Interface ITopicViews
This feature allows a client session to manage topic views.
Inherited Members
Namespace: PushTechnology.ClientInterface.Client.Features
Assembly: Diffusion.Client.dll
Syntax
public interface ITopicViews : IFeature
Remarks
A topic view maps one part of a 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 including publication of partial values, expanding a single topic value into many topics, changing topic values, inserting values from other topics, throttling the rate of publication, and applying a fixed delay to the publication.
A topic view can also map topics from another server (in a different cluster). This capability is referred to as 'remote topic views'. The view can specify the server that the source topics are hosted on in terms of a remote server (see IRemoteServers for details of how to create and maintain remote servers).
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.
Topic view specifications
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 topic view specification comprises three main parts:
- The mapping part which specifies the source topics to map from and the mappings to target reference topics.
- Optional transformations which transform the topic value in some way.
- Optional options which specify other changes that the view may apply.
Mapping comprises:
- The source topic clause identifying the source topics that the view can apply to.
- The path mapping clause which determines how reference topic paths are derived from the source topic paths, and when expanding to more than one reference topic, from where the values are obtained.
Transformations can be:
- patch transformation(s) specifying that a JSON patch is applied to the reference topic value.
- process transformations that allow conditional processing and/or calculations to be applied to the reference topic value.
- insert transformation(s) specifying that values from other topics are inserted into the reference topic value.
Options can be:
- The topic property mapping clause determines how reference topic properties are derived from source topic properties.
- The value mapping clause determines how reference topic values are derived from source topic or expanded values.
- The throttle clause constrains the rate at which each reference topic is updated when its source topic is updated.
- The delay by clause causes a change to a view's source topic to be delayed by a fixed time before it is reflected in reference topics.
- The separator clause can define a replacement path separator for values extracted using the scalar or expand directives.
- The type clause can specify that the reference topic that is created is of a different TopicType from the selected source topic.
Mapping
Source topic clause
The source topic clause begins with the
map
keyword
and is followed by a topic selector.
These topic selectors follow the same Parse(String)
parsing rules as other topic selectors.
When evaluating a topic view, all topics in the topic tree that match the source topic selector are considered (excluding ROUTING topics. However, if a view specification uses some feature that can only be applied to JSON topics then only JSON topics will be selected.
Reference topics are valid source topics. In particular, 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 source topic of a routing topic subscription.
From clause
The from
clause optionally follows the source topic clause. It begins with
the from
keyword and is followed by a remote server name. The name
refers to a remote server created using the IRemoteServers feature.
The presence of the clause indicates that the source topics will be selected from the specified server and not from the local server.
Further details regarding the processing of remote topic views are given below.
Path mapping clause
The paths of reference topics are derived from the source topic according to the path mapping clause. 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 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
.
Transformations
Transformations are specified after the mapping and before any options. Transformations can only be applied to JSON topics.
Transformations are applied to the value extracted from the source topic in the order specified. There can be any number of transformations interspersed with one another and the value from one will be that which is input to the next. The only restriction is that all insert transformations must occur at the end, after any others.
A transformation is applied to the current value within a view processing chain, so if a transformation occurs after an expand then it will be applied to each expanded value.
Process transformations
Process transformations may be used to apply conditional processing to a value (optionally determining whether a reference topic is created) and/or change the value in some way (for example, by applying some calculation to a field within the value).
The format of a process transformation is:-
process {statement}
Where the statement can be:-
-
Operation(s).
One or more operations separated by ';'. -
A conditional statement.
Comprising one or more conditions with operations to perform if they are satisfied.
For example, the following topic view specification could be used to write a field into the value of the reference topic :
map ?a// to b/<path(1)> process {set(/Name, 'John')};
The following example shows a simple conditional statement which would only
generate reference topics if the value of field /Price
was
greater than 50 :
map ?a// to b/<path(1)> process {if '/Price gt 50' continue};
And the following shows a more complex statement which would set a field
according to the value of the input field /Price
:
map ?a// to b/<path(1)> process {if '/Price lt 50' set(/Tier, 1) elseif '/Price gt 50' set(/Tier, 2)};
Process operations
The following operations are supported :
Operation | Description |
---|---|
set(pointer, value) | Sets the field indicated by the JSON
pointer to an absolute value. If the field does not exist it
will be created. The value can be an integer (e.g. 123), a String
(e.g. "XYZ"), or a boolean (e.g. true or false).
