Class TopicSpecificationProperty

Contains all valid ITopicSpecification property keys.

Inheritance

Object

TopicSpecificationProperty

Namespace: PushTechnology.ClientInterface.Client.Topics.Details

Assembly: Diffusion.Client.dll

Syntax

public static class TopicSpecificationProperty : object

Fields

Compression

Key of the topic property that allows the compression policy to be set on a per-topic basis.

Declaration

public static readonly string Compression

Field Value

Type	Description
String	Topic property key for compression.

Remarks

Compression reduces the bandwidth required to broadcast topic updates to subscribed sessions, at the cost of increased server CPU.

Changes to a topic's value are published to each subscribed session as a sequence of topic messages. A topic message can carry the latest value or the difference between the latest value and the previous value (a delta). The compression policy determines if and how published topic messages are compressed. Topic messages are not exposed through the client API; the client library handles decompression and decodes deltas automatically, passing reconstructed values to the application.

The compression policy for a topic is specified by setting this property to one of several values:

• off
• low
• medium
• high

The policies are listed in the order of increasing compression and increasing CPU cost. off disables compression completely for the topic and requires no additional CPU; high compresses the topic messages to the smallest number of bytes, but has the highest CPU cost. Generally some compression is beneficial, so the default value for this property is low.

Prior to version 6.4, only two values were allowed: true (equivalent to medium, and the previous default policy) and false (equivalent to off). These values are still supported.

This property is only one factor that determines whether a topic message will be compressed. Other factors include:

• Compression must be enabled in the server configuration.
• The client library must support the server's compression scheme. In this release, the server supports zlib compression, and also allows compression to be disabled on a per-connector basis. From 6.4, all client libraries are capable of zlib compression. A JavaScript client may or may not support zlib compression, depending on whether the zlib library can be loaded. The zlib library is packaged separately to reduce the download size of the core library.

Added in version 6.2.

Conflation

Key of the topic property that describes the conflation policy of the topic.

Declaration

public static readonly string Conflation

Field Value

Type	Description
String	Topic property key for conflation.

Remarks

The policy specifies how the server manages queued topic updates. Conflation is applied individually to each session queue.

Conflation is the process of merging or discarding topic updates queued for a session to reduce the server memory footprint and network data. The server will conflate sessions that have a large number of queued messages to meet configured queue size targets. The sessions with the largest queues are typically slow consumers or have been disconnected – both will benefit from conflation. This property allows conflation behavior to be tuned on a topic-by-topic basis.

Supported policies are:

• off
• conflate
• unsubscribe
• always

The default policy used when the property is not specified and the topic type is not time series is "conflate". The default policy used when the property is not specified and the topic type is time series is "off".

The policy "off" disables conflation for the topic. This policy disables all conflation for the topic, so topic updates will never be merged or discarded.

The policy "conflate" automatically conflates topic updates when back pressure is detected by the server. This policy is ignored for stateless, single value and record topics.

The policy "unsubscribe" automatically unsubscribes the topic when back pressure is detected by the server. The unsubscription is not persisted to the cluster. If a session fails over to a different server it will be resubscribed to the topic.

The policy "always" automatically conflates topic updates as they are queued for the session. This is an eager policy that ensures only the latest update is queued for the topic, minimizing the server memory and network bandwidth used by the session. This policy is ignored for stateless, single value and record topics.

The "conflate" and "unsubscribe" policies are applied when the server detects back pressure for a session. The server configuration places limits on the data queued for each session. If these limits are breached, the server will conflate the session queue to attempt to reduce its size. If the session queue still exceeds the limits after conflation, the session will be terminated.

Conflation of stateless, single value and record topics is configured using server-side configuration. This configuration describes how topic updates should be merged. Like the "always" policy conflation specified this way is applied when queuing a topic update. The policy "off" prevents this conflation being applied. All other policies allow conflation specified this way to happen. The "unsubscribe" policy will still unsubscribe topics that use conflation specified this way.

The policies "conflate" and "always" are not supported for time series topics as they would cause missing events. Attempts to enable these policies with time series topics will cause the creation of the topic to fail, reporting that the specification is invalid.

