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The NATS Command Line Interface

A command line utility to interact with and manage NATS.

This utility replaces various past tools that were named in the form nats-sub and nats-pub, adds several new capabilities and support full JetStream management.

Features

Installation

Releases are published to GitHub where zip, rpm and debs for various operating systems can be found.

Installation via go install

The nats cli can be installed directly via go install. To install the latest version:

go install github.com/nats-io/natscli/nats@latest

To install a specific release:

go install github.com/nats-io/natscli/nats@v0.0.33

macOS installation via Homebrew

For macOS brew can be used to install the latest version:

brew tap nats-io/nats-tools
brew install nats-io/nats-tools/nats

Arch Linux installation via yay

For Arch users there is an AUR package that you can install with:

yay natscli

Installation from the shell

The following script will install the latest version of the nats cli on Linux and macOS:

curl -sf https://binaries.nats.dev/nats-io/natscli/nats@latest | sh

Nightly docker images

Nightly builds are included in the synadia/nats-server:nightly Docker images.

Configuration Contexts

The nats CLI supports multiple named configurations, for the rest of the document we'll interact via demo.nats.io. To enable this we'll create a demo configuration and set it as default.

First we add a configuration to capture the default localhost configuration.

nats context add localhost --description "Localhost"

Output

NATS Configuration Context "localhost"

  Description: Localhost
  Server URLs: nats://127.0.0.1:4222

Next we add a context for demo.nats.io:4222 and we select it as default.

nats context add nats --server demo.nats.io:4222 --description "NATS Demo" --select

Output

NATS Configuration Context "nats"

  Description: NATS Demo
  Server URLs: demo.nats.io:4222

These are the contexts, the * indicates the default

nats context ls

Output

Known contexts:

   localhost           Localhost
   nats*               NATS Demo

The context is selected as default, use nats context --help to see how to add, remove and edit contexts.

To switch to another context we can use:

nats ctx select localhost

To switch context back to previous one, we can use context previous subcommand:

nats ctx -- -

Configuration file

nats-cli stores contextes in ~/.config/nats/context. Those contextes are stored as JSON documents. You can find the description and expected value for this configuration file by running nats --help and look for the global flags.

JetStream management

For full information on managing JetStream please refer to the JetStream Documentation

As of nats-server v2.2.0 JetStream is GA.

Publish and Subscribe

The nats CLI can publish messages and subscribe to subjects.

Basic Behaviours

We will subscribe to the cli.demo subject:

nats sub cli.demo

Output

12:30:25 Subscribing on cli.demo

We can now publish messages to the cli.demo subject.

First we publish a single message:

nats pub cli.demo "hello world"

Output

12:31:20 Published 11 bytes to "cli.demo"

Next we publish 5 messages with a counter and timestamp in the format message 5 @ 2020-12-03T12:33:18+01:00:

nats pub cli.demo "message {{.Count}} @ {{.TimeStamp}}" --count=5

Output

12:33:17 Published 33 bytes to "cli.demo"
12:33:17 Published 33 bytes to "cli.demo"
12:33:17 Published 33 bytes to "cli.demo"
12:33:18 Published 33 bytes to "cli.demo"
12:33:18 Published 33 bytes to "cli.demo"

We can also publish messages read from STDIN:

echo hello|nats pub cli.demo

Output

12:34:15 Reading payload from STDIN
12:34:15 Published 6 bytes to "cli.demo"

Finally, NATS supports HTTP style headers and the CLI behaves like curl:

nats pub cli.demo 'hello headers' -H Header1:One -H Header2:Two

Output

12:38:44 Published 13 bytes to "cli.demo"

The receiver will show:

nats sub cli.demo

Output

[#47] Received on "cli.demo"
Header1: One
Header2: Two

hello headers

match requests and replies

We can print matching replay-requests together

nats sub --match-replies cli.demo

Output

[#48] Received on "cli.demo" with reply "_INBOX.12345"