For example :
If a hierarchic pointer is specified, the parent object or array must exist. |
set(pointer, calc calculation) | Sets the field indicated by the JSON
pointer to a value which is the result of the specified
calculation. If the field does not exist it will be created. The
calculation can include fields within the input value. The calculation is
specified as a quoted string. See below for a detailed description of
calculations.
For example :
If a hierarchic pointer is specified, the parent object or array must exist. |
remove(pointer) | Removes the json item at the specified pointer. Unlike JSON patch, if the item does not exist the operation does not fail. |
continue | This is a special operation that indicates that the topic view evaluation should continue with the value as it is. This is only for use with conditional statements as the default behavior of a conditional statement is not to proceed if no condition is satisifed. |
set(/Amount, calc "/Value * /Number"); remove(/Value); remove (/Number)
In this case the operations are all performed on the original value, creating a chain of deltas which are only applied to the original value at the end. If the 'set' operation fails no reference topic would be generated, however, processing will continue if the fields specified in the 'remove's are not present.
Process calculations
A calculation may be specified as the value of a set operation. A calculation is a simple arithmetic calculation upon integer fields. If applied to a non integer field the evaluation will not proceed. Floating point calculations are not supported.
Arithmetic operators supported are +
, -
,
*
and /
.
Examples of calculations are:-
set(/Value, calc "/Value* 2")
set(/Result, calc "/Value / 2")
set(/Bonus, calc "/Salary + 1000")
set(/Bonus, calc "/Salary + 1000 + /Age* 10")
Standard operator precedence is applied, so in the last example above we have
(/Salary + 1000 + (/Age * 10))
not
((/Salary + 1000 + /Age) * 10)
. Brackets may be used to override
this.
Process conditional statements
A conditional statement is made up of an if clause, optionally followed by one or more elseif clauses and an optional final else clause.
The if clause takes the form :
if condition operation(s)
Where the condition is a quoted string as described in detail below and the operation(s) is as described previously. If the condition is satisfied, the operations are applied to the value and the process is complete. If the condition is not satisfied, processing moves on to any elseif or else clauses that follow, but if there are none, the topic view evaluation does not proceed and no reference topic is created.
An elseif (else if) clause takes the form :
elseif condition operation(s)
If the condition is satisfied, the operations are applied to the value and the process is complete. If the condition is not satisfied, processing moves on to any elseif or else clauses that follow, but if there are none, the topic view evaluation does not proceed and no reference topic is created. Note that elseif can be abbreviated to elsf.
An else clause takes the form :
else operation(s)
And will only be reached if no previous if or elseif conditions were satisfied. If reached then the operations are applied to the value and the topic view evaluation proceeds. The continue operation may be used to proceed with an unchanged value.
Process conditions
A condition is of the form:
pointer operator [constant/pointer]
Where pointer is a JSON pointer, operator is a relational operator and constant is a string, integer, or boolean value.
for example:-
/Age > 40
/Name = "Bill"
/Age > /RetirementAge
/Manager eq true
Operators allowed are:
Operator | Variant | Description | Supported JSON types |
---|---|---|---|
= | eq | Equals | All |
> | gt | Greater than | Integer only |
< | lt | Less than | Integer only |
!= | ne | Not equals | All |
>= | ge | Greater than or equal | Integer only |
<= | le | Less than or equal | Integer only |
|
or or
&
or and
For example:
/Age = 50 or /Age > 80
/Age gt 50 & /Department eq "Accounts"
Normal boolean precedence applies but brackets can be used to control
precedence. For example:
(/Age > 50 or /Department eq "Accounts") and /Band > 3
Boolean 'not' is also allowed :
not (/Age < 65 or /Retired eq false)
Patch transformations
Patch transformations indicate that a JSON patch is to be applied to the value.
