Topic specifications provide the information required to create a topic. Topics can be created from a topic specification using TopicControl.add.
A topic is specified in terms of its type and a map of optional property settings which can alter the default behavior of the topic.
Depending on the topic type, some properties must be included in the specification when creating a topic and some properties have no effect. All topic specification property values must be supplied as strings.
The required and optional properties for each topic type are set out in the following table.
TIME_SERIES topics have restricted values for the
CONFLATION property. They are only allowed to have the values
ROUTING topics are references to other topics, and have no value of their
own. Instead, they reflect the value of the appropriate source topic.
Observed behavior depends on the values of the
VALIDATE_VALUES properties that are set on the
A map of the topic properties.
The topic type.
Key of the topic property that allows the compression policy to be set on a per-topic basis.
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:
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
Prior to version 6.4, only two values were allowed:
medium, and the previous default policy) and
false (equivalent to
off). These values are still
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.
Key of the topic property that specifies the conflation policy of the topic. 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.
The supported policies are:
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 disables conflation for the topic. This policy
disables all conflation for the topic, so topic updates will never be
merged or discarded.
conflate automatically conflates topic updates when
back pressure is detected by the server.
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.
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, minimising the server memory and
network bandwidth used by the session.
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 can be disabled on a session-by-session basis. If conflation is disabled for a session the policy will not be applied to topic updates queued for the session but will be for other sessions that have conflation enabled.
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.
Key of the topic property that specifies a topic should not retain its last value.
By default, a topic (other than a
ROUTING topic) will retain its latest
value. The latest value will be sent to new subscribers. Setting this
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 DONT_RETAIN_VALUE to
true also disables delta streams,
regardless of the PUBLISH_VALUES_ONLY value. If subsequent values are likely to be
related, delta streams usually provide performance benefits (see PUBLISH_VALUES_ONLY). Consider
leaving DONT_RETAIN_VALUE 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 DONT_RETAIN_VALUE to
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.
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 authorisation rules in the security store.
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:
$Principal is "<i>name</i>"
where name is the name of the principal. Single quotes may be used instead of double quotes and special characters can be escaped using escape if required.
The purpose of this property is to allow a client to create topics on behalf of other users. This can be used in conjunction with the REMOVAL property so that such topics are removed when there are no longer any sessions for the named principal.
specification.withProperty(diffusion.topics.TopicSpecification.OWNER, "$Principal is 'myPrincipal'") .withProperty(diffusion.topics.TopicSpecification.REMOVAL, "when no session has '$Principal is \"myPrincipal\"' for 5s");
Key of the topic property that can be used to prevent a topic from being persisted when the server is configured to enable persistence.
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
Key of the topic property that specifies the topic delivery priority.
The supported delivery priorities are:
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
high priority topics will be delivered first, followed by
default priority topics, followed by updates for
low priority topics. Topic subscription and unsubscription
notifications are also delivered according to the topic delivery
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.
Key of the topic property that specifies whether a topic should publish only values.
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 ValueStream to automatically apply the delta to a local copy of the topic value to calculate the new value.
Setting PUBLISH_VALUES_ONLY 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 PUBLISH_VALUES_ONLY to
Also consider setting PUBLISH_VALUES_ONLY to
true if the network
capacity is high and the bandwidth savings of deltas are not required.
Key of the topic property that specifies a removal policy for automatic removal of the topic (and/or other topics).
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 (
The natural evaluation order of the operators may be changed by
surrounding with parentheses (e.g. (condition
remove clause is optional. It provides a TopicSelector
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
REMOVAL property for one of them, using a
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:
||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 RFC_1123 date time format. Either single or double quotes may be used.|
|subscriptions less than||
||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.
|no updates for||
||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:
|no session has||
||Removal should occur when no sessions satisfy 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.
||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
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 convenience method escape is provided to escape such
characters in a value. The expression is validated only by the server and
therefore if an invalid expression is specified it will be reported as an
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 that created it closes.
when no session has '$Principal is "Alice"'
The topic will be removed when there is no session with the principal 'Alice'.
when no session has '$Principal is "Alice"' for 10m
The topic will be removed when there have been 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 Account 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
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 conditions 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.
Automatic topic removal is supported for a topic that is replicated
across the local cluster, and for a topic with 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.
Key of the topic property that specifies the 'tidy on unsubscribe' option for a topic.
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.
Key of the topic property that specifies the event data type for a time series topic.
The value is the type name of a data type.
Key of the topic property that specifies the range of events retained by a time series topic.
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.
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.
If a range expression contains multiple constraints, the constraint that selects the smallest range is used.
||The five most recent events|
||All events that are no more than ten seconds older than the latest event|
||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
LIMIT 5 LAST 10S.
Key of the topic property that specifies the range of time series topic events to send to new subscribers.
The property value is a time series range expression, following the format used for TIME_SERIES_RETAINED_RANGE.
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 time series feature documentation.
Key of the topic property indicating whether a topic should validate inbound values.
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
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
server validation of inbound values is performed. This is the recommended
setting as there is a performance overhead to validation and values
produced through the data type API will not
be invalid anyway.
Returns a new TopicSpecification with the specified property set to the supplied value.
the new property key
the new property value
a new TopicSpecification with the specified property set.