[#48] Matched reply on "_INBOX.12345"

sub --match-replies --dump subject.name

Output
X.json
X_reply.json

JetStream

When receiving messages from a JetStream Push Consumer messages can be acknowledged when received by passing --ack, the message metadata is also produced:

nats sub js.out.testing --ack

Output

12:55:23 Subscribing on js.out.testing with acknowledgement of JetStream messages
[#1] Received JetStream message: consumer: TESTING > TAIL / subject: js.in.testing / delivered: 1 / consumer seq: 568 / stream seq: 2638 / ack: true
test JS message

Queue Groups

When subscribers join a Queue Group the messages are randomly load shared within the group. Perform the following subscribe in 2 or more shells and then publish messages using some of the methods shown above, these messages will only be received by one of the subscribers at a time.

nats sub cli.demo --queue=Q1

Service Requests and Creation

NATS supports a RPC mechanism where a service received Requests and replies with data in response.

nats reply 'cli.weather.>' "Weather Service"

Output

12:43:28 Listening on "cli.weather.>" in group "NATS-RPLY-22"

In another shell we can send a request to this service:

nats request "cli.weather.london" ''

Output

12:46:34 Sending request on "cli.weather.london"
12:46:35 Received on "_INBOX.BJoZpwsshQM5cKUj8KAkT6.HF9jslpP" rtt 404.76854ms
Weather Service

This shows that the service round trip was 404ms, and we can see the response Weather Service.

To make this a bit more interesting we can interact with the wttr.in web service:

nats reply 'cli.weather.>' --command "curl -s wttr.in/{{2}}?format=3"

Output

12:47:03 Listening on "cli.weather.>" in group "NATS-RPLY-22"

We can perform the same request again:

nats request "cli.weather.{london,newyork}" '' --raw

Output

london: 🌦 +7°C
newyork: ☀️ +2°C

Now the nats CLI parses the subject, extracts the {london,newyork} from the subjects and calls curl, replacing {{2}} with the body of the 2nd subject token - {london,newyork}.

Translating message data using a converter command

Additional to the raw output of messages using nats sub and nats stream view you can also translate the message data by running it through a command.

The command receives the message data as raw bytes through stdin and the output of the command will be the shown output for the message. There is the additional possibility to add the filter subject by using {{Subject}} as part of the arguments for the tranlation command.

Examples for using the translation feature:

Here we use the jq tool to format our json message payload into a more readable format:

We subscribe to a subject that will receive json data.

nats sub --translate 'jq .' cli.json

Now we publish some example data.

nats pub cli.json '{"task":"demo","duration":60}'

The Output will show the message formatted.

23:54:35 Subscribing on cli.json
[#1] Received on "cli.json"
{
  "task": "demo",
  "duration": 60
}

Another example is creating hex dumps from any message to avoid terminal corruption.

By changing the subscription into:

nats sub --translate 'xxd' cli.json

We will get the following output for the same published msg:

00:02:56 Subscribing on cli.json
[#1] Received on "cli.json"
00000000: 7b22 7461 736b 223a 2264 656d 6f22 2c22  {"task":"demo","
00000010: 6475 7261 7469 6f6e 223a 3630 7d         duration":60}

Examples for using the translation feature with template:

A somewhat artificial example using the subject as argument would be:

nats sub --translate "sed 's/\(.*\)/{{Subject}}: \1/'" cli.json

Output

00:22:19 Subscribing on cli.json
[#1] Received on "cli.json"
cli.json: {"task":"demo","duration":60}

The translation feature makes it possible to write specialized or universal translators to aid in debugging messages in streams or core nats.

Benchmarking and Latency Testing

Benchmarking and latency testing is key requirement for evaluating the production preparedness of your NATS network.

