transmitted in a binary form for increased efficiency and can be transmitted as a structural
delta to reduce the amount of data sent. JSON topics are stateful: each topic stores a JSON
value the Diffusion™ server.
Why use a JSON topic?
JSON is a human-readable, industry-standard format for
other platforms. For more information about JSON, see http://www.json.org/.
A JSON topic enables multiple fields to be maintained in
the same topic as part of a composite data type. All updates made at the same time to parts
of a JSON topic are sent out to the client together. This
enables a set of parts to be treated as a transactional group.
Deltas of change are calculated at the Diffusion server such that only those
parts that have changed since the last update are sent out to the subscribed clients. These
structural deltas ensure that the minimum amount of data is sent to clients.
The current value of the topic is cached on the client. When deltas are sent, the client
can automatically apply these deltas to the value to calculate the new value.
As of Diffusion 6.4, JSON topics support partial updates
using the JSON Patch standard. This enables a client to update part of the data within a topic,
without needing to know the full value. In addition, a partial update can provide
performance benefits if the JSON value is large.
If your data structure is too complex to be represented by a topic tree or might make the
topic tree structure difficult to manage, it might be more appropriate to represent part of
the data structure inside a JSON topic as JSON objects.
The value of the topic is transmitted as Concise Binary Object Representation (CBOR). For more
information about CBOR, see http://cbor.io/.
The value of the topic is accessible both as JSON and CBOR.
Properties of a JSON topic
When you create a JSON
topic you can specify the following
properties in the topic specification:
- By default, delta streaming is enabled.
If this property is set to true, delta streaming is disabled and
all values are published in full.
If there is little or no relationship between one value published to a topic and the next,
delta streams will not reduce the amount of data transmitted. For such topics, it is
better to set PUBLISH_VALUES_ONLY.
- If set to true, the latest value of the topic is not retained by
the Diffusion server or the client that publishes it.
New clients that subscribe do not receive an initial value. No value will be returned for fetch operations that select the topic.
For time series topics, if DONT_RETAIN_VALUE is set to true,
time series events are still retained, but the latest value is not stored separately.
The DONT_RETAIN_VALUE property is useful for applications like a
feed of news items, or for values that are only valid at the moment of publication.
You can combine this with VALIDATE_VALUES.
Using DONT_RETAIN_VALUE reduces the topic memory footprint, but disables delta streaming.
Disabling delta streaming is likely to increase the bandwidth used unless subsequent values are unrelated.
This property replaces the obsolete stateless topic type which was removed in Diffusion 6.2.
- If set to true, when a session unsubscribes from
the topic, any updates for the topic that are still queued for the session are
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. This property is disabled by default.
- If set to true, the topic rejects updates that would
create invalid instances of the topic's data type.
If set to anything other than
true, no validation is performed and all values are streamed to
subscribing clients whether they are valid data or not.
Validation has a performance overhead and is disabled by default.
Note: If validation is disabled and the value provided is not valid, the client might
produce errors or other unexpected behavior. The exact error varies depending on the
client platform. To avoid this, use the client-side validation method provided by
the Diffusion API.
- Used to set a conflation policy for the topic. Conflation can
merge or discard topic updates to reduce server memory and network data usage.
The supported values are:
The default is "conflate".
See Using conflation for details of the policies.
- Used to set a security principal as the owner of the topic.
The principal receives READ_TOPIC, MODIFY_TOPIC
and UPDATE_TOPIC privileges.
The format of the property value is:
$Principal is "name"
where name is the name of the principal.
- Used to set a topic removal policy.
The property is an expression which defines the conditions which will trigger automatic removal
of the topic or of a set of topics specified with an optional selector.
See Removing topics automatically for details of the expression format.
- Used to disable persistence for a topic.
If set to "false", the topic will not be persisted, even if persistence
is enabled for the server.
Considerations when using a JSON topic
It is possible to store any data in a JSON topic, even if it is not valid JSON. However,
this can cause problems when clients receive unexpected values.
You can validate JSON in the client, or on the server by setting VALIDATE_VALUES to true.
to and from a Diffusion JSON value.
Only JSON topics support partial updates.
The topic view expansion feature can only be used with JSON topics.
Diffusion JSON values are transmitted as a CBOR representation.
Applications can also access the CBOR binary data directly, and it is often more efficient
to do so than to first convert the data to JSON text. For example, Java™ applications can use the third party Jackson library to map
the CBOR data directly to Java objects.