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annotate doc/developer.rst @ 4035:9c76678a39e2
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author | Goffi <goffi@goffi.org> |
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date | Fri, 07 Apr 2023 15:18:05 +0200 |
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doc: developer documentation explaining storage and pubsub cache
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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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30 |
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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) |
3828 | 75 * it is desirable to do queries on parsed data. Indeed the parsed data are 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 |