Benchmarking

Here we'll run these benchmarks against a local server instead of demo.nats.io.

nats context select localhost

Output

NATS Configuration Context "localhost"

  Description: Localhost
  Server URLs: nats://127.0.0.1:4222

We can benchmark core NATS publishing performance, here we publish 10 million messages from 5 concurrent publishers. By default messages are published as quick as possible without any acknowledgement or confirmations:

nats bench test --msgs=10000000 --pub 5

Output

01:30:14 Starting benchmark [msgs=10,000,000, msgsize=128 B, pubs=5, subs=0, js=false, stream=benchstream  storage=memory, syncpub=false, pubbatch=100, jstimeout=30s, pull=false, pullbatch=100, request=false, reply=false, noqueue=false, maxackpending=-1, replicas=1, purge=false]
Finished      0s [================================================] 100%
Finished      0s [================================================] 100%
Finished      0s [================================================] 100%
Finished      0s [================================================] 100%
Finished      0s [================================================] 100%

Pub stats: 14,047,987 msgs/sec ~ 1.67 GB/sec
 [1] 3,300,540 msgs/sec ~ 402.90 MB/sec (2000000 msgs)
 [2] 3,306,601 msgs/sec ~ 403.64 MB/sec (2000000 msgs)
 [3] 3,296,538 msgs/sec ~ 402.41 MB/sec (2000000 msgs)
 [4] 2,813,752 msgs/sec ~ 343.48 MB/sec (2000000 msgs)
 [5] 2,811,227 msgs/sec ~ 343.17 MB/sec (2000000 msgs)
 min 2,811,227 | avg 3,105,731 | max 3,306,601 | stddev 239,453 msgs

Adding --sub 2 will start two subscribers on the same subject and measure the rate of messages:

nats bench test --msgs=10000000 --pub 5 --sub 2

Output

...
01:30:52 Starting benchmark [msgs=10,000,000, msgsize=128 B, pubs=5, subs=2, js=false, stream=benchstream  storage=memory, syncpub=false, pubbatch=100, jstimeout=30s, pull=false, pullbatch=100, request=false, reply=false, noqueue=false, maxackpending=-1, replicas=1, purge=false]
01:30:52 Starting subscriber, expecting 10,000,000 messages
01:30:52 Starting subscriber, expecting 10,000,000 messages
Finished      6s [================================================] 100%
Finished      6s [================================================] 100%
Finished      6s [================================================] 100%
Finished      6s [================================================] 100%
Finished      6s [================================================] 100%
Finished      6s [================================================] 100%
Finished      6s [================================================] 100%

NATS Pub/Sub stats: 4,906,104 msgs/sec ~ 598.89 MB/sec
 Pub stats: 1,635,428 msgs/sec ~ 199.64 MB/sec
  [1] 328,573 msgs/sec ~ 40.11 MB/sec (2000000 msgs)
  [2] 328,147 msgs/sec ~ 40.06 MB/sec (2000000 msgs)
  [3] 327,411 msgs/sec ~ 39.97 MB/sec (2000000 msgs)
  [4] 327,318 msgs/sec ~ 39.96 MB/sec (2000000 msgs)
  [5] 327,283 msgs/sec ~ 39.95 MB/sec (2000000 msgs)
  min 327,283 | avg 327,746 | max 328,573 | stddev 520 msgs
 Sub stats: 3,271,233 msgs/sec ~ 399.32 MB/sec
  [1] 1,635,682 msgs/sec ~ 199.67 MB/sec (10000000 msgs)
  [2] 1,635,616 msgs/sec ~ 199.66 MB/sec (10000000 msgs)
  min 1,635,616 | avg 1,635,649 | max 1,635,682 | stddev 33 msgs

JetStream testing can be done by adding the --js flag. You can for example measure first the speed of publishing into a stream

nats bench js.bench --js --pub 2 --msgs 1000000 --purge

Output

01:37:36 Starting benchmark [msgs=1,000,000, msgsize=128 B, pubs=2, subs=0, js=true, stream=benchstream  storage=memory, syncpub=false, pubbatch=100, jstimeout=30s, pull=false, pullbatch=100, request=false, reply=false, noqueue=false, maxackpending=-1, replicas=1, purge=true]
01:37:36 Purging the stream
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%

Pub stats: 415,097 msgs/sec ~ 50.67 MB/sec
 [1] 207,907 msgs/sec ~ 25.38 MB/sec (500000 msgs)
 [2] 207,572 msgs/sec ~ 25.34 MB/sec (500000 msgs)
 min 207,572 | avg 207,739 | max 207,907 | stddev 167 msgs

