azaiter / distributed-hash-table

A proof of concept of a simple cloud-native distributed hash table implementation, each peer is a docker container within a contained docker-compose network.

Simple Distributed Hash Table

Author: Abdulrahman (Zack) Zaiter

This program is a proof of concept of simple cloud-native distributed hash table implementation, each peer is a docker container within a contained docker-compose network.

Application Design:

• Assumes a circular arrangement of peers.
• Each peer will hold a small subset of the totality of the (key, value) pairs.
• Keys are content names (e.g., names of movies, albums, and software, or file), and the value is be the any data at which the content is stored; in this case, an example key-value pair is (Led Zeppelin IV, Some Value).
• Each Peer should provide the following services to a user:
  • Insert a (content name, data) pair into the DHT – we are assuming that the user has a content at his key that they want the DHT to be aware of.
  • The user enters the content name and their data
  • The peer should then store the (content name, data) pair in one of the peers in the DHT
• Retrieve data values corresponding to a given content name
  • The user enters a content name
  • The peer should return the list of all the data values where the content is available
  • The user should contact one of the can do whatever with the data .
• Maintain DHT when a peer:
  • Abruptly leaves the DHT
  • Wants to join the DHT

Requirements:

• Docker and Docker-Compose
• That's it.

To Run:

• docker-compose down; docker-compose build; docker-compose up
• That's it

Current design vs. Other designs:

There are 5 points that are worth to be discussed:

1- Nodes/Peers initialization process:

The book Computer Networking: A Top-Down Approach by Kurose, sixth edition does not mention any algorithm or means for the nodes/peers to connect and be aware of each other without using broadcast address, In this application, the initialization and configuration of the peers has to be done in the INIT_CONFIG environment variable to know current/previous/subsequent1/subsequent2 peers/nodes.

2- Consensus of peers/states:

The algorithm does not guarantee a stable and highly available state of the peers as it does not have any mechanism to do so, it distributes the work based on the key to available nodes in the peers chain. One advantage that the current application have over the book's algorithm is that every node/peer is capable of serving the set and get commands.

3- Data:

The data in a specific peer gets lost if the peer decides to leave, the data is distributed and not replicated.

4- Re-arrangement of keys:

If a node decides to join/leave, the data the exists inside the already existing nodes/peers will stay as is and a re-arrangement is needed for optimal keys distribution, this is not accounted for in this application or the algorithm in the book.

5- Collision of hashes

The algorithm in the book does not mention how to handle collision on hash handling and assumes that the keys are collision free despite that the algorithm used for hashing is extremely prone to collisions. The way I handled the collisions in this application is that for each hash key, I created an internal hash table that takes the original string key and an array of values, therefore, the searching algorithm will look if the hash exists, if so, it looks for the internal key, if it exists, it returns the array of values it has saved.

Sample output (comments on execution starts with #):

# Initialization
dht16_1  | Node dht16 on index 16 listening on port 15000!
dht12_1  | Node dht12 on index 12 listening on port 15000!
dht4_1   | Node dht4 on index 4 listening on port 15000!
dht1_1   | Node dht1 on index 1 listening on port 15000!
dht8_1   | Node dht8 on index 8 listening on port 15000!
dht16_1  | Started the heartbeating process.
dht12_1  | Started the heartbeating process.
dht4_1   | Started the heartbeating process.
dht1_1   | Started the heartbeating process.
dht8_1   | Started the heartbeating process.

