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Synapse is Airbnb's new system for service discovery. Synapse solves the problem of automated fail-over in the cloud, where failover via network re-configuration is impossible. The end result is the ability to connect internal services together in a scalable, fault-tolerant way.


Synapse emerged from the need to maintain high-availability applications in the cloud. Traditional high-availability techniques, which involve using a CRM like pacemaker, do not work in environments where the end-user has no control over the networking. In an environment like Amazon's EC2, all of the available workarounds are suboptimal:

  • Round-robin DNS: Slow to converge, and doesn't work when applications cache DNS lookups (which is frequent)
  • Elastic IPs: slow to converge, limited in number, public-facing-only, which makes them less useful for internal services
  • ELB: Again, public-facing only, and only useful for HTTP

One solution to this problem is a discovery service, like Apache Zookeeper. However, Zookeeper and similar services have their own problems:

  • Service discovery is embedded in all of your apps; often, integration is not simple
  • The discovery layer itself is subject to failure
  • Requires additional servers/instances

Synapse solves these difficulties in a simple and fault-tolerant way.

How Synapse Works

Synapse runs on your application servers; here at Airbnb, we just run it on every box we deploy. The heart of synapse is actually HAProxy, a stable and proven routing component. For every external service that your application talks to, we assign a synapse local port on localhost. Synapse creates a proxy from the local port to the service, and you reconfigure your application to talk to the proxy.

Synapse comes with a number of watchers, which are responsible for service discovery. The synapse watchers take care of re-configuring the proxy so that it always points at available servers. We've included a number of default watchers, including ones that query zookeeper and ones using the AWS API. It is easy to write your own watchers for your use case, and we encourage submitting them back to the project.

Example Migration

Let's suppose your rails application depends on a Postgres database instance. The database.yaml file has the DB host and port hardcoded:

  database: mydb
  port: 5432

You would like to be able to fail over to a different database in case the original dies. Let's suppose your instance is running in AWS and you're using the tag 'proddb' set to 'true' to indicate the prod DB. You set up synapse to proxy the DB connection on localhost:3219 in the synapse.conf.yaml file. Add a hash under services that looks like this:

     name: "default-db"
     host: ""
     port: 5432
    method: "awstag"
    tag_name: "proddb"
    tag_value: "true"
    port: 3219
    server_options: "check inter 2000 rise 3 fall 2"
    frontend: mode tcp
    backend: mode tcp

And then change your database.yaml file to look like this:

  database: mydb
  host: localhost
  port: 3219

Start up synapse. It will configure HAProxy with a proxy from localhost:3219 to your DB. It will attempt to find the DB using the AWS API; if that does not work, it will default to the DB given in default_servers. In the worst case, if AWS API is down and you need to change which DB your application talks to, simply edit the synapse.conf.json file, update the default_servers and restart synapse. HAProxy will be transparently reloaded, and your application will keep running without a hiccup.


Add this line to your application's Gemfile:

gem 'synapse'

And then execute:

$ bundle

Or install it yourself as:

$ gem install synapse

Don't forget to install HAProxy prior to installing Synapse.


Synapse depends on a single config file in JSON format; it's usually called synapse.conf.json. The file has two main sections. The first is the services section, which lists the services you'd like to connect. The second is the haproxy section, which specifies how to configure and interact with HAProxy.

Configuring a Service

The services are a hash, where the keys are the name of the service to be configured. The name is just a human-readable string; it will be used in logs and notifications. Each value in the services hash is also a hash, and should contain the following keys:

  • discovery: how synapse will discover hosts providing this service (see below)
  • default_servers: the list of default servers providing this service; synapse uses these if no others can be discovered
  • haproxy: how will the haproxy section for this service be configured

Service Discovery

We've included a number of watchers which provide service discovery. Put these into the discovery section of the service hash, with these options:


The stub watcher is useful in situations where you only want to use the servers in the default_servers list. It has only one option:

  • method: stub

This watcher retrieves a list of servers from zookeeper. It takes the following options:

  • method: zookeeper
  • path: the zookeeper path where ephemeral nodes will be created for each available service server
  • hosts: the list of zookeeper servers to query

The watcher assumes that each node under path represents a service server. Synapse attempts to decode the data in each of these nodes using JSON and also using Thrift under the standard Twitter service encoding. We assume that the data contains a hostname and a port for service servers.


This watcher retrieves a list of docker containers via docker's HTTP API. It takes the following options:

  • method: docker
  • servers: a list of servers running docker as a daemon. Format is {"name":"...", "host": "..."[, port: 4243]}
  • image_name: find containers running this image
  • container_port: find containers forwarding this port
  • check_interval: how often to poll the docker API on each server. Default is 15s.
AWS EC2 tags

This watcher retrieves a list of Amazon EC2 instances that have a tag with particular value using the AWS API. It takes the following options:

  • method: ec2tag
  • tag_name: the name of the tag to inspect. As per the AWS docs, this is case-sensitive.
  • tag_value: the value to match on. Case-sensitive.

Additionally, you MUST supply server_port_override in the haproxy section of the configuration as this watcher does not know which port the backend service is listening on.

The following options are optional, provided the well-known AWS_ environment variables shown are set. If supplied, these options will be used in preference to the AWS_ environment variables.