And then you can for example measure the speed of receiving (i.e. replay) the messages from the stream using ordered push consumers

nats bench js.bench --js --sub 4 --msgs 1000000

Output

01:40:05 Starting benchmark [msgs=1,000,000, msgsize=128 B, pubs=0, subs=4, js=true, stream=benchstream  storage=memory, syncpub=false, pubbatch=100, jstimeout=30s, pull=false, pullbatch=100, request=false, reply=false, noqueue=false, maxackpending=-1, replicas=1, purge=false]
01:40:05 Starting subscriber, expecting 1,000,000 messages
01:40:05 Starting subscriber, expecting 1,000,000 messages
01:40:05 Starting subscriber, expecting 1,000,000 messages
01:40:05 Starting subscriber, expecting 1,000,000 messages
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%

Sub stats: 1,522,920 msgs/sec ~ 185.90 MB/sec
 [1] 382,739 msgs/sec ~ 46.72 MB/sec (1000000 msgs)
 [2] 382,772 msgs/sec ~ 46.73 MB/sec (1000000 msgs)
 [3] 382,407 msgs/sec ~ 46.68 MB/sec (1000000 msgs)
 [4] 381,060 msgs/sec ~ 46.52 MB/sec (1000000 msgs)
 min 381,060 | avg 382,244 | max 382,772 | stddev 698 msgs

Similarily you can benchmark synchronous request-reply type of interactions using the --request and --reply flags. For example you can first start one (or more) replier(s)

nats bench test --sub 2 --reply

And then run a benchmark with one (or more) synchronous requester(s)

nats bench test --pub 10 --request

Output

03:04:56 Starting benchmark [msgs=100,000, msgsize=128 B, pubs=10, subs=0, js=false, stream=benchstream  storage=memory, syncpub=false, pubbatch=100, jstimeout=30s, pull=false, pullbatch=100, request=true, reply=false, noqueue=false, maxackpending=-1, replicas=1, purge=false]
03:04:56 Benchmark in request-reply mode
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%
Finished      2s [================================================] 100%

Pub stats: 40,064 msgs/sec ~ 4.89 MB/sec
 [1] 4,045 msgs/sec ~ 505.63 KB/sec (10000 msgs)
 [2] 4,031 msgs/sec ~ 503.93 KB/sec (10000 msgs)
 [3] 4,034 msgs/sec ~ 504.37 KB/sec (10000 msgs)
 [4] 4,031 msgs/sec ~ 503.92 KB/sec (10000 msgs)
 [5] 4,022 msgs/sec ~ 502.85 KB/sec (10000 msgs)
 [6] 4,028 msgs/sec ~ 503.59 KB/sec (10000 msgs)
 [7] 4,025 msgs/sec ~ 503.22 KB/sec (10000 msgs)
 [8] 4,028 msgs/sec ~ 503.59 KB/sec (10000 msgs)
 [9] 4,025 msgs/sec ~ 503.15 KB/sec (10000 msgs)
 [10] 4,018 msgs/sec ~ 502.28 KB/sec (10000 msgs)
 min 4,018 | avg 4,028 | max 4,045 | stddev 7 msgs

There are numerous other flags that can be set to configure size of messages, using push or pull JetStream consumers and much more, see nats bench --help.

Latency

Latency is the rate at which messages can cross your network, with the nats CLI you can connect a publisher and subscriber to your NATS network and measure the latency between the publisher and subscriber.

nats latency --server-b localhost:4222 --rate 500000

Output

==============================
Pub Server RTT : 64µs
Sub Server RTT : 70µs
Message Payload: 8B
Target Duration: 5s
Target Msgs/Sec: 500000
Target Band/Sec: 7.6M
==============================
HDR Percentiles:
10:       57µs
50:       94µs
75:       122µs
90:       162µs
99:       314µs
99.9:     490µs
99.99:    764µs
99.999:   863µs
99.9999:  886µs
99.99999: 1.483ms
100:      1.483ms
==============================
Actual Msgs/Sec: 499990
Actual Band/Sec: 7.6M
Minimum Latency: 25µs
Median Latency : 94µs
Maximum Latency: 1.483ms
1st Sent Wall Time : 3.091ms
Last Sent Wall Time: 5.000098s
Last Recv Wall Time: 5.000168s

Various flags exist to adjust message size and target rates, see nats latency --help

Super Cluster observation

NATS publish a number of events and have a Request-Reply API that expose a wealth of internal information about the state of the network.

