Web Client 8.0.2

Subscription

Subscription

Class representing a Subscription to be submitted to a Lightstreamer Server. It contains subscription details and the listeners needed to process the real-time data.
After the creation, a Subscription object is in the "inactive" state. When a Subscription object is subscribed to on a LightstreamerClient object, through the LightstreamerClient#subscribe method, its state becomes "active". This means that the client activates a subscription to the required items through Lightstreamer Server and the Subscription object begins to receive real-time events.


A Subscritpion can be configured to use either an Item Group or an Item List to specify the items to be subscribed to and using either a Field Schema or Field List to specify the fields.
"Item Group" and "Item List" are defined as follows:

  • "Item Group": an Item Group is a String identifier representing a list of items. Such Item Group has to be expanded into a list of items by the getItems method of the MetadataProvider of the associated Adapter Set. When using an Item Group, items in the subscription are identified by their 1-based index within the group.
    It is possible to configure the Subscription to use an "Item Group" using the Subscription#setItemGroup method.
  • "Item List": an Item List is an array of Strings each one representing an item. For the Item List to be correctly interpreted a LiteralBasedProvider or a MetadataProvider with a compatible implementation of getItems has to be configured in the associated Adapter Set.
    Note that no item in the list can be empty, can contain spaces or can be a number.
    When using an Item List, items in the subscription are identified by their name or by their 1-based index within the list.
    It is possible to configure the Subscription to use an "Item List" using the Subscription#setItems method or by specifying it in the constructor.

"Field Schema" and "Field List" are defined as follows:
  • "Field Schema": a Field Schema is a String identifier representing a list of fields. Such Field Schema has to be expanded into a list of fields by the getFields method of the MetadataProvider of the associated Adapter Set. When using a Field Schema, fields in the subscription are identified by their 1-based index within the schema.
    It is possible to configure the Subscription to use a "Field Schema" using the Subscription#setFieldSchema method.
  • "Field List": a Field List is an array of Strings each one representing a field. For the Field List to be correctly interpreted a LiteralBasedProvider or a MetadataProvider with a compatible implementation of getFields has to be configured in the associated Adapter Set.
    Note that no field in the list can be empty or can contain spaces.
    When using a Field List, fields in the subscription are identified by their name or by their 1-based index within the list.
    It is possible to configure the Subscription to use a "Field List" using the Subscription#setFields method or by specifying it in the constructor.

Constructor

new Subscription(subscriptionMode, itemsopt, fieldsopt)

Creates an object to be used to describe a Subscription that is going to be subscribed to through Lightstreamer Server. The object can be supplied to LightstreamerClient#subscribe and LightstreamerClient#unsubscribe, in order to bring the Subscription to "active" or back to "inactive" state.
Note that all of the methods used to describe the subscription to the server can only be called while the instance is in the "inactive" state; the only exception is Subscription#setRequestedMaxFrequency.

Parameters:
Name Type Attributes Description
subscriptionMode String

the subscription mode for the items, required by Lightstreamer Server. Permitted values are:

  • MERGE
  • DISTINCT
  • RAW
  • COMMAND
items String | Array.<String> <optional>

an array of Strings containing a list of items to be subscribed to through the server. In case of a single-item subscription the String containing the item name can be passed in place of the array; both of the following examples represent a valid subscription:
new Subscription(mode,"item1",fieldList);
new Subscription(mode,["item1","item2"],fieldList);
It is also possible to pass null (or nothing) and specify the "Item List" or "Item Group" later through Subscription#setItems and Subscription#setItemGroup. In this case the fields parameter must not be specified.
fields Array.<String> <optional>

An array of Strings containing a list of fields for the items to be subscribed to through Lightstreamer Server.
It is also possible to pass null (or nothing) and specify the "Field List" or "Field Schema" later through Subscription#setFields and Subscription#setFieldSchema. In this case the items parameter must not be specified.
Throws:

Methods

addListener(listener)

Adds a listener that will receive events from the Subscription instance.
The same listener can be added to several different Subscription instances.

Lifecycle: a listener can be added at any time.

