marathon-lb

Marathon-lb is a tool for managing HAProxy, by consuming Marathon's app state. HAProxy is a fast, efficient, battle-tested, highly available load balancer with many advanced features which power a number of high-profile websites.

Features

• Stateless design: no direct dependency on any third-party state store like ZooKeeper or etcd (except through Marathon)
• Idempotent and deterministic: scales horizontally
• Highly scalable: can achieve line-rate per instance, with multiple instances providing fault-tolerance and greater throughput
• Real-time LB updates, via Marathon's event bus
• Support for Marathon's health checks
• Multi-cert TLS/SSL support
• Zero-downtime deployments
• Per-service HAProxy templates
• DC/OS integration
• Automated Docker image builds (mesosphere/marathon-lb)
• Global HAProxy templates which can be supplied at launch
• Supports IP-per-task integration, such as Project Calico

Getting started

• Using marathon-lb
• Advanced features of marathon-lb
• Securing your service with TLS/SSL (blog post)

Take a look at the marathon-lb wiki for example usage, templates, and more.

Architecture

The marathon-lb script marathon_lb.py connects to the marathon API to retrieve all running apps, generates a HAProxy config and reloads HAProxy. By default, marathon-lb binds to the service port of every application and sends incoming requests to the application instances.

Services are exposed on their service port (see Service Discovery & Load Balancing for reference) as defined in their Marathon definition. Furthermore, apps are only exposed on LBs which have the same LB tag (or group) as defined in the Marathon app's labels (using HAPROXY_GROUP). HAProxy parameters can be tuned by specify labels in your app.

To create a virtual host or hosts the HAPROXY_{n}_VHOST label needs to be set on the given application. Applications with a vhost set will be exposed on ports 80 and 443, in addition to their service port. Multiple virtual hosts may be specified in HAPROXY_{n}_VHOST using a comma as a delimiter between hostnames.

All applications are also exposed on port 9091, using the X-Marathon-App-Id HTTP header. See the documentation for HAPROXY_HTTP_FRONTEND_APPID_HEAD in the templates section

You can access the HAProxy statistics via :9090/haproxy?stats, and you can retrieve the current HAProxy config from the :9090/_haproxy_getconfig endpoint.

Deployment

The package is currently available from the universe. To deploy marathon-lb on the public slaves in your DC/OS cluster, simply run:

dcos package install marathon-lb

To configure a custom ssl-certificate, set the dcos cli option ssl-cert to your concatenated cert and private key in .pem format. For more details see the HAProxy documentation.

For further customization, templates can be added by pointing the dcos cli option template-url to a tarball containing a directory templates/. See comments in script on how to name those.

Docker

Synopsis: docker run -e PORTS=$portnumber --net=host mesosphere/marathon-lb sse|event|poll ...

You must set PORTS environment variable to allow haproxy bind to this port. Syntax: docker run -e PORTS=9090 mesosphere/marathon-lb sse [other args]

You can pass in your own certificates for the SSL frontend by setting the HAPROXY_SSL_CERT environment variable. If you need more than one certificate you can specify additional ones by setting HAPROXY_SSL_CERT0 - HAPROXY_SSL_CERT100.

sse mode

In SSE mode, the script connects to the marathon events endpoint to get notified about state changes. This only works with Marathon 0.11.0 or newer versions.

Syntax: docker run mesosphere/marathon-lb sse [other args]

event mode

In event mode, the script registers a HTTP callback in marathon to get notified when state changes.

Syntax: docker run mesosphere/marathon-lb event callback-addr:port [other args]

poll mode

If you can't use the HTTP callbacks, the script can poll the APIs to get the schedulers state periodically.

Syntax: docker run mesosphere/marathon-lb poll [other args]

To change the poll interval (defaults to 60s), you can set the POLL_INTERVAL environment variable.

Direct invocation

You can also run the update script directly. To generate an HAProxy configuration from Marathon running at localhost:8080 with the marathon_lb.py script, run:

$ ./marathon_lb.py --marathon http://localhost:8080 --group external

It is possible to pass --auth-credentials= option if your Marathon requires authentication:

$ ./marathon_lb.py --marathon http://localhost:8080 --auth-credentials=admin:password

This will refresh haproxy.cfg, and if there were any changes, then it will automatically reload HAProxy. Only apps with the label HAPROXY_GROUP=external will be exposed on this LB.

marathon_lb.py has a lot of additional functionality like sticky sessions, HTTP to HTTPS redirection, SSL offloading, virtual host support and templating capabilities.

To get the full documentation run:

$ ./marathon_lb.py --help

Providing SSL certificates

You can provide your SSL certificate paths to be placed in frontend marathon_https_in section with --ssl-certs.

