Eon Mode concepts

• 1: Eon Mode architecture
• 2: Namespaces and shards
• 3: Subclusters
• 4: Elasticity
• 5: Data integrity and high availability in an Eon Mode database
• 6: Stopping, starting, terminating, and reviving Eon Mode database clusters

1 - Eon Mode architecture

Eon Mode separates the computational resources from the storage layer of your database. This separation gives you the ability to store your data in a single location. You can elastically vary the number of nodes connected to that location according to your computational needs. Adjusting the size of your cluster does not interrupt analytic workloads.

You can create an Eon Mode database either in a cloud environment, or on-premises on your own systems.

Eon Mode is suited to a range of needs and data volumes. Because compute and storage are separate, you can scale them separately.

Communal storage

Instead of storing data locally, Eon Mode uses a single communal storage location for all data and the catalog (metadata). Communal storage is the database's centralized storage location, shared among the database nodes. Communal storage is based on an object store, such as Amazon's S3 in the cloud or a PureStorage FlashBlade appliance in an on-premises deployment. In either case, Vertica relies on the object store to maintain the durable copy of the data.

Communal storage has the following properties:

• Communal storage in the cloud is more resilient and less susceptible to data loss due to storage failures than storage on disk on individual machines.

• Any data can be read by any node using the same path.

• Capacity is not limited by disk space on nodes.

• Because data is stored communally, you can elastically scale your cluster to meet changing demands. If the data were stored locally on the nodes, adding or removing nodes would require moving significant amounts of data between nodes to either move it off of nodes that are being removed, or onto newly-created nodes.

Communal storage locations are listed in the STORAGE_LOCATIONS system table with a SHARING_TYPE of COMMUNAL.

Within communal storage, the data is grouped into one or more namespaces. Namespaces segment their associated data into a specific number of portions called shards. Shards are how Vertica divides data among the nodes in the database. Nodes subscribe to particular shards, with subscriptions balanced among the nodes. When loading or querying data, each node is responsible for the data in the shards it subscribes to. See Namespaces and shards for more information.

Depot storage

A potential drawback of communal storage is its speed, especially in cloud environments. Accessing data from a shared cloud location is slower than reading it from local disk. Also, the connection to communal storage can become a bottleneck if many nodes are reading data from it at once. To improve data access speed, the nodes in an Eon Mode database maintain a local disk cache of data called the depot. When executing a query, the nodes first check whether the data it needs is in the depot. If it is, then the node finishes the query using the local copy of the data. If the data is not in the depot, the node fetches the data from communal storage, and saves a copy in the depot.

The node stores newly-loaded data in the depot before sending it to communal storage. See Loading Data below for more details.

By default, Vertica sets the maximum size of the depot to be 60% of the total disk space of the filesystem that stores the depot. You can adjust the size of the depot if you wish. Vertica limits the size of the depot to a maximum of 80% of the filesystem that contains it. This upper limit ensures enough disk space for other uses, such as temporary files that Vertica creates during data loads.

Each node also stores a local copy of the database catalog.

Loading data

In Eon Mode, COPY statements usually write to read optimized store (ROS) files in a node's depot to improve performance. The COPY statement segments, sorts, and compresses for high optimization. Before the statement commits, Vertica ships the ROS files to communal storage.

Because a load is buffered in the depot on the node executing the load, the size of your depot limits the amount of data you can load in a single operation. Unless you perform multiple loads in parallel sessions, you are unlikely to encounter this limit.

At load time, the participating nodes write files to the depot and synchronously send them to communal storage. The data is also sent to all nodes that subscribe to the shard into which the data is being loaded. This mechanism of sending data to peers at load time improves performance if a node goes down, because the cache of the peers who take over for the down node is already warm. The file compaction mechanism (mergeout) puts its output files into the cache and also uploads them to the communal storage.

The following diagram shows the flow of data during a COPY statement.

