VoltDB, the company, is now Volt Active Data. The in-memory database VoltDB continues to be the foundation of our product offering. This software release updates messages and other references to provide the most accurate and up-to-date access to product resources.

Our partner, DataDog, offers monitoring services that let you combine metrics from multiple products into a single view of your business systems. Their support for Volt Active Data was recently improved and expanded. For more information, see their original blog post announcing Volt support or visit their website.

In earlier Operator 1.7.x releases, when starting XDCR using nodePorts the external IP addresses were not always being passed correctly and, as a result, the nodePort was not properly initiated. This issue has been resolved.

The following limitations in previous versions have been resolved:

VoltDB is now beta testing the use of Java 17 for running VoltDB. This means you can use Java 8, 11, or 17 to run both the VoltDB server product and client API. Additional testing and validation is ongoing and Java 17 is expected to be ready for general release (and production use) in V11.4.

A new Java client interface, Client2, was introduced as a beta feature in VoltDB V11.2. Testing is complete and this API is now generally available for production use. See the Client2 chapter in the VoltDB Performance and Customization Guide for an introduction to the interface and the Javadoc for details.

One feature of the new Client2 interface is the ability to specify a priority for individual procedure calls, which is now supported by the server as well. Priorities increase the frequency of high priority transactions running versus low priority transactions. You can specify a priority when calling a stored procedure, or you can specify a priority for classes of transactions, such as snapshot and database replication (DR) in the configuration file when you enable prioritization. See the chapter on Prioritizing Transactions in the VoltDB Performance and Customization Guide for information on enabling and specifying priorities.

It is possible to pass mutable parameters most notably arrays to stored procedures. If you modify those parameters before passing them to a SQL statement, there is no guarantee that the results will be deterministic on all copies of the partition. Consequences can be unpredictable results, node crashes, or even data corruption.

If you are sure you do not modify such arrays in your stored procedures, you can tell VoltDB to not copy mutable parameters in the configuration file. See the appendix on Server Configuration Options in the VoltDB Administrator's Guide for details.

The VoltDB Operator and Helm charts now support Kubernetes V1.19.x through V22.x.

A number of functional improvements have been made to the VoltDB Management Center (VMC), including:

The following limitations in previous versions have been resolved:

VoltDB writes informational, warning, and error messages to log files in a subfolder of the database root directory. By default, the log files "roll", creating a new file and renaming the old ones, on a daily basis. Previously, old log files were never deleted. Starting with this release, the default Log4J configuration file has been revised and now retains up to 30 log files. Older files are deleted. You can customize the configuration to change either or both the number of files retained or how frequently a new file is created. See the section on "Managing VoltDB Log Files" in the VoltDB Administrator's Guide for details.

Note that if you use a customized Log4J configuration, your customization will take precedence and override the new behavior.

By default, snapshots created manually or through periodic snapshots are stored in the snapshots folder in the database root directory. When you reinitialize the root directory, using the --force flag to delete existing data, all files are deleted except the snapshot directory, which is renamed snapshots.1, snapshots.2, etc.

Snapshots can be quite large, since they save the entire contents of the database. And previously, VoltDB saved and renamed all of the extant snapshot folders. To avoid consuming unnecessary disk space, the voltadmin init --force command now only saves the two most recent snapshot folders by default. You can specify a different retention value with the --retain flag on the voltadmin init command. See the description of the voltadmin command in the Using VoltDB guide for details.

When you shutdown the database with the voltadmin shutdown --save command, a final snapshot is created saving the database contents. Final snapshots are useful, particularly for saving and restoring the database when upgrading the VoltDB software. VoltDB does not delete the final snapshot after it is restored as a safety precaution, in case you decide the restored database has a problem or is not configured correctly. Saving the final snapshot gives you an opportunity to manually restore the database to another location in case of an emergency.

However, once the final snapshot is restored and operational, the snapshot is no longer needed. Consequently, VoltDB now deletes all but the last two final snapshots when you create a new snapshot with the voltadmin shutdown --save command. This is a fixed limit.

