Campbell Scientific Java PakBus Software Development Kit - Manual (Page 13)

Java PakBus® Software Development Kit
In this version, the class declaration was changed so that class Example was
made to implement the GetTableDefsClient interface and a new
on_complete() was added. We also added the line that adds a new instance of
GetTableDefsTran. When the newly added transaction has completed its
work, the client’s on_complete() notification will be invoked. There is a
common pattern here used for all transactions:
1. The application adds a new transaction object to a station. The
constructor for most transaction objects is going to require an object
that implements a transaction-specific interface.
2. When the transaction is added, the Datalogger object will generate a
unique transaction number that will identify all messages sent by the
transaction. It will also send a request to the network for the
transaction object to have “focus”. This “focus” mechanism is used to
serialize transaction access to the network and prevents the application
from flooding the PakBus® network with simultaneous requests.
3. Once the transaction has gained focus, it will usually send a command
message and then wait for the response associated with that command.
If too much time elapses (determined by the round trip time assigned
to the station) without receiving the response, the transaction will
automatically retry the command message up to three times before
reporting the transaction as a failure.
4. The transaction will post the message to the Datalogger object which
will in turn post the message to the network after assigning the
destination neighbour and PakBus® addresses. The network will then
send the message using the output stream.
5. The datalogger (that is, the physical device) will process the message
and send a response.
6. The response is received by the network and is sent to the appropriate
Datalogger object based upon the source PakBus® address.
7. The device will route the response to the appropriate transaction based
upon the response message's transaction number.
8. Depending upon the transaction type, the transaction may or may not
be over when the first response is received. It may need to send
further commands before the entire operation can be considered
complete. The above steps will be repeated until the transaction is
complete. At that point, the transaction will send a notification signal
to the application by calling the client’s on_complete() method and
removing itself from the Datalogger object’s list of active
transactions.
The next sections will discuss specific types of transactions supported by the
API.
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