August 6, 2018
This document provides information about known issues and limitations to the current release of VoltDB. If you encounter any problems not listed below, please be sure to report them to firstname.lastname@example.org. Thank you.
The process for upgrading from the recent versions of VoltDB is as follows:
Shutdown the database, creating a final snapshot (using voltadmin shutdown --save).
Upgrade the VoltDB software.
Restart the database (using voltdb start).
Support for upgrading using shutdown --save was only added in V6.8. If you are upgrading from older versions of VoltDB, you will need to save and restore the snapshot manually. The procedure to do that is as follows:
Place the database in admin mode (using voltadmin pause).
Perform a manual snapshot of the database (using voltadmin save --blocking).
Shutdown the database (using voltadmin shutdown).
Upgrade the VoltDB software.
Initialize a new database root directory (using the voltdb init --force action).
Start the database in admin mode (using the voltdb start --pause action).
Restore the snapshot created in Step #2 (using voltadmin restore).
Return the database to normal operations (using voltadmin resume).
For customers upgrading from V6.x or earlier releases of VoltDB, please see the V6.0 Upgrade Notes.
For customers upgrading from V5.x or earlier releases of VoltDB, please see the V5.0 Upgrade Notes.
For customers upgrading from V4.x or earlier releases of VoltDB, please see the V4.0 Upgrade Notes.
Users of previous versions of VoltDB should take note of the following changes that might impact their existing applications.
The following limitations in previous versions have been resolved:
Reuse of HTTP connections to improve performance of JSON API and VMC
Previously, if security was enabled for a VoltDB database, JSON calls to the database (through the HTTP port) had to specify the username and password on each call. This could result in unnecessary and detrimental overhead as each call was authenticated. VoltDB now uses Keep-Alive, or persistent, HTTP connections. This means the connection is only authenticated once at the beginning of the session until the session ends or the connection becomes inactive (that is, no call is made) for the timeout period (10 seconds by default). Note that, even after a timeout, the session can be reestablished by passing the username and password again.
Use of Keep-Alive connections provides a significant performance improvement to both the JSON API and the web-based VoltDB Management Center, which uses the JSON interface. Note, however, for customer applications using the JSON interface, you must capture and return the session cookie that is provided by the JSON interface to take advantage of the performance improvement.
The following limitations in previous versions have been resolved:
The following limitation in previous versions has been resolved:
Support for multiple SQL statements in simple stored procedures
Simple stored procedures are stored procedures that can be declared as individual SQL statements without any further processing. They are declared using the CREATE PROCEDURE AS statement. You can now create simple procedures with multiple SQL statements by enclosing the statements in a BEGIN-END clause. For example:
CREATE PROCEDURE ReadyToShip AS BEGIN INSERT INTO shipments SELECT order_num, item, customer FROM orders WHERE item=?; SELECT COUNT(*) FROM shipments; END;
Enhancements to the Analysis tab in VoltDB Management Center
The Analysis tab, which was added to the web-based VoltDB Management Center in 7.6, has been extended and now provides analysis of both procedure execution and table storage. Within the Analysis tab, after running the data gathering, you can choose between Data and Procedures to view statistics regarding the distribution of the tables and the execution of stored procedures.
New and improved Go client
The VoltDB Go client has been revised to significantly improve performance, remove known defects and keep up to date with advances in the Go language. A description of the changes are available on the VoltDB website.
New fraud detection example application
The VoltDB software kit includes a new sample application. The Fraud Detection example application
demonstrates how easy it is to integrate Apache Kafka and VoltDB to create a fast data pipeline with data analysis
and reporting. See the README in the
Improved performance for distributed tasks within VoltDB
Recent enhancements to VoltDB improve the overall performance of certain distributed tasks. Specifically, concurrent read-only multi-partition stored procedures execute more rapidly due to better distribution of the procedures among the execution sites on each server.
Enhanced voltadmin status command
The voltadmin utility has been enhanced to use a topology-aware client interface when providing ongoing cluster status. This means that when using the voltadmin status command to monitor a K-safe cluster, the voltadmin connection is maintained even if the database server to which the command was initially submitted fails.
