Channel Version Revision Published Runs on
latest/stable 66 66 08 Nov 2021
Ubuntu 20.04 Ubuntu 18.04 Ubuntu 16.04 Ubuntu 14.04
latest/candidate 64 64 14 Oct 2021
Ubuntu 20.04 Ubuntu 18.04 Ubuntu 16.04 Ubuntu 14.04
latest/edge 37 37 11 Nov 2020
Ubuntu 16.04 Ubuntu 14.04
juju deploy haproxy
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20.04 18.04 16.04 14.04


This charm deploys a reverse proxy in front of other services. You can use this to load balance existing deployments.


juju deploy haproxy
juju deploy my-web-app
juju relate my-web-app:website haproxy:reverseproxy
juju add-unit my-web-app

Reverse Proxy Relation

The reverse proxy relation is used to distribute connections from one frontend port to many backend services (typically different Juju units). You can use haproxy just like this, but typically in a production service you would frontend this service with apache2 to handle the SSL negotiation, etc. See the "Website Relation" section for more information about that.

When your charm hooks into reverseproxy you have two general approaches which can be used to notify haproxy about what services you are running.

  1. Single-service proxying or 2) Multi-service or relation-driven proxying.
  1. Single-Service Proxying

In this case, your website relation will join underneath a single listen stanza in haproxy. This stanza will have one service entry for each unit connecting. By convention, this is typically called "website". The following is an example of a relation-joined or changed hook:

# hooks/website-relation-joined

relation-set "hostname=$(unit-get private-address)"
relation-set "port=80"

# Set an optional service name, allowing more config-based
# customization
relation-set "service_name=my_web_app"

If you set the service_name relation setting, the configuration services yaml mapping will be consulted to lookup 3 other options based on service name.

  • {service_name}_servers - sets the server line in the listen stanza explicitly.
  • {service_name}_server_options - Will append to the charm-generated server line for for each joining unit in the reverseproxy relation.
  • {service_name}_service_options - expected to be a list of strings. Will set each item as an option under the listen stanza.
  1. Relation-Driven Proxying

In this relation style, your charm should specify these relation settings directly as relation variables when joining reverseproxy. Your charm's website-relation-changed hook would look something like this:

# hooks/website-relation-changed

host=$(unit-get private-address)

relation-set "services=
- { service_name: my_web_app,
    service_port: 80,
    service_options: [mode http, balance leastconn],
    servers: [[my_web_app_1, $host, $port, option httpchk GET / HTTP/1.0],
              [... optionally more servers here ...]]}
- { ... optionally more services here ... }

Once set, haproxy will union multiple servers stanzas from any units joining with the same service_name under one backend stanza, which will be the default backend for the service (requests against the given service_port on the haproxy unit will be forwarded to that backend). Note that service-options and server_options will be overwritten, so ensure they are set uniformly on all services with the same name.

If you need additional backends, possibly handling ACL-filtered requests, you can add a 'backends' entry to a service stanza. For example in order to redirect to a different backend all requests to URLs starting with '/foo', you could have:

relation-set "services=
- { service_name: my_web_app,
    service_port: 80,
    service_options: [mode http, acl foo path_beg -i /foo, use_backend foo if foo],
    servers: [[my_web_app_1, $host, $port, option httpchk GET / HTTP/1.0],
              [... optionally more servers here ...]]
    - { backend_name: foo,
        servers: [[my_web_app2, $host, $port2, option httpchk GET / HTTP/1.0],
                  [... optionally more servers here ...]]}}

In all cases if your service needs to know the public IP(s) of the haproxy unit(s) it relates to, or the value of the default SSL certificate set on or generated by the haproxy service, you can look for the 'public-address' and 'ssl_cert' keys on your relation, which are set by the haproxy service as soon as it joins the reverseproxy relation.

Website Relation

The website relation is the other side of haproxy. It can communicate with charms written like apache2 that can act as a front-end for haproxy to take of things like ssl encryption. When joining a service like apache2 on its reverseproxy relation, haproxy's website relation will set an all_services variable that conforms to the spec laid out in the apache2 charm.

