Filter Middleware

The filter middleware is a Swift middleware that executes storage filters that intercept object flows to run computations or perform transformations on them.

Two differentiated kinds of filters are supported:

  • Storlet filters: Java classes that implement the IStorlet interface and are able to intercept and modify the data flow of GET/PUT requests in a secure and isolated manner.
  • Native filters: python classes that can intercept GET/PUT requests at all the possible life-cycle stages offered by Swift.

Crystal filters

As depicted in the diagram above, filters can be installed both in the proxy and the object storage server. Several filters can be executed for the same request (e.g. compression+encryption). The execution order of filters can be configured and does not depend on the kind of filter: a storlet can be applied before a native filter and vice versa.

Filter classes must be registered through Crystal controller API, that also provides means to configure the filter pipeline and to control the server where they will be executed.

A convenient web dashboard is also available to simplify Crystal controller API calls.

There is a repository that includes some filter samples for compression, encryption, caching, bandwidth differentiation, …

Storlet filters

In order to use Storlet filters, it is necessary to install the storlet engine to Swift.

The code below is an example of a storlet filter:

public class ExampleStorlet implements IStorlet {
     * The invoke method intercepts the data flow of GET/PUT requests, offering
     * the input and output stream to perform calculations/modifications.
    public void invoke(ArrayList<StorletInputStream> inStreams,
            ArrayList<StorletOutputStream> outStreams,
            Map<String, String> parameters,
            StorletLogger logger) throws StorletException {

        StorletInputStream sis = inStreams.get(0);
        InputStream is = sis.getStream();
        HashMap<String, String> metadata = sis.getMetadata();

        StorletObjectOutputStream sos = (StorletObjectOutputStream)outStreams.get(0);
        OutputStream os = sos.getStream();

        # The parameters map contains:
        # 1) the special parameter "reverse", that is "True" when the filtering process
        #    should be reversed (e.g. decompression in the compression filter)
        # 2) Other custom parameters that can be configured through the Controller API
        String reverse = parameters.get("reverse");
        String customParam = parameters.get("custom_param");

        byte[] buffer = new byte[65536];
        int len;

        try {
            while(( != -1) {

                // ...
                // Filter code should be placed here to run calculations on data
                // or perform modifications
                // ...

                os.write(buffer, 0, len);
        } catch (IOException e) {
            logger.emitLog("Example Storlet - raised IOException: " + e.getMessage());

The StorletInputStream is used to stream object’s data into the storlet. An instance of the class is provided whenever the Storlet gets an object as an input. Practically, it is used in all storlet invocation scenarios to stream in the object’s data and metadata. To consume the data call getStream() to get a on which you can just read(). To consume the metadata call the getMetadata() method.

In all invocation scenarios the storlet is called with an instance of StorletObjectOutputStream. Use the setMetadata() method to set the Object’s metadata. Use getStream() to get a on which you can just write() the content of the object. Notice that setMetadata() must be called. Also, it must be called before writing the data.

The StorletLogger class supports a single method called emitLog(), and accepts a String.

For more information on writing and deploy Storlets, please refer to Storlets documentation.

Native filters

The code below is an example of a native filter:

class NativeFilterExample(object):

    def __init__(self, global_conf, filter_conf, logger):
        # The constructor receives the configuration parameters and the logger
        self.logger = logger

    # This method is called by the middleware to allow filters to intercept GET/PUT requests life-cycle
    def execute(self, req_resp, crystal_iter, request_data):
        method = request_data['method']

        if method == 'get':
            if isinstance(req_resp, Request):
                # ...
                # Filter code for GET requests should be placed here
                # ...
            elif isinstance(req_resp, Response):
                # ...
                # Filter code for GET responses should be placed here (the response includes
                # the object data in this phase)
                # ...
        elif method == 'put':
            if isinstance(req_resp, Request):
                # ...
                # Filter code for PUT requests should be placed here (the request includes
                # the object data in this phase)
                # ...
            elif isinstance(req_resp, Response):
                # ...
                # Filter code for PUT responses should be placed here
                # ...

        return crystal_iter

The execute() method is called by the middleware at all life-cycle stages of the request/response. The req_resp parameter can be the swift.common.swob.Request or swift.common.swob.Response depending on the life-cycle phase the method is called. Upon registering the filter through Crystal controller, you can specify which server and life-cycle phase the filter will be called at, depending on the type of required computation or data-manipulation. For example, a caching filter should be executed at proxy servers, intercepting both the PUT and GET requests before reaching the object server (at request phase).

The crystal_iter parameter is an iterator of the data stream to be processed. The execute() method must return the crystal_iter or a modified data stream because the successive filters must receive an iterator to operate correctly, in turn.

The request_data parameter is a dictionary that contains the following keys:

  • ‘app’: ‘proxy-server’ or ‘object-server’
  • ‘api_version’: the Swift API version
  • ‘account’: the tenant name
  • ‘container’: the container name
  • ‘object’: the object name
  • ‘method’: ‘put’ or ‘get’