Cross datacenter replication (XDCR) records any conflicts that occur during active-active replication in the XDCR conflict logs. These logs give you the opportunity to review, analyze, and potentially correct any issues that arise from conflicting transactions. However, over time, subsequent transactions will make correcting old conflicts impractical.

How long you need to save the conflict logs is a business decision. Therefore, by default, VoltDB maintains the old behavior of keeping all conflict logs. (Except in the case of Kubernetes, where a default of 30 days is applied.) You can specify a retention policy in terms of how long to keep log files by adding the conflictretention attribute to the <dr> element in the configuration file:

<dr id="1" role="xdcr" conflictretention="14d" >

The retention period can be specified in seconds, minutes, hours, or days (using the signifier s, m, h, or d). Alternately, you can configure the VOLTDB_XDCR_CONFLICTS export connector, which overrides the conflictretention attribute.

Similarly to XDCR conflict logs, a retention policy can be applied to the output files of any file export connector. By default, all output files are retained. However, you can use the retention property of the connector <configuration> to specify a time limit, after which old files are deleted. For example, the following configuration retains export files for 14 days:

<export>
   <configuration enabled="true" target="log" type="file">
     <property name="retention">14d</property>
     <property name="type">csv</property>
     <property name="nonce">MyLogs</property>
  </configuration>
</export>

See the section on "The File Export Connector" in the Using VoltDB guide for details.

The VoltDB Operator has added support for running VoltDB databases in Kubernetes on the OpenShift platform from Red Hat. This expands VoltDB's Kubernetes compatibility to three of the major cloud platforms: AWS, Google Cloud, and OpenShift. For those interested in using OpenShift, please be sure to read the operational note later in this document for specific instructions on handling Transparent Huge Pages (THP) on the OpenShift platform.

The setTopologyChangeAware property in the VoltDB Java client takes care of maintaining connections to all nodes of the cluster, even if servers drop out or new servers are added. However, traditionally, it was the application's responsibility to explicitly create the initial connections. To simplify the process, the client API adds a new method, createAnyConnection, that lets you provide a list of servers and creates one connection to the first available server on the list, and then lets the topology awareness property establish the remaining connections based on the actual current topology of the cluster. See the Javadoc for more information.

The following limitations in previous versions have been resolved:

The following limitation in previous versions has been resolved:

VoltDB V11.1 includes the Beta release of a new Java client library. Client2 provides a modern, robust and extensible interface to VoltDB for client applications written in Java. For example, Client2 uses Completable Futures to return information from asynchronous procedure calls, providing a more structured way of handling both successful and unsuccessful calls. The new API will also be the main platform for new client development in the future, starting with the ability to assign a priority for individual requests. Client-side priority queuing is available in the beta release and allows the client to prioritize certain procedure calls over others. See the javadoc for details.

As with all beta releases, the new capabilities are believed to work as described. However, further testing and customer feedback is required before we can recommend its use for production. We encourage you to try the new client interface and provide feedback to support@voltdb.com. Thank you.

Helm and the VoltDB Operator make configuring and running VoltDB in Kubernetes easier. However, there are certain management operations that are awkward to perform using Helm properties alone. VoltDB provides a new Helm plugin, now in Beta release, to simplify these additional VoltDB management tasks.

The Helm voltadmin plugin provides commands, similar to regular voltadmin commands, to perform management activities on a running cluster. For its initial release the Helm voltadmin plugin supports the following commands:

Additional commands will be added in future releases. Once installed, you use the plugin on the Helm command line by specifying the plugin name (voltadmin), the name of the Helm release with the --release option, the command, and then any additional arguments. You can use helm voltadmin --help to get a list of all the commands and options. For example, the following command resets the XDCR connection between the cluster in Helm release mydb and the cluster with a DR ID of 1:

$ helm voltadmin --release=mydb dr reset --cluster=1

You can install the beta release of the voltadmin plugin with the following command:

$ helm plugin install https://storage.googleapis.com/voltdb-kubernetes-charts/voltadmin-1.4.0-beta.tgz

The Prometheus agent for VoltDB has a new argument available when starting the agent from the shell command. The --credentials argument lets you specify a text file containing the authentication credentials for accessing the database when security is enabled. The file must define two properties, username and password. For example:

$ cat $HOME/mycreds.txt
username: admin
password: mySpecialPassword
$ ./voltdb/tools/monitoring/prometheus/voltdb-prometheus \
   --credentials $HOME/mycreds.txt

Using the --credentials argument instead of --username and --password avoids exposing your credentials on the command line to the ps command. Note that the file path must be specified as a full pathname, not a relative path.

