Durability and Persistence
[RTI Connext API Reference]

APIs related to RTI Connext Durability and Persistence. More...

RTI Connext offers the following mechanisms for achieving durability and persistence:

Durable Writer History

Durable Reader State

Data Durability

To use any of these features, you need a relational database, which is not included with RTI Connext. Supported databases are listed in the Release Notes.

These three features can be used separately or in combination.

Durable Writer History

This feature allows a DDSDataWriter to locally persist its local history cache so that it can survive shutdowns, crashes and restarts. When an application restarts, each DDSDataWriter that has been configured to have durable writer history automatically loads all the data in its history cache from disk and can carry on sending data as if it had never stopped executing. To the rest of the system, it will appear as if the DDSDataWriter had been temporarily disconnected from the network and then reappeared.

See also:: Configuring Durable Writer History

Durable Reader State

This feature allows a DDSDataReader to locally persists its state and remember the data it has already received. When an application restarts, each DDSDataReader that has been configured to have durable reader state automatically loads its state from disk and can carry on receiving data as if it had never stopped executing. Data that had already been received by the DDSDataReader before the restart will be suppressed so it is not sent over the network.

Data Durability

This feature is a full implementation of the OMG DDS Persistence Profile. The DURABILITY QoS lets an application configure a DDSDataWriter such that the information written by the DDSDataWriter survives beyond the lifetime of the DDSDataWriter. In this manner, a late-joining DDSDataReader can subscribe and receive the information even after the DDSDataWriter application is no longer executing. To use this feature, you need RTI Persistence Service -- an optional product that can be purchased separately.

Durability and Persistence Based on Virtual GUID

Every modification to the global dataspace made by a DDSDataWriter is identified by a pair (virtual GUID, sequence number).

The virtual GUID (Global Unique Identifier) is a 16-byte character identifier associated with a DDSDataWriter or DDSDataReader; it is used to uniquely identify this entity in the global data space.
The sequence number is a 64-bit identifier that identifies changes published by a specific DDSDataWriter.

Several DDSDataWriter entities can be configured with the same virtual GUID. If each of these DDSDataWriter entities publishes a sample with sequence number '0', the sample will only be received once by the DDSDataReader entities subscribing to the content published by the DDSDataWriter entities.

Global Dataspace Changes

RTI Connext also uses the virtual GUID (Global Unique Identifier) to associate a persisted state (state in permanent storage) to the corresponding DDS entity.

For example, the history of a DDSDataWriter will be persisted in a database table with a name generated from the virtual GUID of the DDSDataWriter. If the DDSDataWriter is restarted, it must have associated the same virtual GUID to restore its previous history.

Likewise, the state of a DDSDataReader will be persisted in a database table whose name is generated from the DDSDataReader virtual GUID

History/State Persistence Based on the Virtual GUID

A DDSDataWriter's virtual GUID can be configured using DDS_DataWriterProtocolQosPolicy::virtual_guid. Similarly, a DDSDataReader's virtual GUID can be configured using DDS_DataReaderProtocolQosPolicy::virtual_guid.

The DDS_PublicationBuiltinTopicData and DDS_SubscriptionBuiltinTopicData structures include the virtual GUID associated with the discovered publication or subscription.

Refer to the User's Manual for additional use cases.

See also:: DDS_DataWriterProtocolQosPolicy::virtual_guid DDS_DataReaderProtocolQosPolicy::virtual_guid.

Configuring Durable Writer History

To configure a DDSDataWriter to have durable writer history, use the PROPERTY QoS policy associated with the DDSDataWriter or the DDSDomainParticipant.

Properties defined for the DDSDomainParticipant will be applied to all the DDSDataWriter objects belonging to the DDSDomainParticipant, unless the property is overwritten by the DDSDataWriter.

See also:: DDS_PropertyQosPolicy

The following table lists the supported durable writer history properties.

