nats.docs/developing-with-nats/receiving/drain.md

# Draining Messages Before Disconnect

A feature recently added across the NATS client libraries is the ability to drain connections or subscriptions. Closing a connection, or unsubscribing from a subscription, are generally considered immediate requests. When you close or unsubscribe the library will halt messages in any pending queue or cache for subscribers. When you drain a subscription or connection, it will process any inflight and cached/pending messages before closing.

Drain provides clients that use queue subscriptions with a way to bring down applications without losing any messages. A client can bring up a new queue member, drain and shut down the old queue member, all without losing messages sent to the old client. Without drain, there is the possibility of lost messages due to delivery timing.

The libraries can provide drain on a connection or on a subscriber, or both.

For a connection the process is essentially:

1. Drain all subscriptions
2. Stop new messages from being published
3. Flush any remaining published messages
4. Close

The API for drain can generally be used instead of close:

As an example of draining a connection:

{% tabs %}
{% tab title="Go" %}
```go
wg := sync.WaitGroup{}
wg.Add(1)

errCh := make(chan error, 1)

// To simulate a timeout, you would set the DrainTimeout()
// to a value less than the time spent in the message callback,
// so say: nats.DrainTimeout(10*time.Millisecond).

nc, err := nats.Connect("demo.nats.io",
    nats.DrainTimeout(10*time.Second),
    nats.ErrorHandler(func(_ *nats.Conn, _ *nats.Subscription, err error) {
        errCh <- err
    }),
    nats.ClosedHandler(func(_ *nats.Conn) {
        wg.Done()
    }))
if err != nil {
    log.Fatal(err)
}

// Just to not collide using the demo server with other users.
subject := nats.NewInbox()

// Subscribe, but add some delay while processing.
if _, err := nc.Subscribe(subject, func(_ *nats.Msg) {
    time.Sleep(200 * time.Millisecond)
}); err != nil {
    log.Fatal(err)
}

// Publish a message
if err := nc.Publish(subject, []byte("hello")); err != nil {
    log.Fatal(err)
}

// Drain the connection, which will close it when done.
if err := nc.Drain(); err != nil {
    log.Fatal(err)
}

// Wait for the connection to be closed.
wg.Wait()

// Check if there was an error
select {
case e := <-errCh:
    log.Fatal(e)
default:
}
```
{% endtab %}

{% tab title="Java" %}
```java
Connection nc = Nats.connect("nats://demo.nats.io:4222");

// Use a latch to wait for a message to arrive
CountDownLatch latch = new CountDownLatch(1);

// Create a dispatcher and inline message handler
Dispatcher d = nc.createDispatcher((msg) -> {
    String str = new String(msg.getData(), StandardCharsets.UTF_8);
    System.out.println(str);
    latch.countDown();
});

// Subscribe
d.subscribe("updates");

// Wait for a message to come in
latch.await();

// Drain the connection, which will close it
CompletableFuture<Boolean> drained = nc.drain(Duration.ofSeconds(10));

// Wait for the drain to complete
drained.get();
```
{% endtab %}

{% tab title="JavaScript" %}
```javascript
let nc = NATS.connect({url: "nats://demo.nats.io:4222"});
let inbox = createInbox();
let counter = 0;
nc.subscribe(inbox, () => {
    counter++;
});

nc.publish(inbox);
nc.drain((err)=> {
    if(err) {
        t.log(err);
    }
    t.log('connection is closed:', nc.closed);
    t.log('processed', counter, 'messages');
    t.pass();
    // the snippet is running as a promise in a test
    // and calls resolve to pass the test
    resolve();
});
```
{% endtab %}

{% tab title="Python" %}
```python
import asyncio
from nats.aio.client import Client as NATS

async def example(loop):
    nc = NATS()

    await nc.connect("nats://127.0.0.1:4222", loop=loop)

    async def handler(msg):
        print("[Received] ", msg)
        await nc.publish(msg.reply, b'I can help')

        # Can check whether client is in draining state
        if nc.is_draining:
            print("Connection is draining")

    await nc.subscribe("help", "workers", cb=handler)
    await nc.flush()

    requests = []
    for i in range(0, 10):
        request = nc.request("help", b'help!', timeout=1)
        requests.append(request)

    # Wait for all the responses
    responses = []
    responses = await asyncio.gather(*requests)

    # Gracefully close the connection.
    await nc.drain()

    print("Received {} responses".format(len(responses)))
```
{% endtab %}

