Cassandra is a distributed storage system that can spread over thousands of nodes and store terabytes of structured data. Cassandra was developed at Facebook to solve performance issues during searches in Facebook Inbox. Cassandra can provide a highly available service without a single point of failure.

Cassandra borrows some of its architecture choices from Google's BigTable and Amazon's Dynamo . In some ways, Cassandra resembles the traditional databases, yet it does not support the relational data model completely.

Data Model

Cassandra uses a tabular data structure like relational databases. A table in Cassandra is a distributed multi-dimensional map indexed by a key. Every row operation is atomic per replica, no matter how many columns are being read or written into. Cassandra groups columns into sets called column family. A column family is of two types - Simple and Super (column family within column family). Cassandra can sort columns using name or time. Time-based sorting is useful when used in a setting like Inbox searches where most recent messages should be displayed first.

Cassandra provides following methods -

1. $insert(table, key, rowMutation)$
2. $get(table, key, tableName)$
3. $delete(table, key, tableName)$

Architecture

A data storage system should satisfy some requirements - data persistence, scalability, membership, failure detection and handling, data partitioning, request routing, among others. Cassandra uses a variety of techniques to solve these issues.

Routing

Any node can attend an incoming read/write request with a key. Each node in the Cassandra cluster knows about other nodes. The serving node uses this information and the request's key to determine the route of the request to the appropriate node.

In the case of write requests, the system routes the request to all replicas and waits for a quorum of replicas to acknowledge the completion of the writes.

For reads, the system either routes the requests to the nearest replica with required data or forwards the request to all the replicas and waits for a quorum of responses before replying. Which method, the system utilizes, depends on the consistency requirements of the client.

Partitioning

Cassandra uses consistent hashing with an order-preserving hash. The consistent hash function is used to hash the data key to generate identifiers. The output range of the hash function is treated as a ring (the largest hash value generated wraps around the smallest hash value). Each node is assigned a random hash, which becomes its position on the ring. Each node is responsible for the region in the ring between it and its predecessor node.

For each data item with a key, its hash is generated using the key. The ring is traversed clockwise, and the first node with the position hash value greater than the item's hash value is assigned to the data item. This node is deemed the coordinator of the key.

However, consistent hashing can result in an imbalance in the load distribution and non-uniformity of data distribution. Cassandra solves these issues by periodically analyzing the load information in the ring and repositioning the lightly loaded nodes on the ring to alleviate high load nodes.

Replication

Each data item is replicated at $N$ nodes in the ring. As mentioned above, each data item's key is assigned to a coordinator node, which is responsible for the replication of data items falling within its range (the region between it and its predecessor node). For fault tolerance, in addition to locally storing keys, the coordinator node replicates these keys at the $(N-1)$ replicas on the ring. Replication policies such as Rack aware, Rack unaware, Datacenter aware are used for this purpose.

Leader Election

Cassandra uses Zookeeper for leader election and fault tolerance. Whenever a new node joins the cluster, it contacts the leader who tells them what ranges the node is responsible for. The metadata about ranges of a node is cached locally as well as on the Zookeeper instance.

Membership

Cassandra uses Scuttlebutt - an anti-entropy Gossip based protocol to disseminate the membership information inside the ring. Whenever a new node joins the system, it calculates a token for itself. This token is gossiped around the cluster. That's how each node in the system knows about the membership and positions of other nodes in the system.

Failure Detection

Failure detection is a mechanism by which a node can locally determine if any other node in the system is up or down. Cassandra uses a modified version of $\phi$-Accrual Failure Detector. The basic idea is that the failure detection module emits the suspicion level of a node instead of a binary up/down status. This suspicion level is $\phi$. The idea is to represent $\phi$ on a dynamically adjustable scale, which reflects network and load conditions at the monitored nodes.

Every node in the system maintains a sliding window of inter-arrival times of gossip messages from other nodes in the cluster. The distribution of these inter-arrival times is determined, and $\phi$ is calculated. Cassandra uses exponential distribution as an approximation for determining $\phi$. The original $\phi$-Accrual Failure Detection Algorithm used Gaussian distribution.

Scaling

Whenever a new node joins the system, its token is generated such that it falls within the range of an existing heavily loaded node. This results in the new node splitting the range of the old node. The old node transfers some of its data to the new node using kernel-kernel copying techniques.

Local Persistence

Cassandra uses a commit log as well as an in-memory data structure to store the data. Each write is first committed to the commit log. Only after successful write into the commit log, the data is saved in the in-memory data structure. When the in-memory data structure crosses a predefined threshold, it is dumped to the disk along with an index file for fast lookups. A merge process runs periodically to merge these disk files.

A read operation first queries the in-memory store. If data is not found there, then a disk lookup is required. To avoid looking into multiple files, a bloom filter, summarizing the keys in the file, is also used. The bloom filter can also be used to check the key existence.

Paper Link:- Cassandra - A Decentralized Structured Storage System