Cluster Architecture
This article provides an overview of the architecture of an Exasol cluster.
Database layer
An Exasol database runs in a cluster that contains one or more nodes. Each node is a host machine that has its own CPUs and main memory (RAM). The total number of CPUs and the total amount of RAM in a cluster determines the compute power.
A cluster can have any number of reserve nodes on standby in addition to the active data nodes. A reserve node is a node that has the same basic configuration as a data node, but it is not active and contains no data. Each active node holds a mirror of the data on the neighboring node. If one of the active nodes fails, the reserve node will automatically take over the role of the failed node. Data is then copied to the new node from a mirror on one of the other active nodes. For more details, see also Redundancy.
For information about how to manage clusters and nodes, see Nodes Management.
For information about the failover mechanisms in Exasol 8, see Fail Safety.
Shared-nothing architecture
Exasol is a massively parallel processing (MPP) database designed on a shared-nothing architecture (SN). This means that data is distributed across all nodes in the cluster and constantly synchronized between the nodes. All nodes operate on an equal basis, there is no master node. This processing paradigm is also known as SPMD (single program multiple data).
When a query is sent to the database, the query is first accepted by the node that the client is connected to and then distributed to all nodes in the cluster over the private network. Algorithms optimize the query, determine the best plan of action, and generate indexes as needed. The database then processes the partial results based on the local datasets and deliver the global results back to the user through the original connection.
For information about how to manage Exasol databases, see Database Management.
Storage layer
Data storage in Exasol is managed by a distributed storage engine, EXAStorage. Data is stored in data volumes that can be located on either local or remote storage services. Database backups are stored on separate backup volumes, which can also be created either in the cluster or on remote storage.
For information about how to create and manage volumes, see Storage Management.
BucketFS
BucketFS is a synchronous file system that is available on all database nodes in an Exasol clusters, and which is used for storing files such as drivers and scripts for user-defined functions (UDF scripts). Each node in the cluster can connect to the BucketFS service and will see the same content as the other nodes. The data stored in BucketFS is replicated locally on each node and automatically synchronized.
All configured data disks in Exasol have a preinstalled BucketFS service with a default bucket. You can create additional BucketFS services and buckets as needed.
The data in BucketFS is not part of the database backups and must be backed up separately.
For more information about BucketFS, see BucketFS.
Network layer
An Exasol cluster is normally configured with two networks using separate physical interfaces: a private network for internal communication between the nodes in the cluster, and a public network that allows connections to the database from outside of the cluster.
In Exasol 8, the physical networks must be configured using the network management interfaces on the Linux hosts. You can then specify the private and public IP addresses for the nodes in the deployment configuration.
For information about network requirements and configuration, see Installation.
Software
Exasol 8 provides several administration interfaces that enable you to deploy and manage your databases and clusters using both built-in and external tools. The administration interfaces are available on all nodes in a cluster.
For more information about the administration interfaces, see Administration Tools.