For most of these features you will need a System Account enabled, most of these commands are run against that account.

I create a system context before running these commands and pass that to the commands.

Lifecycle Events

nats event --context system

Output

Listening for Client Connection events on $SYS.ACCOUNT.*.CONNECT
Listening for Client Disconnection events on $SYS.ACCOUNT.*.DISCONNECT
Listening for Authentication Errors events on $SYS.SERVER.*.CLIENT.AUTH.ERR

[12:18:35] [puGCIK5UcWUxBXJ52q4Hti] Client Connection

   Server: nc1-c1
  Cluster: c1

   Client:
                 ID: 17
               User: one
               Name: NATS CLI Version development
            Account: one
    Library Version: 1.11.0  Language: go
               Host: 172.21.0.1

[12:18:35] [puGCIK5UcWUxBXJ52q4Hw8] Client Disconnection

   Reason: Client Closed
   Server: nc1-c1
  Cluster: c1

   Client:
                 ID: 17
               User: one
               Name: NATS CLI Version development
            Account: one
    Library Version: 1.11.0  Language: go
               Host: 172.21.0.1

   Stats:
      Received: 0 messages (0 B)
     Published: 1 messages (0 B)
           RTT: 1.551714ms

Here one can see a client connected and disconnected shortly after, several other system events are supported.

If an account is running JetStream the nats event tool can also be used to look at JetStream advisories by passing --js-metric --js-advisory

These events are JSON messages and can be viewed raw using --json or in Cloud Events format with --cloudevent, finally a short version of the messages can be shown:

nats event --short

Output

Listening for Client Connection events on $SYS.ACCOUNT.*.CONNECT
Listening for Client Disconnection events on $SYS.ACCOUNT.*.DISCONNECT
Listening for Authentication Errors events on $SYS.SERVER.*.CLIENT.AUTH.ERR
12:20:58 [Connection] user: one cid: 19 in account one
12:20:58 [Disconnection] user: one cid: 19 in account one: Client Closed
12:21:00 [Connection] user: one cid: 20 in account one
12:21:00 [Disconnection] user: one cid: 20 in account one: Client Closed
12:21:00 [Connection] user: one cid: 21 in account one

Super Cluster Discovery and Observation

When a cluster or super cluster of NATS servers is configured with a system account a wealth of information is available via internal APIs, the nats tool can interact with these and observe your servers.

A quick view of the available servers and your network RTT to each can be seen with nats server ping:

nats server ping

Output

nc1-c1                                                       rtt=2.30864ms
nc3-c1                                                       rtt=2.396573ms
nc2-c1                                                       rtt=2.484994ms
nc3-c2                                                       rtt=2.549958ms
...

---- ping statistics ----
9 replies max: 3.00 min: 1.00 avg: 2.78

A general server overview can be seen with nats server list:

nats server list

Output

+----------------------------------------------------------------------------------------------------------------------------+
|                                                      Server Overview                                                       |
+--------+------------+-----------+---------------+-------+------+--------+-----+---------+-----+------+--------+------------+
| Name   | Cluster    | IP        | Version       | Conns | Subs | Routes | GWs | Mem     | CPU | Slow | Uptime | RTT        |
+--------+------------+-----------+---------------+-------+------+--------+-----+---------+-----+------+--------+------------+
| nc1-c1 | c1         | localhost | 2.2.0-beta.34 | 1     | 97   | 2      | 2   | 13 MiB  | 0.0 | 0    | 5m29s  | 3.371675ms |
| nc2-c1 | c1         | localhost | 2.2.0-beta.34 | 0     | 97   | 2      | 2   | 13 MiB  | 0.0 | 0    | 5m29s  | 3.48287ms  |
| nc3-c1 | c1         | localhost | 2.2.0-beta.34 | 0     | 97   | 2      | 2   | 14 MiB  | 0.0 | 0    | 5m30s  | 3.57123ms  |
| nc1-c3 | c3         | localhost | 2.2.0-beta.34 | 0     | 96   | 2      | 2   | 15 MiB  | 0.0 | 0    | 5m29s  | 3.655548ms |
...
+--------+------------+-----------+---------------+-------+------+--------+-----+---------+-----+------+--------+------------+
|        | 3 Clusters | 9 Servers |               | 1     | 867  |        |     | 125 MiB |     | 0    |        |            |
+--------+------------+-----------+---------------+-------+------+--------+-----+---------+-----+------+--------+------------+

+----------------------------------------------------------------------------+
|                              Cluster Overview                              |
+---------+------------+-------------------+-------------------+-------------+
| Cluster | Node Count | Outgoing Gateways | Incoming Gateways | Connections |
+---------+------------+-------------------+-------------------+-------------+
| c1      | 3          | 6                 | 6                 | 1           |
| c3      | 3          | 6                 | 6                 | 0           |
| c2      | 3          | 6                 | 6                 | 0           |
+---------+------------+-------------------+-------------------+-------------+
|         | 9          | 18                | 18                | 1           |
+---------+------------+-------------------+-------------------+-------------+

Data from a specific server can be accessed using it's server name or ID:

nats server info nc1-c1

Output

Server information for nc1-c1 (NBNIKFCQZ3J6I7JDTUDHAH3Z3HOQYEYGZZ5HOS63BX47PS66NHPT2P72)

Process Details:

         Version: 2.2.0-beta.34
      Git Commit: 2e26d919
      Go Version: go1.14.12
      Start Time: 2020-12-03 12:18:00.423780567 +0000 UTC
          Uptime: 10m1s

Connection Details:

   Auth Required: true
    TLS Required: false
            Host: localhost:10000
     Client URLs: localhost:10000
                  localhost:10002
                  localhost:10001

Limits:

        Max Conn: 65536
        Max Subs: 0
     Max Payload: 1.0 MiB
     TLS Timeout: 2s
  Write Deadline: 10s

Statistics:

       CPU Cores: 2 1.00%
          Memory: 13 MiB
     Connections: 1
   Subscriptions: 0
            Msgs: 240 in 687 out
           Bytes: 151 KiB in 416 KiB out
  Slow Consumers: 0

Cluster:

            Name: c1
            Host: 0.0.0.0:6222
            URLs: nc1:6222
                  nc2:6222
                  nc3:6222

Super Cluster:

            Name: c1
            Host: 0.0.0.0:7222
        Clusters: c1
                  c2
                  c3

Additional to this various reports can be generated using nats server report, this allows one to list all connections and subscriptions across the entire cluster with filtering to limit the results by account etc.

Additional raw information in JSON format can be retrieved using the nats server request commands.

Monitoring

The nats server check command provides numerous monitoring utilities that supports the popular Nagios exist code based protocol, a format compatible with Prometheus textfile format and a human friendly textual output.

Using these tools one can create monitors for various aspects of NATS Server, JetStream and KV.

Stream and Consumer monitoring

The nats server check stream and nats server check consumer commands can be used to monitor the health of Streams and Consumers.

We'll cover the flags below but since version 0.2.0 these commands support auto configuration from Metadata on the Stream and Consumer. For example if the command accepts --msgs-warn then the metadata io.nats.monitor.msgs-warn can be used to set the same value. Calling the check command without passing the value on the command will use the metadata value instead.

Streams

The stream check command allows the health of a stream to be monitored including Sources, Mirrors, Cluster Health and more.

To perform end to end health checks on a stream it is suggested that canary messages are published regularly into the stream with clients detecting those and discarding them after ACK.

The nats server check message command can be used to check such canary messages exist in the stream, how old they are and if the content is correct. We suggest using this in complex Sourcing and Mirroring setups to perform an additional out-of-band health check on the flow of messages. This includes checking timestamps on the messages.