By default, the VoltDB Prometheus agent reports statistics for all time — since the database started. You can now tell the agent to report statistics in increments, so that each timestamp reports on activity since the last data was reported. For example, if data is collected every 10 seconds, the statistics for transactions per second is the average TPS over the last 10 seconds, rather than the average since the server started. To report incremental statistics, set the Helm property metrics.delta=true in Kubernetes or use the --delta option when starting the Prometheus agent:

$ ./voltdb/tools/monitoring/prometheus/voltdb-prometheus --delta

VoltDB now lets you set a DSCP priority level for network packets associated with database replication (DR) by defining the environment variable DD_SOCKET_TRAFFIC_CLASS to the DSCP value you wish to use prior to starting the VoltDB process. For example, The following commands start the VoltDB server with the DR network traffic DSCP priority set to 3:

$ export VOLTDB_OPTS="-DDR_SOCKET_TRAFFIC_CLASS=3"
$ voltdb start -D ~/mydb

For Kubernetes, the value can be set using the helm property cluster.clusterSpec.env.VOLTDB_OPTS.

The latest VoltDB Operator and Helm charts add support for Kubernetes up to version 1.21. The officially supported versions are Kubernetes 1.16.2 through 1.21.x.

The following limitations in previous versions have been resolved:

VoltDB Topics provide the intelligent streaming of VoltDB's existing import and export capabilities, but with the flexibility of Kafka-like streams. Topics allow for both inbound and outbound streaming to multiple client producers and consumers. They also use the existing Kafka interface to simplify integration into existing infrastructure. But most importantly, they allow for intelligent processing and manipulation of the data as it passes through the pipeline.

VoltDB topics, which were released as a beta feature in V10.2, are now ready for production use. See the chapter on Streaming Data in the Using VoltDB manual for more information.

The VoltDB command line tools have been upgraded to use Python version 3. Python 3 is commonly available on modern operating systems and so simplifies the process of configuring platforms for VoltDB.

Upgrades to the VoltDB Operator for Kubernetes provide two important new features:

The VoltDB Java client interface has been updated with the following improvements:

The VoltDB Management Center (VMC) web-based console for VoltDB has been updated to the latest versions of the jQuery libraries to address security vulnerabilities. The current library versions for VMC are JQuery v3.5.1 and jQuery-UI v1.12.1.

VoltDB reserves the subfolder "segments" under the command log directory for storing the actual command log files. Do not add, remove, or modify any files in this directory. In particular, do not set the command log snapshot directory to a subfolder "segments" of the command log directory, or else the server will hang on startup.

Because DR data is buffered on the master database and then delivered asynchronously to the replica, there is always the danger that data does not reach the replica if a master node stops. This situation is mitigated in a K-safe environment by all copies of a partition buffering on the master cluster. Then if a sending node goes down, another node on the master database can take over sending logs to the replica. However, if multiple nodes go down and rejoin in rapid succession, it is possible that some buffered DR data — from transactions when one or more nodes were down — could be lost when another node with the last copy of that buffer also goes down.

If this occurs and the replica recognizes that some binary logs are missing, DR stops and must be restarted.

To avoid this situation, especially when cycling through nodes for maintenance purposes, the key is to ensure that all buffered DR data is transmitted before stopping the next node in the cycle. You can do this using the @Statistics system procedure to make sure the last ACKed timestamp (using @Statistitcs DR on the master cluster) is later than the timestamp when the previous node completed its rejoin operation.