The format of a patch transformation is
patch 'patch string'
The patch string should be formatted according to the JSON Patch standard (see RFC 6902: JavaScript Object Notation (JSON) Patch).
Patches are a sequence of JSON Patch operations contained in an array. They are applied as an atomic update to the previous value if the resulting update is successfully calculated. The following patch will check the value at a specific key and update if the expected value is correct:
[{"op":"test", "path":"/price", "value" : 22}, {"op":"add", "path":"/price", "value": 23}]
The available operations are:
-
Add:
{"op": "add", "path": "/a/b/c", "value": [ "foo", "bar" ]}
-
Remove:
{"op": "remove", "path": "/a/b/c"}
-
Replace:
{"op": "replace", "path": "/a/b/c", "value": 43}
-
Move:
{"op": "move", "from": "/a/b/c", "path": "/a/b/d"}
-
Copy:
{"op": "copy", "from": "/a/b/c", "path": "/a/b/e"}
-
Test:
{"op": "test", "path": "/a/b/c", "value": "foo"}
The test operation checks that the CBOR representation of the value of a topic is identical to the value provided in the patch after converting it to CBOR. If the value is represented differently as CBOR, commonly due to different key ordering, then the patch will return the index of the failed operation. e.g the values
{"foo": "bar", "count": 43}
and
{"count": 43, "foo": "bar"}
are unequal despite semantic equality due to the differences in a byte for byte comparison.
The following patch clause would add the 'price' field and remove the 'name' field from an input JSON object.
patch '[{"op":"add", "path":"/price", "value" : 22}, {"op":"remove", "path":"/name"}]'
Patches can only be applied to JSON arrays or objects and if they fail to apply, no resulting reference topic will be created by the view. If an update patch fails, any previously created reference topic would be removed.
Insert transformations
Insert transformations are used to insert a value from another topic into the current value.
The name of the topic to insert from can be specified in a similar way to the path mapping in that constants, path directives, and scalar directives (but not expand directives) may be used.
The value from the insertion topic (if found) is inserted into the 'current' value at a specified key position. The current value may be the source topic value, the value output from expand directives (in which case the insertion applies to each value), or the value from a previous transformation. Insertion topics may be JSON, STRING, INT64, or DOUBLE.
If, when evaluating a topic view, the insertion topic is not found (or the specified key within it is not found), or it is of an unsupported topic type, an optional default value may be inserted, but if no default is specified then no insertion takes place and the value is passed to the derived reference topic unchanged.
The values of the insertion topics are only taken at the point when the source topic is evaluated against the topic view (i.e. when the source topic is updated). Changes to the value of the insertion topic that occur in the interim are not applied to the derived reference topics.
The format of an insert transformation is
insert path [key fromKey] at insertionKey
[default defaultValue]
The path is specified in exactly the same way as for the path mapping clause, except it may not contain expand directives. path directives operate on the path of the source topic, whereas scalar directives operate on the 'current' value as defined previously.
key is optional and can specify a fromKey which is a JSON pointer indicating the data within the insertion topic that is to be inserted. If no key is specified then the whole of the insertion topic value is inserted.
at specifies the insertionKey which is a JSON pointer
indicating where to insert the insertion topic value. If a value already
exists at the specified key then it is replaced otherwise it is inserted.
Insertion can only occur if the parent of the key exists in the value and is
of a compatible type. Array pointers may only be used to replace existing
entries or append one greater than the last entry. The special pointer value
/-
may be used to append to the end of an existing array.
default is optional and may be used to specify a string defaultValue to be inserted if the insertion topic does not exist, it is of an unsupported topic type, or the specified key within it could not be found.
