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Sidecar Sidecar

The main repo for this project is the NinesStack fork

Sidecar is a dynamic service discovery platform requiring no external coordination service. It's a peer-to-peer system that uses a gossip protocol for all communication between hosts. Sidecar health checks local services and announces them to peer systems. It's Docker-native so your containerized applications work out of the box. It's designed to be Available, Partition tolerant, and eventually consistent—where "eventually" is a very short time window on the matter of a few seconds.

Sidecar is part of a small ecosystem of tools. It can stand entirely alone or can also leverage:

  • Lyft's Envoy Proxy - In less than a year it is fast becoming a core microservices architecture component. Sidecar implements the Envoy proxy SDS, CDS, LDS (V1) and gRPC (V2) APIs. These allow a standalone Envoy to be entirely configured by Sidecar. This is best used with NinesStack's Envoy proxy container.

  • haproxy-api - A separation layer that allows Sidecar to drive HAproxy in a separate container. It also allows a local HAproxy to be configured against a remote Sidecar instance.

  • sidecar-executor - A Mesos executor that integrates with Sidecar, allowing your containers to be health checked by Sidecar for both service health and service discovery. Also supports a number of extra features including Vault integration for secrets management.

  • sidecar-dns - a working, but WIP, project to serve DNS SRV records from Sidecar services state.

Overview in Brief

Services communicate to each other through a proxy (Envoy or HAproxy) instance on each host that is itself managed and configured by Sidecar. This is, in effect, a half service mesh where outbound connections go through the proxy, but inbound requests do not. This has most of the advantages of service mesh with a lot less complexity to manage. It is inspired by Airbnb's SmartStack. But, we believe it has a few advantages over SmartStack:

  • Eventually consistent model - a better fit for real world microservices
  • Native support for Docker (works without Docker, too!)
  • No dependence on Zookeeper or other centralized services
  • Peer-to-peer, so it works on your laptop or on a large cluster
  • Static binary means it's easy to deploy, and there is no interpreter needed
  • Tiny memory usage (under 20MB) and few execution threads means its very light weight

See it in Action: We presented Sidecar at Velocity 2015 and recorded a YouTube video demonstrating Sidecar with Centurion, deploying services in Docker containers, and seeing Sidecar discover and health check them. The second video shows the current state of the UI which is improved since the first video.

YouTube Video YouTube Video2

Complete Overview and Theory

Sidecar Architecture

Sidecar is an eventually consistent service discovery platform where hosts learn about each others' state via a gossip protocol. Hosts exchange messages about which services they are running and which have gone away. All messages are timestamped and the latest timestamp always wins. Each host maintains its own local state and continually merges changes in from others. Messaging is over UDP except when doing anti-entropy transfers.

There is an anti-entropy mechanism where full state exchanges take place between peer nodes on an intermittent basis. This allows for any missed messages to propagate, and helps keep state consistent across the cluster.

Sidecar hosts join a cluster by having a set of cluster seed hosts passed to them on the command line at startup. Once in a cluster, the first thing a host does is merge the state directly from another host. This is a big JSON blob that is delivered over a TCP session directly between the hosts.

Now the host starts continuously polling its own services and reviewing the services that it has in its own state, sleeping a couple of seconds in between. It announces its services as UDP gossip messages every couple of seconds, and also announces tombstone records for any services which have gone away. Likewise, when a host leaves the cluster, any peers that were notified send tombstone records for all of its services. These eventually converge and the latest records should propagate everywhere. If the host rejoins the cluster, it will announce new state every few seconds so the services will be picked back up.

There are lifespans assigned to both tombstone and alive records so that:

  1. A service that was not correctly tombstoned will go away in short order
  2. We do not continually add to the tombstone state we are carrying

Because the gossip mechanism is UDP and a service going away is a higher priority message, each tombstone is sent twice initially, followed by once a second for 10 seconds. This delivers reliable messaging of service death.

Timestamps are all local to the host that sent them. This is because we can have clock drift on various machines. But if we always look at the origin timestamp they will at least be comparable to each other by all hosts in the cluster. The one exception to this is that if clock drift is more than a second or two, the alive lifespan may be negatively impacted.

Running it

You can download the latest release from the GitHub Releases page.

If you'd rather build it yourself, you should install the latest version of the Go compiler. Sidecar has not been tested with gccgo, only the mainstream Go compiler.

It's a Go application and the dependencies are all vendored into the vendor/ directory so you should be able to build it out of the box.