Bulk operations, such as large deletes, inserts, or updates are possible within a single stored procedure. However, if the binary logs generated for DR are larger than 45MB, the operation will fail. To avoid this situation, it is best to break up large bulk operations into multiple, smaller transactions. A general rule of thumb is to multiply the size of the table schema by the number of affected rows. For deletes and inserts, this value should be under 45MB to avoid exceeding the DR binary log size limit. For updates, this number should be under 22.5MB (because the binary log contains both the starting and ending row values for updates).

There are a number of VoltDB features that help manage the database by constraining memory size and resource utilization. These features are extremely useful in avoiding crashes as a result of unexpected or unconstrained growth. However, these features could interfere with the normal operation of DR when passing data from one cluster to another, especially if the two clusters are different sizes. Therefore, as a general rule of thumb, DR overrides these features in favor of maintaining synchronization between the two clusters.

Specifically, DR ignores any resource monitor limits defined in the deployment file when applying binary logs on the consumer cluster. This means, for example, if the replica database in passive DR has less memory or fewer unique partitions than the master, it is possible that applying binary logs of transactions that succeeded on the master could cause the replica to run out of memory. Note that these resource monitor limits are applied on any original transactions local to the cluster (for example, transactions on the master database in passive DR).

Database Replication (DR) now supports replication across clusters of different sizes. However, if the replica cluster is smaller than the master cluster, it may require a significantly larger Java heap setting. Specifically, if the replica has fewer unique partitions than the master, each partition on the replica must manage the incoming binary logs from more partitions on the master, which places additional pressure on the Java heap.

A simple rule of thumb is that the worst case scenario could require an additional P * R * 20MB space in the Java heap , where P is the number of sites per host on the replica server and R is the ratio of unique partitions on the master to partitions on the replica. For example, if the master cluster is 5 nodes with 10 sites per host and a K factor of 1 (i.e. 25 unique partitions) and the replica cluster is 3 nodes with 8 sites per host and a K factor of 1 (12 unique partitions), the Java heap on the replica cluster may require approximately 320MB of additional space in the heap:

An alternative is to reduce the size of the DR buffers on the master cluster by setting the DR_MEM_LIMIT Java property. For example, you can reduce the DR buffer size from the default 10MB to 5MB using the VOLTDB_OPTS environment variable before starting the master cluster.

Changing the DR buffer limit on the master from 10MB to 5MB proportionally reduces the additional heap size needed. So in the previous example, the additional heap on the replica is reduced from 320MB to 160MB.

The voltadmin status --dr command provides real-time status on the state of database replication (DR). Normally, this includes the status of the current cluster as well as other clusters in the DR environment. (For example, both the master and replica in passive DR or all clusters in XDCR.) However, if the clusters are configured to use different port numbers for the client port, VoltDB cannot reach the other clusters and the command hangs until it times out waiting for a response from the other clusters.

Part of the replication process for XDCR is to verify that the record's starting and ending states match on both clusters, otherwise known as conflict resolution. To do that, XDCR must find the record first. Finding uniquely indexed records is efficient; finding non-unique records is not and can impact overall database performance.

To make you aware of possible performance impact, VoltDB issues a warning if you declare a table as a DR table and it does not have a unique index.

You cannot start XDCR if both databases already have data in the DR tables. Only one of the two participating databases can have preexisting data when DR starts for the first time.

When starting XDCR for the first time, where one database already contains data, a snapshot of that data is sent to the other database. While receiving and processing that snapshot, the receiving database is paused. That is, it is in read-only mode. Once the snapshot is completed and the two database are synchronized, the receiving database is automatically unpaused, resuming normal read/write operations.

Normally, XDCR will automatically restart where it left off after one of the clusters stops and recovers from its command logs (using the voltdb recover command). However, if the workload is predominantly multi-partition write transactions, a failed cluster may not be able to restart XDCR after it recovers. In this case, XDCR must be restarted from scratch, using the content from one of the clusters as the source for synchronizing and recreating the other cluster (using the voltdb create --force command) without any content in the DR tables.

TRUNCATE TABLE is optimized to delete all data from a table rather than deleting tuples row by row. This means that the binary log does not identify which rows are deleted. As a consequence, a TRUNCATE TABLE statement and a simultaneous write operation to the same table can produce a conflict that the XDCR clusters cannot detect or report in the conflict log.

