5.9. Partitions

5.9.1. Introduction

The PARTITION QoS provides a way to control which Entities will match—and thus communicate with—which other Entities. It can be used to prevent Entities that would have otherwise matched from talking to each other. Much in the same way that only applications within the same DDS domain will communicate with each other, only Entities that belong to the same partition can talk to each other.

The PARTITION QoS applies to Publishers and Subscribers. DataWriters and DataReaders belong to the partitions as set in the QoS of the Publishers and Subscribers that created them.

The PARTITION QoS consists of a set of partition names that identify the partitions of which the Entity is a member. These names can be concrete (e.g., ExamplePartition) or regular expression strings (e.g, Example*), and two Entities are considered to be in the same partition if one of the Entities has a concrete partition name matching one of the concrete or regular expression partition names of the other Entity (see Pattern matching for PARTITION names). By default, DataWriters and DataReaders (through their Publisher/Subscriber parents), belong to a single partition whose name is the empty string, “”.

Conceptually, each partition name can be thought of as defining a “visibility plane” within the DDS domain. DataWriters will make their data available on all of the visibility planes that correspond to their Publisher’s partition names, and the DataReaders will see the data that is placed on all of the visibility planes that correspond to their Subscriber’s partition names.

Figure 5.4 illustrates the concept of PARTITION QoS at the Publisher and Subscriber level. In this figure, all DataWriters and DataReaders belong to the same DDS domain ID and DomainParticipant partition, and they use the same Topic. DataWriter1 is configured to belong to three partitions: partition_A, partition_B, and partition_C. DataWriter2 belongs to partition_C and partition_D.

../_images/Partitions.png

Figure 5.4 Controlling Visibility of Data with PARTITION QoS

Similarly, DataReader1 is configured to belong to partition_A and partition_B, and DataReader2 belongs only to partition_C. Given this topology, the data written by DataWriter1 is visible in partitions A, B, and C. The oval tagged with the number “S1” represents one DDS data sample written by DataWriter1.

Similarly, the data written by DataWriter2 is visible in partitions C and D. The oval tagged with the number “S2” represents one DDS data sample written by DataWriter2.

The result is that the data written by DataWriter1 will be received by both DataReader1 and DataReader2, but the data written by DataWriter2 will only be visible by DataReader2.

Publishers and Subscribers always belong to a partition. By default, Publishers and Subscribers belong to a single partition whose name is the empty string, “”. If you set the PARTITION QoS to be an empty set, Connext Micro will assign the Publisher or Subscriber to the default partition, “”. Thus, for the example above, without using the PARTITION QoS on any of the entities, DataReaders 1 and 2 would have received all data samples written by DataWriters 1 and 2.

5.9.2. Rules for PARTITION matching

The PARTITION QosPolicy associates a set of partition names with the entity (Publisher or Subscriber). The partition names are concrete names (e.g., ExamplePartition) or regular expression strings (e.g, Example*).

With regard to the PARTITION QoS, a DataWriter will communicate with a DataReader if and only if the following conditions apply:

  1. The DataWriter and DataReader belong to DomainParticipants bound to the same DDS domain ID.

  2. The DataWriter and DataReader have matching Topics. That is, each is associated with a Topic with the same name and compatible data type.

  3. The QoS offered by the DataWriter is compatible with the QoS requested by the DataReader.

Matching partition names is done by string pattern matching, and partition names are case-sensitive.

Note

Failure to match partitions (on Publisher or Subscriber) is not considered an incompatible QoS and does not trigger any listeners or change any status conditions.

5.9.3. Pattern matching for PARTITION names

You may add strings that are regular expressions to the PARTITION QosPolicy. A PARTITION.name is a regular expression if it contains any of the following unescaped special characters: *, ?, [, ], !, or ^. The PARTITION.name strings can be “concrete” names or regular expression strings; a PARTITION.name element that is a regular expression will only match against concrete strings found in a PARTITON.name element of a different Entity’s PARTITION QosPolicy.

If a PARTITION QoS only contains regular expressions, then the Entity will be assigned automatically to the default partition with the empty string name (“”). Thus, a PARTITION QoS that only contains the string * matches another Entity’s PARTITION QoS that also only contains the string *, not because the regular expression strings are identical, but because they both belong to the default “” partition.

For more on regular expressions, see Regular Expression Matching below.

Two Entities are considered to have a partition in common if the sets of partitions associated with them have:

  • At least one concrete partition name in common

  • A regular expression in one Entity that matches a concrete partition name in another Entity

The programmatic representation of the PARTITION QoS is shown in Table 5.4. The QosPolicy contains the single string sequence, name. Each element in the sequence can be a concrete name or a regular expression. The Entity will be assigned to the default “” partition if the sequence is empty, or if the sequence contains only regular expressions.

