so-5.4.0 New type of mbox - multi-producer and single-consumer

Until v.5.4.0 there was only one type of mbox in SObjectizer: multi-producer/multi-consumer (MPMC). Mboxes of these types are created by create_local_mbox() and create_local_mbox(name) methods of class so_environment_t.

MPMC mbox is like a bulletin board: everyone can pin a message to it and everyone can read a pinned message. MPMC mboxes are used to implement publish-subscribe model in SObjectizer. A message type is used as a topic.

MPMC mboxes are good when 1-to-many or many-to-many agents interaction is required. But they are too expensive for the case of 1-to-1 interaction. It is because message passing via MPMC mbox consists of two phases:

• on the first phase there is a lookup for subscriber list for the specified message type;
• on the second phase there is a check for possibility of handling this message type in the current subscriber’s state.

For the 1-to-1 agents interaction the first phase is redundant. The 1-to-1 interaction will be more efficient if a message is directly passed to the subscriber’s queue without looking up the subscriber list for the message type.

This is a goal for a new type of mboxes in SObjectizer 5.4.0 -- direct and efficient support for 1-to-1 agents interaction.

The new type of mbox is a multi-producer/single-consumer (MPSC) message box. These mboxes are automatically created by SObjectizer for every agent. When a constructor of agent_t finishes its work there is an MPSC mbox for the agent. That mbox can be accessed via agent_t::so_direct_mbox() method. That mbox belongs to the agent for which mbox is created. That agent is the owner of the MPSC mbox.

New MPSC mbox implements mbox_t interface just as existing MPMC mbox. References to the MPSC mboxes can be stored by mbox_ref_t objects. References to MPSC mboxes in form of mbox_ref_t can be passed to other agents or can be stored anywhere user wants.

The main difference between MPSC and MPMC mboxes is the inability to subscribe to the messages from the mbox except for the owner of mbox.

There is one significant trait of MPSC mbox in comparison with MPMC mbox: because there is no the first phase (lookup for a list of subscribers by the message type) it is possible to push a message to agent’s queue even if the agent is not subscribed to it. This message will be rejected on the second phase when the checking for ability of message processing is performed.

After introduction of MPSC mboxes there is a question for a SObjectizer’s user: which type of mboxes should be used and to what end?

Old MPMC mboxes must be used when an agent is a producer of some information which could be processed by several different agents. Or, when there are several producers for information and several consumers for that information.

New MPSC mboxes must be used when agent is a receiver of some agent-specific information and/or agent-specific commands. For example, the message about the need to reconfigure an agent is agent-specific and it is better to send it to agent’s MPSC mbox than to common MPMC mbox shared between several agents.

Another case for MPSC mboxes is a channel creation for information processing. For example, agent-meter receives data about air temperature and sends it directly to agent-controller, which collects data from several meters and then sends a data package to agent-analyzer, which performs the initial information checking and normalizing. And then the agent-analyzer sends preprocessed data directly to agent-processor. There could be several groups of meter agents, controller agents and so on. However, because they use different mboxes for information exchange they will work independently without any influence from one to another.