These settings can then be used when crafting your vhost template to make sure traffic goes to the correct haproxy listener which will in turn forward the traffic to the correct backend server/port

SSL Termination

You can turn on SSL termination by using the ssl_cert/ssl_key service configuration options and then using the crts key in the services yaml, e.g.:

# hooks/website-relation-changed

host=$(unit-get private-address)

relation-set "services=
- { service_name: my_web_app,
    service_options: [mode http, balance leastconn],
    crts: [DEFAULT]
    servers: [[my_web_app_1, $host, $port, option httpchk GET / HTTP/1.0],
              [... optionally more servers here ...]]}
- { ... optionally more services here ... }

The DEFAULT keyword means use the certificate set with ssl_cert/ssl_key (or alternatively you can inline different base64-encode certificates).

Note that in order to use SSL termination you need haproxy 1.5 or later, which is not available in stock trusty, but you can get it from trusty-backports setting the source configuration option to backports or to whatever PPA/archive you wish to use.


Telegraf is recommended for monitoring HAProxy. To do so, deploy the following:

juju deploy haproxy
juju deploy telegraf
juju relate telegraf:juju-info haproxy # For standard telegraf host metrics
juju relate telegraf:haproxy haproxy   # For HAProxy-specific metrics

You can then get metrics for your HAProxy instance(s) by visiting http://${unit_up}:9103/metrics.


The following steps are needed for testing and development of the charm, but not for deployment:

sudo apt-get install python-software-properties
sudo add-apt-repository ppa:cjohnston/flake8
sudo apt-get update
sudo apt-get install python-mock python-flake8 python-nose python-nosexcover python-testtools charm-tools

To run the tests:

make build

... will run the unit tests, run flake8 over the source to warn about formatting issues and output a code coverage summary of the 'hooks.py' module.

Known Limitations and Issues

  • Expand Single-Service section as this has not been fully tested.
  • Trigger website-relation-changed when the reverse-proxy relation changes


Many of the haproxy settings can be altered via the standard juju configuration settings. Please see the config.yaml file as each is fairly clearly documented.


This charm supports sending metrics to statsd.

This is done by setting config values (metrics_target being the primary one) to a host/port of a (UDP) statsd server.

This could instead be done using a relation, but it is common to have one statsd server that serves multiple environments. Once juju supports cross-environment relations then that will be the best way to handle this configuration, as it will work in either scenario.

peering_mode and the indirection layer

If you are going to spawn multiple haproxy units, you should pay special attention to the peering_mode configuration option.

active-passive mode

The peering_mode option defaults to "active-passive" and in this mode, all haproxy units ("peers") will proxy traffic to the first working peer (i.e. that passes a basic layer4 check). What this means is that extra peers are working as "hot spares", and so adding units doesn't add global bandwidth to the haproxy layer.

In order to achieve this, the charm configures a new service in haproxy that will simply forward the traffic to the first working peer. The haproxy service that actually load-balances between the backends is renamed, and its port number is increased by one.

For example, if you have 3 working haproxy units haproxy/0, haproxy/1 and haproxy/2 configured to listen on port 80, in active-passive mode, and haproxy/2 gets a request, the request is routed through the following path :

haproxy/2:80 ==> haproxy/0:81 ==> [backends]

In the same fashion, if haproxy/1 receives a request, it's routed in the following way :

haproxy/1:80 ==> haproxy/0:81 ==> [backends]

If haproxy/0 was to go down, then all the requests would be forwarded to the next working peer, i.e. haproxy/1. In this case, a request received by haproxy/2 would be routed as follows :

haproxy/2:80 ==> haproxy/1:81 ==> [backends]

This mode allows a strict control of the maximum number of connections the backends will receive, and guarantees you'll have enough bandwidth to the backends should an haproxy unit die, at the cost of having less overall bandwidth to the backends.

active-active mode

If the peering_mode option is set to "active-active", then any haproxy unit will be independent from each other and will simply load-balance the traffic to the backends. In this case, the indirection layer described above is not created.

This mode allows increasing the bandwidth to the backends by adding additional units, at the cost of having less control over the number of connections that they will receive.

HAProxy Project Information