Source code for examples.sharding.separate_schema_translates
"""Illustrates sharding using a single database with multiple schemas,where a different "schema_translates_map" can be used for each shard.In this example we will set a "shard id" at all times."""from__future__importannotationsimportdatetimeimportosfromsqlalchemyimportcreate_enginefromsqlalchemyimportForeignKeyfromsqlalchemyimportinspectfromsqlalchemyimportselectfromsqlalchemy.ext.horizontal_shardimportset_shard_idfromsqlalchemy.ext.horizontal_shardimportShardedSessionfromsqlalchemy.ormimportDeclarativeBasefromsqlalchemy.ormimportMappedfromsqlalchemy.ormimportmapped_columnfromsqlalchemy.ormimportrelationshipfromsqlalchemy.ormimportsessionmakerecho=Trueengine=create_engine("sqlite://",echo=echo)withengine.connect()asconn:# use attached databases on sqlite to get "schemas"foriinrange(1,5):ifos.path.exists("schema_%s.db"%i):os.remove("schema_%s.db"%i)conn.exec_driver_sql('ATTACH DATABASE "schema_%s.db" AS schema_%s'%(i,i))db1=engine.execution_options(schema_translate_map={None:"schema_1"})db2=engine.execution_options(schema_translate_map={None:"schema_2"})db3=engine.execution_options(schema_translate_map={None:"schema_3"})db4=engine.execution_options(schema_translate_map={None:"schema_4"})# create session function. this binds the shard ids# to databases within a ShardedSession and returns it.Session=sessionmaker(class_=ShardedSession,shards={"north_america":db1,"asia":db2,"europe":db3,"south_america":db4,},)# mappings and tablesclassBase(DeclarativeBase):pass# table setup. we'll store a lead table of continents/cities, and a secondary# table storing locations. a particular row will be placed in the database# whose shard id corresponds to the 'continent'. in this setup, secondary rows# in 'weather_reports' will be placed in the same DB as that of the parent, but# this can be changed if you're willing to write more complex sharding# functions.classWeatherLocation(Base):__tablename__="weather_locations"id:Mapped[int]=mapped_column(primary_key=True)continent:Mapped[str]city:Mapped[str]reports:Mapped[list[Report]]=relationship(back_populates="location")def__init__(self,continent:str,city:str):self.continent=continentself.city=cityclassReport(Base):__tablename__="weather_reports"id:Mapped[int]=mapped_column(primary_key=True)location_id:Mapped[int]=mapped_column(ForeignKey("weather_locations.id"))temperature:Mapped[float]report_time:Mapped[datetime.datetime]=mapped_column(default=datetime.datetime.now)location:Mapped[WeatherLocation]=relationship(back_populates="reports")def__init__(self,temperature:float):self.temperature=temperature# define sharding functions.# we'll use a straight mapping of a particular set of "country"# attributes to shard id.shard_lookup={"North America":"north_america","Asia":"asia","Europe":"europe","South America":"south_america",}defshard_chooser(mapper,instance,clause=None):"""shard chooser. this is primarily invoked at persistence time. looks at the given instance and returns a shard id note that we need to define conditions for the WeatherLocation class, as well as our secondary Report class which will point back to its WeatherLocation via its 'location' attribute. """ifisinstance(instance,WeatherLocation):returnshard_lookup[instance.continent]else:returnshard_chooser(mapper,instance.location)defidentity_chooser(mapper,primary_key,*,lazy_loaded_from,**kw):"""identity chooser. given a primary key identity, return which shard we should look at. in this case, we only want to support this for lazy-loaded items; any primary query should have shard id set up front. """iflazy_loaded_from:# if we are in a lazy load, we can look at the parent object# and limit our search to that same shard, assuming that's how we've# set things up.return[lazy_loaded_from.identity_token]else:raiseNotImplementedError()defexecute_chooser(context):"""statement execution chooser. given an :class:`.ORMExecuteState` for a statement, return a list of shards we should consult. """ifcontext.lazy_loaded_from:return[context.lazy_loaded_from.identity_token]else:return["north_america","asia","europe","south_america"]# configure shard chooserSession.configure(shard_chooser=shard_chooser,identity_chooser=identity_chooser,execute_chooser=execute_chooser,)defsetup():# create tablesfordbin(db1,db2,db3,db4):Base.metadata.create_all(db)defmain():setup()# save and load objects!tokyo=WeatherLocation("Asia","Tokyo")newyork=WeatherLocation("North America","New York")toronto=WeatherLocation("North America","Toronto")london=WeatherLocation("Europe","London")dublin=WeatherLocation("Europe","Dublin")brasilia=WeatherLocation("South America","Brasila")quito=WeatherLocation("South America","Quito")tokyo.reports.append(Report(80.0))newyork.reports.append(Report(75))quito.reports.append(Report(85))withSession()assess:sess.add_all([tokyo,newyork,toronto,london,dublin,brasilia,quito])sess.commit()t=sess.get(WeatherLocation,tokyo.id,identity_token="asia",)assertt.city==tokyo.cityassertt.reports[0].temperature==80.0# select across shardsasia_and_europe=sess.execute(select(WeatherLocation).filter(WeatherLocation.continent.in_(["Europe","Asia"]))).scalars()assert{c.cityforcinasia_and_europe}=={"Tokyo","London","Dublin",}# optionally set a shard id for the query and all related loadersnorth_american_cities_w_t=sess.execute(select(WeatherLocation).filter(WeatherLocation.city.startswith("T")).options(set_shard_id("north_america"))).scalars()# Tokyo not included since not in the north_america shardassert{c.cityforcinnorth_american_cities_w_t}=={"Toronto",}# the Report class uses a simple integer primary key. So across two# databases, a primary key will be repeated. The "identity_token"# tracks in memory that these two identical primary keys are local to# different shards.newyork_report=newyork.reports[0]tokyo_report=tokyo.reports[0]assertinspect(newyork_report).identity_key==(Report,(1,),"north_america",)assertinspect(tokyo_report).identity_key==(Report,(1,),"asia")# the token representing the originating shard is also available# directlyassertinspect(newyork_report).identity_token=="north_america"assertinspect(tokyo_report).identity_token=="asia"if__name__=="__main__":main()