$ go build

Or you can run it like this:

$ go run *.go --cluster-ip <boostrap_host>

You always need to supply at least one IP address or hostname with the --cluster-ip argument (or via the SIDECAR_SEEDS environment variable). If are running solo, or are the first member, this can be your own hostname. You may specify the argument multiple times to have multiple hosts. It is recommended to use more than one when possible.

Note: --cluster-ip will overwrite the values passed into the SIDECAR_SEEDS environment variable.

Running in a Container

The easiest way to deploy Sidecar to your Docker fleet is to run it in a container itself. Instructions for doing that are provided.

Nitro Software maintains builds of the Docker container image on Docker Hub. Note that the README describes how to configure this container.


Sidecar configuration is done through environment variables, with a few options also supported on the command line. Once the configuration has been parsed, Sidecar will use Rubberneck to print out the values that were used. The environment variable are as follows. Defaults are in bold at the end of the line:

  • SIDECAR_LOGGING_LEVEL: The logging level to use (debug, info, warn, error) info

  • SIDECAR_LOGGING_FORMAT: Logging format to use (text, json) text

  • SIDECAR_DISCOVERY: Which discovery backends to use as a csv array (static, docker) [ docker ]

  • SIDECAR_SEEDS: csv array of IP addresses used to seed the cluster.

  • SIDECAR_CLUSTER_NAME: The name of the Sidecar cluster. Restricts membership to hosts with the same cluster name.

  • SIDECAR_BIND_PORT: Manually override the Memberlist bind port 7946

  • SIDECAR_ADVERTISE_IP: Manually override the IP address Sidecar uses for cluster membership.

  • SIDECAR_EXCLUDE_IPS: csv array of IPs to exclude from interface selection [ ]

  • SIDECAR_STATS_ADDR: An address to send performance stats to. none

  • SIDECAR_PUSH_PULL_INTERVAL: How long to wait between anti-entropy syncs. 20s

  • SIDECAR_GOSSIP_MESSAGES: How many times to gather messages per round. 15

  • SIDECAR_DEFAULT_CHECK_ENDPOINT: Default endpoint to health check services on /version

  • SERVICES_NAMER: Which method to use to extract service names. In both cases it will fall back to image name. (docker_label, regex) docker_label.

  • SERVICES_NAME_MATCH: The regexp to use to extract the service name from the container name.

  • SERVICES_NAME_LABEL: The Docker label to use to identify service names ServiceName

  • DOCKER_URL: How to connect to Docker if Docker discovery is enabled. unix:///var/run/docker.sock

  • STATIC_CONFIG_FILE: The config file to use if static discovery is enabled static.json

  • LISTENERS_URLS: If we want to statically configure any event listeners, the URLs should go in a csv array here. See Listeners section below for more on dynamic listeners.

  • HAPROXY_DISABLE: Disable management of HAproxy entirely. This is useful if you need to run without a proxy or are using something like haproxy-api to manage HAproxy based on Sidecar events. You should also use this setting if you are using Envoy as your proxy.

  • HAPROXY_RELOAD_COMMAND: The reload command to use for HAproxy sane defaults

  • HAPROXY_VERIFY_COMMAND: The verify command to use for HAproxy sane defaults

  • HAPROXY_BIND_IP: The IP that HAproxy should bind to on the host

  • HAPROXY_TEMPLATE_FILE: The source template file to use when writing HAproxy configs. This is a Go text template. views/haproxy.cfg

  • HAPROXY_CONFIG_FILE: The path where the haproxy.cfg file will be written. Note that if you change this you will need to update the verify and reload commands. /etc/haproxy.cfg

  • HAPROXY_PID_FILE: The path where HAproxy's PID file will be written. Note that if you change this you will need to update the verify and reload commands. /var/run/

  • HAPROXY_USER: The Unix user under which HAproxy should run haproxy

  • HAPROXY_GROUP: The Unix group under which HAproxy should run haproxy

  • HAPROXY_USE_HOSTNAMES: Should we write hostnames in the HAproxy config instead of IP addresses? false

  • ENVOY_USE_GRPC_API: Enable the Envoy gRPC API (V2) true

  • ENVOY_BIND_IP: The IP that Envoy should bind to on the host

  • ENVOY_USE_HOSTNAMES: Should we write hostnames in the Envoy config instead of IP addresses? false

  • ENVOY_GRPC_PORT: The port for the Envoy API gRPC server 7776


Sidecar requires both TCP and UDP protocols be open on the port configured via SIDECAR_BIND_PORT (default 7946) through any network filters or firewalls between it and any peers in the cluster. This is the port that the gossip protocol (Memberlist) runs on.


