Below is a list of terms and their applicability within the context of Riak:
Active Anti-Entropy (AAE)
A continuous background process that compares and repairs any divergent, missing, or corrupted replicas. Unlike read repair, which is only triggered when data is read, the active anti-entropy system ensures the integrity of all data stored in Riak. This is particularly useful in clusters containing “cold data”: data that may not be read for long periods of time, potentially years. Furthermore, unlike the repair command, active anti- entropy is an automatic process, requiring no user intervention and is enabled by default in Riak 1.3.
A Bucket is a container and keyspace for data stored in Riak, with a set of common properties for its contents (the number of replicas, or n_val, for instance). Buckets are accessed at the top of the URL hierarchy under “riak”, e.g.
* Take a more in-depth look at Bucket Operations
A Riak cluster is a 160-bit integer space which is divided into equally-sized partitions. Each vnode in the Riak Ring is responsible for one of these partitions.
Consistent hashing is a technique used to limit the reshuffling of keys when a hash-table data structure is rebalanced (when slots are added or removed). Riak uses consistent hashing to organize its data storage and replication. Specifically, the vnodes in the Riak Ring responsible for storing each object are determined using the consistent hashing technique.
Riak uses a “gossip protocol” to share and communicate ring state and bucket properties around the cluster. Whenever a node changes its claim on the ring, it announces its change via this protocol. Each node also periodically sends its current view of the ring state to a randomly-selected peer, in case any nodes missed previous updates.
Hinted handoff is a technique for dealing with node failure in the Riak cluster in which neighboring nodes temporarily take over storage operations for the failed node. When the failed node returns to the cluster, the updates received by the neighboring nodes are handed off to it.
Hinted handoff allows Riak to ensure database availability. When a node fails, Riak can continue to handle requests as if the node were still there.
Keys are unique object identifiers in Riak and are scoped within buckets.
Lager is an Erlang/OTP framework that ships as Riak's default logger.
Links are metadata attached to objects in Riak. These links make establishing relationships between objects in Riak as simple as adding a Link header to the request when storing the object.
Riak's MapReduce gives developers the capability to perform more powerful queries over the data stored in their key/value data.
A node is analogous to a physical server. Nodes run a certain number of vnodes, each of which claims a partition in the Riak Ring key space.
An object is another name for a value.
Partitions are the spaces into which a Riak cluster is divided. Each vnode in Riak is responsible for a partition. Data are stored on a set number of partitions determined by the n_val setting, with the target partitions chosen statically by applying consistent hashing to an object's key.
Quorum in Riak has two meanings:
- The quantity of replicas that must respond to a read or write request before it is considered successful. This is defined as a bucket property or as one of the relevant parameters to a single request (R,W,DW,RW).
- A symbolic quantity for the above,
quorum, which is equivalent to
n_val / 2 + 1. With Riak's default settings, this is
Read repair is an anti-entropy mechanism Riak uses to optimistically update stale replicas when they reply to a read request with stale data.
Replicas are copies of data stored in Riak. The number of replicas required for both successful reads and writes is configurable in Riak and should be set based on your application's consistency and availability requirements.
Riak Core is the modular distributed systems framework that serves as the foundation for Riak's scalable architecture.
Riak KV is a component application of Riak and provides a key/value datastore and features MapReduce, lightweight data relations, and several different client APIs.
Riak Pipe is the processing layer that powers Riak's MapReduce. It's best described as “UNIX pipes for Riak.”
Riak Search is a distributed, scalable, failure-tolerant, real-time, full-text search engine built around Riak Core and tightly integrated with Riak KV.
The Riak Ring is a 160-bit integer space. This space is equally divided into partitions, each of which is claimed by a vnode, which themselves reside on actual physical server nodes.
Secondary Indexing in Riak gives developers the ability to tag an object stored in Riak with one or more values which can then be queried.
Riak is most-easily described as a key/value store. In Riak, “values” are opaque BLOBS (binary large objects), identified with a unique key, that can be any type of data, though some programming advantages are gained by using JSON.
Riak utilizes vector clocks (or vclock) to handle version control. Since any node in a Riak cluster is able to handle a request, and not all nodes need to participate, data versioning is required to keep track of a current value. When a value is stored in Riak, it is tagged with a vector clock and establishes the initial version. When it is updated, the client provides the vector clock of the object being modified so that this vector clock can be extended to reflect the update. Riak can then compare vector clocks on different versions of the object and determine certain attributes of the data.
Vnodes, or “virtual nodes” are responsible for claiming a partition in the Riak Ring, and they coordinate requests for these partitions. Vnodes reside on physical nodes in a Riak cluster, and the number of vnodes per physical node is determined by the total number of vnodes and the number of active physical nodes in the cluster. Riak balances the assignment of vnodes across the active physical nodes.