Querying data

Vertica uses a slightly different process to plan queries in Eon Mode to incorporate the sharding mechanism and remote storage. Instead of using a fixed-segmentation scheme to distribute data to each node, Vertica uses the sharding mechanism to segment the data in each namespace into a specific number of shards that at least one (and usually more) nodes subscribes to. When the optimizer selects a projection, the layout for the projection is determined by the participating subscriptions for the session. The optimizer generates query plans that are equivalent to those in Enterprise Mode. It selects one of the nodes that subscribes to each shard to participate in query execution.

Vertica first tries to use data in the depot to resolve a query. When the data in the depot cannot resolve the query, Vertica reads from the communal storage. You could see an impact on query performance when a substantial number of your queries read from the communal storage. If this is the case, then you should consider re-sizing your depot or use depot system tables to get a better idea of what is causing the issue. You can use ALTER_LOCATION_SIZE to change depot size.

Workload isolation and scaling

Eon Mode lets you define subclusters that divide up your nodes to isolate workloads from one another. You can also use subclusters to ensure that scaling down your cluster does not result in Vertica going into read-only mode to maintain data integrity. See Subclusters for more information.

2 - Namespaces and shards

In Eon Mode, Vertica stores data communally in a shared data storage location, such as in S3 when running on AWS. All nodes are capable of accessing all of the data in the communal storage location. In order for nodes to divide the work of processing queries, Vertica must divide the data between them in some way. It breaks the data in communal storage into segments called shards. Each node in your database subscribes to a subset of the shards in the communal storage location. The shards in your communal storage location are similar to a collection of segmented projections in an Enterprise Mode database.

The number of shards that data is segmented into depends on the namespace to which the data belongs. Namespaces are a collection of schemas and tables in your database that are grouped under a common name and segmented into the number of shards defined by that namespace. In Eon Mode databases, namespaces represent the top-level data structure in the Vertica object hierarchy. Every table and schema in the database belongs to a namespace. By default, the database contains a single default namespace, default_namespace, which is created on database creation with the shard count specified during setup. For details about creating and managing additional namespaces, see Managing namespaces.

When you create a table or schema, you can specify the namespace under which the object is created. If no namespace is specified, the table or schema is created under the default namespace.

The recommended shard count for a namespace depends primarily on the size of the tables it will store and the expected complexity of its query workload. Generally, the database performs best when large tables with complex analytic workloads are segmented with a larger shard count, and small tables with simpler query workloads are segmented with a smaller shard count. Aligning table size and query complexity to shard count also minimizes catalog overhead. If your database contains namespaces with a range of shard counts, you can create tables and schemas under the namespace that best fits these basic sizing guidelines, or those of your particular use case. For a more detailed exploration of shard count and various use cases, see Configuring your Vertica cluster for Eon Mode.

For the best performance, the number of shards you choose for a namespace should be no greater than twice the number of nodes. At most, you should limit the shard:node ratio to no greater than 3:1. MC warns you to take all aspects of shard count into consideration. The number of shards should always be a multiple (or an even divisor) of the number of nodes in your database. See Choosing shard and initial node counts for more information.

For efficiency, Vertica transfers metadata about shards directly between database nodes. This peer-to-peer transfer applies only to metadata; the actual data that is stored on each node gets copied from communal storage to the node's depot as needed.

Shard subscriptions and k-safety

When K-safety is 1 or higher (high availability), each shard has more than one node subscribing to it in each subcluster. One of the subscribers is responsible for executing queries involving the shard. The other subscribers act as backups. If the main subscriber shuts down or is stopped, then another subscriber takes its place. See Data integrity and high availability in an Eon Mode database for more information.

Each shard in a namespace has a primary subscriber. This subscriber is a primary node that maintains the data in the shard by planning Tuple Mover operations on it. This node can delegate executing these actions to another node in the database cluster. See Tuple mover for more information about these operations. If the primary subscriber node is stopped or goes down, Vertica chooses another primary node that subscribes to the shard as the shard's primary subscriber. If all of the primary nodes that subscribe to a shard go down or are stopped, your database goes into read-only mode to maintain data integrity. Any primary node that is the sole subscriber to a shard is a critical node.

