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Diffusion Apple API
6.4.10
Unified Client Library for iOS, tvOS and OS X / macOS
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Diffusion Apple API
6.4.10
Unified Client Library for iOS, tvOS and OS X / macOS
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This feature allows a client session to manage topic views.
A topic view maps one part of the server's topic tree to another. It dynamically creates a set of reference topics from a set of source topics, based on a declarative topic view specification. The capabilities of topic views range from simple mirroring of topics within the topic tree to advanced capabilities that include publication of partial values, expanding a single topic value into many topics and throttling the rate of publication.
Each reference topic has a single source topic and has the same topic type as its source topic. Reference topics are read-only (they cannot be updated), nor can they be created or removed directly. Otherwise, they behave just like standard topics. A client session can subscribe to a reference topic, and can fetch the reference topic's current value if it has one.
The source topics of a topic view are defined by a topic selector. One or more reference topics are created for each source topic, according to the topic view. If a source topic is removed, reference topics that are derived from it will automatically be removed. If a topic is added that matches the source topic selector of a topic view, corresponding reference topics will be created. Removing a topic view will remove all of its reference topics.
The following is a simple topic view specification that mirrors all topics below the path a to reference topics below the path b.
map ?a// to b/<path(1)>
A topic view with this specification will map a source topic at the path a/x/y/z to a reference topic at the path b/x/y/z. The specification is simple, so the reference topic will exactly mirror the source topic.
A general topic view specification comprises several parts:
The source topic clause begins with the map keyword and is followed by a topic selector. These topic selectors follow the same parsing rules as other topic selectors.
When evaluating a topic view, topics in the topic tree that match the source topic selector are considered, with the following exceptions:
Both slave and reference topics are valid source topics. In paerticular, chaining of topic views is supported; that is, a reference topic created by one topic view can be the source topic of another topic view. Additionally, a reference topic can be the master topic of a slave topic, or the source topic of a routing topic subscription.
Prefer topic views to slave topics which are now deprecated. Individual topics can be mirrored by creating a slave topic, but maintaining slave topics for a branch of the topic tree quickly becomes tedious. A topic view will maintain such a branch automatically, and provides more sophisticated mapping options.
The paths of reference topics are derived from the source topic according to the topic view path mapping. The path mapping allows the source topic path and the value of the source topic to determine the path of the reference topic. In addition the path mapping can include expand directives which allow objects and arrays in JSON source topic values to be expanded to produce many reference topics.
A path mapping clause begins with the to keyword and is followed by a path mapping template. A path mapping template is a topic path with embedded directives. Directives are evaluated when creating the topic reference and substituted into the topic path. Directives are delimited by angle brackets (<, >) and consist of the name of the directive and a list of parameters. The parameter list is comma-separated and surrounded by parentheses ((, )).
The following path mapping directives are supported:
Source path directives extract a portion of the source path and are parameterized by the index of the start part of the source path and the number of parts to include. The number of parts parameter is optional – if it is missing, the selection extends to the end of the source path. The syntax is <path(start, number)>, or <path(start)> when the number of parts parameter is omitted.
For example, given the source path a/b/c/d, the source path directive <path(1, 2)> is mapped to the reference topic path b/c, and the source path directive <path(2)> is mapped to the reference topic path c/d.
Source value directives are only applied to JSON source topics; if the path mapping contains a source value directive, non-JSON topics matching the source topic selector are ignored. Source value directives use the keyword scalar and are parameterized by a single JSON pointer that extracts a scalar value from the source (or expanded) value. A scalar value is a string, a number, true, false, or null, that is, anything other than an array or a object. If the JSON pointer does not refer to a scalar value in the source (or expanded) value, no reference topic will be created. This includes cases where the JSON pointer refers to an array or an object), or when no part of the source value is selected.
Deriving the reference topic paths from part of the source topic value effectively creates a secondary index on the value. For source value directives to work efficiently, the selected scalar values should be relatively stable. If an update to the source topic changes the selected scalar value, the corresponding reference topic will be removed and a new reference topic will be created.
