Connection Diagrams of The IPython ZMQ Cluster#
This is a quick summary and illustration of the connections involved in the ZeroMQ based IPython cluster for parallel computing.
The IPython cluster consists of a Controller, and one or more each of clients and engines. The goal of the Controller is to manage and monitor the connections and communications between the clients and the engines. The Controller is no longer a single process entity, but rather a collection of processes - specifically one Hub, and 4 (or more) Schedulers.
It is important for security/practicality reasons that all connections be inbound to the controller processes. The arrows in the figures indicate the direction of the connection.
The Controller consists of 1-5 processes. Central to the cluster is the Hub, which monitors engine state, execution traffic, and handles registration and notification. The Hub includes a Heartbeat Monitor for keeping track of engines that are alive. Outside the Hub are 4 Schedulers. These devices are very small pure-C MonitoredQueue processes (or optionally threads) that relay messages very fast, but also send a copy of each message along a side socket to the Hub. The MUX queue and Control queue are MonitoredQueue ØMQ devices which relay explicitly addressed messages from clients to engines, and their replies back up. The Balanced queue performs load-balancing destination-agnostic scheduling. It may be a MonitoredQueue device, but may also be a Python Scheduler that behaves externally in an identical fashion to MQ devices, but with additional internal logic. stdout/err are also propagated from the Engines to the clients via a PUB/SUB MonitoredQueue.
Once a controller is launched, the only information needed for connecting clients and/or
engines is the IP/port of the Hub’s
ROUTER socket called the Registrar. This socket
handles connections from both clients and engines, and replies with the remaining
information necessary to establish the remaining connections. Clients use this same socket for
querying the Hub for state information.
The heartbeat process has been described elsewhere. To summarize: the Heartbeat Monitor
publishes a distinct message periodically via a
PUB socket. Each engine has a
zmq.FORWARDER device with a
SUB socket for input, and
DEALER socket for output.
SUB socket is connected to the
PUB socket labeled ping, and the
connected to the
ROUTER labeled pong. This results in the same message being relayed
back to the Heartbeat Monitor with the addition of the
DEALER prefix. The Heartbeat
Monitor receives all the replies via an
ROUTER socket, and identifies which hearts are
still beating by the
zmq.IDENTITY prefix of the
DEALER sockets, which information
the Hub uses to notify clients of any changes in the available engines.
The controller has at least three Schedulers. These devices are primarily for
relaying messages between clients and engines, but the Hub needs to see those
messages for its own purposes. Since no Python code may exist between the two sockets in a
queue, all messages sent through these queues (both directions) are also sent via a
PUB socket to a monitor, which allows the Hub to monitor queue traffic without
interfering with it.
For tasks, the engine need not be specified. Messages sent to the
ROUTER socket from the
client side are assigned to an engine via ZMQ’s
DEALER round-robin load balancing.
Engine replies are directed to specific clients via the IDENTITY of the client, which is
received as a prefix at the Engine.
ROUTER is used for both in and output sockets in the device. Clients must
specify the destination by the
zmq.IDENTITY of the
ROUTER socket connected to
the downstream end of the device.
At the Kernel level, both of these
ROUTER sockets are treated in the same way as the
socket in the serial version (except using ZMQStreams instead of explicit sockets).
Execution can be done in a load-balanced (engine-agnostic) or multiplexed (engine-specified)
manner. The sockets on the Client and Engine are the same for these two actions, but the
scheduler used determines the actual behavior. This routing is done via the
the upstream sockets in each MonitoredQueue.
On the kernels, stdout/stderr are captured and published via a
PUB socket. These
sockets all connect to a
SUB socket input of a MonitoredQueue, which subscribes to all
messages. They are then republished via another
PUB socket, which can be
subscribed by the clients.
The hub’s registrar
ROUTER socket also listens for queries from clients as to queue status,
and control instructions. Clients connect to this socket via an
DEALER during registration.
The Hub publishes all registration/unregistration events via a
PUB socket. This
allows clients to stay up to date with what engines are available by subscribing to the
feed with a
SUB socket. Other processes could selectively subscribe to just
registration or unregistration events.