One of my rake tasks failed with an interesting error this morning.

1. uncaught exception: error: { "$err" : "not master and slaveok=false", "code" : 13435 }

Qu’est-ce que c’est?

The problem is that I am connecting to a slave in a replica set and trying to execute a write operation that must happen on a master node. That’s because I was lazy and was executing command-line update queries, such as this one. I was being lazy. Shame on me.

1. system "mongo #{db_host}:#{db_port}/#{db_name} -u #{db_user} -p#{db_password} --eval 'db.widgets.drop()'"

This is run in a Rake task. Lets replace this with some Ruby code, the way it’s ought to be.

1. db = Mongo::Connection.new(db_host, db_port).db(db_name)
2. db.authenticate(db_user, db_password) unless (db.user.nil? || db.user.blank?)      
3. db.collection("widgets").drop()

It’s actually a lot cleaner, I am not sure why I was hung up on the command line thing. Unfortunately it doesn’t fix our problem. In a replica set we need to use a ReplSetConnection that will automatically load-balance requests and send writes to the master. It takes a list of hosts, something like

1. db_connection = Mongo::ReplSetConnection.new(db_host_list).db(db_name)
2. db_connection.authenticate(db_user, db_password)
3. db_connection("widgets").drop()

Lets try to write something usable in all of our rake tasks. What I have is a YML file with the Heroku MongoHQ configuration. The first environment is a replica set, while the second is a single MongoDB.

1. production:
2.   config:
3.     MONGOHQ_URL:          "mongodb://heroku:password@replica.mongohq.com:12345/production-name"
4.     MONGOHQ_DATABASE:     "db-name"
5.     MONGOHQ_HOST_LIST:    "[['node0.replica.mongohq.com', 12345], ['node1.replica.mongohq.com', 12345]]"
6.     MONGOHQ_PASSWD:       "password"
7.     MONGOHQ_USER:         "heroku"
8.  
9. staging:
10.   config: &default
11.     MONGOHQ_URL:          "mongodb://heroku:password@small.mongohq.com:12345/staging-name"

We can write a basic connect  method that picks up the right configuration, as a Rake task.

1. namespace :mongohq do
2.     
3.     def heroku_config(env = Rails.env)
4.       @@config ||= YAML.load_file(Rails.root.join("config/heroku.yml")).symbolize_keys
5.       config_env = @@config[env.to_sym]
6.       raise "missing '#{env}' section in config/heroku.yml" if config_env.nil?
7.       config_env["config"]
8.     end
9.     
10.     def parse_mongohq_url(url)
11.       uri = URI.parse(url)
12.       [ uri, uri.path.gsub("/", "") ]
13.     end
14.     
15.     # connect to a MongoDB
16.     def mongohq_connect(env = Rails.env)
17.       config = heroku_config(env)
18.       if ! config["MONGOHQ_HOST_LIST"].blank?
19.         mongohq_host_list = eval(config["MONGOHQ_HOST_LIST"])
20.         puts "[#{Time.now}] connecting to #{config["MONGOHQ_DATABASE"]} on #{eval(config["MONGOHQ_HOST_LIST"])}"
21.         db_connection = Mongo::ReplSetConnection.new(* mongohq_host_list).db(config["MONGOHQ_DATABASE"])
22.         db_connection.authenticate(config["MONGOHQ_USER"], config["MONGOHQ_PASSWD"])
23.         db_connection
24.       elsif ! config["MONGOHQ_URL"].blank?
25.         puts "[#{Time.now}] connecting to #{config["MONGOHQ_URL"]}"
26.         db, db_name = parse_mongohq_url(config["MONGOHQ_URL"])
27.         db_connection = Mongo::Connection.new(db.host, db.port).db(db_name)
28.         db_connection.authenticate(db.user, db.password) unless (db.user.nil? || db.user.blank?)
29.         db_connection
30.       else
31.         raise "missing MONGOHQ_URL or MONGOHQ_HOST_LIST for #{env} environment"
32.       end
33.     end
34.     
35. end

Now, our rake tasks can call mongohq_connect(:production) or mongohq_connect(:staging) without having to worry about the kind of setup we have.