Therefore, do not use TRUNCATE TABLE with XDCR. Instead, explicitly delete all rows with a DELETE statement and a filter. For example, DELETE * FROM table WHERE column=column ensures all deleted rows are identified in the binary log and any conflicts are accurately reported. Note that DELETE FROM table without a WHERE clause is not sufficient, since its execution plan is optimized to equate to TRUNCATE TABLE.

VoltDB provides a way to generate a deterministically unique ID within a stored procedure using the getUniqueId method. This method guarantees uniqueness within the current cluster. However, the method could generate the same ID on two distinct database clusters. Consequently, when using XDCR, you should combine the return values of VoltProcedure.getUniqueId with VoltProcedure.getClusterId, which returns the current cluster's unique DR ID, to generate IDs that are unique across all clusters in your environment.

In an XDCR environment involving three or more clusters, command logging is used to ensure the durability of the XDCR "conversations" between clusters. If not, when a cluster stops, the remaining clusters can be at different stages of their conversation with the downed cluster, resulting in divergence.

For example, assume there are three clusters (A, B, and C) and cluster B is processing binary logs faster than cluster C. If cluster A stops, cluster B will have more binary logs from A than C has. You can think of B being "ahead" of C. With command logging enabled, when cluster A restarts, it will continue its XDCR conversations and cluster C will catch up with the missing binary logs. However, without command logging, when A stops, it must restart from scratch. There is no mechanism for resolving the difference in binary logs processed by clusters B and C before the failure.

This is why command logging is required to ensure the durability of XDCR conversations in a multi-cluster (that is , three or more) XDCR environment. The alternative, if not using command logging, is to restart all but one of the remaining clusters to ensure they are starting from the same base point.

There are two ways to remove an cluster from an XDCR relationship: you can use voltadmin drop on a running cluster to remove it from the XDCR network, or you can use voltadmin dr reset from the remaining clusters to remove a cluster that is no longer available. But you should not use voltadmin dr reset to remove a cluster that is still running and connected to the network. Resetting a running cluster will break DR for that cluster, but will result in errors on the remaining clusters and leave their DR queues for the reset cluster in an ambiguous state. Ultimately, this can result in the removed cluster not being able to rejoin the XDCR network at a later time.

TTL, or time to live, is a feature that automatically deletes old records based on a timestamp or integer column. For replicated tables, the process of checking whether records need to be deleted is performed as a write transaction — even if no rows are deleted. As a consequence, any replicated DR table with TTL defined will generate frequent DR log entries, whether there are any changes or not, significantly increasing DR traffic.

Because of the possible performance impact this behavior can have on the database, use of TTL with replicated tables and DR is not recommended at this time.

A bug in the Apache Kafka client can result in file descriptors being allocated but not released if the producer.type attribute is set to "sync" (which is the default). The consequence is that the system eventually runs out of file descriptors and the VoltDB server process will crash.

Until this bug is fixed, use of synchronous Kafka export is not recommended. The workaround is to set the Kafka producer.type attribute to "async" using the VoltDB export properties.

If, while Kafka import is enabled, the Kafka broker that VoltDB is connected to stops (for example, if the server crashes or is taken down for maintenance), some messages may be lost between Kafka and VoltDB. To ensure no data is lost, we recommend you disable VoltDB import before taking down the associated Kafka broker. You can then re-enable import after the Kafka broker comes back online.

There is an issue with the Kafka importer where, if multiple nodes in the cluster fail and restart, the importer can lose track of some of the data that was being processed when the nodes failed. Normally, these pending imports are replayed properly on restart. But if multiple nodes fail, it is possible for some in-flight imports to get lost. This issue will be addressed in an upcoming release.

When entering a multi-line statement at the sqlcmd prompt, if a line ends in a comment (indicated by two hyphens) and the comment contains an unmatched single quote character, the following lines of input are not interpreted correctly. Specifically, the comment is incorrectly interpreted as continuing until the next single quote character or a closing semi-colon is read. This is most likely to happen when reading in a schema file containing comments. This issue is specific to the sqlcmd utility.