3 - Subclusters

Because Eon Mode separates compute and storage, you can create subclusters within your cluster to isolate work. For example, you might want to dedicate some nodes to loading data and others to executing queries. Or you might want to create subclusters for dedicated groups of users (who might have different priorities). You can also use subclusters to organize nodes into groups for easily scaling your cluster up and down.

Every node in your Eon Mode database must belong to a subcluster. This requirement means your database must always have at least one subcluster. When you create a new Eon Mode database, Vertica creates a subcluster named default_subcluster that contains the nodes you create on database creation. If you add nodes to your database without assigning them to a subcluster, Vertica adds them to the default subcluster. You can choose to designate another subcluster as the default subcluster, or rename default_subcluster to something more descriptive. See Altering subcluster settings for more information.

Using subclusters for work isolation

Database administrators are often concerned about workload management. Intense analytics queries can consume so many resources that they interfere with other important database tasks, such as data loading. Subclusters help you prevent resource issues by isolating workloads from one another.

In Eon Mode, by default, queries only run on nodes in the subcluster that contains the initiator node. For example, consider the two subclusters shown in the following diagram. If you are connected to Node 4, your queries would run on nodes 4 through 9.

Similarly, queries started on Node 1 only run on nodes 1 through 3.

This isolation lets you configure your database cluster to prevent workloads from interfering with each other. You can assign subclusters to specific tasks such as loading data, performing in-depth analytics, and short-running dashboard queries. You can also create subclusters for different groups in your organization, so their workloads do not interfere with one another. You can tune the settings of each subcluster (resource pools, for example) to match their specific workloads.

Subcluster types

There are two types of subclusters: primary and secondary.

Primary subclusters form the core of your Vertica database. They are responsible for planning the maintenance of the data in the communal storage. Your primary subclusters must always be running. If all of your primary subclusters shut down, your database shuts down because it cannot maintain the data in communal storage without a primary subcluster.

Usually, you have just a single primary subcluster in your database. You can choose to have multiple primary subclusters. Additional primary subclusters can make your database more resilient to having primary nodes fail. However, additional primary subclusters make your database less scalable. You usually do not dynamically add or remove nodes from primary subclusters or shut them down to scale your database. In most cases, a single primary subcluster is enough.

Secondary subclusters are designed for dynamic scaling: you add and remove or start and stop these subclusters based on your analytic needs. They are not essential for maintaining your database's data. So, you can easily add, remove, and scale up or down secondary subclusters without impacting the database's ability to run normally.

The nodes in the subcluster inherit their primary or secondary status from the subcluster that contains them; primary subclusters contain primary nodes and secondary subclusters contain secondary nodes.

Subcluster types and elastic scaling

The most important difference between primary and secondary subclusters is their impact on how Vertica determines whether the database is K-Safe and has a quorum. Vertica only considers the nodes in primary subclusters when determining whether all of the shards in the database have a subscribing node. It also only considers primary nodes when determining whether more than half the nodes in the database are running (also known as having a quorum of primary nodes). If either of these conditions is not met, the database goes into read-only mode to prevent data corruption. See Data integrity and high availability in an Eon Mode database for more information about how Vertica maintains data integrity.

Vertica does not consider the secondary nodes when determining whether the database has shard coverage or a quorum of nodes. This fact makes secondary subclusters perfect for managing groups of nodes that you plan to expand and reduce dynamically. You can stop or remove an entire subcluster of secondary nodes without forcing the database into read-only mode.

Minimum subcluster size for K-safe databases

In a K-safe database, subclusters must have at least three nodes in order to operate. Each subcluster tries to maintain subscriptions to all shards in the database. If a subcluster has less than three nodes, it cannot maintain redundant shard coverage where each shard has at least two subscribers in the subcluster. Without redundant coverage, the subcluster cannot continue processing queries if it loses a node. Vertica returns an error if you attempt to rebalance shards in a subcluster with less than three nodes in a K-safe database.

