Synapse

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.

Motivation

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 it 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

Lets suppose your rails application depends on a Postgre database instance. The database.yaml file has the DB host and port hardcoded:

production:
  database: mydb
  host: mydb.example.com
  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.json file. Add a hash under services that looks like this:

{"services":
    {
      "name": "proddb",
      "local_port": 3219,
      "server_options": "check inter 2000 rise 3 fall 2",
      "default_servers": [
        {
          "name": "default-db",
          "host": "mydb.example.com",
          "port": 5432
        }
      ],
      "discovery": {
        "method": "awstag",
        "tag": "proddb",
        "value": "true"
      },
      "listen": [
      ]
    },
...

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

production:
  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.

Installation

Add this line to your application's Gemfile:

gem 'synapse'

And then execute:

$ bundle

Or install it yourself as:

$ gem install synapse

Configuration

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 none 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:

Stub

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

• method: stub

Zookeeper

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.

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 haproxy Section

This section is it's own hash, which should contain the following keys:

• port: the port (on localhost) where HAProxy will listen for connections to the service.
• 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.
• listen: additional lines passed to the HAProxy config in the listen stanza of this service

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

Contributing

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_relative "./base"
module Synapse
  class NewWatcher < BaseWatcher
    def start
      # write code which begins running service discovery
    end

    private
    def validate_discovery_opts
      # here, validate any required options in @discovery
    end
  end
end

3. Implement the start and validate_discovery_opts methods
4. 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.