Added in version 6.1

DontRetainValue

Key of the topic property that specifies a topic should not retain its last value.

Declaration

public static readonly string DontRetainValue

Field Value

Type	Description
String	Topic property key for don't retain value.

Remarks

By default, a topic will retain its latest value. The latest value will be sent to new subscribers. Setting this property to `true` disables this behavior. New subscribers will not be sent an initial value. No value will be returned for fetch operations that select the topic. This is useful for data streams where the values are only transiently valid.

Setting this property to `true` also disables delta streams, regardless of the PublishValuesOnly property value. If subsequent values are likely to be related, delta streams usually provide performance benefits (PublishValuesOnly). Consider leaving this property set to `false` to benefit from delta streams, even if there is no other requirement to retain the last value.

Bearing in mind the performance trade-offs of disabling delta streams, there are two reasons to consider setting DontRetainValue to `true`. First, it stops the server and each subscribed client from keeping a copy of the value, reducing their memory requirements. Second, when a topic has a high update rate and is replicated across a cluster, it can significantly improve throughput because the values need not be persisted to the cluster.

Time series topics ignore this property and always retain the latest value.

Implemented in Version 6.0.

Owner

Key of the topic property that allows the creator of a topic to extend READ_TOPIC, MODIFY_TOPIC, and UPDATE_TOPIC permissions to a specific principal, in addition to the permissions granted by the authorization rules in the security store.

Declaration

public static readonly string Owner

Field Value

Type	Description
String	Topic property key for owner.

Remarks

A session that has authenticated using the principal can update and remove the topic, so the principal can be considered the topic owner. To fetch or subscribe to the topic, the principal must also be granted the SELECT_TOPIC permission by the security store rules.

This may be used in the following cases: 1) A session creates a topic and makes its own principal the owner. 2) A session creates a topic and makes another principal the owner.

The format of the property value is: $Principal is "name"

where name is the name of the principal. Single quotes may be used instead of double quotes and special characters can be escaped.

The purpose of this property is to allow a client to create topics on behalf of other users. This can be used in conjunction with the Removal property so that such topics are removed when there are no longer any sessions for the named principal.

For example:

specification.WithProperty(
    TopicSpecificationProperty.OWNER, "$Principal is 'myPrincipal'" )
    .WithProperty( TopicSpecificationProperty.REMOVAL, "when no session has '$Principal is \"myPrincipal\"' for 5s" );

Added in version 6.1

Persistent

Key of the topic property that can be used to prevent a topic from being persisted when the server is configured to enable persistence.

Declaration

public static readonly string Persistent

Field Value

Type	Description
String	Topic property key for persistence.

Remarks

By default, a topic will be persisted if persistence is enabled at the server and the topic type supports persistence.

Setting Persistent to false will prevent the topic from being persisted.

Added in version 6.1

Priority

Key of the topic property that specifies the topic delivery priority.

Declaration

public static readonly string Priority

Field Value

Type	Description
String	Topic property key for priority.

Remarks

The supported delivery priorities are:

• low
• default
• high

The delivery priority affects the order of topic updates sent to a subscribed client session. When there are multiple topic updates for topics with different priorities in a session's outbound queue, updates for high priority topics will be delivered first, followed by updates for default priority topics, followed by updates for low priority topics. Topic subscription and unsubscription notifications are also delivered according to the topic delivery priority.

Using different delivery priorities is most beneficial when there is a large backlog of queued updates to deliver to a client session. On lightly loaded systems, updates typically remain in the outbound queue for a few milliseconds and so there is a lower chance of topic updates being reordered based on their priority. The backlog will be larger if the topic update rate is higher; the server or the client are more heavily loaded; the client session becomes temporarily disconnected; or if there is poor network connectivity between the server and the client.

Messages from the server to the client that are not topic updates, for example ping requests and responses, are queued with the default delivery priority.

Added in version 6.4

PublishValuesOnly

Key of the topic property that specifies whether a topic should publish only values.

