3. Cassandra PV Archiver 3.2

3. Cassandra PV Archiver 3.2
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The Cassandra PV Archiver 3.2 adds a few new configuration options and updates its dependencies to their respective newest versions. It is compatible with the Cassandra PV Archiver 3.1.x, meaning that it can operate on data stored by the Cassandra PV Archiver 3.1.x and the APIs supported by the Cassandra PV Archiver 3.1.x are fully supported.

Due to newly introduced configuration options, configuration files for version 3.2.x are not compatible with version 3.1.x. However, configuration files for version 3.1.x remain compatible with version 3.2.x.

The following improvements have been made in this release:

The memory consumption when generating decimated samples based on pre-existing source samples has been reduced significantly. This is important when adding new decimation levels to a large number of channels. In earlier versions, the sample decimation process could allocate so much memory that the heap space would be exhausted, resulting in an OutOfMemoryError. Two new configuration options have been introduced for controlling the memory consumption of the sample decimation process: throttling.sampleDecimation.maxFetchedSamplesInMemory and throttling.sampleDecimation.maxRunningFetchOperations.
The implementation of AbstractObjectResultSet has been improved in order to avoid unnecessary copy operations. This change should improve the performance when reading samples from the database. In order to profit from this change, control-system supports using AbstractObjectResultSet for implementing their sample result sets should change the result set’s fetchNextPage() method to return a SizedIterator instead of a regular Iterator. This change has already been implemented for the ResultSetBasedObjectResultSet, so control-system supports using this class (like the Channel Access control-system support) will automatically profit from this improvement.
The cassandra.fetchSize option has been introduced in order to control the default fetch size used for queries. Usually, the default fetch size of the Cassandra driver should be fine, but users wanting to fine-tune the fetch size can now do so.
The server.interNodeCommunicationRequestTimeout option has been introduced in order to control the timeout for requests sent from one archiving server to another one. This timeout has been significantly increased in version 3.1.2, but now it is possible to increase it even further if necessary or to choose a shorter timeout if sufficient.
The way how the throttling of statements executed on the Cassandra cluster is handled has been improved. In older versions, the limit for read statements would only apply for the initial execution of a statement. If a statement later fetched more results (because there were more rows than the configured fetch size), this fetch operation would not count towards the limit. Now, a fetch operation is treated like a read statement and fully counts towards the limit. This means that in certain situations it might be possible to slightly raise the limit for read statements.
The naming scheme for the MBeans exposed via JMX has been changed so that all MBeans of the archiving server are in a single domain. This means that clients using JMX to monitor the archiving server have to be changed to use the new object names. The JMX interface is not considered a public API and thus might again change in future versions.

There also was one bug that has been fixed in this release:

The way how write operations to the generic_data_store table were handled was unsafe because light-weight transactions (LWTs) were mixed with regular updates. This could theoretically lead to invalid data if writes were happening very rapidly or server clocks had an extremely large clock skew. As data is only rarely written to this table (once when the archiving cluster is initialized and every time the administrator’s password is changed), this bug was very unlikely to cause any actual problems.

The Cassandra driver has been updated to version 3.2.0 in this release. That version includes a change to how user-defined types (UDTs) are handled when using the schema builder to create a table. Control-system supports using the schema builder to create a table with UDT columns might have to be changed to use the schema builder’s addUDTColumn(…) method with a parameter constructed using SchemaBuilder.frozen(…) instead of using addColumn(…) with an instance of UserType.

3.1. Cassandra PV Archiver 3.2.1

Version 3.2.1 is a bugfix release that fixes an issue introduced in the 3.2.0 release.

This issue caused the archiving server not to start if the configuration file contained a line for the sample-decimation throttling options, but not any actual options. Unfortunately, the default configuration file provided with the distribution contained such a line so that the archiving server would not start with its default configuration file.

Starting with version 3.2.1, the archiving server accepts such a line in the configuration file and thus will work with the unchanged configuration file.

3.2. Cassandra PV Archiver 3.2.2

Version 3.2.2 is a bugfix release that fixes an issue with the throttling mechanism introduced in the 3.2.0 release.

This issue caused the throttling mechanism to not work correctly in certain cases, thus fetching (significantly) more samples than allowed by the specified limit.

3.3. Cassandra PV Archiver 3.2.3

Version 3.2.3 is a bugfix release that fixes an issue with the generation of decimated samples.

Due to this issue, the generation of decimated samples would permanently stop if there was an error while reading source samples from the database. The code will now catch such an error and retry periodically.

3.4. Cassandra PV Archiver 3.2.4

Version 3.2.4 is a bugfix release that fixes a regression introduced with the 3.2.0 release and improves the initialization logic of the archiving service.

The regression would cause the generation process for decimated samples to be brought to a halt when more than 100 source samples were used to generate a single decimated sample. This regression would only affect the generation of decimated samples from source samples in the database (typically when a new decimation level was added). When generating decimated samples as new samples were written, the regression would not be triggered.

The initialization logic of the archiving service has been improved so that it will be run again if there is an error while trying to initialize the channels. Before, the initialization would only be attempted again when the server went offline (because the database was unavailable) and then back online. This had the consequence that a very brief database problem (possibly caused by the database being overloaded) might cause the initialization logic to fail without the server going offline, resulting in a server that was online, but did not have any active channels.

3.5. Cassandra PV Archiver 3.2.5

Version 3.2.5 is a bugfix release that further reduces the memory consumption when generating decimated samples. This release also contains a small improvement that helps to avoid spending CPU cycles on generating decimated samples (that will not be written anyway) after a channel has been destroyed.

Thanks to these improvements, the memory consumption when generating decimates samples from source samples already stored in the database (e.g. when adding a new decimation level) should be significantly reduced, resulting in a better performance and potentially avoiding memory exhaustion.

3.6. Cassandra PV Archiver 3.2.6

Version 3.2.6 is a bugfix release with a few minor fixes.

A bug in the Channel Access control-system support that affected the monitorMask configuration option has been fixed. Due to this bug, setting the monitorMask option would actually change the value of the metaDataMonitorMask option and thus there was no way to use a non-default value for the monitorMask option.

There also was a typo in the post-installation script of the Debian package that could cause the package installation to fail in certain rare circumstances.

A small improvement has been made to the logging of the component that generates decimated samples. Now, the stack trace of an exception will be logged with the error message, making it easier to find the cause of a problem.

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