Sidecar supports both Docker-based discovery and a discovery mechanism where you publish services into a JSON file locally, called "static". These can then be advertised as running services just like they would be from a Docker host. These are configured with the SIDECAR_DISCOVERY environment variable. Using both would look like:

export SIDECAR_DISCOVERY=static,docker

Zero or more options may be supplied. Note that if nothing is in this section, Sidecar will only participate in a cluster but will not announce anything.

Configuring Docker Discovery

Sidecar currently accepts a single option for Docker-based discovery, the URL to use to connect to Docker. Ideally this will be the same machine that Sidecar runs on because it makes assumptions about addresses. By default it will use the standard Docker Unix domain socket. You can change this with the DOCKER_URL env var. This needs to be a url that works with the Docker client.

Note that Sidecar only supports a single URL, unlike the Docker CLI tool.

NOTE Sidecar can now use the normal Docker environment variables for configuring Docker discovery. If you unset DOCKER_URL entirely, it will fall back to trying to use environment variables to configure Docker. It uses the standard variables like DOCKER_HOST, TLS_VERIFY, etc.

Docker Labels

When running Docker discovery, Sidecar relies on Docker labels to understand how to handle a service it has discovered. It uses these to:

  1. Understand how to map container ports to proxy ports. ServicePort_XXX
  2. How to name the service. ServiceName=
  3. How to health check the service. HealthCheck and HealthCheckArgs
  4. Whether or not the service is a receiver of Sidecar change events. SidecarListener
  5. Whether or not Sidecar should entirely ignore this service. SidecarDiscovery
  6. Envoy or HAproxy proxy behavior. ProxyMode

Service Ports Services may be started with one or more ServicePort_xxx labels that help Sidecar to understand ports that are mapped dynamically. This controls the port on which the proxy will listen for the service as well. If I have a service where the container is built with EXPOSE 80 and I want my proxy to listen on port 8080 then I will add a Docker label to the service in the form:


With dynamic port bindings, Docker may then bind that to 32767 but Sidecar will know which service and port that belongs.

Health Checks If you services are not checkable with the default settings, they need to have two Docker labels defining how they are to be health checked. To health check a service on port 9090 on the local system with an HttpGet check, for example, you would use the following labels:


The currently available check types are HttpGet, External and AlwaysSuccessful. External checks will run the command specified in the HealthCheckArgs label (in the context of a bash shell). An exit status of 0 is considered healthy and anything else is unhealthy. Nagios checks work very well with this mode of health checking.

Excluding From Discovery Additionally, it can sometimes be nice to exclude certain containers from discovery. This is particularly useful if you are running Sidecar in a container itself. This is accomplished with another Docker label like so:


Proxy Behavior By default, HAProxy or Envoy will run in HTTP mode. The mode can be changed to TCP by setting the following Docker label:


You may also enable Websocket support where it's available (e.g. in Envoy) by setting:


Templating In Labels You sometimes need to pass information in the Docker labels which is not available to you at the time of container creation. One example of this is the need to identify the actual Docker-bound port when running the health check. For this reason, Sidecar allows simple templating in the labels. Here's an example.

If you have a service that is exposing port 8080 and Docker dynamically assigns it the port 31445 at runtime, your health check for that port will be impossible to define ahead of time. But with templating we can say:

--label HealthCheckArgs="http://{{ host }}:{{ tcp 8080 }}/"

This will then fill the template fields, at call time, with the current hostname and the actual port that Docker bound to your container's port 8080. Querying of UDP ports works as you might expect, by calling {{ udp 53 }} for example.

Note that the tcp and udp method calls in the templates refer only to ports mapped with ServicePort labels. You will need to use the port number that you expect the proxy to use.

Configuring Static Discovery

Static Discovery requires an entry in the SIDECAR_DISCOVERY variable of static. It will then look for a file configured with STATIC_CONFIG_FILE to export services. This file is usually static.json in the current working directory of the process.

A static discovery file might look like this:

        "Service": {
            "Name": "some_service",
            "Image": "bb6268ff91dc42a51f51db53846f72102ed9ff3f",
            "Ports": [
                    "Type": "tcp",
                    "Port": 10234,
					"ServicePort": 9999
			"ProxyMode": "http",
        "Check": {
            "Type": "HttpGet",
            "Args": "http://:10234/"

Here we've defined both the service itself and the health check to use to validate its status. It supports a single health check per service. You should supply something in place of the value for Image that is meaningful to you. Usually this is a version or git commit string. It will show up in the Sidecar web UI.

