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Resources for Dicoogle users and developers.

Storage Plugins

Data storage stands as one of the primary requirements of a PACS archive. Medical imaging storage requirements have become increasingly demanding, to the point that relying on a server’s local file system has become inadequate in some use cases. Delegating storage to cloud services, for instance, have been thoroughly discussed in recent years.

Storage plugins (or storage providers) represent a particular “place” where files can be read and stored, and sometimes also define how these files are stored and read. The Dicoogle Storage API serves the following two purposes:

  • To make an abstraction over the underlying storage technology, thus being able to use and evaluate different sources of data storage (e.g. cloud storage services such as Amazon S3) and different forms of persistence (e.g. using a document-oriented database instead of a file system). With this common API, DICOM object reading and storing becomes possible, regardless of the underlying technology.
  • To augment storage and retrieval procedures with certain algorithms, such as for anonymization, compression, and encryption.

Programmatically, the storage interface is currently defined as below:

/** Storage plugin interface. These types of plugins provide an abstraction to reading and writing from
 * files or data blobs.
public interface StorageInterface extends DicooglePlugin {    
     * Gets the scheme URI of this storage plugin.
     * @see URI
     * @return a string denoting the scheme that this plugin associates to
    public String getScheme();
     * Provides a means of iteration over existing objects at a specified location.
     * This method is particularly nice for use in for-each loops.
     * The provided scheme is not relevant at this point, but the developer must avoid calling
     * this method with a path of a different schema.
     * <pre>
     * for(StorageInputStream dicomObj :"file://dataset/")){
     *      // use dicomObj here
     * }
     * </pre>
     * @param location the location to read
     * @param parameters a variable list of extra parameters for the retrieve
     * @return an iterable of storage input streams
     * @see StorageInputStream
    public Iterable<StorageInputStream> at(URI location, Object ... parameters);
     * Stores a DICOM object into the storage.
     * @param dicomObject Object to be Stored
     * @param parameters a variable list of extra parameters for the retrieve
     * @return The URI of the previously stored Object.
    public URI store(DicomObject dicomObject, Object ... parameters);

     * Stores a new element into the storage.
     * @param inputStream an input stream with the contents to be stored
     * @param parameters a variable list of extra parameters for the retrieve
     * @return the URI of the stored data
     * @throws IOException if an I/O error occurs
    public URI store(DicomInputStream inputStream, Object ... parameters) throws IOException;
    /** Removes an element at the given URI.
    public void remove(URI location);

    /** Lists the elements at the given location in the storage's file tree.
     * Unlike `StorageInterface#at`, this method is not recursive and
     * can yield intermediate URIs representing other directories rather than
     * objects.
    public default Stream<URI> list(URI location) throws IOException { ... }

Storing files

The two store method overloads are used to store new objects. The only difference between the overloads is that one takes a DicomInputStream, whereas the other takes a DicomObject. Unless the storage can take advantage of streamed processing, it is usual to read the stream into an object (using DicomInputStream#readDicomObject) and call the other method overload:

public URI store(DicomInputStream inputStream, Object ... parameters) {
    DicomObject obj = inputStream.readDicomObject();
    return, parameters);

A unique identifier for the object is to be created by the storage and returned when successful. The stored object should then become accessible by using the same URI. A traditional file storage might create a hierarchical URI based on the DICOM object’s meta-data (so as to categorize files by modality, study, series, and so on), and serialize it into a file with its path defined by the URI. Therefore, a valid URI would be, for example:

Remember when we indexed a directory in "Using Dicoogle"?

This is the approach made by the provided file storage plugin, which makes indexing of existing files easier: you can infer the URI of a file just by looking at its path in the system! Although it is useful to have a trivial mapping such as this one, this behaviour is not required for Dicoogle storages in general.

Fetching files

The at method introduces a new abstraction for files in a Dicoogle storage provider. The StorageInputStream interface, despite the name, represents an item in storage (often a DICOM file). An ordinary Java input stream can be obtained by calling getInputStream(). The code below would allow you to read a file as DICOM data. Indexers and other plugins may instead interpret the file as images, or arbitrary binary data, depending on their purpose.

Iterable<StorageInputStream> it ="file:/data/X/000.dcm");
StorageInputStream f =; // expect one file
DicomObject dcm = new DicomInputStream(f.getInputStream()).readObject();
// use DICOM object

Storage plugins are required to implement the logic behind obtaining a list of files by URI, as well as a class type to be used as a StorageInputStream. The following should be kept in mind:

  • If the storage is hierarchical, calling at for a parent resource must recursively yield all files inside that directory. This also means that"file:/") would give us all files in storage.
  • As stores may end up having millions of entries, it is recommended to build a lazy iterable of the files upon a call to at. For instance, the iterator may keep a queue of directories yet to be expanded to their respective children.
  • Never return null from this method, and always return a valid iterable object, even if it has to be an empty one.

Other methods

getScheme() should constantly return a string compatible with a URI scheme. A basic file storage plugin would use the file scheme like this:

public String getScheme() {
    return "file";

list() was introduced in Dicoogle 3, enabling storage interfaces to provide a list of entries at a particular position in the storage tree. Note that this is different from the method at(): list is optional and provides a shallow list of files and directories directly below the given directory, whereas at is required and provides a full list of all files (leaf items) in storage at the given base directory. Implementers may ignore this method, but implemeting it may grant additional features to end applications. Consumers of this method should catch UnsupportedOperationException to handle situations in which the method is not implemented.

handles(URI) is a deprecated method, which was made to verify whether the given URI can relate to an item (existent or inexistent) in this storage. Since this was meant to be based uniquely on the scheme of the URI, implementers should ignore it, and consumers should instead check for equality of the scheme:

boolean handles = Objects.equals(storageInterface.getScheme(), uri.getScheme());