Over the next few releases, certain older, obsolete, or unused features of VoltDB will be deprecated. This means that the feature, although still operational, is no longer recommended for use and may be removed from the product in some future major release.
For example, recent performance work has minimized the speed difference between "safe" and "fast" consistency modes, if not eliminating it entirely. As a result, the "fast" option is being deprecated and will be removed in a future release. Note that this change requires no modifications to customer applications and is transparent to users.
No security changes in this release.
In addition to the new features and capabilities described above, the following limitations in previous versions have been resolved:
The following issue has been corrected:
Race condition interferes with schema updates and statistics reporting
Since VoltDB 7.1, there has been a race condition triggered by frequent schema changes (including updates to stored procedure classes or configuration options). The schema update could conflict with the gathering of database statistics, resulting in a number of different symptoms, including:
This issue has been resolved.
It is now possible to define your own functions for use in SQL queries and data manipulation statements. User-defined functions are written as Java methods, which are then declared using the CREATE FUNCTION statement:
CREATE FUNCTION season FROM METHOD myapp.calendar.DateToSeason;
Data loading utilities stay connected
VoltDB has a number of utilities for loading data from external sources — CSV files, JDBC databases, and Kafka streams — as well as a generic bulk loader API for creating your own loading utility. Previously, if the loaders lost contact with all of the servers in the VoltDB cluster, the loading process would stop. Now, the utilities will wait, periodically retrying the connection, until the VoltDB cluster becomes accessible again.
This new behavior makes the loaders more resilient to failure, both of the external source and the VoltDB
database. However, if you wish to revert to the previous behavior, you can add the
Better mechanism for specifying delimiters in the file export configuration
It is possible to export files with different delimiters than the standard comma-separated delimiters using
the file export connector. However, it has been difficult to use certain non-standard characters in the past due to
the way the XML configuration option for the
Now, special and non-printing characters should be entered as Java string literals. For example, the new line character can be entered as "\n". Also any XML special characters, such as ampersand and greater-than, should be encoded as HTML entities. See the section on the "File Export Connector" in the Using VoltDB manual for details.
Improved visibility of continuation lines in sqlcmd interface
The sqlcmd utility lets you continue SQL statements across multiple lines and the statement is not processed until you enter the closing semi-colon and press return. However, previously there was no indication that the utility was still waiting for more of the statement. Now the utility indents the command line prompt to show when it is still waiting for more input. For example:
$ sqlcmd 1> SELECT o.order_id, o.item, c.customer_id 2> FROM orders as o,customers as c 3> WHERE c.customer_id = o.customer_id 4> AND o.ship_date > NOW(); 5>
Enhanced logging of schema changes
All changes to the database schema and/or the configuration are now recorded in the log file. The log records the time of the change, as well as the username and IP address of the person who changed it.
New analysis tab in VoltDB Management Center
The web-based VoltDB Management Center has a new tab, Analysis, that lets you analyze the performance of a running database. The Analysis tab takes a snapshot of run-time database performance and provides several views of how the stored procedures are performing, including average execution time, frequency, and the total impact (that is, frequency times execution time. By clicking on a specific stored procedure, you can see further details on each statement within the procedure.
Further improvements to the performance of schema updates
Over the past few releases, significant effort has been focused on reducing the performance impact of database modifications, such as schema changes, loading and unloading stored procedure classes, and modifying the database configuration. In particular, this release minimizes the amount of time transactions must wait while the changes are actually applied across the cluster.
The following changes have been made to improve security and eliminate potential threats:
In addition to the new features and capabilities described above, the following limitations in previous versions have been resolved:
Support for Java boxed datatypes
Java boxed datatypes such as Short, Integer, and Long are now supported as input to stored procedures. Previously, only primitive datatypes were supported. Using boxed types, you can pass Java null values directly to VoltDB and have them interpreted correctly.
