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remark about replication
author Sebastien Jodogne <s.jodogne@gmail.com>
date Fri, 07 Oct 2016 08:07:24 +0200
parents 38fb7bf24794
children 365427cebb64
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1 .. _orthanc-storage:
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3 How does Orthanc store its database?
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4 ====================================
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5
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6 Storage folder
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7 --------------
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8
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9 **By default** (i.e. if no database plugin such as :ref:`PostgreSQL
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10 <postgresql>` is used), Orthanc stores all the DICOM files it receives
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11 in a folder called ``OrthancStorage`` on the filesystem. Orthanc also
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12 associates each incoming DICOM file with a JSON file that summarizes
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13 all its DICOM tags, which speeds up subsequent processing by avoiding
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14 a costly DICOM parsing.
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16 More generally, the ``OrthancStorage`` folder contains a set of
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17 so-called **attachments**, that may correspond to either a DICOM file,
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18 a JSON file, or any user-defined file. Internally, each attachment is
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19 automatically associated with an `universally unique identifier (UUID)
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20 <https://en.wikipedia.org/wiki/Universally_unique_identifier>`__.
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21 Orthanc can be configured to compress these files on-the-fly in order
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22 to save disk space (cf. the ``StorageCompression`` :ref:`configuration
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23 option <configuration>`).
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25 To reduce the number of files in a single directory (which is
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26 something that some operating systems might not like), a 3-level
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27 hierarchy of directories is created to store the attachments: The
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28 first two hexadecimal characters of the UUID give the first-level
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29 folder, and the two next characters give the second-level folder.
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32 SQLite index
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33 ------------
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34
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35 Inside the same ``OrthancStorage`` folder, Orthanc maintains a `SQLite
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36 database <https://en.wikipedia.org/wiki/SQLite>`__ called ``index``
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37 that **indexes** all these attachments. The database records, for each
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38 attachment, its compression method, and its MD5 hashes before and
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39 after compression in order to detect disk corruption (cf. the
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40 ``StoreMD5ForAttachments`` :ref:`configuration option
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41 <configuration>`).
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43 One attachment must be associated with one :ref:`DICOM resource
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44 <model-world>` (patient, study, series, or instance). Incoming DICOM
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45 files and associated JSON summary are associated with one
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46 instance-level resource, but user-defined attachments can be
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47 associated with any kind of resource.
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49 Given one DICOM resource, all of its child attachments are identified
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50 by a number between 0 and 65535. Identifiers <= 1023 are reserved for
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51 the Orthanc core, whereas identifiers >= 1024 can be user-defined for
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52 external applications.
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55 Direct access
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56 -------------
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57
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58 Directly accessing the content of the ``OrthancStorage`` folder and
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59 the content of the SQLite database is strongly discouraged for several
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60 reasons:
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61
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62 * This internal organization is only true when no database plugin is
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63 used (e.g. the :ref:`PostgreSQL plugin <postgresql>` can be
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64 configured to store the attachments inside a database).
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65 * Orthanc can be configured to compress the attachments before writing
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66 them on the disk (cf. the ``StorageCompression`` option).
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67 * By directly reading the content of ``OrthancStorage``, you bypass
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68 all the locking mechanisms used by Orthanc, which might result in
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69 data corruption.
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70 * One SQLite database should be accessed by at most one process at any
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71 time to avoid any problem (e.g. with NFS filesystems), for reasons
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72 that are `explained in the SQLite FAQ
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73 <https://www.sqlite.org/faq.html#q5>`__. Orthanc will stop if it
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74 receives the ``SQLITE_BUSY`` status.
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76 As a consequence, it is **HIGHLY recommended NOT to directly access**
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77 the ``OrthancStorage`` folder and the SQLite database. Use the
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78 :ref:`REST API <rest>` instead, which contains primitives to access
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79 the attachments (cf. the ``.../attachments/...`` URIs).
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80
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81 The only exception to this rule is for **read-only access when Orthanc
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82 is stopped**, e.g. as a part of a :ref:`backup <backup>` or
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83 :ref:`upgrade/replication <replication>` process.