A fix for this condition is planned for an upcoming point release

VoltDB currently intercepts assertions as part of its handling of stored procedures. Attempts to use assertions in stored procedures for debugging or to find programmatic errors will not work as expected.

The UPPER() and LOWER() functions return a string converted to all uppercase or all lowercase letters, respectively. However, for the initial release, these functions only operate on characters in the ASCII character set. Other case-sensitive UTF-8 characters in the string are returned unchanged. Support for all case-sensitive UTF-8 characters will be included in a future release.

There is a problem with how the math library used to build the C++ client library handles large decimal values on 32-bit operating systems. As a result, the C++ library cannot serialize and pass Decimal datatypes reliably on these systems.

Note that the C++ client interface can send and receive Decimal values properly on 64-bit platforms.

Enabling SNMP can take up to 2 minutes to complete. This delay does not always occur and can vary in length. If SNMP is enabled when the database server starts, the delay occurs after the server logs the message "Initializing SNMP" and before it attempts to connect to the cluster. If you enable SNMP while the database is running, the delay can occur when you issue the voltadmin update command or modify the setting in the VoltDB Management Center Admin tab. This issue results from a Java constraint related to secure random numbers used by the SNMP library.

With VoltDB V7.0, cross datacenter replication (XDCR) supports multiple clusters in an XDCR network. However, the VoltDB Management Center currently reports on only one such connection per cluster. In the future, the Management Center will provide monitoring and statistics for all connections to the current cluster.

Shutting down a VoltDB cluster by specifying a replica count of zero should shut down the cluster and remove the pods on which it ran. However, on very rare occasions Kubernetes does not delete the pods. As a result, the cluster cannot be restarted. This is an issue with Kubernetes. The workaround is to manually delete the pods before restarting the cluster.

The property cluster.clusterSpec.additionalVolumes lets you specify additional resources to include in the server classpath. However, if you specify an invalid or misconfigured volume, Helm will not be able to start the cluster and the process will stall.

The Helm --set-file argument lets you set the value of a property as the contents of a file. However, if the contents of the file are binary, they can become corrupted if you try to resize the cluster with the helm upgrade command, using the --reuse-values argument. For example, this can happen if you use --set-file to assign a JAR file of stored procedure classes to the cluster.config.classes property.

This is a known issue for Kubernetes and Helm. The workaround is either to explicitly include the --set-file argument again on the helm upgrade command. Or you can include the content through a different mechanism. For example, you can include class files by mounting them on a separate volume that you then include with the cluster.clusterSpec.additionalVolumes property.

VoltDB works in IPv4, IPv6, and mixed network environments. Although the examples in the documentation use IPv4 addresses, you can use IPv6 when configuring your database, making connections through applications, or using the VoltDB command line utilities, such as voltdb and voltadmin. When specifying IPv6 addresses on the command line or in the configuration file, be sure to enclose the address in square brackets. If you are specifying both an IPv6 address and port number, put the colon and port number after the square brackets. For example:

voltadmin status --host=[2001:db8:85a3::8a2e:370:7334]:21211

Any time the database schema or stored procedures are changed, the data table showing stored procedure statistics at the bottom of the Monitor section of the VoltDB Management Center get reset. As soon as new invocations of the stored procedures occur, the statistics table will show new values based on performance after the schema update. Until invocations occur, the procedure table is blank.

The ASSUMEUNIQUE attribute is designed for identifying columns in partitioned tables where the column values are known to be unique but the table is not partitioned on that column, so VoltDB cannot verify complete uniqueness across the database. Using interactive DDL, you can create a table with a column marked as ASSUMEUNIQUE, but if you try to partition the table on the ASSUMEUNIQUE column, you receive an error. The solution is to drop and add the column using the UNIQUE attribute instead of ASSUMEUNIQUE.