$ ./marathon_lb.py --marathon http://localhost:8080 --group external --ssl-certs /etc/ssl/site1.co,/etc/ssl/site2.co

If you are using the script directly, you have two options:

• Provide nothing and config will use /etc/ssl/cert.pem as the certificate path. Put the certificate in this path or edit the file for the correct path.
• Provide --ssl-certs command line argument and config will use these paths.

If you are using the provided run script or Docker image, you have three options:

• Provide your certificate text in HAPROXY_SSL_CERT environment variable. Contents will be written to /etc/ssl/cert.pem. Config will use this path unless you specified extra certificate paths as in the next option.
• Provide SSL certificate paths with --ssl-certs command line argument. Your config will use these certificate paths.
• Provide nothing and it will create self-signed certificate on /etc/ssl/cert.pem and config will use it.

Skipping configuration validation

You can skip the configuration file validation (via calling HAProxy service) process if you don't have HAProxy installed. This is especially useful if you are running HAProxy on Docker containers.

$ ./marathon_lb.py --marathon http://localhost:8080 --group external --skip-validation

Using HAProxy maps for backend lookup.

You can use HAProxy maps to speed up web application (vhosts) to backend lookup. This is very useful for large installations where the traditional vhost to backend rules comparison takes considerable time since it sequentially compares each rule. HAProxy map creates a hash based lookup table so its fast compared to the other approach, this is supported in marathon-lb using --haproxy-map flag.

$ ./marathon_lb.py --marathon http://localhost:8080 --group external --haproxy-map

Currently it creates a lookup dictionary only for host header (both HTTP and HTTPS) and X-Marathon-App-Id header. But for path based routing and auth, it uses the usual backend rules comparison.

API endpoints

Marathon-lb exposes a few endpoints on port 9090 (by default). They are:

Endpoint	Description
:9090/haproxy?stats	HAProxy stats endpoint. This produces an HTML page which can be viewed in your browser, providing various statistics about the current HAProxy instance.
:9090/haproxy?stats;csv	This is a CSV version of the stats above, which can be consumed by other tools. For example, it's used in the zdd.py script.
:9090/_haproxy_health_check	HAProxy health check endpoint. Returns 200 OK if HAProxy is healthy.
:9090/_haproxy_getconfig	Returns the HAProxy config file as it was when HAProxy was started. Implemented in getconfig.lua.
:9090/_haproxy_getvhostmap	Returns the HAProxy vhost to backend map. This endpoint returns HAProxy map file only when --haproxy-map flag is enabled, it returns a empty string otherwise. Implemented in getvhostmap.lua.
:9090/_haproxy_getpids	Returns the PIDs for all HAProxy instances within the current process namespace. This literally returns $(pidof haproxy). Implemented in getpids.lua. This is also used by the zdd.py script to determine if connections have finished draining during a deploy.

HAProxy configuration

App labels

App labels are specified in the Marathon app definition. These can be used to override HAProxy behaviour. For example, to specify the external group for an app with a virtual host named service.mesosphere.com:

{
  "id": "http-service",
  "labels": {
    "HAPROXY_GROUP":"external",
    "HAPROXY_0_VHOST":"service.mesosphere.com"
  }
}

Some labels are specified per service port. These are denoted with the {n} parameter in the label key, where {n} corresponds to the service port index, beginning at 0.

See the configuration doc for the full list of labels.

Templates

Marathon-lb searches for configuration files in the templates/ directory. The templates/ directory is located in a relative path from where the script is run. Some templates can also be overridden per app service port. You may add your own templates to the Docker image, or provide them at startup.

See the configuration doc for the full list of templates.

Overridable templates

Some templates may be overridden using app labels, as per the labels section. Strings are interpreted as literal HAProxy configuration parameters, with substitutions respected (as per the templates section). The HAProxy configuration will be validated for correctness before reloading HAProxy after changes. Note: Since the HAProxy config is checked before reloading, if an app's HAProxy labels aren't syntactically correct, HAProxy will not be reloaded and may result in stale config.

Here is an example for a service called http-service which requires that http-keep-alive be disabled:

{
  "id": "http-service",
  "labels":{
    "HAPROXY_GROUP":"external",
    "HAPROXY_0_BACKEND_HTTP_OPTIONS":"  option forwardfor\n  no option http-keep-alive\n  http-request set-header X-Forwarded-Port %[dst_port]\n  http-request add-header X-Forwarded-Proto https if { ssl_fc }\n"
  }
}

The full list of per service port templates which can be specified are documented here.