The following insert transformation would cause the whole value of the topic named AnyTopic to be inserted into the current value at key /T, assuming that the current value is an object.
insert AnyTopic at /T
The following insert transformation would cause the whole value of the topic named AnyTopic to be inserted into the current value at key /T/MyKey, assuming that an object with the key T exists in the current value.
insert AnyTopic at /T/MyKey
The following insert transformation would cause the whole value of the topic named AnyTopic to be appended to the array at the key T in the current value.
insert AnyTopic at /T/-
The following insert transformation would cause the value at the key name within the topic named AnyTopic to be appended to the array at the key T in the current value.
insert AnyTopic key /name at /T/-
In the above examples no insertion would take place if the insertion value was not found, but a default value can be specified to insert into the current value in this case.
insert AnyTopic at /T/- default "unknown"
The path of the topic to insert from can be built from parts of the source topic path and/or scalar values within the current value. For example:
insert AC/<path(1,1)>/<scalar(/myval)> at /T
Options
Options are specified after the mapping and any transformations. Any number of options may be specified but any particular option may only be specified once.
Topic property mapping
The ITopicSpecification 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 topic property mapping option.
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 |
---|---|---|---|
Compression | Copied from source topic specification | Yes | |
Conflation | Copied from source topic specification | Yes | |
DontRetainValue | Copied from source topic specification | Yes | |
Owner | Not set | No | |
Persistent | 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. |
Priority | Copied from source topic specification | Yes | |
PublishValuesOnly | Copied from source topic specification | Yes | |
Removal | Not set | No | Reference topics cannot be removed directly. |
Schema | Copied from source topic specification | No | A RECORD_V2 reference topic has the same schema as its source topic. |
TidyOnUnsubscribe | Copied from source topic specification | Yes | |
TimeSeriesEventValueType | Copied from source topic specification | No | A TIME_SERIES reference topic has the same value type as its source topic. |
TimeSeriesRetainedRange | Copied from source topic specification | Yes, with restrictions | A topic property mapping cannot increase the time series retained range by overriding the TimeSeriesRetainedRange property. The retained range of a reference time series topic will be constrained to be no greater than that of its source topic. |
TimeSeriesSubscriptionRange | Copied from source topic specification | Yes | |
ValidateValues | Not set | No | A reference topic reflects updates to its source topic. It cannot reject updates. |
A topic property option 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
Topic value option
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 and/or
modified by transformations. For JSON
source topics or TIME_SERIES topics with
a JSON
event type, the value option can be applied to extract part of the
resulting value (the latest value in the case of TIME_SERIES topics).
A topic value option begins with 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
current value to copy.
For example, given a current value of
{
"account" : "1234",
"balance" : { "amount" : 12.57, "currency" : "USD" }
}
and the value option as <value(/balance)>
,
the reference topic value will be
{
"amount" : 12.57,
"currency" : "USD"
}
Value mappings that follow expand directives and/or transformations apply to the current derived 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)>
Value mappings that follow expand directives apply to the expanded value and not the source topic value.
Throttle option
The throttle option 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 option 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 option 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 throttle option 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 option 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 option provides topic-scoped conflation.
The throttle option 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.
Delay option
The delay option causes a change to a view's source topic to be delayed by a fixed time before it is reflected in reference topics. Topic additions, updates, and removals are all delayed. Delays can range from one second to many days.
Such a publication delay is a useful way to devalue topic data so it can be given away to non-paying users.
The delay option has the form delay by duration
, where
duration 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
changes to a source topic are delayed by five minutes before they are
reflected in the corresponding reference topic.
map? a// to b/<path(1)> delay by 5 minutes
Views with the delay option specified initially create reference topics in an unpublished state. The topics are published once the delay time has expired. A topic in the unpublished state prevents a lower priority topic view from creating a reference topic with the same path. Sessions with the rights to read the source topic can browse unpublished topics using the WithUnpublishedDelayedTopics fetch request option.
Separator option
Views can use the scalar and expand directives in path mappings to extract
text from the source value. By default, any /
characters in the text
are interpreted as path separators and will introduce extra levels in
reference topic paths. If this is undesirable, the separator option can be
used to replace any /
characters produced by the scalar and expand
directives with a constant string. The replacement can contain path
separators but must have no empty path segments (//
).
