Concurent or Single process : data access to ONE-writer, MULTIPLE-readers Store with RAY

one-writer-many-readers.py

"""

This example shows how can you handle a data store that can have only ONE WRITER, but MULTIPLE READERS.
KV stores and SQLite come to mind. Also Text files, right ?


There are Three scenarios not TWO :

	1. Writer writes directly
	2. Reader uses Writer in the same process 
	3. Reader uses Writer in the remote process 


"""

class Store(object):

	def __init__(self, store_name, root, is_writer=False, writer=None):
		assert (is_writer and writer is None) or (not is_writer and writer is not None), 'Reader or Writer' 
		self.root = root
		self.data =  self.root + '/data'

		self.store_name = store_name

		#writer? : by name or if locally as object
		self.is_writer = is_writer
    
    #writer is a string i.e. name not reference
		if isinstance(writer, str) : 
			self.writer_name = writer
			self.writer = ray.get_actor(self.writer_name)
		else :	
			self.writer = writer

		if self.is_writer :
			log('kv',f'ReadWrite store : {self.store_name} ...')
			self.store = .... open read-write ....
		else :
			log('kv',f'ReadOnly store : {self.store_name} ...')
			self.store = .... open read-only ....

	#Single writer logic
	def set(self,key,value): 
		if self.is_writer :#direct write 
			log('kvset', f'local-writer-set> {key} => {value}')
			self.store.kv_set(key, value) # !!!original store method
		else : # writer in remote process
			if str(self.writer).startswith('Actor') :
				log('kvset', f'remote-set> {key} => {value}')
				self.writer.set.remote(key, value)
			else : #writer in the same process
				log('kvset', f'local-reader-set> {key} => {value}')
				self.writer.set(key, value)


@ray.remote  #Ray Actor
class RemoteStore(Store): pass


#------------- later ------------------------------------

# in this example both are Remote. 
# if you use many actors you should probably create the Writer as remote, 
# because you need to use Named-writer to pass the name around

class Runner(object):

	def __init__(self, store_name, nreaders=2, writer_name='writer'):

		self.writer = RemoteStore.options(name=writer_name).remote(store_name, is_writer=True)
		self.readers = [ 
			RemoteStore.remote(store_name, is_writer=False, writer=writer_name) 
			for _ in range(nreaders) 
		]
		self.pool = ActorPool(self.readers)

      ... do something ...