How to start Xandra as child process

Phoenix Forum Questions / Help
,
1

I am trying to use Xandra for my Phoenix project for which I am connecting to Cassandra like

{:ok, conn} = Xandra.start_link([
  nodes: ["localhost:9042"]
])

Every time I am suppose to run a query I need to start_link, Is there a way I can provide all connection details in config file and it runs once the app is started?

Thanks

2

Anyone with help is appreciated.

3

Not in the config file, but in (one of) the supervisor(s) of your application. See here.

However, how one obtains a conn datatype to pass to all of the other functions when using Xandra (or another DBConnection-implementation for that matter) inside a supervision tree is something I cannot immediately figure out.

Maybe library author @whatyouhide can shed some light on this?

EDIT: It’s probably {Xandra, name: MyXandraConnection}, as now the connection GenServer is registered node-wide with that particular name. Then you can use MyXandraConnection everywhere you passed a conn PID before.

4

Thanks @Qqwy for the details analysis but I am relatively new to this stuff.

I am not sure where the connection details would go where it will be started the first time.

5

Probably something like {Xandra, name: MyXandraConnection, other: option, here: probably}

6

You can can read the configuration from the application environment manually:

# in config/config.exs (or in an environment-specific config file)
config :my_app, :my_xandra, options

and then:

# in my_app/application.ex
xandra_config = Application.fetch_env!(:my_app, :my_xandra)

children = [
  # ...,
  {Xandra, xandra_config}
]
1 Like
7

Thank you @whatyouhide for the solution but I am still getting few errors with this approach.

dev.exs

config :my_app, :xandra,
  nodes: System.get_env("CASSANDRA_HOST"),
  authentication: {Xandra.Authenticator.Password, 
        [
          username: System.get_env("CASSANDRA_USER"),
          password: System.get_env("CASSANDRA_PASSWORD")
        ]
  },
  pool: DBConnection.Poolboy,
  pool_size: 10

application.exs

children = [
      # Start the Ecto repository
      supervisor(MyApp.Repo, []),
      # Start the endpoint when the application starts
      supervisor(MyApp.Web.Endpoint, []),
      worker(Cachex, [:app_cache, []]),
      worker(MyApp.Scheduler, []),
      {Xandra, Application.fetch_env!(:my_app, :xandra)}
    ]

Stacktrace

(Mix) Could not start application my_app: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: Xandra
    ** (EXIT) an exception was raised:
        ** (ArgumentError) multi-node use requires Xandra.Cluster instead of Xandra
            (xandra) lib/xandra.ex:1027: Xandra.parse_nodes_option/1
            (xandra) lib/xandra.ex:1010: Xandra.convert_nodes_options_to_address_and_port/1
            (xandra) lib/xandra.ex:336: Xandra.start_link/1
            (stdlib) supervisor.erl:379: :supervisor.do_start_child_i/3
            (stdlib) supervisor.erl:365: :supervisor.do_start_child/2
            (stdlib) supervisor.erl:349: anonymous fn/3 in :supervisor.start_children/2
            (stdlib) supervisor.erl:1157: :supervisor.children_map/4
            (stdlib) supervisor.erl:315: :supervisor.init_children/2
            (stdlib) gen_server.erl:374: :gen_server.init_it/2
            (stdlib) gen_server.erl:342: :gen_server.init_it/6
            (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
49: :proc_lib.init_p_do_apply/3

Then I changed the info in application.exs to

children = [
      # Start the Ecto repository
      supervisor(MyApp.Repo, []),
      # Start the endpoint when the application starts
      supervisor(MyApp.Web.Endpoint, []),
      worker(Cachex, [:app_cache, []]),
      worker(MyApp.Scheduler, []),
      {Xandra.Cluster, Application.fetch_env!(:my_app, :xandra)}
    ]

I got new exception

Could not start application my_app: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: Xandra.Cluster
    ** (EXIT) an exception was raised:
        ** (Protocol.UndefinedError) protocol Enumerable not implemented for "localhost:9042". This protocol is implemented for: Ecto.Adapters.SQL.Stream, Postgrex.Stream, Xandra.PageStream, Xandra.Page, DBConnection.Stream, DBConnection.PrepareStream, Timex.Interval, HashSet, Range, Map, Function, List, Stream, Date.Range, HashDict, GenEvent.Stream, MapSet, File.Stream, IO.Stream
            (elixir) /Users/sahilpaudel/.kiex/builds/elixir-git/lib/elixir/lib/enum.ex:1: Enumerable.impl_for!/1
            (elixir) /Users/sahilpaudel/.kiex/builds/elixir-git/lib/elixir/lib/enum.ex:141: Enumerable.reduce/3
            (elixir) lib/enum.ex:3015: Enum.map/2
            (xandra) lib/xandra/cluster.ex:204: Xandra.Cluster.start_link/1
            (stdlib) supervisor.erl:379: :supervisor.do_start_child_i/3
            (stdlib) supervisor.erl:365: :supervisor.do_start_child/2
            (stdlib) supervisor.erl:349: anonymous fn/3 in :supervisor.start_children/2
            (stdlib) supervisor.erl:1157: :supervisor.children_map/4
            (stdlib) supervisor.erl:315: :supervisor.init_children/2
            (stdlib) gen_server.erl:374: :gen_server.init_it/2
            (stdlib) gen_server.erl:342: :gen_server.init_it/6
            (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

My env file

export CASSANDRA_HOST=localhost:9042
export CASSANDRA_USER=
export CASSANDRA_PASSWORD=

Thanks.

8

The nodes option requires a list of strings, but you are reading a single string from the system environment. Use nodes: [System.get_env("CASSANDRA_HOST")].

1 Like
9

Great, it worked.

Final question how can I execute a query now?

Thanks.

10

You need to pass the name option to give a name to the Xadnra pool (such as name: MyXandra) and then you can use Xandra.execute or similar functions to execute queries. Refer to the docs for more information :slight_smile:

1 Like