See also

• Eon Mode architecture
• Adding and removing nodes from subclusters
• Altering subcluster settings
• Configuring your Vertica cluster for Eon Mode
• Creating sandboxes

4 - Elasticity

Elasticity refers to the ability for you adjust your database to changing workload demands by adding or removing nodes. When your database experiences high demand, you can add new nodes or start stopped nodes to increase the amount of compute available. When your database experiences lower demands (such as during holidays or weekends) you can stop or terminate nodes to save money. You can also gradually add nodes over time as your database demands grow.

All nodes in an Eon Mode database belong to a subcluster. By choosing which subclusters get new nodes, you can affect how the new nodes impact your database. There are two goals you can achieve when adding nodes to your database:

• Improve query throughput: higher throughput means your database processes more queries simultaneously. You often want to improve throughput when you have a workload of "dashboard queries": many relatively short-running queries. In this case, speeding up the processing of individual queries is not as important as having more queries run in parallel.

• Improve query performance: higher query performance means that your complex in-depth analytic queries complete faster.

Scaling for query throughput

To scale for query throughput, add additional nodes to your database in one or more new subclusters. Subclusters independently process queries: a query only runs on the nodes in the subcluster containing the initiator node. By adding one or more subclusters, your database can process more queries at the same time. For the best performance, add the same number of nodes to each new subcluster as there are shards in the database. For example, if you have 6 shards in your database, add 6 nodes to each new subcluster you create.

To take advantage of the improved throughput offered by the new subclusters, clients must connect to them. The best way to ensure your users take advantage of the subclusters you have added for throughput scaling is to create connection load balancing policies that spread client connections across the all nodes in all of these subclusters. See Connection load balancing policies for more information.

Subclusters also organize nodes into groups that can easily be stopped or started together. This feature makes expanding and shrinking your database easier. See Starting and stopping subclusters for details.

Scaling for query performance

To improve the performance of individual queries in a subcluster, add more nodes to it. Queries perform faster when there is more computing power available to process them.

Adding nodes is especially effective if your subcluster has less nodes than there are shards in the database. In this case, nodes are responsible for processing data in multiple shards. When you add more nodes, the newly-added nodes take over responsibility for some of the shards. With less data to process, each node finishes their part of the query faster, resulting in better overall performance. For the best performance, make the number of nodes in the subcluster an even divisor of (or equal to) the number of shards in the database. For example, in a 12-shard database, make the number of nodes in the subcluster 3, 6, or 12.

You can further improve query performance by adding more nodes than there are shards in the database. When nodes outnumber shards, multiple nodes in the subcluster subscribe to the same shard. In this case, when processing a query, Vertica uses a feature called elastic crunch scaling (ECS) to have all of the nodes in the subcluster take part in the query. ECS assigns a subset of the data in each shard to the shard's subscribers. For example, in six-node subcluster in a a three-shard database, each shard has two subscribers. ECS assigns each of the subscribers half of the data in the shard to process during queries. In most cases, with less data to process, the nodes finish executing the query faster. When adding more nodes than shards to a subcluster, make the number of nodes a multiple of the number of shards to ensure an even distribution. For example, in a three-shard database, make the number of nodes in the subcluster 6, 9, 12, and so on.

Using different subclusters for different query types

You do not have to choose one form of elasticity over the other in your database. You can create a group of subclusters to improve query throughput and one or more subclusters that improve query performance. The difference between the two subcluster types is mainly the number of subclusters you create and the number of nodes they contain. To improve throughput, add a multiple subclusters that contain a number of nodes that is equal to or less than the number of shards in the database. The more subclusters you add, the greater the throughput you achieve. To improve query performance, add one or more subclusters where the number of nodes is a multiple of the number of shards in the database.

Once you have created your set of subclusters, you must have clients connect to the correct subcluster for the types of queries they will run. For clients executing frequent, simple dashboard queries, create a connection load balancing policy that connects them to nodes in the throughput scaling subclusters. For clients running more complex analytic queries, create another load balancing policy that connects them to nodes in the performance scaling subcluster.

For details on scaling your Eon Mode database, see Scaling your Eon Mode database.