# Inset a key-value pair
dht12_1  | Sending Some Great Book 1:Some Great Value 1 of hash 4 into node [email protected] on index 16
dht16_1  | Sending Some Great Book 1:Some Great Value 1 of hash 4 into node [email protected] on index 4
dht4_1   | Inserting Some Great Book 1:Some Great Value 1 of hash 4 into node [email protected] on index 4
dht12_1  | Sending Some Great Book 2:Some Great Value 2 of hash 5 into node [email protected] on index 16
dht16_1  | Sending Some Great Book 2:Some Great Value 2 of hash 5 into node [email protected] on index 1
dht1_1   | Sending Some Great Book 2:Some Great Value 2 of hash 5 into node [email protected] on index 4
dht4_1   | Sending Some Great Book 2:Some Great Value 2 of hash 5 into node [email protected] on index 8
dht8_1   | Inserting Some Great Book 2:Some Great Value 2 of hash 5 into node [email protected] on index 8
dht12_1  | Sending Some Great Book 2:Some Great Value 3 of hash 5 into node [email protected] on index 16
dht16_1  | Sending Some Great Book 2:Some Great Value 3 of hash 5 into node [email protected] on index 1
dht1_1   | Sending Some Great Book 2:Some Great Value 3 of hash 5 into node [email protected] on index 4
dht4_1   | Sending Some Great Book 2:Some Great Value 3 of hash 5 into node [email protected] on index 8
dht8_1   | Inserting Some Great Book 2:Some Great Value 3 of hash 5 into node [email protected] on index 8
dht12_1  | Sending Some Great Book 2:Some Great Value 4 of hash 5 into node [email protected] on index 16
dht16_1  | Sending Some Great Book 2:Some Great Value 4 of hash 5 into node [email protected] on index 1
dht1_1   | Sending Some Great Book 2:Some Great Value 4 of hash 5 into node [email protected] on index 4
dht4_1   | Sending Some Great Book 2:Some Great Value 4 of hash 5 into node [email protected] on index 8
dht8_1   | Inserting Some Great Book 2:Some Great Value 4 of hash 5 into node [email protected] on index 8


# Get a key
dht1_1   | Searching for Some Great Book in node [email protected] that has index 1...
dht1_1   | Key Some Great Book NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht4_1   | Searching for Some Great Book in node [email protected] that has index 4...
dht4_1   | Key Some Great Book NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht8_1   | Searching for Some Great Book in node [email protected] that has index 8...
dht8_1   | Key Some Great Book NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht12_1  | Searching for Some Great Book in node [email protected] that has index 12...
dht12_1  | Key Some Great Book NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht16_1  | Searching for Some Great Book in node [email protected] that has index 16...
dht16_1  | Key Some Great Book NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht1_1   | The searching process for Some Great Book reached a loop, sending NOT FOUND to client
dht1_1   | Searching for Some Great Book 1 in node [email protected] that has index 1...
dht1_1   | Key Some Great Book 1 NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht4_1   | Searching for Some Great Book 1 in node [email protected] that has index 4...
dht4_1   | Key Some Great Book 1 found in node [email protected] that has index 4...
dht1_1   | Searching for Some Great Book 2 in node [email protected] that has index 1...
dht1_1   | Key Some Great Book 2 NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht4_1   | Searching for Some Great Book 2 in node [email protected] that has index 4...
dht4_1   | Key Some Great Book 2 NOT found in node [email protected] ... Sending to subsequent node [email protected]...
dht8_1   | Searching for Some Great Book 2 in node [email protected] that has index 8...
dht8_1   | Key Some Great Book 2 found in node [email protected] that has index 8...

# Leave command
dht4_1   | a LEAVE command was sent to node [email protected].
dht1_1   | Heartbeat from dht1 to dht4 failed, changing nodes chain.
dht1_1   | Subsequent 1 hostname is now: [email protected]
dht1_1   | Subsequent 2 hostname is now: [email protected]
dht16_1  | Changed SUBSEQUENT2_HOSTNAME to [email protected] with index of 8.
dht8_1   | Changed PREVIOUS_HOSTNAME to [email protected] with index of 1.

# Join command
dht4_1   | a JOIN command was sent to node [email protected].
dht1_1   | Changed SUBSEQUENT2_HOSTNAME to [email protected] with index of 8.
dht1_1   | Changed SUBSEQUENT1_HOSTNAME to [email protected] with index of 4.
dht8_1   | Changed PREVIOUS_HOSTNAME to [email protected] with index of 4.
dht16_1  | Changed SUBSEQUENT2_HOSTNAME to [email protected] with index of 4.