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.
# Go client
go get github.com/nats-io/go-nats-streaming
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
sub, _ := sc.Subscribe("foo", func(m *stan.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))
}, stan.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))
}, stan.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))
}, stan.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))
}, stan.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))
}, stan.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 *stan.Msg) {
fmt.Printf("Received a message: %s\n", string(m.Data))
}, stan.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 *stan.Msg) {
fmt.Printf("Received a message: %s\n", string(m.Data))
}, stan.DurableName("my-durable"))
...
// client receives messages 41-current
All subscriptions with the same queue name (regardless of the connection they originate from) will form a queue group. Each message will be delivered to only one subscriber per queue group, using queuing semantics. You can have as many queue groups as you wish.
Normal subscribers will continue to work as expected.
A queue group is automatically created when the first queue subscriber is created. If the group already exists, the member is added to the group.
sc, _ := stan.Connect("test-cluster", "clientid")
// Create a queue subscriber on "foo" for group "bar"
qsub1, _ := sc.QueueSubscribe("foo", "bar", qcb)
// Add a second member
qsub2, _ := sc.QueueSubscribe("foo", "bar", qcb)
// Notice that you can have a regular subscriber on that subject
sub, _ := sc.Subscribe("foo", cb)
// A message on "foo" will be received by sub and qsub1 or qsub2.
Note that once a queue group is formed, a member's start position is ignored when added to the group. It will start receive messages from the last position in the group.
Suppose the channel foo
exists and there are 500
messages stored, the group
bar
is already created, there are two members and the last
message sequence sent is 100
. A new member is added. Note its start position:
sc.QueueSubscribe("foo", "bar", qcb, stan.StartAtSequence(200))
This will not produce an error, but the start position will be ignored. Assuming
this member would be the one receiving the next message, it would receive message
sequence 101
.
There are two ways of leaving the group: closing the subscriber's connection or
calling Unsubscribe
:
// Have qsub leave the queue group
qsub.Unsubscribe()
If the leaving member had un-acknowledged messages, those messages are reassigned to the remaining members.
There is no special API for that. Once all members have left (either calling Unsubscribe
,
or their connections are closed), the group is removed from the server.
The next call to QueueSubscribe
with the same group name will create a brand new group,
that is, the start position will take effect and delivery will start from there.
As described above, for non durable queue subscribers, when the last member leaves the group, that group is removed. A durable queue group allows you to have all members leave but still maintain state. When a member re-joins, it starts at the last position in that group.
A durable queue group is created in a similar manner as that of a standard queue group,
except the DurableName
option must be used to specify durability.
sc.QueueSubscribe("foo", "bar", qcb, stan.DurableName("dur"))
A group called dur:bar
(the concatenation of durable name and group name) is created in
the server. This means two things:
- The character
:
is not allowed for a queue subscriber's durable name. - Durable and non-durable queue groups with the same name can coexist.
// Non durable queue subscriber on group "bar"
qsub, _ := sc.QueueSubscribe("foo", "bar", qcb)
// Durable queue subscriber on group "bar"
durQsub, _ := sc.QueueSubscribe("foo", "bar", qcb, stan.DurableName("mydurablegroup"))
// The same message produced on "foo" would be received by both queue subscribers.
The rules for non-durable queue subscribers apply to durable subscribers.
As for non-durable queue subscribers, if a member's connection is closed, or if
Unsubscribe
its called, the member leaves the group. Any unacknowledged message
is transferred to remaining members. See Closing the Group for important difference
with non-durable queue subscribers.
The last member calling Unsubscribe
will close (that is destroy) the
group. So if you want to maintain durability of the group, you should not be
calling Unsubscribe
.
So unlike for non-durable queue subscribers, it is possible to maintain a queue group with no member in the server. When a new member re-joins the durable queue group, it will resume from where the group left of, actually first receiving all unacknowledged messages that may have been left when the last member previously left.
NATS Streaming subscriptions do not support wildcards.
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
aw, _ := time.ParseDuration("60s")
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))
}, stan.SetManualAckMode(), stan.AckWait(aw))
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 outstanding 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
sc.Subscribe("foo", func(m *stan.Msg) {
fmt.Printf("Received message #: %s\n", 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
}, stan.SetManualAckMode(), stan.MaxInflight(25))
Unless otherwise noted, the NATS source files are distributed under the Apache Version 2.0 license found in the LICENSE file.