--lag-critical=MSGS Critical threshold to allow for lag on any source or mirror. Lag is how many tasks the source or mirror is behind, this means the mirror or source do not have complete data and would require fixing.

--seen-critical=DURATION Critical threshold for how long ago the source or mirror should have been seen. During network outages or problems with the foreign Stream this time would increase. The duration can be a string like 5m.

--min-sources=SOURCES, --max-sources=SOURCES Minimum and Maximum number of sources to expect, this allow you to monitor that in a dynamically configured environment that the set number of sources are configured.

--peer-expect=SERVERS Number of cluster replicas to expect, again allowing an assertion that the configuration does not change unexpectedly

--peer-lag-critical=OPS Critical threshold to allow for cluster peer lag, any RAFT peer that is further behind than this number of operations will result in a critical error

--peer-seen-critical=DURATION Critical threshold for how long ago a cluster peer should have been seen, this is sumular to the lag on Sources and Mirrors but checks the lag in the Raft cluster.

--msgs-warn=MSGS and --msgs-critical=MSGS Checks the number of messages in the stream, if warn is smaller than critical the check will alert for fewer messages than the thresholds.

--subjects-warn=SUBJECTS and --subjects-critical=SUBJECTS Checks the number of subjects in the stream. If warn is bigger than critical the logic will be inverted ensuring that no more than the thresholds exist in the stream.

Consumers

The consumer check is concerned with message flow through a consumer and have various adjustable thresholds in duration and count to detect stalled consumers, consumers with no active clients, consumers with slow clients or ones where processing the messages are failing.

A suggested pattern is publishing canary messages into the stream regularly, perhaps with the header Canary: 1 set, and having applications just ACK and discard those messages. This way even in idle times the end to end flow of messages can be monitored.

--outstanding-ack-critical=-1 Maximum number of outstanding acks to allow, this allow you to alert on the scenario where clients consuming messages are slow to process messages and the number of outstanding acks are growing. Once this hits the configured max the consumer will stall.

--waiting-critical=-1 Maximum number of waiting pulls to allow

--unprocessed-critical=-1 Maximum number of unprocessed messages to allow, this indicates how far behind the end of the stream the consumer is, in work queue scenarios this will indicate a alert if the amount of outstanding work grows.

--last-delivery-critical=0s This is the time duration since the last delivery to a client, if this number grows it could mean there are no messages to deliver or no clients to deliver messages to.

--last-ack-critical=0s This is the duration of time since the last message was acknowledged, this duration might indicate that no messages are being successfully processed.

--redelivery-critical=-1 Alerts on the number of redeliveries currently in flight, a high number means many clients are doing NAKs or not completing message processing within the allowed Ack window.

Schema Registry

We are adopting JSON Schema to describe the core data formats of events and advisories - as shown by nats event. Additionally all the API interactions with the JetStream API is documented using the same format.

These schemas can be used using tools like QuickType to generate stubs for various programming languages.

The nats CLI allows you to view these schemas and validate documents using these schemas.

nats schema ls

Output

Matched Schemas:

  io.nats.jetstream.advisory.v1.api_audit
  io.nats.jetstream.advisory.v1.consumer_action
  io.nats.jetstream.advisory.v1.max_deliver
...

The schemas can be limited using a regular expression, try nats schema ls request to see all API requests.

Schemas can be viewed in their raw JSON or YAML formats using nats schema info io.nats.jetstream.advisory.v1.consumer_action, these schemas include descriptions about each field and more.

Finally, if you are interacting with the API using JSON request messages constructed using languages that is not supported by our own management libraries you can use this tool to validate your messages:

nats schema validate io.nats.jetstream.api.v1.stream_create_request request.json

Output

Validation errors in request.json:

  retention: retention must be one of the following: "limits", "interest", "workqueue"
  (root): Must validate all the schemas (allOf)

Here I validate request.json against the Schema that describes the API to create Streams, the validation indicates that I have an incorrect value in the retention field.