New functions to convert between radians and degrees
Two new SQL functions have been added, RADIANS() and DEGREES(), that let you convert floating-point values between radians and degrees.
Improvements to offset tracking for the kafkaloader utility and Kafka import
The kafkaloader utility and built-in Kafka importer have been modified to improve the tracking of the current offset between Kafka and VoltDB. By default, these features now use a manual commit policy to ensure the current offset in the Kafka message queue matches the records successfully inserted into the VoltDB database, rather than tracking just what messages have been successfully read from Kafka. You can, if you wish, revert to an automatic commit policy by setting the appropriate property in the command line argument or import configuration.
Support for SSL and Kafka 0.10.2
Kafka 0.10.2 supports secure socket layer (SSL) encryption. Using the 0.10.2 version of the kafkaloader
utility (kafkaloader10), you can connect to SSL-secured Kafka brokers by putting the appropriate SSL properties
in the Kafka configuration file specified in the
security.protocol=SSL ssl.truststore.location=/Home/jdoe/mytruststore.jks ssl.truststore.password=mytrustpasswd ssl.keystore.location=/Home/jdoe/mykeystore.jks ssl.keystore.password=mykeypasswd ssl.key.password=passwd
Ability to write Kafka messages to two or more tables
The Kafka import connector is now capable of writing messages from a single topic to multiple tables, without requiring a custom insert procedure. If you create two configurations with the same Kafka topic ID but different group IDs, you can have a single topic written to two different tables. For example, the following configuration writes records from the Kafka employees topic to both the EMP and MGR tables.
<import> <configuration type="kafka" enabled="true"> <property name="topics">employees</property> <property name="groupid">voltdbemp</property> <property name="procedure">EMP.insert</property> </configuration> <configuration type="kafka" enabled="true"> <property name="topics">employees</property> <property name="groupid">voltdbmgr</property> <property name="table">MGR.insert</property> </configuration> </import>
New @Statistics selector QUEUE
A new selector has been added to the @Statistics system procedure. The QUEUE selector returns statistics on the number of tasks in each partition's process queue and the average and maximum time tasks were waiting in the queue. See the description of the @Statistics system procedure in the Using VoltDB manual for details.
Improvements to INSERT INTO... SELECT to reduce temporary memory usage
Performing an INSERT INTO... SELECT can produce many records, depending on the results of the SELECT. In the past, it was easy to use up all of the available temporary memory that VoltDB allocates for transactions, resulting in the statement failing and rolling back. The execution plans for these statements have been rewritten to perform each insert inline, significantly reducing the amount of temporary memory required and increasing the number of rows that can be processed in a single statement.
More flexibility in placement of COUNT(*) in CREATE VIEW
Previously, the CREATE VIEW statement selection list had to begin with any columns listed in the GROUP BY statement, followed by COUNT(*), and then any other aggregate functions. The restriction on the placement of COUNT(*) has been removed and it can now appear anywhere in the selection list after the GROUP BY columns.
Ability to use absolute paths to identify custom import formatters
By default, when using a custom import formatter, VoltDB looks for the formatter JAR file in the
Dynamic connections to clusters for the kafkaloader and jdbcloader utilities
The kafkaloader and jdbcloader utilities have been updated to use the auto-reconnecting client library, so they can adjust to changes in the database cluster configuration dynamically.
New message logged when processes run more than 10 seconds
If a task blocks a process queue for more than 10 seconds, an informational message is written to the server
log. You can adjust the threshold at which these messages are written by setting the configuration element
Improved partition management when cluster nodes fail
When a node fails on a K-safe cluster, management of one or more of the database partitions (known as mastership) may need to be re-assigned to the remaining nodes. In previous versions, after several failures and rejoins mastership of the partitions could become unevenly dispersed in the cluster. Reassignment has now been improved to maintain an even distribution in the cluster.
In addition to the new features and capabilities described above, the following limitations in previous versions have been resolved:
New DR DROP command
There is a new voltadmin command to safely remove a single cluster from a multi-cluster XDCR environment without disrupting the connection between the remaining clusters. The DR DROP command tells a running cluster to send any queued binary logs, stop DR and inform the remaining clusters to remove it from the XDCR environment. See the description of the voltadmin utility in the Using VoltDB manual for details and an explanation of the difference between DR DROP and DR RESET.
