Get data from a stream layer

You can read messages from a stream layer if you subscribe to it.

To get data from the stream layer:

  1. Create the OlpClientSettings object.

    For instructions, see Create platform client settings.

  2. Create the StreamLayerClient object with the HERE Resource Name (HRN) of the catalog that contains the layer, layer ID, and platform client settings from step 1.

    olp::dataservice::read::StreamLayerClient client(
         client::HRN catalog, std::string layer_id,
         client::OlpClientSettings settings);
    
  3. Create the SubscribeRequest object with the serial or parallel subscription type.

    • If your application should read smaller volumes of data using a single subscription, use the serial subscription type.

    • If your application should read large volumes of data in a parallel manner, use the parallel subscription type and subscription ID.

    auto request = olp::dataservice::read::SubscribeRequest()
                      .WithSubscriptionMode(olp::dataservice::read::SubscribeRequest::SubscriptionMode::kSerial));
    
  4. Call the Subscribe method with the SubscribeRequest parameter.

    client::CancellableFuture<SubscribeResponse> Subscribe(
       SubscribeRequest request);
    

    You receive a subscription ID from the requested subscription to the selected layer.

  5. Call the Poll method.

    client::CancellableFuture<PollResponse> Poll();
    

    You get messages with the layer data and partition metadata. The Poll method also commits the offsets, so you can continue polling new messages.

    If the data size is less than 1 MB, the data field is populated. If the data size is greater than 1 MB, you get a data handle that points to the object stored in the blob store.

  6. If the data size is greater than 1 MB, call the GetData method with the Messages instance.

     client::CancellableFuture<DataResponse> GetData(
         const model::Message& message);
    

You get data from the requested partition.

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