Here is an example replacing all path separators with an alternate character:
map ?a/path/ to b/<scalar(/x/y)> separator '%'
If the value at '/x/y' in the source topic was foo/bar, this would cause the reference topic to be created at /b/foo%bar instead of /b/foo/bar.
Preserve topics option
The default behavior for a topic view is that only the reference topics that can be derived from the current value of the source topic are maintained. This applies to views using directives that derive the path of the reference topic(s) from a value within the source topic (e.g. scalar or expand). For example if a view uses a scalar directive to determine the path of the reference topic and the source topic is updated such that the selected scalar value changes then the previously created reference topic will be removed and a new one created. Similarly an expand directive may create many reference topics from the source value but if the source is updated then only those reference topics that still have a corresponding value within the source value will be updated (or new ones added if not in the previous value), whilst others will be removed.
There are situations where an application may not want reference topics to be removed because of source value changes. In such cases the 'preserve topics' option may be used to indicate that all reference topics created by a view should be retained (and updated by later updates that apply to it) until either the source topic or the creating view are removed.
For example:
map ?a/path/ to b/<expand()> preserve topics
Topic type option
The 'type' option can be used to specify the TopicType of the target reference topic. If the current source value of type indicated by the source topic's type can be converted to the target type, a reference topic of the specified type will be created.
For example:
map ?a/ to b/<path(1)> type STRING
The specified type must be one of the supported target types (STRING, INT64, DOUBLE, JSON, TIME_SERIES, or BINARY), case insensitive.
The following table describes the supported conversions from the source topic type (the left column) to the supported target types. The number in brackets indicates a note at the foot of the table describing exactly how the conversion is processed. Where there is no number, no conversion is necessary and the derived source value is simply mapped to the target reference topic as if the 'type' option was not specified. Where there is no entry for the source topic type in the left column, no conversion is supported and topics of unsupported types will not be processed by the topic view. An 'x' indicates that the conversion is not supported.
In all cases the value being processed will be the 'current' value as derived from other mappings within the topic view (e.g. expand) which is not necessarily the value of the source topic.
Source topic | STRING | INT64 | DOUBLE | JSON | TIME_SERIES | BINARY |
---|---|---|---|---|---|---|
STRING | ( 1 ) | ( 1 ) | ( 2 ) | ( 3 ) | x | |
INT64 | ( 4 ) | ( 5 ) | ( 2 ) | ( 3 ) | x | |
DOUBLE | ( 4 ) | ( 6 ) | ( 2 ) | ( 3 ) | x | |
JSON | ( 7 ) | ( 7 ) | ( 7 ) | ( 3 ) | x | |
TIME_SERIES | ( 8 ) | ( 8 ) | ( 8 ) | ( 8 ) | ( 9 ) | BINARY | x | x | x | x | ( 9 ) |
Conversion notes:-
- STRING to number conversions will only occur if the value of the string can be converted to the target number type. If the string cannot be converted then no reference topic will be created.
- Primitive types to JSON will result in a JSON topic containing just the scalar value.
- Conversions to TIME_SERIES will result in a time series topic with an event type matching the source topic. Every update to the source topic will result in a new value being appended to the reference time series topic. It is not possible to convert to a time series topic with a different event type from the source topic.
- Conversions from number types to STRING will result in a simple string representation of the number in the reference topic value.
- INT64 to DOUBLE conversions perform a simple conversion. For example, 123 becomes 123.0.
- DOUBLE to INT64 conversions perform rounding to the nearest integer value. For example 12.51 becomes 13.
- JSON to primitive type conversions only occur if the JSON value is a scalar which can be read as a string and converted to the target type. Currently, only string and integer scalar values can be read as a string (i.e. not doubles). The string representation of the value will be converted in the same way as specified for STRING to other primitive types. If the JSON value is a structure or cannot be converted then no conversion takes place and no reference topic will be created.
- The conversion of TIME_SERIES to other types follows the same rules as for conversion from the source topic type that matches the source time series topic's event value type. So if the time series event type is 'double' then the conversion rules from source topic type DOUBLE to the target type will apply. Each value appended to the source time series topic will result in an update to the reference topic. If a failure to convert occurs at any point then the reference topic would be removed and only recreated if a value is appended that can be converted.