Support for alternate character sets in the csvloader utility
New column added to the @Statistics DRPRODUCER selector output
A new column, QUEUE_GAP, has been added to the return values of the DRPRODUCER selector of the @Statistics system procedure. This column indicates if there is a gap between the last acknowledged transaction and the next transaction queued for transmission to the DR consumer. See the description of the @Statistics system procedure in the Using VoltDB manual for details.
The sqlcmd FILE directive now supports processing multiple files in a single batch
Previously, the sqlcmd FILE directive let you process a single file containing SQL
statements (and other sqlcmd directives). The directive has been extended to support a list of
files, separated by spaces. If you include the
Support for converting Java BigDecimal to integer VoltDB datatypes
Java BigDecimal values are now allowable input to VoltDB integer datatypes, including TINYINT, SHORTINT, INTEGER, and BIGINT. As with any datatype conversion, if the BigDecimal value overflows the target integer type it will generate a runtime error. See the appendix on datatype compatibility in the Using VoltDB manual for details.
The voltdb init --schema option now ignores unsupported sqlcmd directives
Optimization of runtime schema changes
As part of an ongoing effort to optimize and reduce the impact of changes to the running database — specifically changes to the schema, stored procedures, or configuration — the internal code to implement these actions is being reorganized. One consequence of this change is that the name of internal procedures reported by the @Statistics system procedure may change. For example, previously, schema changes were reported by the @Statistics INITIATOR and PROCEDURE selectors as the UpdateApplicationCatalog procedure. Now these actions are executed by the UpdateCore procedure.
Hash indexes are now deprecated
VoltDB uses tree indexes by default. It has been possible to request a hash index by including the string "hash" in the index name. However, testing shows that in most cases, tree indexes are more effective or at least as effective as hash indexes. Consequently, hash indexes are now deprecated and will be removed at an upcoming major release.
New online upgrade process for VoltDB software
There is a new process available for upgrading VoltDB software on a running cluster using XDCR without requiring a second set of cluster hardware. Upgrading the software between two XDCR clusters is the easiest method. However, if you do not want or have the resources for a second separate cluster, the new process uses XDCR and K-safety to split an existing cluster into two and upgrade the two halves through XDCR. See the section on "Performing an Online Upgrade with Limited Hardware" in the VoltDB Administrator's Guide for details.
New @Statistics selector for measuring latency
There is a new selector for the @Statistics system procedure, LATENCY, which provides periodic latency statistics. The statistics are generated every five seconds and provide both maximum and various levels of percentile measurement up to 99.999%. See the description of the @Statistics system procedure in the Using VoltDB manual for details.
Support for Kerberos when using the VoltDB command line utilities
Kerberos support has been extended to provide integration with the VoltDB command line utilities, such as sqlcmd and voltadmin. See the discussion of "Accessing the Database from the Command Line" in the Kerberos section of the Using VoltDB manual for details.
Beta release of kafkaloader utility support for Kafka version 0.10.2
There is a new version of the kafkaloader utility, named kafkaloader10, that works with Kafka version 0.10.2. There are enough differences between Kafka 0.8.2 and 0.10.2 that two separate utilities will be maintained for the foreseeable future.
Customers interested in using kafkaloader10 are encouraged to try this beta release and
provide us feedback. Note however, it has not yet received the rigorous testing of the default 0.8.2 version of
kafkaloader and should not be considered production-ready quite yet. See the online help for
kafkaloader10 in the
Ability to choose a preferred source for a new XDCR connection
It is now possible to select one cluster in a multi-cluster XDCR configuration as the preferred host of the
initial synchronizing snapshot when the current cluster joins the environment. You use the
<dr id="3" role="xdcr" > <connection source="svra,svrb,svrc" preferred-source="2" /> </dr>
New Import tab in the VoltDB Management Center
There is a new tab available in the VoltDB Management Center that helps you monitor the progress of import connectors. The new Import tab is only visible when import connectors are configured and active.
