Predict on Binary Inputs via KServe API

GRPC

KServe API allows sending the model input data in a variety of formats inside the InferTensorContents objects or in raw_input_contents field of ModelInferRequest.

When the data is sent in the bytes_contents field of InferTensorContents and input datatype is set to BYTES, such input is interpreted as a binary encoded image. The BYTES datatype is dedicated to binary encoded images and if it’s set, the data must be placed in bytes_contents or in raw_input_contents if batch size is equal to 1.

Note, that while the model metadata reports the inputs shape with layout NHWC, the binary data must be sent with shape: [N] with datatype: BYTES. Where N represents number of images converted to string bytes.

When sending data in the array format, the shape and datatype gives information on how to interpret bytes in the contents. For binary encoded data, the only information given by the shape field is the amount of images in the batch. On the server side, the bytes in each element of the bytes_contents field are loaded, resized to match model input shape and converted to the OpenVINO-friendly array format by OpenCV.

HTTP

JPEG / PNG encoded images

KServe API also allows sending binary encoded data via HTTP interface. The tensor binary data is provided in the request body, after JSON object. While the JSON part contains information required to route the data to the target model and run inference properly, the data itself, in the binary format is placed right after the JSON.

For binary inputs, the parameters map in the JSON part contains binary_data_size field for each binary input that indicates the size of the data on the input. Since there’s no strict limitations on image resolution and format (as long as it can be loaded by OpenCV), images might be of different sizes. Therefore, to send a batch of different images, specify their sizes in binary_data_size field as a list with sizes of all images in the batch. The list must be formed as a string, so for example, for 3 images in the batch, you may pass - "9821,12302,7889". If the request contains only one input binary_data_size parameter can be omitted - in this case whole buffer is treated as a input image.

For HTTP request headers, Inference-Header-Content-Length header must be provided to give the length of the JSON object, and Content-Length continues to give the full body length (as HTTP requires). See an extended example with the request headers, and multiple images in the batch:

On the server side, the binary encoded data is loaded using OpenCV which then converts it to OpenVINO-friendly data format for inference.

The structure of the response is specified Inference Response specification.

Raw data

Above section described how to send JPEG/PNG encoded image via REST interface. Data sent like this is processed by OpenCV to convert it to OpenVINO-friendly format. Many times data is already available in OpenVINO-friendly format and all you want to do is to send it and get the prediction.

With KServe API you can also send raw data in a binary representation via REST interface. That way the request gets smaller and easier to process on the server side, therefore using this format is more effecient when working with RESTful API, than providing the input data in a JSON object. To send raw data in the binary format, you need to specify datatype other than BYTES and data shape, should match the input shape (also the memory layout should be compatible).

For the Raw Data binary inputs binary_data_size parameter can be omitted since the size of particular input can be calculated from its shape.

Usage examples

Sample clients that use binary inputs via KFS API can be found here (REST sample)/(GRPC sample) Also, see the (README)

Recommendations:

Sending the data in binary format can significantly simplify the client code and it’s preprocessing load. With the REST API client, only curl or the requests python package is needed. In case the original input data is jpeg or png encoded, there is no preprocessing needed to send the request.

Binary data can significantly reduce the network utilization. In many cases it allows reducing the latency and achieve very high throughput even with slower network bandwidth.