{% tab title="Ruby" %}
```ruby
NATS.start(drain_timeout: 1) do |nc|
  NATS.subscribe('foo', queue: "workers") do |msg, reply, sub|
    nc.publish(reply, "ACK:#{msg}")
  end

  NATS.subscribe('bar', queue: "workers") do |msg, reply, sub|
    nc.publish(reply, "ACK:#{msg}")
  end

  NATS.subscribe('quux', queue: "workers") do |msg, reply, sub|
    nc.publish(reply, "ACK:#{msg}")
  end

  EM.add_timer(2) do
    next if NATS.draining?

    # Drain gracefully closes the connection.
    NATS.drain do
      puts "Done draining. Connection is closed."
    end
  end
end
```
{% endtab %}

{% tab title="TypeScript" %}
```typescript
let sub = await nc.subscribe('updates', (err, msg) => {
    t.log('worker got message', msg.data);
}, {queue: "workers"});
// [end drain_sub]
nc.flush();

await nc.drain();
// client must close when the connection drain resolves
nc.close();
```
{% endtab %}

{% tab title="C" %}
```c
static void
onMsg(natsConnection *conn, natsSubscription *sub, natsMsg *msg, void *closure)
{
    printf("Received msg: %s - %.*s\n",
           natsMsg_GetSubject(msg),
           natsMsg_GetDataLength(msg),
           natsMsg_GetData(msg));

    // Add some delay while processing
    nats_Sleep(200);

    // Need to destroy the message!
    natsMsg_Destroy(msg);
}

static void
closeHandler(natsConnection *conn, void *closure)
{
    cond_variable cv = (cond_variable) closure;

    notify_cond_variable(cv);
}

(...)


natsConnection      *conn      = NULL;
natsOptions         *opts      = NULL;
natsSubscription    *sub       = NULL;
natsStatus          s          = NATS_OK;
cond_variable       cv         = new_cond_variable(); // some fictuous way to notify between threads.

s = natsOptions_Create(&opts);
if (s == NATS_OK)
    // Setup a close handler and pass a reference to our condition variable.
    s = natsOptions_SetClosedCB(opts, closeHandler, (void*) cv);
if (s == NATS_OK)
    s = natsConnection_Connect(&conn, opts);

// Subscribe
if (s == NATS_OK)
    s = natsConnection_Subscribe(&sub, conn, "foo", onMsg, NULL);

// Publish a message
if (s == NATS_OK)
    s = natsConnection_PublishString(conn, "foo", "hello");

// Drain the connection, which will close it when done.
if (s == NATS_OK)
    s = natsConnection_Drain(conn);

// Wait for the connection to be closed
if (s == NATS_OK)
    cond_variable_wait(cv);

(...)

// Destroy objects that were created
natsSubscription_Destroy(sub);
natsConnection_Destroy(conn);
natsOptions_Destroy(opts);
```
{% endtab %}
{% endtabs %}

The mechanics of drain for a subscription are simpler:

1. Unsubscribe
2. Process all cached or inflight messages
3. Clean up

The API for drain can generally be used instead of unsubscribe:

