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Replication is the Key for Scalability & High Availability
December 18, 2014
One of the interesting things about attending industry events, like AWS re:Invent, is identifying common trends that arise in conversations. Recent conversations point to a renewed interest in “enterprise ready replication” for NoSQL databases.
As business data continues to grow, there is an entirely new set of challenges that are presented related to availability, scalability, and fault-tolerance. While most NoSQL databases work at small scale, availability is often compromised as applications reach full production or peak capacity. Having the right replication functionality is key to ensuring that availability requirements are not compromised as your system grows.
“Replication” may mean different things based on context. In this case, we are referring to the movement of data in a database cluster — or across datacenters — for the purpose of redundancy or data locality. If your database experience began in an RDBMS context, then replication implies a specific contextual understanding of multi-master transactional deployment and, perhaps, shipping transaction logs between incremental backups in a hot/warm database configuration. In contrast, for those who began in the NoSQL era, the term may evoke images of replica-sets on a sharded infrastructure and the operational overhead associated therewith.
In a distributed NoSQL database, like Riak, the term replication is used to encompass two distinct concepts. First, intra-cluster replication for high availability and fault tolerance within the datacenter; and second, multi-datacenter replication for data locality and global availability. There is none of the complexity of log shipping or dealing with a sharded infrastructure.
Intra-cluster Replication
Data replication is a core feature of Riak’s basic architecture. Riak was designed to operate as a clustered system containing multiple nodes (commodity servers or cloud instances). The replication implementation allows data to live on multiple machines at once, with a single write request, in case a node in the cluster goes down or is unavailable due to issues like network partitioning.
Intra-cluster replication is fundamental and automatic in Riak, so that your data is always available. All data stored in Riak is replicated to a number of nodes in the cluster according to a configurable parameter (n_val) set in a buckets bucket type.
With the default n_val setting of 3, there are always three copies of all data. These copies will be on three different partitions/vnodes. A detailed explanation and analysis of this replication capability is discussed in the Riak documentation – Understanding replication by example.
In the case of intra-cluster replication, or what we would refer to simply as “replication”, data distribution ensures redundant data such that high availability is maintained in a failure state.
Multi-Datacenter Replication
In contrast to intra-cluster replication, multi-datacenter replication (a feature of Riak Enterprise) is a critical part of modern application infrastructures. Riak Enterprise offers multi-datacenter replication features so that data stored in Riak can be replicated to multiple sites (vs. multiple servers in the same site).
As we are all aware, understanding application latency (for an end user) begins with the realization data can’t travel faster than the speed of light. So, inherently, as source information moves further from it’s consumption latency is introduced. As such, there is a set amount of latency for a customer connecting to your application hosted in New York when they are accessing the application from San Francisco. This latency profile increases, and becomes more complex, as the geographic distribution of your customer base increases.
With multi-datacenter replication in Riak Enterprise, data can be replicated across locations and geographic areas providing for disaster recovery, data locality, compliance with regulatory requirements, the ability to “burst” peak loads into public cloud infrastructure, amongst others.
Riak’s multi-datacenter replication is masterless. One cluster acts as a primary, or source, cluster. The primary cluster handles replication requests from one or more secondary, or sink, clusters (generally located in datacenters in other regions or countries). If the datacenter with the primary cluster goes down, a secondary cluster can automatically take over as the primary cluster.
More architectural strategies for multi-datacenter implementations, are covered in the Basho whitepaper entitled Riak Enterprise: Multi-Datacenter Replication – A Technical Overview & Use Cases or in the Basho Documentation section Multi-Datacenter Replication: v3 Architecture.
In Summary
Replication, inside a cluster, is a core design tenant of Riak. This is used to provide the availability and fault-tolerance characteristics — with a low operational overhead — that many unstructured data workloads demand.
Multi-datacenter replication, while related, is an entirely different approach and architecture to enable the geographic distribution of data to solve for redundancy, geo-data locality, etc.
Replication is an important scalability feature of any database deployment. Ensuring that your NoSQL database replicates data in a way that is scalable, operationally simple and achieves your business objectives is key to your success.
Why Your Riak Cluster Should Have At Least Five Nodes
April 26, 2012
Here at Basho we want to make sure that your Riak implementations are set up from the beginning to succeed. While you can use the Riak Fast Track to quickly set up a 3-node dev/test environment, we recommend that all production deployments use a minimum of 5 nodes, ensuring you benefit from the architectural principles that underpin Riak’s availability, fault-tolerance and scaling properties.
TL;DR: Deployments of five nodes or greater will provide a foundation for the best performance and growth as the cluster expands. Since Riak scales linearly with the addition of more nodes, users find improved performance, reliability, and throughput with larger clusters. Smaller deployments can compromise the fault-tolerance of the system: with a “sane” replication requirement for availability (we default to three copies), node failures in smaller clusters mean that replication requirements may not be met. This can result in degraded performance and risk of data loss. Additionally, clusters smaller than five nodes mean that with a sane replication requirement of 3, a high percentage (75-100% of the nodes) will need to respond to each request, putting undue load on the cluster that may degrade performance.
Let’s take a closer look in the scenario of a three- and four-node cluster.
Performance and Fault Tolerance Concerns in a 3-Node Cluster
To ensure that the cluster is always available to respond to read and write requests, Basho recommends a “sane default” for data replication: three copies of the data on three different nodes. The default configuration of Riak requires four nodes at minimum to insure no single node holds more than one copy of any particular piece of data. (In future versions of Riak we’ll be able to guarantee that each replica is living on a separate physical node. At this point it’s almost at 100%, but we won’t tell you it’s guaranteed until it is.) While it is possible to change the settings to ensure that the three replicas are on distinct nodes in a three node cluster, you still run into issues of replica placement during a node failure or network partition.
In the event of node failure or a network partition in a three-node cluster, the default requirement for replication remains three but there are only two nodes available to service requests. This will result in degraded performance and carries a risk of data loss.
Performance and Fault Tolerance Concerns in a 4-Node Cluster
With a requirement of three replicas, any one request for a particular piece of data from a 4-node cluster will require a response from 75 – 100% of the nodes in the cluster, which may result in degraded performance. In the event of node failure or a network partition in a 4-node cluster, you are back to the issues we outline above.
What if I want to change the replication default?
If using a different data replication number is right for your implementation, just make sure to use a cluster of N +2 nodes where N is the number of replicas for the reasons outlined above.
Going With 5 Nodes
As you add nodes to a Riak cluster that starts with 5 nodes, the percentage of the cluster required to service each request goes down. Riak scales linearly and predictably from this point on. When a node is taken out of service or fails, the number of nodes remaining is large enough to protect you from data loss.
So do your development and testing with smaller clusters, but when it comes to production, start with five nodes.
Happy scaling.