Improved performance and reduced bandwidth use in the VoltDB Management Center
The VoltDB Management Center (VMC) has been optimized to reduce its bandwidth use and thereby improve overall performance.
Ability to export table and column names in lowercase for JDBC targets
By default, VoltDB sends SQL statements through the JDBC export connector using all uppercase table and column
names. However, a few databases (most notably, PostgreSQL) are case-sensitive. There is a new property for the JDBC
connector that tells VoltDB to use lowercase table and column names. The
Deprecation notice: Java client API method updateApplicationCatalog()
There is a method in the VoltDB Java client API,
Ability to initialize root directory with schema and procedure classes
It is now possible to add a starting schema and procedure classes when you initialize the database root
directory. The new options
$ voltdb init --config=deployment.xml \ --schema=myschema.sql --classes=myprocs.jar
Enhancements to the DR reset command for managing XDCR environments
In multi-cluster XDCR environments, it is now possible to drop one cluster from the network without disrupting replication between the remaining clusters by using the voltadmin dr reset --cluster command. See the description of the voltadmin command in the Using VoltDB manual for details.
DR support for @SwapTables
Use of the @SwapTables system procedure is now supported by database replication (DR). Swapping tables on a producer cluster acts like a schema change, causing the consumer cluster to pause until you execute the same @SwapTables statement on the consumer. See the description of the @SwapTables system procedure in the Using VoltDB manual for details.
Support for subqueries in data manipulation language (DML) statements
Data manipulation language (DML) statements, such as INSERT, UPDATE, and DELETE, now support subqueries. See the description of the individual statements in the appendix of supported SQL statements in the Using VoltDB manual for details.
New voltadmin status command
There is a new command, voltadmin status, that reports the current status of a running
cluster. Information in the output includes the number and IP addresses of the servers in the cluster and if any
nodes are missing from a K-safe cluster. You can also add the
New voltdb collect syntax
The syntax for the voltdb collect command has been enhanced to be consistent with the other
voltdb commands, such as get, init, and
start. The voltdb collect command now let's you specify the location of the
root directory using the
Improved logging of security events and configuration changes
VoltDB now provides more thorough logging of security events. Specifically, every authorization failure is logged, each user who successfully authenticates is logged, and every configuration change is logged prior to the change being made.
New documentation of custom exporters, importers, and formatters
A new chapter on writing custom exporters, importers and formatters has been added to the VoltDB Guide to Performance and Customization.
Support for TLS encryption on external ports
VoltDB now supports Transport Layer Security (TLS) — the recommended upgrade from Secure Socket Layer
(SSL) encryption — for all externally accessible ports, not just the HTTP port. With TLS enabled, data passing
through both the client and admin ports is encrypted. If encryption is enabled for the client and admin ports, you
must use the
To support the new options, the configuration of TLS in the deployment file has been reorganized. Customers who previously enabled TLS encryption for the HTTP port only will need to change their configuration files. For example, the old and new syntax for enabling TLS on the HTTP port is:
<httpd> <https enabled="true"> [ . . . ] </https> </httpd>
<ssl enabled="true"> [ . . . ] </ssl>
Ability to retrieve the schema, procedures, and configuration from a database
Three new voltdb commands have been added that allow you to extract the schema, stored procedures, and configuration from the local database root directory. These commands can be used whether the database is running or not. The new commands are:
New statistics provide performance details about individual statements within a procedure.
A new selector, PROCEDUREDETAIL, for the @Statistics system procedure provides detailed performance information about the execution of individual statements within stored procedures. See the description of the @Statistics system procedure in the Using VoltDB manual for details.
New system procedure lets you swap two tables
A new system procedure, @SwapTables, lets you swap the contents of two tables. The two tables must have the identical schema (that is, the same column names, datatypes, and constraints) and cannot have views defined. See the description of the @SwapTables system procedure in the Using VoltDB manual for details.