{% tabs %}
{% tab title="Go" %}
```go
    nc, err := nats.Connect("demo.nats.io")
    if err != nil {
        log.Fatal(err)
    }
    defer nc.Close()

    done := sync.WaitGroup{}
    done.Add(1)

    count := 0
    errCh := make(chan error, 1)

    msgAfterDrain := "not this one"

    // Just to not collide using the demo server with other users.
    subject := nats.NewInbox()

    // This callback will process each message slowly
    sub, err := nc.Subscribe(subject, func(m *nats.Msg) {
        if string(m.Data) == msgAfterDrain {
            errCh <- fmt.Errorf("Should not have received this message")
            return
        }
        time.Sleep(100 * time.Millisecond)
        count++
        if count == 2 {
            done.Done()
        }
    })

    // Send 2 messages
    for i := 0; i < 2; i++ {
        nc.Publish(subject, []byte("hello"))
    }

    // Call Drain on the subscription. It unsubscribes but
    // wait for all pending messages to be processed.
    if err := sub.Drain(); err != nil {
        log.Fatal(err)
    }

    // Send one more message, this message should not be received
    nc.Publish(subject, []byte(msgAfterDrain))

    // Wait for the subscription to have processed the 2 messages.
    done.Wait()

    // Now check that the 3rd message was not received
    select {
    case e := <-errCh:
        log.Fatal(e)
    case <-time.After(200 * time.Millisecond):
        // OK!
    }
```
{% endtab %}

{% tab title="Java" %}
```java
Connection nc = Nats.connect("nats://demo.nats.io:4222");

// Use a latch to wait for a message to arrive
CountDownLatch latch = new CountDownLatch(1);

// Create a dispatcher and inline message handler
Dispatcher d = nc.createDispatcher((msg) -> {
    String str = new String(msg.getData(), StandardCharsets.UTF_8);
    System.out.println(str);
    latch.countDown();
});

// Subscribe
d.subscribe("updates");

// Wait for a message to come in
latch.await();

// Messages that have arrived will be processed
CompletableFuture<Boolean> drained = d.drain(Duration.ofSeconds(10));

// Wait for the drain to complete
drained.get();

// Close the connection
nc.close();
```
{% endtab %}

{% tab title="JavaScript" %}
```javascript
let nc = NATS.connect({url: "nats://demo.nats.io:4222"});
let inbox = createInbox();
let counter = 0;
let sid = nc.subscribe(inbox, () => {
    counter++;
});

nc.publish(inbox);
nc.drainSubscription(sid, (err)=> {
    if(err) {
        t.log(err);
    }
    t.log('processed', counter, 'messages');
});
nc.flush(() => {
    nc.close();
    t.pass();
    resolve();
});
```
{% endtab %}

{% tab title="Python" %}
```python
import asyncio
from nats.aio.client import Client as NATS

async def example(loop):
    nc = NATS()

    await nc.connect("nats://127.0.0.1:4222", loop=loop)

    async def handler(msg):
        print("[Received] ", msg)
        await nc.publish(msg.reply, b'I can help')

        # Can check whether client is in draining state
        if nc.is_draining:
            print("Connection is draining")

    sid = await nc.subscribe("help", "workers", cb=handler)
    await nc.flush()

    # Gracefully unsubscribe the subscription
    await nc.drain(sid)
```
{% endtab %}

{% tab title="Ruby" %}
```ruby
# There is currently no API to drain a single subscription, the whole connection can be drained though via NATS.drain
```
{% endtab %}

{% tab title="TypeScript" %}
```typescript
let sub = await nc.subscribe('updates', (err, msg) => {
    t.log('worker got message', msg.data);
}, {queue: "workers"});
```
{% endtab %}

{% tab title="C" %}
```c
natsConnection      *conn      = NULL;
natsSubscription    *sub       = NULL;
natsStatus          s          = NATS_OK;

s = natsConnection_ConnectTo(&conn, NATS_DEFAULT_URL);

// Subscribe
if (s == NATS_OK)
    s = natsConnection_Subscribe(&sub, conn, "foo", onMsg, NULL);

// Publish 2 messages
if (s == NATS_OK)
{
    int i;
    for (i=0; (s == NATS_OK) && (i<2); i++)
    {
        s = natsConnection_PublishString(conn, "foo", "hello");
    }
}

// Call Drain on the subscription. It unsubscribes but
// wait for all pending messages to be processed.
if (s == NATS_OK)
    s = natsSubscription_Drain(sub);

(...)

// Destroy objects that were created
natsSubscription_Destroy(sub);
natsConnection_Destroy(conn);
```
{% endtab %}
{% endtabs %}

Because draining can involve messages flowing to the server, for a flush and asynchronous message processing, the timeout for drain should generally be higher than the timeout for a simple message request/reply or similar.