Table 5.4 DDS_PartitionQosPolicy

Type

Field Name

Description

DDS_StringSeq

name

Empty by default.

There can be up to 64 names, with a maximum of 256 characters (including the NUL terminator), summed across all names.

You can have one long partition string of 256 chars, or multiple shorter strings that add up to 256 or fewer characters. For example, you can have one string of 4 chars and one string of 252 chars.

5.9.3.1. Regular Expression Matching

The SQL expression format provided by Connext Micro supports the relational operator MATCH. It may only be used with string fields. The right-hand operator is a string pattern, which specifies a template that the left-hand field must match.

MATCH is case-sensitive. The following characters have special meaning, unless escaped by the escape character: ,\/?*[]-^!\%.

The pattern allows limited “wild card” matching under the rules in Table 5.5.

The syntax is similar to the POSIX® fnmatch syntax. (See http://www.opengroup.org/onlinepubs/000095399/functions/fnmatch.html.)

Table 5.5 Wild Card Matching

Character

Meaning

,

NOT SUPPORTED

A , separates a list of alternate patterns. The field string is matched if it matches one or more of the patterns.

/

NOT SUPPORTED

A / in the pattern string matches a / in the field string. It separates a sequence of mandatory substrings.

?

A ? in the patterns tring matches any single non-special characters in the field string.

*

A * in the pattern string matches 0 or more non-special characters in the field string.

%

NOT SUPPORTED

This special character is used to designate filter expression parameters.

Escape character for special characters.

[charlist]

Matches any one of the characters in charlist.

[!charlist] or [^charlist]

Matches any one of the characters not in charlist.

[s-e]

Matches any character from s to e, inclusive.

[!s-e] or [^s-e]

Matches any character not in the interval s to e.

Note

To use special characters as regular characters in regular expressions, you must escape them using the character \. For example, A[ is considered a malformed expression and the result is undefined.

5.9.4. Example

The PARTITION QosPolicy is useful to control which DataWriters can communicate with which DataReaders and vice versa—even if all of the DataWriters and DataReaders are for the same Topic. This facility is useful for creating temporary separation groups among Entities that would otherwise be connected to and exchange data each other.

The code below illustrates how to set the PARTITION QosPolicy on a Publisher:

struct DDS_PublisherQos pub_qos = DDS_PublisherQos_INITIALIZER;
   DDS_StringSeq_set_maximum(&pub_qos.partition.name,2);
   DDS_StringSeq_set_length(&pub_qos.partition.name,2);
   *DDS_StringSeq_get_reference(&pub_qos.partition.name,0) = DDS_String_dup("partition1");
   *DDS_StringSeq_get_reference(&pub_qos.partition.name,1) = DDS_String_dup("partition2");

publisher = DDS_DomainParticipant_create_publisher(application->participant,&pub_qos, NULL,DDS_STATUS_MASK_NONE);
   if (publisher == NULL)
   {
      ...
   }

Using partitions, connectivity can be controlled based on location-based partitioning, access-control groups, or a combination of these and other application-defined criteria. We will examine some of these options via concrete examples.

5.9.4.1. Location-based partitions

Assume you have a set of Topics in a traffic management system such as “TrafficAlert,” “AccidentReport,” and “CongestionStatus.” You may want to control the visibility of these Topics based on the actual location to which the information applies. You can do this by placing the Publisher in a partition that represents the area to which the information applies. This can be done using a string that includes the city, state, and country, such as “USA/California/Santa Clara.” A Subscriber can then choose whether it wants to see the alerts in a single city, the accidents in a set of states, or the congestion status across the US. Some concrete examples are shown in Table 5.6.

Table 5.6 Example of Using Location-Based Partitions

Publisher Partitions

Subscriber Partitions

Result

Specify a single partition name using the pattern:

“<country>/<state>/<city>”

Specify multiple partition names, one per region of interest

Limits the visibility of the data to Subscribers that express interest in the geographical region.

“USA/California/Santa Clara”

(Subscriber partition is irrelevant here.)

Send only information for Santa Clara, California.

(Publisher partition is irrelevant here.)

“USA/California/Santa Clara”

Receive only information for Santa Clara, California.

(Publisher partition is irrelevant here.)

“USA/California/Santa Clara”

“USA/California/Sunnyvale”

Receive information for Santa Clara or Sunnyvale, California.

(Publisher partition is irrelevant here.)