Ability to restart a partial cluster
It is now possible to restart a K-safe cluster with fewer than the original number of nodes. For example, if
one server fails on a five node cluster and you want to restart the cluster but the failed server is not ready, you
can start the cluster with the remaining four nodes by using the
$ voltdb start --count=5 --missing=1
Note that the partial cluster is not fully K-safe to its specified K-safety value until the full complement of nodes rejoin the cluster. See the description of the voltdb command in the Using VoltDB manual for details.
New SQL functions for handling internet addresses
There are four new SQL functions to convert internet addresses from text strings to binary and back again. Two functions (INET_ATON and INET_NTOA) convert IPv4 addresses and two (INET6_ATON and INET6_NTOA) convert IPv6 addresses. See the Appendix of SQL Functions in the Using VoltDB manual for details.
Statistics related to schema changes are no longer reset by each change
Previously, the information gathered by the @Statistics system procedure related to schema changes (from DML statements or calls to @UpdateClasses or @UpdateApplicationCatalog) was reset each time the database schema changed. Schema changes no longer reset the statistics on each invocation. Also, every schema change is recorded, rather than the 5% sampling done for other procedures.
SNMP traps now report database replication (DR) events
VoltDB can send SNMP traps for selected database events, such as node failures or when resource limits are reached. Starting with V7.1, it also reports database replication (DR) events when DR is enabled.
New index optimizations
We continue to extend support for the use of indexes to optimize the execution of SQL statements. Two new uses of indexes are when evaluating:
Enhancements to the kafkaloader utility
Three new features have been added to the kafkaloader utility:
See the online help for the kafkaloader utility for details.
Improved performance when updating procedure classes
Previously, updating the stored procedure classes (using the sqlcmd LOAD CLASSES directive or the @UpdateClasses system procedure) updated all of the classes in the schema. As a result, the more classes in the schema, the longer the update would take, even if the update itself was small. VoltDB now updates only those classes affected by the change, significantly reducing the time required for the update to complete.
Managing heterogeneous clusters in Database Replication (DR)
You can create a DR environment using clusters running different versions of VoltDB. However, to support new functionality (in particular, multi-cluster XDCR) the DR protocol changed in V7.0. That major version acts as a gateway between earlier and later releases. In other words, you can run a V7.0 cluster with earlier versions, or you can run a 7.0 cluster with later versions. However, you cannot run pre-V7.0 clusters with post-V7.0 clusters.
As a consequence, if you want to perform an in-service upgrade (using different version clusters) to upgrade a DR environment using a pre-V7.0 version (say, V6.9) to V7.1 or later, you must upgrade all clusters to V7.0 first, then upgrade to your final target version. Of course, you can always use the normal single-cluster upgrade process to go directly from V6.x to V7.x if you choose.
VoltDB servers no longer support SHA-1
To ensure the security of VoltDB servers, the database no longer accepts SHA-1 for hashed data. All hashed data (such as passwords etc) must be hashed using SHA-256. Note: this is an incompatible change. Some earlier client libraries will not be able to connect to the V7.1 servers because they do not support the more robust SHA-256 hashing.
The following limitations in previous versions have been resolved:
Updated operating system and software requirements
The operating system and software requirements for VoltDB have been updated based on changes to the supported versions of the underlying technologies. Specifically:
Improved configuration and reporting for database replication (DR)
As mentioned earlier, the addition of the
VoltDB now protects against inserting invalid values into TIMESTAMP columns
Previously, VoltDB allowed any 8-byte integer as input to a TIMESTAMP column. However, the TIMESTAMP functions limit the values to those defined by the MIN_VALID_TIMESTAMP() and MAX_VALID_TIMESTAMP() functions. Starting with V7.0, VoltDB no longer accepts values outside of the valid timestamp range for TIMESTAMP columns.
Note that this is an incompatible change and may require changes to applications that are inadvertently inserting invalid timestamp values. If so, you can use the IS_VALID_TIMESTAMP() function to validate input values before inserting them into TIMESTAMP columns.
