YugaByte DB has a similar LSM storage engine design as Cassandra, but with additional benefits to performance and durability: Cassandra uses majority writes to update the commit logs of the replicas. Persistent memory allows Cassandra to have a single unified data model. If all the replicas are up, they will receive write request regardless of their consistency level. The partition index is then scanned to locate the compression offset which is then used to find the appropriate data on disk. Distributed Every node in the cluster has the same role. Separate Cassandra data centers which cater to distinct workloads using the same data, e.g. Over a million developers have joined DZone. Opinions expressed by DZone contributors are their own. High level Domain Expert in TOP MNCs with 8+ Years of Experience. The illustration above outlines key steps that take place when reading data from an SSTable. But first, we need determine what our keys are in general. Each node is assigned a token and is responsible for token values from the previous token (exclusive) to the node's token (inclusive). For a read request, Cassandra requests the data from the required number of replicas and compares their write-timestamp. Having looked at the data model of Cassandra, let's return to its architecture to understand some of its strengths and weaknesses from a distributed systems point of view. There are two kinds of replication strategies in Cassandra. This means you can determine the location of your data in the cluster based on the data. If the partition cache does not contain a corresponding entry the partition key summary is scanned. Apache Cassandra is an open-source, NoSQL database built from the foundation of geographically distributed and fault tolerant data replication. These writes are indexed and written to an in-memory structure called a memtable. Your requirements might differ from the architecture described here. Key Concepts, Data Structures and Algorithms. Cassandra also replicates data according to the chosen replication strategy. State information is exchanged every second and contains information about itself and all other known nodes. It then proceeds to fetch the compressed data on disk and returns the result set. Build a Cassandra cluster with geographically distinct data centers which cater to clients from distinct locations, e.g.a cluster with three data centers in US, EU, and APAC serving local clients with low latency. First, let’s take a high-level look at the main components in the original (unmodified) version of the Cassandra architecture. The first replica for the data is determined by the partitioner. Cassandra also keeps a copy of the bloom filter on disk which enables it to recreate the bloom filter in memory quickly . Cassandra architecture enables transparent distribution of data to nodes. Seed nodes are used during start up to help discover all participating nodes. At a 10000 foot level Cassandra stores data by dividing data evenly around its cluster of nodes. Apache Cassandra Hardware Choices. For example the machine has a power outage before the memtable could get flushed. Lets try and understand Cassandra's architecture by walking through an example write mutation. In our example let’s assume that we have a consistency level of QUORUM and a replication factor of three. The majority is one more than half of the nodes. Introduction to Apache Cassandra Architecture The architecture of Cassandra greatly contributes to its being a database that scales and performs with continuous availability. Please note in CQL (Cassandra Query Language) lingo a Column Family is referred to as a table. Over a period of time a number of SSTables are created. The chosen node is called the coordinator and is responsible for returning the requested data. Features. At the completion of this process the memtable is cleared and the commit log is recycled. Allrights Reserved by acte.in is a Division of. SimpleStrategy places the first replica on the node selected by the partitioner. It reaches its maximum allocated size in memory. This is, roughly speaking, a certain number. When mem-table is full, data is flushed to the SSTable data file. Cassandra also keeps a copy of the bloom filter on disk which enables it to recreate the bloom filter in memory quickly . The concept of requesting a certain number of acknowledgements is called tunable consistency and it can be applied at the individual query level. Also, Handled Around 16+ Projects and Shared his Knowledge by Writing these Blogs for us. In this post I have provided an introduction to Cassandra architecture. Commit log is used for crash recovery. Seeds nodes have no special purpose other than helping bootstrap the cluster using the gossip protocol. In my upcoming posts I will try and explain Cassandra architecture using a more practical approach. There are two main replication strategies used by Cassandra, Simple Strategy and the Network Topology Strategy. JAVA Developer | Openings in HCL Technologies- Apply Now! For example, there are 4 of them (see the picture below). Managed Apache Cassandra database service deployable on the cloud of your choice or on-prem. This data is then merged and returned to the coordinator. Our Service Location: Adambakkam, Adyar, Alwarpet, Arumbakkam, Ashok Nagar, Ambattur, Anna Nagar, Avadi, Aynavaram, Besant Nagar, Chepauk, Chengalpet, Chitlapakkam, Choolaimedu, Chromepet, Egmore, George Town, Gopalapuram, Guindy, Jafferkhanpet, K.K. About Apache Cassandra. Dataset Partitioning: Consistent Hashing. The number of racks in a data center should be in multiples of the replication factor. Cassandra was designed to ful ll the storage needs of the Inbox Search problem. separate data centers to serve client requests and to run analytics jobs. The illustration above outlines key steps when reading data on a particular node. Often Cassandra is deployed in a DC environment and one must replicate data intelligently to ensure no single point of failure. This is  a common case as the compaction operation tries to group all row key related data into as few SSTables as possible. The figure above illustrates dividing a 0 to 255 token range evenly amongst a four node cluster. The partition summary is a subset to the partition index and helps determine the approximate location of the index entry in the partition index. One, two, three: Specified number of replicas must acknowledge the operation. Every SSTable has an associated bloom filter which enables it to quickly ascertain if data for the requested row key exists on the corresponding SSTable. Cassandra has been architected from the ground up to handle large volumes of data while providing high availability. The key thing to keep in mind is that reaching a consistent state often takes microseconds. Documentation for developers and administrators on installing, configuring, and using the features and capabilities of Apache Cassandra scalable open source NoSQL database.