Load-Balancer: A C repository from madalinazanficu

Copyright 2021 <Zanficu Madalina-Valentina 313CA>
My program includes 5 .c files:
1.server.c / server.h
2.load_balancer.c / load_balancer.h
3.LinkedList.c / LinkedList.h 
4.CircularDoublyLinkedList.c / CircularDoublyLinkedList.h 
5.extra.c
+ utils.h for DIE
I will describe the functions from server.c, load_balancer.c and extra.c
* SERVER.C / SERVER.H *
-> implemented the server_memory (which is a hashtable) using Direct Chaining 
   for collsions
-> nodes'data from the lists will conatin strutures str_info 
   (with fields: key, value)

init_server_memory
-> responsible for initializing the server_memory fields
-> allocating memory for the array of list

b.ht_server_has_key
-> search through all the buckets if a key is present in the hashtable
-> return 1 -> true, 0-> key is not present

c.server_store
-> manages to add a new entry (key, value) in hashtable
-> if the key is already present, it is supposed to update the value
-> otherwise, I allocate memory to add a new str_info entry

d_server_remove
-> for removing a (key, value) entry from the hashtable
-> searching for the index_key, after hashing the key, and remove it

e.server_retrieve
-> searching for the value associated with a given key

d_free_server_memory && free_list_inside_data
-> responsible for release the server_memory used
-> firstly eliberated the data inside the list (structures with key, value)
-> after that the lists and the array of lists + server


* LOAD_BALANCER.C / LOAD_BALANCER.H *
-> implemented the load_balancer using a circular doubly linked list 
   in order to have full control over the operations of adding and removing
   of servers from the hash_ring
-> node's data will contain structures -hr_element with: 
                                        server_tag
				         server_id
					 hash of the server_tag 
					 pointer which indicates to a hashtable
					 
-> to optimize the memory usage, objects are not stored on the hash_ring, 
   they are added in the servers'hashtable (pSever-field from the structure)
   
a. init_load_balancer -> allocated memory and created the hash_ring (cdll)

b. loader_add_server
-> adding a new server in the hash_ring, in ascending order according to the hash
-> finding the position where to add, depending on the hash of the server_tag, 
   using "position_in_the_hash_ring" (from extra.c)
-> initializating structures of type hr_element for the original server, 
   and his copies with: server_id (inherited from the original server), 
   server_tag, hash of server_tag, pointer to server_memeory 
   (same for the original and replicas)										
-> call redistribute_server_objects_add function (from extra.c)
   in order to balance the objects on the hash_ring
   
   
c. loader_remove_server
-> removing a server and his duplicates from the hash_ring
-> memory release for the data of the original and copies
-> balancing the objects of the server which will be removed 
   using redistribute_server_objects_remove (extra.c)
-> and one memory release for the common pointer to the server_memory


d. loader_store
-> searching the server (based on his hash) to store (key, value) entry 
   in server_memory

e. loader_retreieve
-> looking in the hash_ring for the server which contain an object with a given key
-> used the server_retrieve function to return the value associated with the key

f. free_load_balancer
-> implemented to release the entry memory used: for the hash_ring (cdll), 
   node's data, pointers to Hashtable, and for the load_balancer as well


* EXTRA.C / EXTRA.H *
a. hash_functions_servers (given)
b. hash_functions_key (given)

c. position in the hash_ring
-> it is used from loader_add_store to find out the position where 
   new_server will be stored
-> linear searching, depending on the hash of the server 
   (elements are in ascending order)
-> in case of hash equality, we will check the server id

d. searching_hash -> linear searching in cdll, for a specific hash
                  -> O(n)- time complexity

e. redistribute_server_object_add
-> responsible for balancing server's objects, when a new server is added
-> set conditions for corner cases such as: 
   Not to distribute objects on the same server.
   If the new server is added on the first position, the hash of the added object 
   can be either smaller than that of the neighbor on the right or bigger than 
   that of the neighbor on the left.
   Otherwise, the object_hash should respect both condition simultaneous.
   
f. redistribute_server_object_remove
-> responsible for balancing the hash_ring server's objects, 
   when a server is removed
-> went through server_memory buckets, and used loader store function to 
   distribute the server's objects to other servers before deleting the server 
   from the system.
madalinazanficu/Load-Balancer