For example, given a source value of
{
"account" : "1234",
"balance" : { "amount" : 12.57, "currency" : "USD" }
}
and the source value directive currency/<scalar(/balance/currency)</account/<scalar(/account)>, the reference topic path will be currency/USD/account/1234.
If the extracted value is a string, it is copied literally to the reference topic path. A value that contains path separators (/) will create a reference topic path with more levels than the path mapping template.
An extracted value of null will be copied to the reference topic path as the string "null".
Expand value directives are only applied to JSON source topics; if the path mapping contains an expand value directive, non-JSON topics matching the source topic selector are ignored.
Expand value directives use the keyword expand and are parameterized by one or two JSON pointers.
The first pointer indicates the element within the value to be expanded, and if omitted, the value is expanded from the root. Expansion of a source topic indicates that every direct child of the element pointed to by the expand pointer will be used to create a new reference topic (or provide input to later expand or scalar directives). For example <expand()> would expand every child item in the source value and <expand(/account)> would expand every child of the account value in the source value. The specified value could be an object, an array or even a scalar value, but a scalar value would expand to only a single new value.
The optional second parameter of the expand directive specifies a pointer to a scalar value within the expanded value which will be used to derive the path fragment of the reference topic path. If the second pointer is not specified or no scalar value is found for the pointer, the path fragment is taken from the key (if the child value is an object) or the index (if the child value is an array). Scalar child values will expand to a reference topic but will not add anything to the generated path. For example expand(,/name)> would expand from the root of the source value and each child value path fragment would be obtained from the scalar value with the key name.
So if a source topic had a value of
{
"values": [1, 5, 7]
}
a path mapping of value<expand(/values)> would expand the value to the following reference topics:-
path value0 with a value of 1
path value1 with a value of 5
path value2 with a value of 7
Expand directives can be nested (i.e. there can be more than one expand directive in a path mapping). In this case a second expand directive will use the value from the previous expand as its source (root) value and not the value of the source topic. This also applies to scalar directives that follow an expand directive.
If expansion causes more than one mapping to the same topic path, only the first encountered will be created and updated.
Expanding source topic values effectively creates secondary indices on the value. For expanded value directives to work efficiently, the value selected for expansion should be relatively stable in terms of the children it contains. If an update to the source topic changes the children of the expanded value, then corresponding reference topics will be removed and created. Updates should generally be limited to changing values within the expanded values.
The topic specification of a reference topic is derived from the topic specification of the source topics. A reference topic has the same topic type as its source topic.
The topic properties of a reference topic are derived from the source topic. Some topic properties can be tuned using the optional topic property mapping clause. The following table describes the behavior for each topic property.
| Source topic property | Reference topic specification default | Can be set by topic property mapping? | Notes |
|---|---|---|---|
compressionPropertyKey | Copied from source topic specification | Yes | |
conflationPropertyKey | Copied from source topic specification | Yes | |
dontRetainValuePropertyKey | Copied from source topic specification | Yes | |
ownerPropertyKey | Not set | No | |
persistentPropertyKey | Not set | No | Reference topics are not persisted. Topic views are persisted, so a reference topic will be recreated on server restart if its source is persistent. |
priorityPropertyKey | Copied from source topic specification | Yes | |
publishValuesOnlyPropertyKey | Copied from source topic specification | Yes | |
removalPropertyKey | Not set | No | Reference topics cannot be removed directly. |
schemaPropertyKey | Copied from source topic specification | No | A recordV2 reference topic has the same schema as its source topic. |
slaveMasterTopicPropertyKey | Not set | No | If a reference topic has a slave topic as its source topic, it indirectly references the slave's master topic. |
tidyOnSubscribePropertyKey | Copied from source topic specification | Yes | |
timeSeriesEventValueTypePropertyKey | Copied from source topic specification | No | A time series reference topic has the same value type as its source topic. |
timeSeriesRetainedRangePropertyKey | Copied from source topic specification | Yes, with restrictions | A topic property mapping cannot increase the time series retained range by overriding the TIME_SERIES_RETAINED_RANGE property. The retained range of a reference time series topic will be constrained to be no greater than that of its source topic. |
timeSeriesSubscriptionRangePropertyKey | Copied from source topic specification | Yes | |
validateValuesPropertyKey | Not set | No | A reference topic reflects updates to its source topic. It cannot reject updates. |
A topic property mapping clause begins with the keywords with properties and consists of a comma-separated list of topic property keys and values, each separated by a colon. For example, the following topic view specification maps all topics below the path a to reference topics below the path b, and disables both conflation and compression for the reference topics.