- BINARY to TIME_SERIES, and vice-versa is supported in the same way as for other time series conversions.
Escaping and quoting special characters
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 contains 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.
Dealing with topic path conflicts
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.
Remote topic views
A remote topic view is one that specifies another server as the location of the source topics using the from clause as shown in the example below:
map? a// from server1 to b/<path(1)>
The server name server1
in this example refers to the name of a
remote server created using the IRemoteServers feature.
In this case, upon establishing a successful connection with the remote server indicated the topic view will create reference topics locally based upon the topics selected by the topic view's selector at the remote server. It is important to note that the selector only refers to topics that match it at the remote server and not on the local server and there is no reason why there could not be a source topic at the remote server that has the same path as an entirely different topic on the local server.
More than one topic view can specify the same remote server.
A remote server only makes a physical connection when it is in use, therefore the first topic view that specifies a remote server will cause it to establish a connection. Similarly, if the last topic view that uses a remote server is removed then the connection will be closed.
It is not necessary for the named remote server definition to exist before creating the topic view, as if it does not then the topic view will simply remain dormant until the remote server is created and a successful connection to the server specified in its URL is established. Similarly, if a remote server that is in use by remote topic views is removed then all of the reference topics created by the topic views will be removed and the topic views will become dormant until the named remote server is created again or the views are changed to name a different remote server.
If a remote topic view selects a ROUTING
topic at the remote server
then local mappings will only be performed if the routing topic mapping at
the remote server is able to establish a mapping for the remote server
connection. The mapping will be done as if from the resolved routing topic.
The rules of precedence for remote topic views are the same as for other topic views. If the remote server for a remote topic view does not exist or does not have an established connection then the remote topic view is not evaluated (i.e. it is as if the source topics for the view did not exist), but if the remote server later connects then the view will be evaluated and rules of precedence will determine whether reference topic will replace those created by earlier views.
Topic view persistence and replication
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 (and remote servers) 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. When remote topic views are in use, each server in the cluster will make a connection to the specified remote server and will separately manage their remote topic views.
A view with a delay clause uses temporary storage to record delayed events. If there is a high volume of updates, temporary per-server disk files will be used to save server memory. The storage is per-server, and does not survive server restart. When a server is started, no data will be published by a view with a delay clause until the delay time has expired.
Access control
The following access control restrictions are applied:
- To list the topic views, a session needs the READ_TOPIC_VIEWS global permission.
- To create, replace, or remove a topic view, a session needs the MODIFY_TOPIC_VIEWS global permission and SELECT_TOPIC permission for the path prefix of the source topic selector.
- Each topic view records the principal and security roles of the session that created it as the topic view security context. When a topic view is evaluated, this security context is used to constrain the creation of reference topics. A reference topic will only be created if the security context has READ_TOPIC permission for the source topic path, and MODIFY_TOPIC permission for the reference topic path. The topic view security context is copied from the creating session at the time the topic view is created or replaced, and is persisted with the topic view. The topic view security context is not updated if the roles associated with the session are changed.
Accessing the feature
This feature may be obtained from an ISession as follows:
var topicViews = session.TopicViews;
This feature is also extended by the ITopics feature. This means it is possible to use the methods described here through the ITopics feature.
Added in version 6.3.
Methods
CreateTopicViewAsync(String, String)
Create a new named topic view.
Declaration
Task<ITopicView> CreateTopicViewAsync(string name, string specification)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
String | specification | The specification of the topic view. |
Returns
Type | Description |
---|---|
Task<ITopicView> | The |
Remarks
If a view with the same name already exists the new view will update the existing view.
If the operation completes successfully, the Task
result will be a
ITopicView instance.
Exceptions
Type | Condition |
---|---|
InvalidTopicViewException | The supplied specification is invalid. Thrown by the returned |
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission or
appropriate path prefix permissions. Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRoutingException | A transient cluster error occurred. Thrown by the returned |
See Also
CreateTopicViewAsync(String, String, CancellationToken)
Create a new named topic view.