The following are known limitations to the current release of VoltDB. Workarounds are suggested where applicable. However, it is important to note that these limitations are considered temporary and are likely to be corrected in future releases of the product.
Do not use the subfolder name "segments" for the command log snapshot directory.
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.
Some DR data may not be delivered if master database nodes fail and rejoin in rapid succession.
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.
Avoid bulk data operations within a single transaction when using database replication
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).
Database replication ignores resource limits
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. DR also ignores any partition row limits defined in the database schema when applying binary logs. 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 and tables row limits are applied on any original transactions local to the cluster (for example, transactions on the master database in passive DR).
Different cluster sizes can require additional Java heap
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:
Sites-per-host * master/replace ratio * 20MB
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.
$ export VOLTDB_OPTS="-DDR_MEM_LIMIT=5"
$ voltdb start
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.
Avoid replicating tables without a unique index.
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.
When starting XDCR for the first time, only one database can contain data.
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.
During the initial synchronization of existing data, the receiving database is paused.
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.
A large number of multi-partition write transactions may interfere with the ability to restart XDCR after a cluster stops and recovers.
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.
A TRUNCATE TABLE transaction will be reported as a conflict with any other write operation to the same table.
When using XDCR, if the binary log from one cluster includes a TRUNCATE TABLE statement and the other cluster performs any write operation to the same table before the binary log is processed, the TRUNCATE TABLE operation will be reported as a conflict. Note that currently DELETE operations always supercede other actions, so the TRUNCATE TABLE will be executed on both clusters.
Exceeding a LIMIT PARTITION ROWS constraint can generate multiple conflicts
It is possible to place a limit on the number of rows that any partition can hold for a specific table using the LIMIT PARTITION ROWS clause of the CREATE TABLE statement. When close to the limit, transactions on either or both clusters can exceed the limit simultaneously, resulting in a potentially large number of delete operations that then generate conflicts when the the associated binary log reaches the other cluster.
Use of the VoltProcedure.getUniqueId method is unique to a cluster, not across clusters.
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.
XDCR cannot be used with deprecated export syntax.
You cannot use cross datacenter replication (XDCR) with the deprecated export syntax, that is the EXPORT TABLE statement. To use XDCR with export, you must use the current CREATE STREAM syntax for declaring the source streams for export targets.
Synchronous export in Kafka can use up all available file descriptors and crash the database.
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.
Data may be lost if a Kafka broker stops during import.
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.
Kafka import can lose data if multiple nodes stop in succession.
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.
Comments containing unmatched single quotes in multi-line statements can produce unexpected results.
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
Do not use assertions in VoltDB stored procedures.
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 currently convert ASCII characters only.
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.
Avoid using decimal datatypes with the C++ client interface on 32-bit platforms.
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 traps can slow down database startup.
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.
The VoltDB Management Center currently reports on only one DR connection.
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.
The following notes provide details concerning how certain VoltDB features operate. The behavior is not considered incorrect. However, this information can be important when using specific components of the VoltDB product.
Schema updates clear the stored procedure data table in the Management Center Monitor section
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.
You cannot partition a table on a column defined as ASSUMEUNIQUE.
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.
Adding or dropping column constraints (UNIQUE or ASSUMEUNIQUE) is not supported by the ALTER TABLE ALTER COLUMN statement.
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;
Do not use UPDATE to change the value of a partitioning column
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.
Certain SQL syntax errors result in the error message "user lacks privilege or object not found".
If you refer to a table or column name that does not exist, VoltDB reports that the "user lacks privilege or object not found". This can happen, for example, if you misspell a table or column name.
Another situation where this occurs is if you mistakenly use double quotation marks to enclose a string
literal (such as
The workaround is, if you receive this error, to look for misspelled table or columns names or string literals delimited by double quotes in the offending SQL statement.
Ambiguous column references no longer allowed.
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 . . .
File Descriptor Limits
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.
Use of Resources in JAR Files
There are two ways to access additional resources in a VoltDB database. You can place the resources in the
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.