Declaration

public static readonly string PublishValuesOnly

Field Value

Type	Description
String	Topic property key for publish only values.

Remarks

By default, a topic that supports delta streams will publish the difference between two values (a delta) when doing so is more efficient than publishing the complete new value. Subscribing sessions can use a IValueStream<TValue> to automatically apply the delta to a local copy of the topic value to calculate the new value.

Setting PublishValuesOnly to `true` disables this behavior so that deltas are never published. Doing so is usually not recommended because it will result in more data being transmitted, less efficient use of network resources, and increased transmission latency. On the other hand, calculating deltas can require significant CPU from the server or, if update streams are used, from the updating client. The CPU cost will be higher if there are many differences between successive values, in which case delta streams confer fewer benefits. If successive values are unrelated to each other, consider setting PublishValuesOnly to `true`. Also consider setting PublishValuesOnly to `true` if the network capacity is high and the bandwidth savings of deltas are not required.

DontRetainValue

Removal

Key of the topic property that specifies a removal policy for automatic removal of the topic (and/or other topics).

Declaration

public static readonly string Removal

Field Value

Type	Description
String	Topic property key for removal.

Remarks

This property is specified as an expression which defines one or more conditions that are to be satisfied before automatic removal occurs.

The expression takes the form: when conditions [remove "selector"].

At least one condition must be supplied. If more than one is supplied, they must be separated by logical operators (and or or). The natural evaluation order of the operators may be changed by surrounding with parentheses (for example (condition and condition)).

The remove clause is optional. It provides a topic selector expression representing the topics to be removed. If a remove clause is specified, the topic with the removal policy will only be removed if its path matches the selector expression. The selector must be surrounded by either double or single quotes.

When many topics have the same removal policy, it is better to set the REMOVAL property for one of them, using a remove clause that selects all of the topics. This is more efficient because it allows the server to avoid evaluating the same condition many times.

The permissions that are applied at the time of removal are those defined by the roles of the principal that created the topic at the time of creation. The roles of that principal may therefore change before the removal with no effect, but if the permissions given to the roles change it may have an effect upon the final removal.

Only one occurrence of each of the following condition types may be included within the expression:

Condition	Format	Usage
time after	time after absoluteTime	Removal should occur after a specified absolute time. Absolute time may be specified as a number of milliseconds since the epoch (00:00:00 on 1 January 1970) or as a quoted date and time formatted in RF-1123 (https://tools.ietf.org/html/rfc1123). Either single or double quotes may be used.
subscriptions less than	[local] subscriptions > n for forPeriod [after afterPeriod]	Removal should occur when the topic has had less than the specified number (n) of subscriptions for a given period (forPeriod) of time. Optionally, an initial period (afterPeriod) may be specified by which to delay the initial checking of this condition. See below for period formats. The optional local keyword restricts evaluation to only count subscriptions from sessions belonging to the local server or cluster, ignoring subscriptions from sessions belonging to downstream remote servers that host fanout replicas of the topic.
no updates for	no updates for forPeriod [after afterPeriod]	Removal should occur when the topic has had no updates for a given period (forPeriod) of time. Optionally, an initial period (afterPeriod) may be specified by which to delay the initial checking of this condition. See below for period formats.

Multiple occurrences of the following condition types may be included within the expression:

Condition	Format	Usage
no [local] session has	no session has criteria [for forPeriod] [after afterPeriod]	Removal should occur when there are no sessions satisfying certain criteria. Optionally the criteria can be required to be satisfied for a period of time (forPeriod). Optionally, an initial period (afterPeriod) can be specified to delay the initial check of the criteria. Session selection criteria are specified as defined in session filters and must be surrounded by single or double quotes. See below for period formats. The optional local keyword restricts evaluation to sessions belonging to the local server or cluster, ignoring sessions belonging to downstream remote servers that host fanout replicas of the topic.
	this session closes	This is a shorthand form of no local session has that may be used to indicate that the topic is to be removed when the session that created it closes.