5 - Data integrity and high availability in an Eon Mode database

The nodes in your Eon Mode database's primary subclusters are responsible for maintaining the data in your database. These nodes (collectively called the database's primary nodes) plan the Tuple Mover mergeout operations that manage the data in the shards. They can also execute these operations if they are the best candidate to do so (see The Tuple Mover in Eon Mode Databases).

The primary nodes can be spread across multiple primary subclusters—they all work together to maintain the data in the shards. The health of the primary nodes is key for your database to continue running normally.

The nodes in secondary subclusters do not plan Tuple Mover operations. They can execute Tuple Mover mergeout operations if a primary node assigns it to them. Your database cluster can lose all of its secondary nodes and still maintain the data in the shards.

Maintaining data integrity the is top goal of your database. If your database loses too many primary nodes, it cannot safely process data. In this case, it goes into read-only mode to prevent data inconsistency or corruption.

High availability (having the database continue running even if individual nodes are lost) is another goal of Vertica. It has several redundancy features to help it prevent downtime. With these features enabled, your database continues to run even if it loses one or more primary nodes.

There are two requirements for the database to continue normal operations: maintaining quorum, and maintaining shard coverage.

Maintaining quorum

The basic requirement for the primary nodes in your Eon Mode database is maintaining a quorum of primary nodes running at all times. To maintain quorum, more than half of the primary nodes (50% plus 1) must be up. For example, in a database with 6 primary nodes, at least 4 of them must be up. If half or more of the primary nodes are down, your database goes into read-only mode to prevent potential data integrity issues. In a database with 6 primary nodes, the database goes into read-only if it loses 3 or more of them. See Read-Only Mode below.

Vertica only counts the primary nodes that are currently part of the database when determining whether the database has quorum. Removing primary nodes cannot result in a loss of quorum. During the removal process, Vertica adjusts the node count to prevent the loss of quorum.

At a minimum, your Eon Mode database must have at least one primary node to function. In most cases, it needs more than one. See Minimum Node Requirements for Eon Mode Database Operation below.

Maintaining shard coverage

In order to continue to process data, your database must be able to maintain the data in its shards. To maintain the data, each shard must have a subscribing primary node that is responsible for running the Tuple Mover on it. This requirement is called having shard coverage. If one or more shards do not have a primary node maintaining its data, your database loses shard coverage and goes into read-only mode (explained below) to prevent possible data integrity issues.

The measure of how resilient your Eon Mode database is to losing a primary node is called its K-safety level. The value K is the number of redundant shard subscriptions your Eon Mode database cluster maintains. It also represents the number of primary nodes in your database that can fail and still be able to run safely. In many cases, your database can lose more than K nodes and still continue to run normally, as long as it maintains shard coverage.

Vertica recommends that your database always have a K-safety value of 1 (K=1). In a K=1 database, each shard has two subscribers: a primary subscriber that is responsible for the shard, and a secondary subscriber that can fill in if the primary subscriber is lost. The primary subscriber is responsible for running the Tuple Mover on the data in the shard. The secondary subscriber maintains a copy of the shard's catalog metadata. so it can fill in if the primary subscriber is lost.

If a shard's primary subscriber fails, the secondary subscriber fills in for it. Because it does not maintain a separate depot for its secondary subscription, the secondary subscriber always directly accesses the shard's data in communal storage. This direct access impacts your database's performance while a secondary subscriber fills in for a primary subscriber. For this reason, always restart or replace a down primary node as soon as possible.

With primary and secondary subscribers in a K=1 database, the loss of a single primary node does not affect the database's ability to process and maintain data. However, if the secondary subscriber fails while standing in for the primary subscriber, your database would lose shard coverage. and be forced to go into read-only mode.

Elastic K-safety

Vertica uses a feature called elastic K-safety to help limit the possibility of shard coverage loss. By default, if either the primary or secondary subscriber to a shard fails, Vertica subscribes an additional primary node to the shard. This subscription takes time to be established, as the newly-subscribed node must get a copy of the shard's metadata. If the shard's sole subscriber fails while the new subscriber is getting the shard's metadata, the database loses shard coverage and can shut down. Once the newly-subscribed node gets a copy of the metadata, it is able to take over maintenance of the shard in case the other subscriber fails. At this point, your database once again has two subscribers for the shard.

