concurrency

Concurrency

ODENOS partly follows Actor Model for concurrency.

However, ODENOS guarantees message sequence (FIFO) for messages destined for a same RemoteObject instance, and supports synchronous messaging:

• Request/Response is synchronous
• Event is asynchronous
• (Synchronous Event is TBD)

Synchronous/asynchronous messaging

RemoteObject a     RemoteObject b  RemoteObject c      RemoteObject d
    |                        |       |                        |
    |<----- EVENT -------------------|                        |
    |                        |       |----- EVENT ----------->|
    |------ REQUEST -------->|
  Timer(30 sec)              |
    |<----- RESPONSE --------|
    |                        |

Note: RESPONSE received after timeout (30sec) is dropped.

ODENOS assigns virtually a single thread to each actor (an instance of ODENOS RemoteObject class), and ODENOS's MessagaDispatcher put a mail in a mailbox for each actor.

Message sequence guarantee

              FIFO incoming queue (mailbox)   FIFO incoming queue (per JVM process)
                            ---------          ---------
[RemoteObject instance] <-- o o o o o <------- o o o o o <------- messaging bus 
            |               ---------          ---------
            |                             [MessageDispatcher]
            |                                 FIFO outgoing queue (per JVM process)
            |                                  ---------
            +--------------------------------> o o o o o --------> messaging bus 
                                               ---------

Messaging within a same JVM process does not use the messaging bus (i.e., Redis) at all. Request breaks into the FIFO incoming queue and Request/Response is performed using the sender's thread. On the other hand, EVENT is sent to a loopback interface and put in the mailbox.

              FIFO incoming queue (mailbox)   FIFO incoming queue (per JVM process)
                            ---------          ---------
[RemoteObject instance] <-- o o o o o <------- o o o o o <------- messaging bus 
            |            ^  ---------          ---------       ^
            |            |                [MessageDispatcher]  |
            |            |                    FIFO outgoing queue (per JVM process)
            |            |                     ---------       |
            +------------|-------------------> o o o o o ------|-> messaging bus         
                         |                     ---------       |
                  Request/Response                             |
                         |                                     |
[RemoteObject instance]<-+                                     |
[RemoteObject instance]--- Event ------------------------------+

Although Actor Model is thought of as a very old technique for concurrency and linear scalability (e.g., a similar technique was used by central office switches (Telephony) some ten years ago), relatively new languages such as Scala adopted the model.

Since ODENOS supports multi-languages (Java, Python, Ruby...), is uses Redis as a messaging bus and ODENOS-original Actor Model implementation.