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doc: developer documentation explaining storage and pubsub cache
author Goffi <goffi@goffi.org>
date Thu, 29 Jul 2021 22:51:01 +0200
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children 432aaba0d7fe
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1 .. _developer:
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2
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3 =======================
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4 Developer Documentation
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5 =======================
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6
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7 This documentation is intended for people who wants to contribute or work with the
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8 internals of the project, it is not for end-users.
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9
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10 Storage
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11 =======
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12
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13 Since version 0.9, Libervia uses SQLAlchemy_ with its Object–Relational Mapping as a
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14 backend to store persistent data, and Alembic_ is used to handle schema and data
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15 migrations.
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16
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17 SQLite_ is currently the only supported database, but it is planned to add support for
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18 other ones (notably PostgreSQL), probably during the development of 0.9 version.
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19
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20 The mapping is done in ``sat.memory.sqla_mapping`` and working with database is done
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21 through high level methods found in ``sat.memory.sqla``.
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22
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23 Before the move to SQLAlchemy, there was a strict separation between database
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24 implementation and the rest of the code. With 0.9, objects mapping to database can be used
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25 and manipulated directly outside of ``sat.memory.sqla`` to take profit of SQLAlchemy
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26 possibilities.
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27
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28 Database state is detected when the backend starts, and the database will be created or
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29 migrated automatically if necessary.
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31 To create a new migration script, ``Alembic`` may be used directly. To do so, be sure to
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32 have an up-to-date database (and a backup in case of troubles), then activate the virtual
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33 environment where Libervia is installed (Alembic needs to access ORM mapping), go to
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34 ``sat/memory/migration`` directory, and enter the following command::
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35
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36 alembic revision --autogenerate -m "some revision message"
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37
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38 This will create a base migration file in ``versions`` directory. Adapt it to your needs,
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39 try to create both ``upgrade`` and ``downgrade`` method whenever possible, and be sure to
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40 test it in both directions (``alembic upgrade head`` and ``alembic downgrade
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41 <previous_revision>``). Please check Alembic documentation for more details.
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42
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43 .. _SQLALchemy: https://www.sqlalchemy.org/
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44 .. _Alembic: https://alembic.sqlalchemy.org/
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45 .. _SQLite: https://sqlite.org
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46
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47 Pubsub Cache
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48 ============
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49
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50 There is an internal cache for pubsub nodes and items, which is done in
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51 ``plugin_pubsub_cache``. The ``PubsubNode`` and ``PubsubItem`` class are the one mapping
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52 the database.
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53
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54 The cache is operated transparently to end-user, when a pubsub request is done, it uses a
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55 trigger to check if the requested node is or must be cached, and if possible returns
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56 result directly from database, otherwise it lets the normal workflow continue and query the
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57 pubsub service.
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58
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59 To save resources, not all nodes are fully cached. When a node is checked, a series of
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60 analysers are checked, and the first one matching is used to determine if the node must be
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61 synchronised or not.
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62
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63 Analysers can be registered by any plugins using ``registerAnalyser`` method:
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64
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65 .. automethod:: sat.plugins.plugin_pubsub_cache.PubsubCache.registerAnalyser
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66
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67 If no analyser is found, ``to_sync`` is false, or an error happens during the caching,
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68 the node won't be synchronised and the pubsub service will always be requested.
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69
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70 Specifying an optional **parser** will store parsed data in addition to the raw XML of the
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71 items. This is more space consuming, but may be desired for the following reasons:
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72
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73 * the parsing is resource consuming (network call or some CPU intensive operations are
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74 done)
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75 * it is desirable to to queries on parsed data. Indeed the parsed data is stored in a
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76 JSON_ field and its keys may be queried individually.
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77
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78 The Raw XML is kept as the cache operates transparently, and a plugin may need raw data, or
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79 an user may be doing a low-level pubsub request.
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80
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81 .. _JSON: https://docs.sqlalchemy.org/en/14/core/type_basics.html#sqlalchemy.types.JSON
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82