  • aws_access_key_id: AWS key or set AWS_ACCESS_KEY_ID in the environment.
  • aws_secret_access_key: AWS secret key or set AWS_SECRET_ACCESS_KEY in the environment.
  • aws_region: AWS region (i.e. us-east-1) or set AWS_REGION in the environment.

Listing Default Servers

You may list a number of default servers providing a service. Each hash in that section has the following options:

  • name: a human-readable name for the default server; must be unique
  • host: the host or IP address of the server
  • port: the port where the service runs on the host

The default_servers list is used only when service discovery returns no servers. In that case, the service proxy will be created with the servers listed here. If you do not list any default servers, no proxy will be created. The default_servers will also be used in addition to discovered servers if the keep_default_servers option is set.

The haproxy Section

This section is its own hash, which should contain the following keys:

  • port: the port (on localhost) where HAProxy will listen for connections to the service. If this is omitted, only a backend stanza (and no frontend stanza) will be generated for this service; you'll need to get traffic to your service yourself via the shared_frontend or manual frontends in extra_sections
  • server_port_override: the port that discovered servers listen on; you should specify this if your discovery mechanism only discovers names or addresses (like the DNS watcher). If the discovery method discovers a port along with hostnames (like the zookeeper watcher) this option may be left out, but will be used in preference if given.
  • server_options: the haproxy options for each server line of the service in HAProxy config; it may be left out.
  • frontend: additional lines passed to the HAProxy config in the frontend stanza of this service
  • backend: additional lines passed to the HAProxy config in the backend stanza of this service
  • listen: these lines will be parsed and placed in the correct frontend/backend section as applicable; you can put lines which are the same for the frontend and backend here.
  • shared_frontend: optional: haproxy configuration directives for a shared http frontend (see below)

Configuring HAProxy

The haproxy section of the config file has the following options:

  • reload_command: the command Synapse will run to reload HAProxy
  • config_file_path: where Synapse will write the HAProxy config file
  • do_writes: whether or not the config file will be written (default to true)
  • do_reloads: whether or not Synapse will reload HAProxy (default to true)
  • global: options listed here will be written into the global section of the HAProxy config
  • defaults: options listed here will be written into the defaults section of the HAProxy config
  • extra_sections: additional, manually-configured frontend, backend, or listen stanzas
  • bind_address: force HAProxy to listen on this address (default is localhost)
  • shared_fronted: (OPTIONAL) additional lines passed to the HAProxy config used to configure a shared HTTP frontend (see below)

Note that a non-default bind_address can be dangerous. If you configure an address:port combination that is already in use on the system, haproxy will fail to start.

HAProxy shared HTTP Frontend

For HTTP-only services, it is not always necessary or desirable to dedicate a TCP port per service, since HAProxy can route traffic based on host headers. To support this, the optional shared_fronted section can be added to both the haproxy section and each indvidual service definition. Synapse will concatenate them all into a single frontend section in the generated haproxy.cfg file. Note that synapse does not assemble the routing ACLs for you; you have to do that yourself based on your needs. This is probably most useful in combination with the service_conf_dir directive in a case where the individual service config files are being distributed by a configuration manager such as puppet or chef, or bundled into service packages. For example:

  shared_frontend: "bind"
  reload_command: "service haproxy reload"
  config_file_path: "/etc/haproxy/haproxy.cfg"
  socket_file_path: "/var/run/haproxy.sock"
   - "daemon"
   - "user    haproxy"
   - "group   haproxy"
   - "maxconn 4096"
   - "log local2 notice"
   - "stats   socket /var/run/haproxy.sock"
   - "log      global"
   - "balance  roundrobin"
    method: "zookeeper"
    path:  "/nerve/services/service1"
    hosts: ""
    server_options: "check inter 2s rise 3 fall 2"
     - "acl is_service1 hdr_dom(host) -i"
     - "use_backend service1 if is_service1"
    backend: "mode http"

    method: "zookeeper"
    path:  "/nerve/services/service2"
    hosts: ""

    server_options: "check inter 2s rise 3 fall 2"
     - "acl is_service1 hdr_dom(host) -i"
     - "use_backend service2 if is_service2
    backend: "mode http"

This would produce an haproxy.cfg much like the following:

backend service1
        mode http
        server check inter 2s rise 3 fall 2

backend service2
        mode http
        server check inter 2s rise 3 fall 2

frontend shared-frontend
        acl is_service1 hdr_dom(host) -i
        use_backend service1 if is_service1
        acl is_service2 hdr_dom(host) -i
        use_backend service2 if is_service2

Non-HTTP backends such as MySQL or RabbitMQ will obviously continue to need their own dedicated ports.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

Creating a Service Watcher

If you'd like to create a new service watcher:

  1. Create a file for your watcher in service_watcher dir
  2. Use the following template:
require 'synapse/service_watcher/base'

module Synapse
  class NewWatcher < BaseWatcher
    def start
      # write code which begins running service discovery

    def validate_discovery_opts
      # here, validate any required options in @discovery
  1. Implement the start and validate_discovery_opts methods
  2. Implement whatever additional methods your discovery requires

When your watcher detects a list of new backends, they should be written to @backends. You should then call @synapse.configure to force synapse to update the HAProxy config.


A transparent service discovery framework for connecting an SOA







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