By default (i.e. if no database plugin such as PostgreSQL or MySQL is used), Orthanc stores all the
DICOM files it receives in a folder called OrthancStorage
on the
filesystem.
More precisely, the OrthancStorage
folder contains a set of
so-called attachments, that may correspond to either a DICOM file,
a JSON file, or any user-defined file. Internally, each attachment is
automatically associated with an universally unique identifier (UUID).
Orthanc can be configured to compress these files on-the-fly in order
to save disk space (cf. the StorageCompression
configuration
option).
To reduce the number of files in a single directory (which is something that some operating systems might not like), a 3-level hierarchy of directories is created to store the attachments: The first two hexadecimal characters of the UUID give the first-level folder, and the two next characters give the second-level folder.
Inside the same OrthancStorage
folder, Orthanc maintains a SQLite
database called index
that indexes all these attachments. The database records, for each
attachment, its compression method, and its MD5 hashes before and
after compression in order to detect disk corruption (cf. the
StoreMD5ForAttachments
configuration option).
One attachment must be associated with one DICOM resource (patient, study, series, or instance). Incoming DICOM files and associated JSON summary are associated with one instance-level resource, but user-defined attachments can be associated with any kind of resource.
Given one DICOM resource, all of its child attachments are identified by a number between 0 and 65535. Identifiers <= 1023 are reserved for the Orthanc core, whereas identifiers >= 1024 can be user-defined for external applications.
Besides the attachments, the database index maintains other information for each DICOM resource, notably the metadata, the history of changes, and an associative map that stores the so-called “main” DICOM tags (to avoid accessing the storage folder are when this is not needed). The SQLite database schema is kept as simple as possible, and can be found in the following two files of the source code of Orthanc: PrepareDatabase.sql and InstallTrackAttachmentsSize.sql.
Directly accessing the content of the OrthancStorage
folder and
the content of the SQLite/MySQL/PostgreSQL database is strongly
discouraged for several reasons:
StorageCompression
option).OrthancStorage
, you bypass
all the locking mechanisms used by Orthanc, which might result in
data corruption.SQLITE_BUSY
status.As a consequence, it is HIGHLY recommended NOT to directly access
the OrthancStorage
folder and the SQLite/MySQL/PostgreSQL
database. Use the REST API instead, which contains
primitives to access the attachments (cf. the .../attachments/...
URIs).
The only exception to this rule is for read-only access when Orthanc is stopped, e.g. as a part of a backup or upgrade/replication process.