You cannot add or remove a column constraint such as UNIQUE or ASSUMEUNIQUE using the ALTER TABLE ALTER COLUMN statement. Instead to add or remove such constraints, you must first drop then add the modified column. For example:

ALTER TABLE employee DROP COLUMN empID;
ALTER TABLE employee ADD COLUMN empID INTEGER UNIQUE;

For partitioned tables, the value of the column used to partition the table determines what partition the row belongs to. If you use UPDATE to change this value and the new value belongs in a different partition, the UPDATE request will fail and the stored procedure will be rolled back.

Updating the partition column value may or may not cause the record to be repartitioned (depending on the old and new values). However, since you cannot determine if the update will succeed or fail, you should not use UPDATE to change the value of partitioning columns.

The workaround, if you must change the value of the partitioning column, is to use both a DELETE and an INSERT statement to explicitly remove and then re-insert the desired rows.

Starting with VoltDB 6.0, ambiguous column references are no longer allowed. For example, if both the Customer and Placedorder tables have a column named Address, the reference to Address in the following SELECT statement is ambiguous:

SELECT OrderNumber, Address FROM Customer, Placedorder
   . . .

Previously, VoltDB would select the column from the leftmost table (Customer, in this case). Ambiguous column references are no longer allowed and you must use table prefixes to disambiguate identical column names. For example, specifying the column in the preceding statement as Customer.Address.

A corollary to this change is that a column declared in a USING clause can now be referenced using a prefix. For example, the following statement uses the prefix Customer.Address to disambiguate the column selection from a possibly similarly named column belonging to the Supplier table:

SELECT OrderNumber, Vendor, Customer.Address
   FROM Customer, Placedorder Using (Address), Supplier
    . . .

VoltDB opens a file descriptor for every client connection to the database. In normal operation, this use of file descriptors is transparent to the user. However, if there are an inordinate number of concurrent client connections, or clients open and close many connections in rapid succession, it is possible for VoltDB to exceed the process limit on file descriptors. When this happens, new connections may be rejected or other disk-based activities (such as snapshotting) may be disrupted.

In environments where there are likely to be an extremely large number of connections, you should consider increasing the operating system's per-process limit on file descriptors.

There are two ways to access additional resources in a VoltDB database. You can place the resources in the /lib folder where VoltDB is installed on each server in the cluster or you can include the resource in a subfolder of a JAR file you add using the sqlcmd LOAD CLASSES directive. Adding resources via the /lib directory is useful for stable resources (such as third-party software libraries) that do not require updating. Including resources (such as XML files) in the JAR file is useful for resources that may need to be updated, as a single transaction, while the database is running.

LOAD CLASSES is used primarily to load classes associated with stored procedures and user-defined functions. However, it will also load any additional resource files included in subfolders of the JAR file. You can remove classes that are no longer needed using the REMOVE CLASSES directive. However, there is no explicit command for removing other resources.

Consequently, if you rename resources or move them to a different location and reload the JAR file, the database will end up having multiple copies. Over time, this could result in more and more unnecessary memory being used by the database. To remove obsolete resources, you must first reinitialize the database root directory, start a fresh database, reload the schema (including the new JAR files with only the needed resources) and then restore the data from a snapshot.

If a server has multiple network interfaces (and therefore multiple IP addresses) VoltDB will, by default, open ports on all available interfaces. You can limit the ports to an single interface in two ways:

Also, when using an IP address to reference a server with multiple interfaces in command line utilities (such as voltadmin stop node), use the @SystemInformation system procedure to determine which IP address VoltDB has selected to identify the server. Otherwise, if you choose the wrong IP address, the command might fail.

For production, VoltDB requires that Transparent Huge Pages (THP) are disabled because they interfere with memory-intensive applications. However, THP may be enabled on OpenShift containers and the containers themselves not have permission to disable them. To overcome this situation, you must run the Helm chart for disabling THP from a privileged container:

$ helm -n kube-system install thp voltdb/transparent-hugepages \
       --set thp.securityContext.privileged=true

The following table describes the compatibility matrix for versions of the VoltDB software, VoltDB Kubernetes Operator, and associated Helm Charts. The table identifies the versions of the Operator and Helm chart required and supported for running the specified version of the VoltDB server software. See the VoltDB Operator Release Notes for additional information about individual releases of the VoltDB Operator.