---
layout: docs
page_title: Connect - Sidecar Service Registration
sidebar_title: Sidecar Service Registration
description: |-
  Sidecar service registrations provide a convenient shorthand for registering a
  sidecar proxy inline with a regular service definition.
---

# Sidecar Service Registration

Connect proxies are typically deployed as "sidecars" that run on the same node
as the single service instance that they handle traffic for. They might be on
the same VM or running as a separate container in the same network namespace.

To simplify the configuration experience when deploying a sidecar for a service
instance, Consul 1.3 introduced a new field in the Connect block of the [service
definition](/docs/agent/services).

To deploy a service and sidecar proxy locally, complete the
[Getting Started guide](https://learn.hashicorp.com/tutorials/consul/get-started-service-networking?utm_source=consul.io&utm_medium=docs).

The `connect.sidecar_service` field is a complete nested service definition on
which almost any regular service definition field can be set. The exceptions are
[noted below](#limitations). If used, the service definition is treated
identically to another top-level service definition. The value of the nested
definition is that _all fields are optional_ with some opinionated defaults
applied that make setting up a sidecar proxy much simpler.

## Minimal Example

To register a service instance with a sidecar, all that's needed is:

```json
{
  "service": {
    "name": "web",
    "port": 8080,
    "connect": { "sidecar_service": {} }
  }
}
```

This will register the `web` service as normal, but will also register another
[proxy service](/docs/connect/proxies) with defaults values used.

The above expands out to be equivalent to the following explicit service
definitions:

```json
{
  "service": {
    "name": "web",
    "port": 8080,
  }
}
{
  "name": "web-sidecar-proxy",
  "port": 20000,
  "kind": "connect-proxy",
  "checks": [
    {
      "Name": "Connect Sidecar Listening",
      "TCP": "127.0.0.1:20000",
      "Interval": "10s"
    },
    {
      "name": "Connect Sidecar Aliasing web",
      "alias_service": "web"
    }
  ],
  "proxy": {
    "destination_service_name": "web",
    "destination_service_id": "web",
    "local_service_address": "127.0.0.1",
    "local_service_port": 8080,
  }
}
```

Details on how the defaults are determined are [documented
below](#sidecar-service-defaults).

-> **Note:** Sidecar service registrations are only a shorthand for registering
multiple services. Consul will not start up or manage the actual proxy processes
for you.

## Overridden Example

The following example shows a service definition where some fields are
overridden to customize the proxy configuration.

```json
{
  "name": "web",
  "port": 8080,
  "connect": {
    "sidecar_service": {
      "proxy": {
        "upstreams": [
          {
            "destination_name": "db",
            "local_bind_port": 9191
          }
        ],
        "config": {
          "handshake_timeout_ms": 1000
        }
      }
    }
  }
}
```

This example customizes the [proxy
upstreams](/docs/connect/registration/service-registration#upstream-configuration-reference)
and some [built-in proxy
configuration](/docs/connect/proxies/built-in).

## Sidecar Service Defaults

The following fields are set by default on a sidecar service registration. With
[the exceptions noted](#limitations) any field may be overridden explicitly in
the `connect.sidecar_service` definition to customize the proxy registration.
The "parent" service refers to the service definition that embeds the sidecar
proxy.

- `id` - ID defaults to being `<parent-service-id>-sidecar-proxy`. This can't
  be overridden as it is used to [manage the lifecycle](#lifecycle) of the
  registration.
- `name` - Defaults to being `<parent-service-name>-sidecar-proxy`.
- `tags` - Defaults to the tags of the parent service.
- `meta` - Defaults to the service metadata of the parent service.
- `port` - Defaults to being auto-assigned from a [configurable
  range](/docs/agent/options#sidecar_min_port) that is
  by default `[21000, 21255]`.
- `kind` - Defaults to `connect-proxy`. This can't be overridden currently.
- `check`, `checks` - By default we add a TCP check on the local address and
  port for the proxy, and a [service alias
  check](/docs/agent/checks#alias) for the parent service. If either
  `check` or `checks` fields are set, only the provided checks are registered.
- `proxy.destination_service_name` - Defaults to the parent service name.
- `proxy.destination_service_id` - Defaults to the parent service ID.
- `proxy.local_service_address` - Defaults to `127.0.0.1`.
- `proxy.local_service_port` - Defaults to the parent service port.

## Limitations

Almost all fields in a [service definition](/docs/agent/services) may be
set on the `connect.sidecar_service` except for the following:

- `id` - Sidecar services get an ID assigned and it is an error to override
  this. This ensures the agent can correctly deregister the sidecar service
  later when the parent service is removed.
- `kind` - Kind defaults to `connect-proxy` and there is currently no way to
  unset this to make the registration be for a regular non-connect-proxy
  service.
- `connect.sidecar_service` - Service definitions can't be nested recursively.
- `connect.proxy` - (Deprecated) [Managed
  proxies](/docs/connect/proxies/managed-deprecated) can't be defined on a
  sidecar.
- `connect.native` - Currently the `kind` is fixed to `connect-proxy` and it's
  an error to register a `connect-proxy` that is also Connect-native.

## Lifecycle

Sidecar service registration is mostly a configuration syntax helper to avoid
adding lots of boiler plate for basic sidecar options, however the agent does
have some specific behavior around their lifecycle that makes them easier to
work with.

The agent fixes the ID of the sidecar service to be based on the parent
service's ID. This enables the following behavior.

- A service instance can _only ever have one_ sidecar service registered.
- When re-registering via API or reloading from configuration file:
  - If something changes in the nested sidecar service definition, the change
    will _update_ the current sidecar registration instead of creating a new
    one.
  - If a service registration removes the nested `sidecar_service` then the
    previously registered sidecar for that service will be deregistered
    automatically.
- When reloading the configuration files, if a service definition changes it's
  ID, then a new service instance _and_ a new sidecar instance will be
  registered. The old ones will be removed since they are no longer found in
  the config files.