Once the down nodes recover and rejoin the subcluster, Vertica removes the subscriptions it added for elastic K-safety. Once all of the nodes rejoin the cluster, the shard subscriptions are the same as they were before the node loss.

With elastic K-safety, your database could manage to maintain shard coverage through the gradual loss of primary nodes, up to the point that it loses quorum. As long as there is enough time for newly-subscribed nodes to gather the shard's metadata, your database is able to maintain shard coverage. However, your database could still be forced into read-only mode due to loss of shard coverage if it lost the primary and secondary subscribers to a shard before a new primary node could complete the process of subscribing to the shard.

Database read-only mode

If your database loses either quorum or primary shard coverage, it goes into read-only mode. This mode prevents potential data corruption that could result when too many nodes are down or unable to reach other nodes in the database. Read-only mode prevents changes being made to the database that require updates to the global catalog.

DML and DDL in read-only mode

In read-only mode, statements that change the global catalog (such as most DML and DDL statements) fail with an error message. For example, executing DDL statements such as CREATE TABLE while the database is in read-only mode results in the following error:

=> CREATE TABLE t (a INTEGER, b VARCHAR);
ERROR 10428:  Transaction commit aborted since the database is currently in read-only mode
HINT:  Commits will be restored when the database restores the quorum

DML statements such as COPY return a different error. Vertica stops them from executing before they perform any work:

=> COPY warehouse_dimension from stdin;
ERROR 10422:  Running DML statements is not possible in read-only mode
HINT:  Running DMLs will be restored when the database restores the quorum

By returning the error early, Vertica avoids performing all of the work required to load data, only to fail when it tries to commit the transaction.

DDL and DML statements that do not affect the global catalog still work. For example, you can create a local temporary table and load data into it while the database is in read-only mode.

Queries in read-only mode

Queries can run on any subcluster that has shard coverage. For example, suppose you have an Eon Mode database with a 3-node primary and a 3-node secondary subcluster. If two of the primary nodes go down, the database loses quorum and goes into read-only mode. The primary subcluster also loses shard coverage, because two of its nodes are down. The remaining node does not have a subscription to at least some of the shards. In this case, queries on the remaining primary node (except for some system table queries) always fail:

=> SELECT * FROM warehouse_dimension;
ERROR 9099:  Cannot find participating nodes to run the query

The secondary subcluster still has shard coverage so you can execute queries on it.

Monitoring read-only mode

Besides noticing DML and DDL statements returning errors, you can determine whether the database has gone into read-only mode by monitoring system tables:

• The NODES system table has a column named is_readonly that becomes true for all nodes when the database is in read-only mode.

  => SELECT node_name, node_state, is_primary, is_readonly, subcluster_name FROM nodes;
  
        node_name       | node_state | is_primary | is_readonly |  subcluster_name
  ----------------------+------------+------------+-------------+--------------------
   v_verticadb_node0001 | UP         | t          | t           | default_subcluster
   v_verticadb_node0002 | DOWN       | t          | t           | default_subcluster
   v_verticadb_node0003 | DOWN       | t          | t           | default_subcluster
   v_verticadb_node0004 | UP         | f          | t           | analytics
   v_verticadb_node0005 | UP         | f          | t           | analytics
   v_verticadb_node0006 | UP         | f          | t           | analytics
  (6 rows)
  

• When the database goes into read-only mode, every node that is still up in the database records a Cluster Read-only event (event code 20). You can find these events by querying the event monitoring system tables such as ACTIVE_EVENTS:

  => \x
  Expanded display is on.
  