Declaration
Task<ITopicView> CreateTopicViewAsync(string name, string specification, CancellationToken cancellationToken)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
String | specification | The specification of the topic view. |
CancellationToken | cancellationToken | The cancellation token used to cancel the current operation. |
Returns
Type | Description |
---|---|
Task<ITopicView> | The |
Remarks
The topic view name cannot be empty.
If a view with the same name already exists the new view will update the existing view.
If the operation completes successfully, the Task
result will be a
ITopicView instance.
Exceptions
Type | Condition |
---|---|
InvalidTopicViewException | The supplied specification is invalid. Thrown by the returned |
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission or
appropriate path prefix permissions. Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRoutingException | A transient cluster error occurred. Thrown by the returned |
GetTopicViewAsync(String)
Get a named Topic View.
Declaration
Task<ITopicView> GetTopicViewAsync(string name)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
Returns
Type | Description |
---|---|
Task<ITopicView> | The |
Remarks
If the named view does not exist the Task
will complete
successfully with a null result.
If the operation completes successfully, the Task
result will be a
named ITopicView if it exists.
Exceptions
Type | Condition |
---|---|
ClusterRoutingException | A transient cluster error occurred. Thrown by the returned |
SessionSecurityException | The calling session does not have READ_TOPIC_VIEWS
permission or appropriate path prefix permissions.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
GetTopicViewAsync(String, CancellationToken)
Get a named Topic View.
Declaration
Task<ITopicView> GetTopicViewAsync(string name, CancellationToken cancellationToken)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
CancellationToken | cancellationToken | The cancellation token used to cancel the current operation. |
Returns
Type | Description |
---|---|
Task<ITopicView> | The |
Remarks
If the named view does not exist the Task
will complete
successfully with a null result.
If the operation completes successfully, the Task
result will be a
named ITopicView if it exists.
Exceptions
Type | Condition |
---|---|
ClusterRoutingException | A transient cluster error occurred. Thrown by the returned |
SessionSecurityException | The calling session does not have READ_TOPIC_VIEWS
permission or appropriate path prefix permissions.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ListTopicViewsAsync()
List all of the topic views that have been created.
Declaration
Task<IReadOnlyCollection<ITopicView>> ListTopicViewsAsync()
Returns
Type | Description |
---|---|
Task<IReadOnlyCollection<ITopicView>> | The |
Remarks
If the operation completes successfully, the Task
result will be a
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have READ_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
See Also
ListTopicViewsAsync(CancellationToken)
List all of the topic views that have been created.
Declaration
Task<IReadOnlyCollection<ITopicView>> ListTopicViewsAsync(CancellationToken cancellationToken)
Parameters
Type | Name | Description |
---|---|---|
CancellationToken | cancellationToken | The cancellation token used to cancel the current operation. |
Returns
Type | Description |
---|---|
Task<IReadOnlyCollection<ITopicView>> | The |
Remarks
If the operation completes successfully, the Task
result will be a
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have READ_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
RemoveTopicViewAsync(String)
Remove a named topic view if it exists.
Declaration
Task<object> RemoveTopicViewAsync(string name)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
Returns
Type | Description |
---|---|
Task<Object> | The |
Remarks
If the named view does not exist the Task
will complete successfully.
If the task completes successfully, the Task
result will be null
.
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRoutingException | A transient cluster error occurred. Thrown by the returned |
See Also
RemoveTopicViewAsync(String, CancellationToken)
Remove a named topic view if it exists.
Declaration
Task<object> RemoveTopicViewAsync(string name, CancellationToken cancellationToken)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
CancellationToken | cancellationToken | The cancellation token used to cancel the current operation. |
Returns
Type | Description |
---|---|
Task<Object> | The |
Remarks
If the named view does not exist the Task
will complete successfully.
If the task completes successfully, the Task
result will be null
.
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRoutingException | A transient cluster error occurred. Thrown by the returned |