Time periods are specified as a number followed (with no intermediate space) by a single letter representing the time unit. The time unit may be s (seconds), m (minutes), h (hours) or d (days). For example, 10 minutes would be specified as 10m.

If quotes or backslashes (\) are required within quoted values such as selectors or session criteria then they may be escaped by preceding with \. The expression is validated only by the server and therefore if an invalid expression is specified it will be reported as an InvalidTopicSpecificationException.

Examples

when time after 1518780068112

The topic will be removed when the date and time indicated by the specified number of milliseconds since the epoch has passed.

when time after "Tue, 3 Jun 2018 11:05:30 GMT"

The topic will be removed when the specified date and time has passed.

when time after "Tue, 3 Jun 2018 11:05:30 GMT" remove "*alpha/beta//"

The topic alpha/beta and all topics subordinate to it will be removed when the specified date and time has passed.

when subscriptions < 1 for 20m

The topic will be removed when it has had no subscriptions for a continuous period of 20 minutes.

when subscriptions < 2 for 20m after 1h

The topic will be removed when it has had less than 2 subscriptions for a continuous period of 20 minutes after one hour has passed since its creation.

when no updates for 3h

The topic will be removed when it has had no updates for a continuous period of 3 hours.

when no updates for 15m after 1d

The topic will be removed when it has had no updates for a continuous period of 15 minutes after one day has passed since its creation.

when this session closes

The topic will be removed when the session creating it closes.

when no session has '$Principal is "Alice"'

The topic will be removed when there are no sessions with the principal 'Alice'.

when no session has '$Principal is "Alice"' for 10m

The topic will be removed when there are no sessions with the principal 'Alice' for a continuous period of 10 minutes.

when no session has 'Department is "Accounts"' for 30m after 2h

The topic will be removed when there have been no sessions from the Accounts department for a continuous period of 30 minutes after 2 hours have passed since its creation.

when time after "Tue, 3 Jun 2018 11:05:30 GMT" and subscriptions < 1 for 30m

The topic will be removed when the specified date and time has passed and the topic has had no subscriptions for a continuous period of 30 minutes after that time.

when time after "Tue, 3 Jun 2018 11:05:30 GMT" and subscriptions < 2 for 10m after 1h

The topic will be removed when the specified date and time has passed and the topic has had less than 2 subscriptions for a continuous period of 10 minutes after that time plus one hour.

when time after "Tue, 3 Jun 2018 11:05:30 GMT" or subscriptions < 2 for 10m after 1h

The topic will be removed when the specified date and time has passed or the topic has had less than 2 subscriptions for a continuous period of 10 minutes after one hour from its creation.

when time after "Tue, 3 Jun 2018 11:05:30 GMT" and (subscriptions < 2 for 10m after 1h or no updates for 20m)

The topic will be removed when the specified date and time has passed and either the topic has had less than 2 subscriptions for a continuous period of 10 minutes after that time plus one hour or it has had no updates for a continuous period of 20 minutes. Note that the parentheses are significant here as without them the topic would be removed if it had had no updates for 20 minutes regardless of the time and subscriptions clause.

Notes and restrictions on use

The after time periods refer to the period since the topic was created or restored from persistence store after a server is restarted. They are designed as a 'grace' period after the topic comes into existence before the related condition starts to be evaluated. When not specified the conditions start to be evaluated as soon as the topic is created or restored.

The server will evaluate conditions on a periodic basis (every few seconds) so the exact removal time will not be precise for low periodic granularity.

The meaning of the for period in a no session has condition is subtly different from its use in other conditions. It does not guarantee that there has been no session satisfying the condition at some point between evaluations, only that when evaluated the given period of time has passed since it was last evaluated and found to have no matching sessions.

subscriptions is the number of subscriptions to a topic. When monitoring across a cluster, the subscriptions less than condition is first checked on the server that owns the topic and if satisfied there then each cluster member is queried to check if the condition has also been satisfied there. The topic will only be removed if the total number of subscriptions across the cluster is less than that specified in the condition.