A further example is available in the fixtures/ directory used by the tests.

Sidecar Events and Listeners

Services which need to know about service discovery change events can subscribe to Sidecar events. Any time a significant change happens, the listener will receive an update over HTTP from Sidecar. There are three mechanisms by which a service can subscribe to Sidecar events:

  1. Add the endpoint in the LISTENERS_URLS env var, e.g.:

    export LISTENERS_URLS="http://localhost:7778/api/update"

    This is an array and can be separated with spaces or commas.

  2. Add a Docker label to the subscribing service in the form SidecarListener=10005 where 10005 is a port that is mapped to a ServicePort with a Docker label like ServicePort_80=10005. This port will then receive all updates on the /sidecar/update endpoint. The subscription will be dynamically added and removed when the service starts or stops.

  3. Add the listener export to the static.json file exposed by static services. The ListenPort is a top-level setting for the Target and is of the form ListenPort: 10005 inside the Target definition.

Monitoring It

The logging output is pretty good in the normal info level. It can be made quite verbose in debug mode, and contains lots of information about what's going on and what the current state is. The web interface also contains a lot of runtime information on the cluster and the services. If you are running HAproxy, it's also recommended that you expose the HAproxy stats port on 3212 so that Sidecar can find it.

Currently the web interface runs on port 7777 on each machine that runs sidecar.

The /ui/services endpoint is a very textual web interface for humans. The /api/services.json endpoint is JSON-encoded. The JSON is still pretty-printed so it's readable by humans.

Sidecar API

Other than the UI that lives on the base URL, there is a minimalist API available for querying Sidecar. It supports the following endpoints:

  • /services.json: This returns a big JSON blob sorted and grouped by service.
  • /state.json: Returns the whole internal state blob in the internal representation order (servers -> server -> service -> instances)
  • /services/<service name>.json: Returns the same format as the /service.json endpoint, but only contains data for a single service.
  • /watch: Inconsistenly named endpoint that returns JSON blobs on a long-poll basis every time the internal state changes. Useful for anything that needs to know what the ongoing service status is.

Sidecar can also be configured to post the internal state to HTTP endpoints on any change event. See the "Sidecar Events and Listeners" section.

Envoy Proxy Support

Envoy uses a very different model than HAproxy and thus Sidecar's support for it is quite different from its support for HAproxy.

When using the REST-based LDS API (V1), Envoy makes requests to a variety of discovery service APIs on a timed basis. Sidecar currently implements three of these: the Cluster Discovery Service (CDS), the Service Discovery Service (SDS), and the Listeners Discovery Service (LDS). When using the gRPC V2 API, Sidecar sends updates to Envoy as soon as possible via gRPC.

Note that the LDS API (V1) has been deprecated by Envoy and it's recommended to use the gRPC-based V2 API.

Nitro builds and supports an Envoy container that is tested and works against Sidecar. This is the easiest way to run Envoy with Sidecar. You can find an example container configuration here if you need to configure it differently from Nitro's recommended setup.

The critical component is that the Envoy proxy needs to be able to talk to the Sidecar API. By default the Nitro container assumes that Sidecar will be running on If your sidecar is addressable on that address, you can start the envoy container with your platform's equivalent of the following Docker command:

docker run -i -t --net host --cap-add NET_ADMIN gonitro/envoyproxy:latest

Note: This assumes host networking mode so that Envoy can freely open and close listeners. Beware that the docker (Linux) bridge network is not reachable on OSX hosts, due to the way containers are run under HyperKit, so we suggest trying this on Linux instead.


Contributions are more than welcome. Bug reports with specific reproduction steps are great. If you have a code contribution you'd like to make, open a pull request with suggested code.

Pull requests should:

  • Clearly state their intent in the title
  • Have a description that explains the need for the changes
  • Include tests!
  • Not break the public API

Ping us to let us know you're working on something interesting by opening a GitHub Issue on the project.

By contributing to this project you agree that you are granting New Relic a non-exclusive, non-revokable, no-cost license to use the code, algorithms, patents, and ideas in that code in our products if we so choose. You also agree the code is provided as-is and you provide no warranties as to its fitness or correctness for any purpose


The logo is used with kind permission from Picture Esk.


Gossip-based service discovery. Docker native, but supports non-container discovery, too.





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