*Durable Writer History Properties*
Property	Description
dds.data_writer.history.plugin_name	Must be set to "dds.data_writer.history.odbc_plugin.builtin" to enable durable writer history in the DataWriter. This property is required.
dds.data_writer.history.odbc_plugin.dsn	The ODBC DSN (Data Source Name) associated with the database where the writer history must be persisted. This property is required.
dds.data_writer.history.odbc_plugin.driver	This property tells RTI Connext which ODBC driver to load. If the property is not specified, RTI Connext will try to use the standard ODBC driver manager library: UnixOdbc (odbc32.dll) on UNIX/Linux systems; the Windows ODBC driver manager (libodbc.so) on Windows systems).
dds.data_writer.history.odbc_plugin.username	Configures the username used to connect to the database. This property is not used if it is unspecified. There is no default value.
dds.data_writer.history.odbc_plugin.password	Configures the password used to connect to the database. This property is not used if it is unspecified. There is no default value.
dds.data_writer.history.odbc_plugin.shared	If set to 1, RTI Connext creates a single connection per DSN that will be shared across DataWriters within the same Publisher. If set to 0 (the default), a DDSDataWriter will create its own database connection. Default: 0 (false)
dds.data_writer.history.odbc_plugin.instance_cache_max_size	These properties configure the resource limits associated with the ODBC writer history caches. To minimize the number of accesses to the database, RTI Connext uses two caches, one for samples and one for instances. The initial and maximum sizes of these caches are configured using these properties. The resource limits initial_instances, max_instances, initial_samples, max_samples and max_samples_per_instance in the DDS_ResourceLimitsQosPolicy are used to configure the maximum number of samples and instances that can be stored in the relational database. Default: DDS_ResourceLimitsQosPolicy::max_instances
dds.data_writer.history.odbc_plugin.instance_cache_init_size	See description above. Default: DDS_ResourceLimitsQosPolicy::initial_instances
dds.data_writer.history.odbc_plugin.sample_cache_max_size	See description above. Default: 32 (the minimum)
dds.data_writer.history.odbc_plugin.sample_cache_init_size	See description above. Default: 32
dds.data_writer.history.odbc_plugin.restore	This property indicates whether or not the persisted writer history must be restored once the DDSDataWriter is restarted. If the value is 0, the content of the database associated with the DDSDataWriter being restarted will be deleted. If the value is 1, the DDSDataWriter will restore its previous state from the database content. Default: 1
dds.data_writer.history.odbc_plugin.in_memory_state	This property determines how much state will be kept in memory by the ODBC writer history in order to avoid accessing the database. When in_memory_state is equal to 1, instance_cache_max_size is always equal to DDS_ResourceLimitsQosPolicy::max_instances (it cannot be changed). In addition, the ODBC writer history will keep in memory a fixed state overhead of 24 bytes per sample. In this operating mode, the ODBC writer history provides the best performance. However, the restore operation will be slower and the maximum number of samples that the writer history can manage will be limited by the available physical memory. If in_memory_state is equal to 0, all the state will be kept in the underlying database. In this operating mode, the maximum number of samples in the writer history will not be limited by the physical memory available unless the underlying database is an in-memory database (TimesTen). Default: 1

Configuring Durable Reader State

To configure a DDSDataReader with durable reader state, use the PROPERTY QoS policy associated with the DDSDataReader or DDSDomainParticipant.

A property defined in the DDSDomainParticipant will be applicable to all the DDSDataReader belonging to the DDSDomainParticipant unless it is overwritten by the DDSDataReader.

See also:: DDS_PropertyQosPolicy

The following table lists the supported durable reader state properties.

*Durable Reader State Properties*
Property	Description
dds.data_reader.state.odbc.dsn	The ODBC DSN (Data Source Name) associated with the database where the DDSDataReader state must be persisted. This property is required.
dds.data_reader.state.filter_redundant_samples	To enable durable reader state, this property must be set to 1. Otherwise, the reader state will not be kept and/or persisted. When the reader state is not maintained, RTI Connext does not filter duplicate samples that may be coming from the same virtual writer. By default, this property is set to 1.
dds.data_reader.state.odbc.driver	This property is used to indicate which ODBC driver to load. If the property is not specified, RTI Connext will try to use the standard ODBC driver manager library: UnixOdbc (odbc32.dll) on UNIX/Linux systems; the Windows ODBC driver manager (libodbc.so) on Windows systems).
dds.data_reader.state.odbc.username	This property configures the username used to connect to the database. This property is not used if it is unspecified. There is no default value.
dds.data_reader.state.odbc.password	This property configures the password used to connect to the database. This property is not used if it is unspecified. There is no default value.
dds.data_reader.state.restore	This property indicates if the persisted DDSDataReader state must be restored or not once the DDSDataReader is restarted. If this property is 0, the previous state will be deleted from the database. If it is 1, the DDSDataReader will restore its previous state from the database content. Default: 1
dds.data_reader.state.checkpoint_frequency	This property controls how often the reader state is stored in the database. A value of N means to store the state once every N samples. A high frequency will provide better performance. However, if the reader is restarted it may receive some duplicate samples. These samples will be filtered by the middleware and they will not be propagated to the application. Default: 1
dds.data_reader.state.persistence_service.request_depth	This property indicates the number of most recent historical samples that the persisted DDSDataReader wants to receive when it starts up. Default: 0

Configuring Data Durability

RTI Connext implements DDS_TRANSIENT_DURABILITY_QOS and DDS_PERSISTENT_DURABILITY_QOS durability using RTI Persistence Service, available for purchase as a separate RTI product.

For more information on RTI Persistence Service, refer to the User's Manual, or the RTI Persistence Service online documentation.

See also:: DURABILITY

Durability and Persistence [RTI Connext API Reference]