map ?a// to b/<path(1)> with properties 'CONFLATION':'off', 'COMPRESSION':'false'
By default, a reference topic's value is a copy of the source topic value, or part of the source value produced by an expand path mapping directive. For JSON source topics, the optional topic value mapping clause can be applied to extract part of the source value, or to further refine the value produced by the expand directive.
A topic value mapping begins the keyword as and is followed by a value directive. A value directive is delimited by angle brackets (<, >), and consists of the value keywords and a single JSON pointer parameter. The JSON pointer selects the part of the source value to copy.
For example, given a source value of
{
"account" : "1234",
"balance" : { "amount" : 12.57, "currency" : "USD" }
}
and the value mapping clause as <value(/balance)>, the reference topic value will be
{
"amount" : 12.57,
"currency" : "USD"
}
Value mappings that follow expand directives apply to the expanded value and not the source topic value.
Topic value mappings only alter the reference topic value; only the path mapping determines whether a reference topic should exist. If the topic value mapping's JSON pointer fails to select anything from the source topic value, the reference topic will have the JSON value null.
Topic value mappings are often used with path value mappings to avoid repeating information in the path and the value. For example:
map ?accounts// to balances/<scalar(/account)> as <value(/balance)>
The optional throttle clause can be used to constrain the rate at which a reference topic is updated when its source topic is updated. The primary application of a throttle clause is to restrict the number of updates sent to reference topic subscribers, reducing network utilization or the processing each subscriber must do. Throttling also restricts the rate at which client sessions can observe changes to reference topic values using the fetch API. The throttle clause has the form:
throttle to X updates every period
where X is a positive integer, and period is a positive integer followed by a time unit which is one of seconds, minutes, or hours. For example, the following topic view specification maps all topics below the path a to reference topics below the path b, but updates the value of each reference topic at most twice every five seconds:
map ?a// to b/<path(1)> throttle to 2 updates every 5 seconds
To improve readability, the throttling clause allows 1 update as an alternative to 1 updates, and every second as an alternative to every 1 seconds (and so on, for other time units). For example, the following topic view specification maps all topics below the path a to reference topics below the path b, but updates the value of each reference topic at most once every hour:
map ?a// to b/<path(1)> throttle to 1 update every minute
The throttle clause is only applied when a source topic is updated more frequently than the configured rate. If a source topic is updated less frequently, updates are passed on unconstrained. If the rate is exceeded, a reference topic will not be updated again until the configured period has expired. At this time, the reference topic will be updated based on the source topic updates that happened in the interim, and a single value will be published. Thus, a throttle clause provides topic-scoped conflation.
The throttle clause is ignored for time series topics because time series updates do not support efficient conflation. Updates to source time series topics are passed on immediately to the corresponding reference topics, regardless of any throttle clause.
Each part of a topic view expression has characters with special significance. Source topic clauses and path mapping clauses are delimited by white space. Directives in path and topic property mapping clauses are delimited by the < and > characters, and each directive parameter is terminated by , or ). Topic property mapping clauses are delimited by white space, and the : and , characters.
Sometimes a topic view must refer to or generate topics with paths that containing special characters, or use a JSON pointer containing special characters. The escape sequence \x can be used to literally insert any character x, with a one exception: \/ cannot be used in path fragments since the path delimiter / is always significant.
Here is an example topic view expression containing escape sequences. It maps the topic path a topic a reference topic with the path another topic.
map a\ topic to another\ topic
Here is an example with a source value directive that uses the JSON pointer /x()/y to extract the target path from the source value. The ) character in the JSON pointer must be escaped so it is not treated as the end of the parameter list.
map ?a// to <scalar(/x(\)/y)>
To insert \, the escape sequence \\ must be used.
