Subscription
This section explains how to subscribe to streaming data and the associated operations through DolphinDB C++ API.
Subscription Option
The DolphinDB C++ API provides three classes to subscribe to streaming data:
ThreadedClient, ThreadPooledClient, and
PollingClient. To quickly find out the appropriate mode for
your scenario, refer to the table provided below.
| Class | Scenario |
|---|---|
| ThreadedClient | produces a single thread that calls for a user-defined handler on incoming message(s) |
| ThreadPooledClient | produces multiple threads that poll and call a user-defined handler simultaneously on each incoming message |
| PollingClient | returns a message queue, from which user can retrieve and process the messages |
Message Class
When the API receives a subscribed message from DolphinDB, it will be encapsulated
into a Message class instance. You can process Message objects with
user-defined function (handler).
The Message class is a subclass of ConstantSP, so
the method of retrieving data from a Message object is the same as reading data from
a ConstantSP object. You can refer to Data Objects for instructions on how to access the data within Message
objects.
There are two unique functions of the Message class.
getSymbol
When subscribing to a heterogeneous stream table, you can use the
getSymbol function to retrieve the symbol identifier. This
symbol can be used to determine the source table from which the received message
originates. For more information, refer to Heterogeneous Stream
Table Subscription.
The function declaration is as follows:
const std::string& getSymbol();getOffset
You can use the getOffset function to obtain the offset of the
message within the table (starting from 0).
The function declaration is as follows:
int getOffset();Note: This function only takes effect when the filter parameter is not specified.