a per subscription basis
• Replay/Restart
• Last Value Semantics

• Please raise questions/issues via the Issue Tracker or via the #stan-preview channel on natsio.slack.com (contact larry@apcera.com or brian@apcera.com for access)

• When an application crashes and immediately tries to reconnect, there is a known issue where the server may prevent it from doing so. This will be fixed shortly.
• Persistence is memory based at this point in development; file based persistence is underway and will be made available soon.
• Time- and sequence-based subscriptions are exact. Requesting a time or seqno before the earliest stored message for a subject will result in an error (in SubscriptionRequest.Error)

# Go client
go get github.com/nats-io/go-nats-streaming

For the Preview, stan-server is provided in binary form for Linux and Mac here

sc, _ := stan.Connect(clusterID, clientID)

// Simple Synchronous Publisher
sc.Publish("foo", []byte("Hello World")) // does not return until an ack has been received from NATS Streaming

// Simple Async Subscriber
sc.Subscribe("foo", func(m *nats.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
})

// Unsubscribe
sub.Unsubscribe()

// Close connection
sc.Close()

NATS Streaming subscriptions are similar to NATS subscriptions, but clients may start their subscription at an earlier point in the message stream, allowing them to receive messages that were published before this client registered interest. The options are described with examples below:

// Subscribe starting with most recently published value
sub, err := sc.Subscribe("foo", func(m *stan.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, StartWithLastReceived())

// Receive all stored values in order
sub, err := sc.Subscribe("foo", func(m *stan.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, DeliverAllAvailable())

// Receive messages starting at a specific sequence number
sub, err := sc.Subscribe("foo", func(m *stan.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, StartAtSequence(22))

// Subscribe starting at a specific time
var startTime time.Time
...
sub, err := sc.Subscribe("foo", func(m *stan.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, StartAtTime(startTime))

// Subscribe starting a specific amount of time in the past (e.g. 30 seconds ago)
sub, err := sc.Subscribe("foo", func(m *stan.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, StartAtTimeDelta(time.ParseDuration("30s")))

Replay of messages offers great flexibility for clients wishing to begin processing at some earlier point in the data stream. However, some clients just need to pick up where they left off from an earlier session, without having to manually track their position in the stream of messages. Durable subscriptions allow clients to assign a durable name to a subscription when it is created. Doing this causes the NATS Streaming server to track the last acknowledged message for that clientID + durable name, so that only messages since the last acknowledged message will be delivered to the client.

sc, _ := stan.Connect("test-cluster", "client-123")

// Subscribe with durable name
sc.Subscribe("foo", func(m *nats.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, DurableName("my-durable"))
...
// client receives message sequence 1-40 
...
// client disconnects for an hour
...
// client reconnects with same clientID "client-123"
sc, _ := stan.Connect("test-cluster", "client-123")

// client re-subscribes to "foo" with same durable name "my-durable"
sc.Subscribe("foo", func(m *nats.Msg) {
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, DurableName("my-durable"))
...
// client receives messages 41-current

The basic publish API (Publish(subject, payload)) is synchronous; it does not return control to the caller until the NATS Streaming server has acknowledged receipt of the message. To accomplish this, a NUID is generated for the message on creation, and the client library waits for a publish acknowledgement from the server with a matching NUID before it returns control to the caller, possibly with an error indicating that the operation was not successful due to some server problem or authorization error.

Advanced users may wish to process these publish acknowledgements manually to achieve higher publish throughput by not waiting on individual acknowledgements during the publish operation. An asynchronous publish API is provided for this purpose:

    ackHandler := func(ackedNuid string, err error) {
        if err != nil {
            log.Printf("Warning: error publishing msg id %s: %v\n, ackedNuid, err.Error())
        } else {
            log.Printf("Received ack for msg id %s\n", ackedNuid)
        }
    }
    
    // can also use PublishAsyncWithReply(subj, replysubj, payload, ah)
    nuid, err := sc.PublishAsync("foo", []byte("Hello World"), ackHandler) // returns immediately
    if err != nil {
        log.Printf("Error publishing msg %s: %v\n", nuid, err.Error())
    }

NATS Streaming offers At-Least-Once delivery semantics, meaning that once a message has been delivered to an eligible subscriber, if an acknowledgement is not received within the configured timeout interval, NATS Streaming will attempt redelivery of the message. This timeout interval is specified by the subscription option AckWait, which defaults to 30 seconds.

By default, messages are automatically acknowledged by the NATS Streaming client library after the subscriber's message handler is invoked. However, there may be cases in which the subscribing client wishes to accelerate or defer acknowledgement of the message. To do this, the client must set manual acknowledgement mode on the subscription, and invoke Ack() on the Msg. ex:

// Subscribe with manual ack mode, and set AckWait to 60 seconds
sub, err := sc.Subscribe("foo", func(m *stan.Msg) {
    m.Ack() // ack message before performing I/O intensive operation
    ...
    fmt.Printf("Received a message: %s\n", string(m.Data))
}, SetManualAckMode(), AckWait(time.ParseDuration("60s")))

A classic problem of publish-subscribe messaging is matching the rate of message producers with the rate of message consumers. Message producers can often outpace the speed of the subscribers that are consuming their messages. This mismatch is commonly called a "fast producer/slow consumer" problem, and may result in dramatic resource utilization spikes in the underlying messaging system as it tries to buffer messages until the slow consumer(s) can catch up.

NATS Streaming provides a connection option called MaxPubAcksInFlight that effectively limits the number of unacknowledged messages that a publisher may have in-flight at any given time. When this maximum is reached, further PublishAsync() calls will block until the number of unacknowledged messages falls below the specified limit. ex:

sc, _ := stan.Connect(clusterID, clientID, MaxPubAcksInFlight(25))

ah := func(nuid string, err error) {
    // process the ack
    ...
}

for i := 1; i < 1000; i++ {
    // If the server is unable to keep up with the publisher, the number of oustanding acks will eventually 
    // reach the max and this call will block
    guid, _ := sc.PublishAsync("foo", []byte("Hello World"), ah) 
}

Rate limiting may also be accomplished on the subscriber side, on a per-subscription basis, using a subscription option called MaxInFlight. This option specifies the maximum number of outstanding acknowledgements (messages that have been delivered but not acknowledged) that NATS Streaming will allow for a given subscription. When this limit is reached, NATS Streaming will suspend delivery of messages to this subscription until the number of unacknowledged messages falls below the specified limit. ex:

// Subscribe with manual ack mode and a max in-flight limit of 25
i := 0
sc.Subscribe("foo", func(m *nats.Msg) {
    fmt.Printf("Received message #%d: %s\n", ++i, string(m.Data))
    ...
    // Does not ack, or takes a very long time to ack
    ...
    // Message delivery will suspend when the number of unacknowledged messages reaches 25
}, SetManualAckMode(), MaxInFlight(25))

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Copyright (c) 2012-2016 Apcera Inc.

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