Automatic topic removal is supported for a topic that is replicated across the local cluster, and for a topic with fanout replicas on downstream remote servers. A `subscriptions less than` condition will be evaluated against the total number of subscriptions across the cluster and on all fanout replicas on downstream remote servers. A `no session has` condition will consider all sessions hosted across the cluster and all sessions hosted by downstream remote servers that have a fanout replica of the topic. The `local` keyword can be used to restrict evaluation to the local cluster, ignoring fanout replicas.

Added in version 6.1

Schema

Key of the topic property that specifies a schema which constrains topic values.

Declaration

public static readonly string Schema

Field Value

Type	Description
String	Topic property key for schema.

Remarks

This property is only used by RECORD_V2 topics. The value is converted to a Diffusion record schema using ParseSchema(String).

Implemented in Version 6.0.

TidyOnUnsubscribe

Key of the topic property that specifies the "tidy on unsubscribe" option for a topic.

Declaration

public static readonly string TidyOnUnsubscribe

Field Value

Type	Description
String	Topic property key for tidy on unsubscribe.

Remarks

By default, if a session unsubscribes from a topic, it will receive any updates for that topic that were previously queued but not sent.

If this property is set to `true`, when a session unsubscribes from the topic, any updates for the topic that are still queued for the session are removed. There is a performance overhead to using this option as the client queue must be scanned to find topic updates to remove. However, it may prove useful for preventing unwanted data being sent to sessions.

Implemented in Version 6.0.

TimeSeriesEventValueType

The key of the topic property that specifies the event data type for a time series topic.

Declaration

public static readonly string TimeSeriesEventValueType

Field Value

Type	Description
String	Topic property key for time series event value type.

Remarks

The value is the TypeName of a data type.

Added in version 6.1.

TimeSeriesRetainedRange

The key of the topic property that specifies the range of events retained by a time series topic.

Declaration

public static readonly string TimeSeriesRetainedRange

Field Value

Type	Description
String	Topic property key for time series retained range.

Remarks

When a new event is added to the time series, older events that fall outside of the range are discarded.

If the property is not specified, a time series topic will retain the ten most recent events.

Time series range expressions

The property value is a time series range expression string composed of one or more constraint clauses. Constraints are combined to provide a range of events from the end of the time series.

Limit constraint: A limit constraint specifies the maximum number of events from the end of the time series.

Last clause: A last constraint specifies the maximum duration of events from the end of the time series. The duration is expressed as an integer followed by one of the following time units.

MS - milliseconds; S - seconds; H - hours;

If a range expression contains multiple constraints, the constraint that selects the smallest range is used.

Property value	Meaning
limit 5	The five most recent events.
last 10s	All events that are no more than ten seconds older than the latest event.
last 10s limit 5	The five most recent events that are no more than ten seconds older than the latest event.

Range expressions are not case sensitive: limit 5 last 10s is equivalent to LIMIT 5 LAST 10S.

Added in version 6.1.

TimeSeriesSubscriptionRange

The key of the topic property that specifies the range of time series topic events to send to new subscribers.

Declaration

public static readonly string TimeSeriesSubscriptionRange

Field Value

Type	Description
String	Topic property key for time series subscription range.

Remarks

The property value is a time series range expression, following the format used for TimeSeriesRetainedRange.

If the property is not specified, new subscribers will be sent the latest event if delta streams are enabled and no events if delta streams are disabled. See the description of Subscription range in the ITimeSeries documentation.

Added in version 6.1.

ValidateValues

Key of the topic property indicating whether a topic should validate inbound values.

Declaration

public static readonly string ValidateValues

Field Value

Type	Description
String	Topic property key for validate values.

Remarks

By default, the server does not validate received values before sending them on to client sessions. Invalid or corrupt values will be stored in the topic and passed on to sessions. If this property is set to `true`, the server will perform additional validation on values to check that they are valid instances of the data type, and if it is not then it will return an error to the updater and not update the topic.

If this value is not set (or set to something other than `true`), no server validation of inbound values is performed. This is the recommended setting as there is a performance overhead to validation and a session using ITopicUpdate cannot send invalid values anyway.