“USA/California/*”

“USA/Nevada/*”

Receive information for California or Nevada.

(Publisher partition is irrelevant here.)

“USA/California/*”

“USA/Nevada/Reno”

“USA/Nevada/Las Vegas”

Receive information for California and two cities in Nevada.

5.9.4.2. Access-control group partitions

Suppose you have an application where access to the information must be restricted based on reader membership to access-control groups. You can map this group-controlled visibility to partitions by naming all the groups (e.g., executives, payroll, financial, general-staff, consultants, external-people) and assigning the Publisher to the set of partitions that represents which groups should have access to the information. The Subscribers specify the groups to which they belong, and the partition-matching behavior will ensure that the information is only distributed to Subscribers belonging to the appropriate groups. Some concrete examples are shown in Table 5.7

Table 5.7 Example of Access-Control Group Partitions

Publisher Partitions

Subscriber Partitions

Result

Specify several partition names, one per group that is allowed access:

Specify multiple partition names, one per group to which the Subscriber belongs.

Limits the visibility of the data to Subscribers that belong to the access-groups specified by the Publisher.

“payroll”

“financial”

(Subscriber partition is irrelevant here.)

Makes information available only to Subscribers that have access to either financial or payroll information.

(Publisher partition is irrelevant here.)

“executives”

“financial”

Gain access to information that is intended for executives or people with access to the finances.

A slight variation of this pattern could be used to confine the information based on security levels.

5.9.5. Properties

This QosPolicy cannot be modified at runtime.

Strictly speaking, this QosPolicy does not have request-offered semantics, although it is matched between DataWriters and DataReaders, and communication is established only if there is a match between partition names.

5.9.6. Resource limits

Before this QoS policy can be used, you must configure the following DomainParticipantResourceLimitsQosPolicy fields:

  • max_partitions: sets the maximum number of partitions for each PARTITION QoS.

  • max_partition_cumulative_characters: sets the maximum number of characters (per DomainParticipant) that can be used for the sum-total length of all partition names. Note that the NUL terminator in each string contributes to the character count.

  • max_partition_string_size: sets the maximum number of characters that can be used for each partition name. This can be set to a value greater than 0 or DDS_LENGTH_UNLIMITED.

  • max_partition_string_allocation: sets the maximum total memory allocated to partition names across all DomainParticipants. This can be set to a value greater than 0 or DDS_LENGTH_UNLIMITED.

Note

All applications in the DDS domain must have the same resource limit values in order to communicate. For example, if two applications have different values, and one application sets the PARTITION QosPolicy to hold more partitions or longer names than set by another application, the matching Entities between the two applications will not connect. This is similar to the restrictions for the GROUP_DATA, USER_DATA, and TOPIC_DATA Qos Policies.

These fields collectively determine how your application manages partition memory. The subsections below explain how to configure your DomainParticipantResourceLimitsQosPolicy for different behaviors.

5.9.6.1. Configuring for runtime allocation

This configuration allows memory for PARTITION.name strings to be allocated and freed during runtime. Each PARTITION.name string can be of any size; however, the sum-total string length of all partition names is still limited by max_partition_cumulative_characters.

For this behavior, set the following:

  • Set max_partition_string_size to DDS_LENGTH_UNLIMITED.

  • Set max_partition_string_allocation to DDS_LENGTH_UNLIMITED.

5.9.6.2. Configuring for preallocated memory

Preallocating memory gives you greater control over memory utilization. There are two possible configurations for preallocated memory: reusable and non-reusable.

5.9.6.2.1. Reusable preallocated memory

In this configuration, memory for PARTITION.name strings is preallocated and will never be freed during operation. However, memory will be reused for PARTITION names that are added, internally deleted, and no longer needed. For example, if the application creates a Publisher with a unique PARTITION name instance and then deletes it, the application will reuse the memory that was storing the unique name (unless there are other uses of that name).

For this behavior, set the following:

  • Set max_partition_string_size to a value greater than 0.

  • Set max_partition_string_allocation to a value greater than 0. This value must be large enough to store every instance of each PARTITION name that will be created or discovered.

Note that each PARTITION name will take up memory equal to max_partition_string_size, regardless of the actual string length.

5.9.6.2.2. Non-reusable preallocated memory

In this configuration, memory for PARTITION.name strings is preallocated and will never be freed or reused.

For this behavior, set the following:

  • Set max_partition_string_size to DDS_LENGTH_UNLIMITED.

  • Set max_partition_string_allocation to a value greater than 0. This value must be large enough to store every instance of each PARTITION name that is created and discovered.

Note that each PARTITION name will take up memory equal to its exact string size.