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Routing

When connecting to a Neo4j cluster, Bolt.Sips will create 3 distinct connection pools, each of them dedicated to one of the following connection types (connection roles):

  • :route - used for getting information from the Neo4j router, such as: routing details about which server is handling what type of role: read/write, and more.
  • :read - used for read-only connections
  • :write - used for write-only connections.

Having the Bolt.Sips configured in routing mode, will enforce your code to clarify what type of connections you want, type you must specify when requesting a Bolt.Sips connection. Example:

rconn = Bolt.Sips.conn(:read)
wconn = Bolt.Sips.conn(:write)
router_conn = Bolt.Sips.conn(:route)

Without being explicit about the connection type, you will receive errors, in case you'll attempt to execute a query that will say: create new nodes, on a server having the role: read or route. This is the only rule you must observe, when using the Bolt.Sips driver with a causal cluster.

Routing walk-through

Let's walk-through a simple experiment with using Bolt.Sips in routing mode and a Neo4j cluster.

If you don't have a local server, or a remote Neo4j cluster available for your tests, you can easily setup your own local playground. All you need is Docker.

We'll use a docker-compose.yml file that you can find in the Bolt.Sips main source repo.

If you have:

Start the Neo4j cluster

In a folder where you have the docker-compose.yml file, start a new shell session and run the following command:

docker-compose up

If this is the first time you run this command, or use Neo4j as a Docker image, then based on the quality of your Internet connection, you'll wait a few seconds while Docker downloads a Neo4j Enterprise image. You'll see something like this:

Creating network "neo4j_lan" with the default driver
Pulling core1 (neo4j:3.5.3-enterprise)...
3.5.3-enterprise: Pulling from library/neo4j
e7c96db7181b: Pull complete
f910a506b6cb: Pull complete
b6abafe80f63: Pull complete
b95a7fd32595: Pull complete
6c09128ad074: Pull complete
648805e5f471: Pull complete
e2790f69a70d: Pull complete
Creating core2 ... done
Creating core3 ... done
Creating core1 ... done
Attaching to core3, core1, core2
core3    | Changed password for user 'neo4j'.
core1    | Changed password for user 'neo4j'.
core2    | Changed password for user 'neo4j'.
core3    | Active database: graph.db
core3    | Directories in use:
core3    |   home:         /var/lib/neo4j
core3    |   config:       /var/lib/neo4j/conf
...

and towards the end of the starting sequence, this:

core2    | 2019-06-17 12:37:59.078+0000 INFO  Remote interface available at http://localhost:7475/
core3    | 2019-06-17 12:37:59.165+0000 INFO  Remote interface available at http://localhost:7476/

Check to see if you can connect to your local Neo4j cluster, as simple as pointing your Internet browser to this url: http://localhost:7474, and if everything was executed successfully, you'll be seeing the familiar Neo4j web interface.

Now let's play with the Bolt.Sipsdriver and our local Neo4j cluster.

Change your elixir test project configuration and modify the config/config.exs file like this (excerpt):

use Mix.Config

config :bolt_sips, Bolt,
  # bolt+routing will be deprecated?!
  # url: "bolt+routing://localhost:7687",
  url: "neo4j://localhost:7687",
  basic_auth: [username: "neo4j", password: "test"],
  pool_size: 10

then start a IEx shell session, from the projects'r main folder: iex -S mix. While inside the IEx session, let's see if our configuration is sound?

iex» Bolt.Sips.info()
%{
  default: %{
    connections: %{
      read: %{"localhost:7688" => 0, "localhost:7689" => 0},
      route: %{
        "localhost:7687" => 0,
        "localhost:7688" => 0,
        "localhost:7689" => 0
      },
      write: %{"localhost:7687" => 0},
      routing_query: %{...},
      ttl: 300,
      updated_at: 1560775628
    },
    user_options: [
      url: "neo4j://localhost:7687",
      pool_size: 10,
      ....
    ]
  }
}

if you see the response above, it means your settings are ready. Without going into much details about the data structure above, the routing details are these:

  read: %{"localhost:7688" => 0, "localhost:7689" => 0},
  write: %{"localhost:7687" => 0},
  route: %{
    "localhost:7687" => 0,
    "localhost:7688" => 0,
    "localhost:7689" => 0
  ttl: 300,
  updated_at: ...

According to the routing information returned by our cluster, we have:

  • two nodes accepting :read commands: localhost:7688 andlocalhost:7689
  • three nodes capabale of responding with routing specific details: localhost:7687 localhost:7688 and localhost:7689
  • one node accepting :write commands; the localhost:7687, respectively.

But don't worry about the gory details, we got you covered :)

Let's run some Cypher queries.

iex» alias Bolt.Sips.Response
iex» alias Bolt.Sips, as: Neo

# obtaining a read(only) connection:
iex» rconn = Neo.conn(:read)
#PID<0.324.0>

# checking if there are any Person nodes "named": Bob?
iex» %Response{results: r} = Neo.query!(rconn, "MATCH (p:Person{name: 'Bob'}) RETURN p")
%Bolt.Sips.Response{
  bookmark: "neo4j:bookmark:v1:tx2",
  fields: ["p"],
  notifications: [],
  plan: nil,
  profile: nil,
  records: [],
  results: [],
  stats: [],
  type: "r"
}

# r is [], meaning: our query found none. So let's create one.
# First we obtain a connection suitable for `write` operations:
iex» wconn = Neo.conn(:write)
#PID<0.384.0>

# and now we can use it for creating a new node:

iex» %Response{results: r} = Neo.query!(wconn, "CREATE (p:Person{name:'Bob'})")
%Bolt.Sips.Response{
  ...
  stats: %{"labels-added" => 1, "nodes-created" => 1, "properties-set" => 1},
  type: "w"
}

# our node was created and has one property set,  w⦿‿⦿t!
# but can we find it? Rerun the previous query using the `read` connection:

iex» Neo.query!(rconn, "MATCH (p:Person{name: 'Bob'}) RETURN p") |> Response.first()
%{
  "p" => %Bolt.Sips.Types.Node{
    id: 20,
    labels: ["Person"],
    properties: %{"name" => "Bob"}
  }
}

# and yessss, our new Person node is in the cluster!
# Do you need its json form, instead? Easy:
iex» Neo.query!(rconn, "MATCH (p:Person{name: 'Bob'}) RETURN p") |>
...» Response.first() |>
...» Bolt.Sips.ResponseEncoder.encode!(:json)
"{\"p\":{\"id\":20,\"labels\":[\"Person\"],\"properties\":{\"name\":\"Bob\"}}}"

But what happens if we try to create a new Person, using our read connection?

iex» Neo.query!(rconn, "CREATE (p:Person{name:'Alice'})")
** (Bolt.Sips.Exception) ... No write operations are allowed directly on this database. Writes must pass through the leader. The role of this server is: FOLLOWER

Neo4j will promptly let us know we can't use that connection for write operations. This is the main difference that you must consider when coding.

Same command executed on the proper (write) connection, will be successful:

iex» Neo.query!(wconn, "CREATE (p:Person{name:'Alice'})")
%Bolt.Sips.Response{
  ...
  stats: %{"labels-added" => 1, "nodes-created" => 1, "properties-set" => 1},
  type: "w"
}