Operational Best Practices

• Use service ports within the reserved range (which is 10000 to 10100 by default). This will prevent port conflicts, and ensure reloads don't result in connection errors.
• Avoid using the HAPROXY_{n}_PORT label; prefer defining service ports.
• Consider running multiple marathon-lb instances. In practice, 2 or more should be used to provide high availability. Note: do not run marathon-lb on every node in your cluster. This is considered an anti-pattern due to the implications of hammering the Marathon API and excess health checking.
• Consider using a dedicated load balancer in front of marathon-lb to permit upgrades/changes. Common choices include an ELB (on AWS) or a hardware load balancer for on-premise installations.
• Use separate marathon-lb groups (specified with --group) for internal and external load balancing. On DC/OS, the default group is external. A simple options.json for an internal load balancer would be:

  {
    "marathon-lb": {
      "name": "marathon-lb-internal",
      "haproxy-group": "internal",
      "bind-http-https": false,
      "role": ""
    }
  }

• For HTTP services, consider setting VHost (and optionally a path) to access the service on ports 80 and 443. Alternatively, the service can be accessed on port 9091 using the X-Marathon-App-Id header. For example, to access an app with the ID tweeter:

$ curl -vH "X-Marathon-App-Id: /tweeter" marathon-lb.marathon.mesos:9091/
*   Trying 10.0.4.74...
* Connected to marathon-lb.marathon.mesos (10.0.4.74) port 9091 (#0)
> GET / HTTP/1.1
> Host: marathon-lb.marathon.mesos:9091
> User-Agent: curl/7.48.0
> Accept: */*
> X-Marathon-App-Id: /tweeter
>
< HTTP/1.1 200 OK

Zero-downtime deployments

Marathon-lb is able to perform canary style blue/green deployment with zero downtime. To execute such deployments, you must follow certain patterns when using Marathon.

The deployment method is described in this Marathon document. Marathon-lb provides an implementation of the aforementioned deployment method with the script zdd.py. To perform a zero downtime deploy using zdd.py, you must:

• Specify the HAPROXY_DEPLOYMENT_GROUP and HAPROXY_DEPLOYMENT_ALT_PORT labels in your app template
  • HAPROXY_DEPLOYMENT_GROUP: This label uniquely identifies a pair of apps belonging to a blue/green deployment, and will be used as the app name in the HAProxy configuration
  • HAPROXY_DEPLOYMENT_ALT_PORT: An alternate service port is required because Marathon requires service ports to be unique across all apps
• Only use 1 service port: multiple ports are not yet implemented
• Use the provided zdd.py script to orchestrate the deploy: the script will make API calls to Marathon, and use the HAProxy stats endpoint to gracefully terminate instances
• The marathon-lb container must be run in privileged mode (to execute iptables commands) due to the issues outlined in the excellent blog post by the Yelp engineering team found here
• If you have long-lived TCP connections using the same HAProxy instances, it may cause the deploy to take longer than necessary. The script will wait up to 5 minutes (by default) for connections to drain from HAProxy between steps, but any long-lived TCP connections will cause old instances of HAProxy to stick around.

An example minimal configuration for a test instance of nginx is included here. You might execute a deployment from a CI tool like Jenkins with:

./zdd.py -j 1-nginx.json -m http://master.mesos:8080 -f -l http://marathon-lb.marathon.mesos:9090 --syslog-socket /dev/null

Zero downtime deployments are accomplished through the use of a Lua module, which reports the number of HAProxy processes which are currently running by hitting the stats endpoint at the /_haproxy_getpids. After a restart, there will be multiple HAProxy PIDs until all remaining connections have gracefully terminated. By waiting for all connections to complete, you may safely and deterministically drain tasks. A caveat of this, however, is that if you have any long-lived connections on the same LB, HAProxy will continue to run and serve those connections until they complete, thereby breaking this technique.

The ZDD script includes the ability to specify a pre-kill hook, which is executed before draining tasks are terminated. This allows you to run your own automated checks against the old and new app before the deploy continues.

Mesos with IP-per-task support

Marathon-lb supports load balancing for applications that use the Mesos IP-per-task feature, whereby each task is assigned unique, accessible, IP addresses. For these tasks services are directly accessible via the configured discovery ports and there is no host port mapping. Note, that due to limitations with Marathon (see mesosphere/marathon#3636) configured service ports are not exposed to marathon-lb for IP-per-task apps.

For these apps, if the service ports are missing from the Marathon app data, marathon-lb will automatically assign port values from a configurable range. The range is configured using the --min-serv-port-ip-per-task and --max-serv-port-ip-per-task options. While port assignment is deterministic, the assignment is not guaranteed if you change the current set of deployed apps. In other words, when you deploy a new app, the port assignments may change.

Contributing

PRs are welcome, but here are a few general guidelines:

• Avoid making changes which may break existing behaviour
• Document new features
• Update/include tests for new functionality
• Use the pre-commit hook to automatically generate docs:

  bash /path/to/marathon-lb/scripts/install-git-hooks.sh