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1 Terminology of advanced features
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2 ================================
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3
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4 .. _peers:
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5
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6 Peers
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7 -----
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8
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9 An "Orthanc peer" is another instance of Orthanc, possibly running on
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10 a remote computer.
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11
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12 Contrarily to a "modality", it is possible to communicate with a peer
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13 through the **HTTP/HTTPS protocol** (i.e. through the REST API of
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14 Orthanc), and not through the DICOM protocol. This implies a much
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15 easier configuration: It is only required to know the URL, the
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16 username and the password to communicate with an Orthanc peer. This
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17 contrasts with DICOM query/retrieve, that is quite complex and that
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18 involves a lot of pitfalls (cf. the FAQ entry about :ref:`troubleshooting
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19 DICOM communications <dicom>`) that can be bypassed if using HTTP.
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20
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21 Furthermore, as HTTP(S) communications are generally not blocked by
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22 firewalls (contrarily to the DICOM protocol that is inherently an
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23 Intranet protocol and that often requires the setup of VPN tunnels),
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24 it is much easier to setup communications of medical images through
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25 the Internet with Orthanc peers.
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26
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27
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28 .. _recycling:
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29
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30 Recycling/Protection
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31 --------------------
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32
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33 Because of its focus on low-end computers, Orthanc implements **disk
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34 space recycling**: The patient that has been stored for the longest
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35 time inside Orthanc can be automatically deleted when the disk space
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36 used by Orthanc grows above a threshold, or when the number of stored
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37 patients grows above a threshold. This feature enables the automated
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38 control of the disk space dedicated to Orthanc.
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39
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40 Recycling is controlled by the ``MaximumStorageSize`` and the
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41 ``MaximumPatientCount`` options in the :ref:`Orthanc configuration
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42 file <configuration>`.
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43
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44 It is possible to prevent important data from being automatically
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45 recycled. This mechanism is called **protection**. Each patient can be
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46 individually protected against recycling by using the
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47 ``Unprotected/Protected`` switch that is available from Orthanc
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48 Explorer.
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49
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50 Note that protection is only available at the patient level. It
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51 protects all the studies/series/instances of the patient against
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52 recycling. The rationale is that we think it is important to keep
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53 available all the data related to one patient. Unwillingly losing a
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54 study/series that is part of the same patient might lead to a loss in
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55 consistency with respect to the medical history of this patient.
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56
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57
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58 .. _compression:
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59
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60 Compression
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61 -----------
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62
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63 If your disk space is limited, besides :ref:`recycling`, you should
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64 also consider using **disk space compression**. When compression is
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65 enabled, Orthanc compresses the incoming DICOM instances on-the-fly
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66 before writing them to the filesystem, using `zlib
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67 <http://en.wikipedia.org/wiki/Zlib>`_. This is useful, because the
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68 images are often stored as raw, uncompressed arrays in DICOM
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69 instances: The size of a typical DICOM instance can hopefully be
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70 divided by a factor 2 through lossless compression. This compression
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71 process is transparent to the user, as Orthanc automatically
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72 decompresses the file before sending it back to the external world.
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73
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74 Compression is controlled by the ``StorageCompression`` option in the
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75 :ref:`Orthanc configuration file <configuration>`.
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