  => SELECT * FROM ACTIVE_EVENTS WHERE event_code = 20;
  -[ RECORD 1 ]-------------+--------------------------------------------------------------------------
  node_name                 | v_verticadb_node0001
  event_code                | 20
  event_id                  | 0
  event_severity            | Critical
  event_posted_timestamp    | 2021-11-22 15:57:24.514475+00
  event_expiration          | 2089-12-10 19:11:31.514475+00
  event_code_description    | Cluster Read-only
  event_problem_description | Cluster cannot perform updates due to quorum loss and can only be queried
  reporting_node            | v_verticadb_node0001
  event_sent_to_channels    | Vertica Log
  event_posted_count        | 1
  -[ RECORD 2 ]-------------+--------------------------------------------------------------------------
  node_name                 | v_verticadb_node0004
  event_code                | 20
  event_id                  | 0
  event_severity            | Critical
  event_posted_timestamp    | 2021-11-22 15:57:24.515022+00
  event_expiration          | 2089-12-10 19:11:31.515022+00
  event_code_description    | Cluster Read-only
  event_problem_description | Cluster cannot perform updates due to quorum loss and can only be queried
  reporting_node            | v_verticadb_node0004
  event_sent_to_channels    | Vertica Log
  event_posted_count        | 1
  -[ RECORD 3 ]-------------+--------------------------------------------------------------------------
  node_name                 | v_verticadb_node0005
  event_code                | 20
  event_id                  | 0
  event_severity            | Critical
  event_posted_timestamp    | 2021-11-22 15:57:24.515019+00
  event_expiration          | 2089-12-10 19:11:31.515019+00
  event_code_description    | Cluster Read-only
  event_problem_description | Cluster cannot perform updates due to quorum loss and can only be queried
  reporting_node            | v_verticadb_node0005
  event_sent_to_channels    | Vertica Log
  event_posted_count        | 1
  -[ RECORD 4 ]-------------+--------------------------------------------------------------------------
  node_name                 | v_verticadb_node0006
  event_code                | 20
  event_id                  | 0
  event_severity            | Critical
  event_posted_timestamp    | 2021-11-22 15:57:24.515172+00
  event_expiration          | 2089-12-10 19:11:31.515172+00
  event_code_description    | Cluster Read-only
  event_problem_description | Cluster cannot perform updates due to quorum loss and can only be queried
  reporting_node            | v_verticadb_node0006
  event_sent_to_channels    | Vertica Log
  event_posted_count        | 1
  

  See Monitoring events.

Recover from read-only mode

To recover from read-only mode, restart the down nodes. Restarting the nodes resolves loss of quorum or loss of primary shard coverage that caused the database to go into read-only mode.

Once the down nodes restart and rejoin the database, Vertica restarts on the nodes that were in read-only mode. This step is necessary to allow the nodes to resubscribe to their shards. During this restart, client connections to these nodes will drop. For example, users connected via vsql to one of the nodes where Vertica is restarting see the message:

server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.

Users using vsql to connect to nodes as Vertica restarts see the message:

vsql: FATAL 4149:  Node startup/recovery in progress. Not yet ready
to accept connections

Once Vertica restarts on the nodes, the database resumes normal operation.

When Vertica sets the K-safety value in an Eon Mode database

When you have three or more primary nodes in your database, Vertica automatically sets the database's K-safety to 1 (K=1). It also automatically configures shard subscriptions so that each node can act as a backup for another node, as described in Maintaining Shard Coverage.

This behavior is different than an Enterprise Mode database, where you must design your database's physical schema to meet several criteria before you can have Vertica mark the database as K-safe. See Difference Between Enterprise Mode and Eon Mode K-safe Designs below for details.

Minimum node requirements for Eon Mode database operation

The K-safety level of your database determines the minimum number of primary nodes it must have:

• When K=0, your database must have at least 1 primary node. Setting K to 0 allows you to have a single-node Eon Mode database. Note that in a K=0 database, the loss of a primary node will result in the database going into read-only mode.
• When K=1 (the most common case), your database must have at least three primary nodes. This number of primary nodes allows Vertica to maintain data integrity if a primary node goes down.
• If you want to manually set the K-safe value to 2 (see Difference Between Enterprise Mode and Eon Mode K-safe Designs below) you must have at least 5 primary nodes.

Vertica prevents you from removing primary nodes if your cluster would fall below the lower limit for your database's K-safety setting. If you want to remove nodes in a database at this lower limit, you must lower the K-safety level using the MARK_DESIGN_KSAFE function and then call REBALANCE_SHARDS.