There is no need to escape white space in JSON pointers directive parameters. However, white space is significant. For example, the following expressions have different topic value mapping clauses since the JSON pointer in the second expression is "/x "; that is, it has a trailing space:
map a to b as <value(/x)> map a to b as <value(/x )>
Instead of using escape sequences, white space characters can be included in source topic clauses and path mapping clauses using quotes. A clause is quoted by wrapping it in single quote (') or double quote (") characters. For example:
map "a topic" to "another topic"
Within a quoted clause, quotes of the same type must be escaped:
map 'alice\'s topic' to 'bob\'s topic'
For consistency, the values in topic property mapping clauses can be escaped or quoted. However, there is no need to do so because none of the valid values for the mappable properties contain special characters.
Reference topics have a lower priority than normal topics created through the API, including replicas of normal topics created by topic replication or fan-out. A reference topic will only be created if no topic or reference topic is already bound to its derived topic path.
Topic views have a precedence based on order of creation. If two topic views define mappings the same topic path, the earliest-created topic view will create a reference topic. If a topic view is updated, it retains its original precedence.
Reference topics are neither replicated nor persisted. They are created and removed based on their source topics. However, topic views are replicated and persisted. A server that restarts will restore topic views during recovery. Each topic view will then create reference topics based on the source topics that have been recovered.
The server records all changes to topic views in a persistent store. Topic views are restored if the server is started.
If a server belongs to a cluster, topic views will be replicated to each server in the cluster. Topic views are evaluated locally within a server. Replicated topic views that select non-replicated source topics can create different reference topics on each server in the cluster.
To list the topic views a session needs READ_TOPIC_VIEW permission. To create or remove a topic view a session needs MODIFY_TOPIC_VIEW permission, SELECT_TOPIC permission for the prefix of the source topic selector and MODIFY_TOPIC permission for the prefix of the path mapping.
The Topic Views feature for a session can be obtained from the session's topicViews property.
Instance Methods | |
| (void) | - createTopicViewWithName:specification:completionHandler: |
| (void) | - listTopicViewsWithCompletionHandler: |
| (void) | - removeTopicViewWithName:completionHandler: |
 Instance Methods inherited from PTDiffusionFeature | |
| (instancetype) | - NS_UNAVAILABLE |
Additional Inherited Members | |
| Class Methods inherited from PTDiffusionFeature | |
| (instancetype) | + NS_UNAVAILABLE |
| Properties inherited from PTDiffusionFeature | |
| PTDiffusionSession * | session |
| - (void) createTopicViewWithName: | (NSString *) | name | |
| specification: | (NSString *) | specification | |
| completionHandler: | (PTDiffusionTopicView *_Nullable) | view | |
| (NSError *_Nullable error) | completionHandler | ||
Create a new named topic view.
If a view with the same name already exists the new view will update the existing view.
| name | The name of the view. |
| specification | The specification of the view using the DSL. |
| completionHandler | Block to be called asynchronously on success or failure. If the operation was successful, the error argument passed to the block will be nil. The completion handler will be called asynchronously on the main dispatch queue. |
| NSInvalidArgumentException | If any argument is nil. |
| - (void) listTopicViewsWithCompletionHandler: | (NSArray< PTDiffusionTopicView * > *_Nullable) | views | |
| (NSError *_Nullable error) | completionHandler | ||
List all the topic views that have been created.
| completionHandler | Block to be called asynchronously on success or failure. If the operation was successful, the error argument passed to the block will be nil and views will contain a list of views sorted by their creation order. The completion handler will be called asynchronously on the main dispatch queue. |
| NSInvalidArgumentException | If completionHandler is nil. |
| - (void) removeTopicViewWithName: | (NSString *) | name | |
| completionHandler: | (NSError *_Nullable error) | completionHandler | |
Remove a named topic view if it exists.
If the named view does not exist then the operation is considered successful and the completion handler is called accordingly.
| name | The name of the view. |
| completionHandler | Block to be called asynchronously on success or failure. If the operation was successful, the error argument passed to the block will be nil. The completion handler will be called asynchronously on the main dispatch queue. |
| NSInvalidArgumentException | If any argument is nil. |
1.8.5