NATS HTTP Round Tripper

This is a Golang http.RoundTripper that uses NATS as a transport.

Included is a http.RoundTripper for clients, a server that uses normal HTTP Handlers and any existing http handler mux and a Caddy Server transport.

Why?

Using NATS as a transport for microservices has a number of advantages, you get a lot of the benefits of a service mesh with no additional infrastructure.

• Load balancing is free with no additional components like load balancers
• Failover is free with no additional components like load balancers
• Geo failover and fallback is free with no additional components like global dns
• Long running connections are used for microservices speeding up connection handling
• NATS support canary deploys, gradual deploys, traffic shaping or more
• NATS, being subject and interest based, does not need any additional infrastructure for service discovery

Moving to NATS as transport for microservices can be a big job, implementing a http.RoundTripper means your existing microservices can be hosted on nats with minimal changes.

You can see it in action on YouTube doing geo failover.

Example?

Server / Microservice

See the Weather Service

The server is a normal HTTP server, no changes to your handlers:

http.HandleFunc("/city", cityHandler)

// allow for gradual migration by listening on HTTP
log.Printf("Listening on 8080")
go http.ListenAndServe(":"+port, nil)

// Listen on NATS
nats, _ := nrt.New(nrt.WithNatsServer("nats.internal"))
go nats.ListenAndServ(context.Background(), "weather.nats", nil)

<-context.Background().Done()

Client

The client is a normal HTTP client, see the bundled nrtget

t, _ := nrt.New(nrt.WithNatsServer("nats.internal"))
c := &http.Client{Transport: t}

req, _ := http.NewRequest("GET", "https://weather.nats/city", bytes.NewBuffer([]byte(`{"city":"london"}`))))
resp, _ := c.Do(req)

defer resp.Body.Close()
body, err := ioutil.ReadAll(resp.Body)

fmt.Print(string(body))

Proxy

I have a Caddy Server 2 extension that you can build using xcaddy, using this you can bridge existing HTTP traffic to a backend managed by NATS.

This extension is just an experiment, the Caddy 2 documentation is non existent so doing the right thing is quite hard. Caddy is really attractive though as it's high speed, easy to extend and support many features like Lets Encrypt right out of the box.

You'd look to use something like this to bridge existing traffic to your backend, or to assist with migrating from a current REST based solution to a NATS one.

{
    order nats last
}

http://:9090 {
        nats /city weather.nats
}

Run this using the environment variables listed below like NATS_CONTEXT for configuration.

With this in place requests to http://example.net:9090/city will be sent to the NATS backend weather.nats microservice.

This has a number of advantages:

• A long-running connection from Caddy to NATS is made significantly reducing the cost overhead of connection management, especially with TLS
• Despite being a single long running connection load balancing and geo failover is supported
• The connection is self managing and will reconnect automatically, supports NATS topology discovery and will keep working through scale ups and downs of the NATS infrastructure
• Massive infrastructure reduction as you don't need any service discovery or LB level configuration to do traffic shaping etc

Traffic Management

Load balancing

Without any additional effort the service will scale horizontally and vertically, simply starting more of the same code will cause the load to be randomly distributed.

Geo Failover

If instances of the service is started in multiple NATS Clustered connected into a Super Cluster then connections made in cluster "east" will automatically failover to cluster "west" and fall back to "east" when workers are available.

Canary Deploys

When running a service the NATS_PREFIX environment variable or the WithPrefix() option can adjust the subjects where the service listen.

Here we run the weather service on a v2 subject:

$ WEATHER_API=your.api.key \
  HTTP_PORT=8088 \
  NATS_CONTEXT=weather \
  NATS_PREFIX=v2 \
  ./weather

Lets assume we have both a v1 and v2 running, and we wish to test 10% of the traffic against v2. Configuring the NATS Server as follows will set this up:

mappings: {
    nrt.requests.weather.nats.*: [
        {destination: v1.requests.weather.nats.$1, weight: 90%}
        {destination: v2.requests.weather.nats.$1, weight: 10%}
    ]
}

With this in place 10% of the traffic will be routed to v2 of the microservice. These settings can be adjusted on the fly to migrate traffic onto v2 gradually.

Fault Injection

Building on the mappings above we can arrange for 10% of the traffic to be lost:

mappings: {
    nrt.requests.weather.nats.*: [
        {destination: v1.requests.weather.nats.$1, weight: 90%}
        {destination: v1.discard.weather.nats.$1, weight: 10%}
    ]
}

Here we ensure no services listens on v1.discard... and those 10% of traffic will be lost.

Design?

A server like weather.nats will subscribe in a queue group to the subject nrt.requests.weather.nats.*, clients will publish a GET request to nrt.requests.weather.nats.get.

The body being sent is the entire HTTP request, exactly as it came in the wire. On the server this is received and turned into a standard http.Request and sent to the handler.

Response is simply a response containing the entire body with some headers added:

Header	Meaning
NRT-ID	A unique request ID set by the server
NRT-Connected-Server	The NATS server the microservice was connected to
NRT-Client	The Client ID of the server connection
NRT-Connected-Cluster	The cluster the server is connected to if clustered

Configuration ?

Several environment variables influence the behavior of the transport, most have matching options to configure the connections:

Variable	Description
NATS_CONTEXT	A context to use as made by the nats context save command, see NATS_ENV
NATS_ENV	Set to true when no nats context is used, configure using the NATS_* variables instead
NATS_URL	Where the nats servers are
NATS_USER	User to connect as, also used as token when password is empty
NATS_PASSWORD	The password to connect with
NATS_CREDS	Path to a nats creds file
NATS_NKEY	NKey to authenticate with
NATS_CERT	Path to a x509 certificate
NATS_KEY	Path to a x509 private key
NATS_CA	Path to a x509 certificate authority certificate chain
NATS_PREFIX	A subject to prefix to the hostname, defaults to nrt

Contact?

R.I.Pienaar / rip@devco.net / @ripienaar