Critical nodes and subclusters

Vertica designates any node or subcluster in the database whose loss would cause the database to go into read-only mode as critical. For example, in an Eon Mode database, when a primary node goes down, nodes with secondary subscriptions to its shards take over maintaining the shards' data. These nodes become critical. Their loss would cause the database to lose shard coverage and be forced to go into read-only mode.

Vertica maintains a list of critical nodes and subclusters in two system tables: CRITICAL_NODES and CRITICAL_SUBCLUSTERS. Before stopping nodes or subclusters, check these tables to ensure the node or subcluster you intend to stop is not critical.

Difference between Enterprise Mode and Eon Mode K-safe designs

In an Enterprise Mode database, you use the MARK_DESIGN_KSAFE function to enable high availability in your database. You call this function after you have designed your database's physical schema to meet all the requirements for K-safe design (often, by running the database designer). If you attempt to mark your database as K-safe when the physical schema does not support the level K-safety you pass to MARK_DESIGN_KSAFE, it returns an error. See Designing segmented projections for K-safety for more information.

In Eon Mode, you do not need to use the MARK_DESIGN_KSAFE because Vertica automatically makes the database K-safe when you have three or more primary nodes. You can use this function to change the K-safety level of your database. In an Eon Mode database, this function changes how Vertica configures shard subscriptions. You can call MARK_DESIGN_KSAFE with any level of K-safety you want. It only has an effect when you call REBALANCE_SHARDS to update the shard subscriptions for the nodes in your database.

6 - Stopping, starting, terminating, and reviving Eon Mode database clusters

If you do not need your Eon Mode database for a period of time, you can choose to stop or terminate its cluster. Stopping or terminating the cluster saves you money when running in cloud environments.

Stopping and starting a database cluster

When you stop your database cluster, you shut down the nodes in the cluster. Shutting down the cluster is an additional step beyond just shutting down the database. When you shut down the cluster in cloud environments, the node's instances no longer run but are still defined in the cloud platform. You can quickly restart the cluster and database when you need to use it again.

Stopping the database cluster is the best option to use when you will not need it for a short to medium time frame. For example, if no one accesses your database on weekends or holidays, you may consider stopping the cluster.

You save money when you shut down your database cluster in cloud environments. Stopped clusters do not consume expensive CPU resources. Stopped clusters can still cost you money, however. If you configured your nodes with persistent local storage, your cloud provider usually still charges a small amount to maintain that storage space.

Terminating and reviving a database cluster

Terminating a database cluster frees up the resources used by the database cluster's nodes.

On a cloud platform, terminating the database cluster deletes the node's virtual machine instances. The database's data and catalog remain stored in communal storage. You can restart the database by reviving it. When you revive a database, you provision a new database cluster and configure it to use the database's data and metadata stored in communal storage.

In an on-premises Eon Mode database, terminating the database cluster usually means shutting down the database and then repurposing the hardware that the nodes ran on.

Terminating the database cluster is the best option for when you will not need the database for an extended period (or if you are unsure whether you will ever need the database again). As long as you do not delete the communal storage location, you can get your database running again by reviving it.

To revive a database, you create a new Vertica Eon Mode cluster and configure it to use the database's communal storage location. The easiest way to revive a database in the cloud is to use the Management Console. It provisions a new Eon Mode cluster for you, and then revives the database onto it.

Reviving a database takes longer than starting a stopped database. Even if you use the MC to automate the process, provisioning a new set of nodes takes much longer than just restarting stopped nodes. When the new nodes start for the first time, they must load data from communal storage from scratch.

Terminating the database cluster can save you more money over simply stopping the database when the database's nodes have persistent local storage. Cloud providers usually charge you a small recurring fee for the space consumed by persistent local storage on the nodes.

See also

• Reviving an Eon Mode database cluster
• Terminating an Eon Mode database cluster
• Eon Mode architecture
• Adding and removing nodes from subclusters
• Altering subcluster settings