Parameters:
Name Type Description
listener SubscriptionListener

An object that will receive the events as shown in the SubscriptionListener interface.
Note that the given instance does not have to implement all of the methods of the SubscriptionListener interface. In fact it may also implement none of the interface methods and still be considered a valid listener. In the latter case it will obviously receive no events.

configure()

Changes the real max frequency of this subscription.
If there is a change, the method SubscriptionListener.onRealMaxFrequency is triggered.
The method SubscriptionListener.onRealMaxFrequency is also triggered if there is a new maximum among the item frequencies of a two-level command subscription.

getCommandPosition() → {Number}

Returns the position of the "command" field in a COMMAND Subscription.
This method can only be used if the Subscription mode is COMMAND and the Subscription was initialized using a "Field Schema".

Lifecycle: This method can be called at any time.

Throws:

if the Subscription mode is not COMMAND or if the SubscriptionListener#onSubscription event for this Subscription was not yet fired.

Type
IllegalStateException
Returns:

the 1-based position of the "command" field within the "Field Schema".

Type
Number

getCommandSecondLevelDataAdapter() → {String}

Inquiry method that can be used to read the second-level Data Adapter name configured through Subscription#setCommandSecondLevelDataAdapter.

Lifecycle: This method can be called at any time.

Returns:

the name of the second-level Data Adapter.

Type
String

getCommandSecondLevelFields() → {Array.<String>}

Inquiry method that can be used to read the "Field List" specified for second-level Subscriptions.

Lifecycle: This method can only be called if the second-level of this Subscription has been initialized using a "Field List"

Throws:

if the Subscription was initialized with a field schema or was not initialized at all.

Type
IllegalStateException
Returns:

the list of fields to be subscribed to through the server.

Type
Array.<String>

getCommandSecondLevelFieldSchema() → {String}

Inquiry method that can be used to read the "Field Schema" specified for second-level Subscriptions.

Lifecycle: This method can only be called if the second-level of this Subscription has been initialized using a "Field Schema".

Throws:

if the Subscription was initialized with a "Field List" or was not initialized at all.

Type
IllegalStateException
Returns:

the "Field Schema" to be subscribed to through the server.

Type
String

getCommandValue(itemIdentifier, keyValue, fieldIdentifier) → {String}

Returns the latest value received for the specified item/key/field combination. This method can only be used if the Subscription mode is COMMAND. Subscriptions with two-level behavior are also supported, hence the specified field can be either a first-level or a second-level one.
It is suggested to consume real-time data by implementing and adding a proper SubscriptionListener rather than probing this method.
Note that internal data is cleared when the Subscription is unsubscribed from.

Lifecycle: This method can be called at any time; if called to retrieve a value that has not been received yet, then it will return null.

Parameters:
Name Type Description
itemIdentifier String

a String representing an item in the configured item list or a Number representing the 1-based position of the item in the specified item group. (In case an item list was specified, passing the item position is also possible).
keyValue String

a String containing the value of a key received on the COMMAND subscription.
fieldIdentifier String

a String representing a field in the configured field list or a Number representing the 1-based position of the field in the specified field schema. (In case a field list was specified, passing the field position is also possible).
Throws:
Returns:

the current value for the specified field of the specified key within the specified item (possibly null), or null if the specified key has not been added yet (note that it might have been added and eventually deleted).

Type
String

getDataAdapter() → {String}

Inquiry method that can be used to read the name of the Data Adapter specified for this Subscription through Subscription#setDataAdapter.

Lifecycle: This method can be called at any time.

Returns:

the name of the Data Adapter; returns null if no name has been configured, so that the "DEFAULT" Adapter Set is used.

Type
String

getFields() → {Array.<String>}

Inquiry method that can be used to read the "Field List" specified for this Subscription.

Lifecycle: This method can only be called if the Subscription has been initialized using a "Field List".

Throws:

if the Subscription was initialized with a "Field Schema" or was not initialized at all.

Type
IllegalStateException
Returns:

the "Field List" to be subscribed to through the server.

Type
Array.<String>

getFieldSchema() → {String}

Inquiry method that can be used to read the field schema specified for this Subscription.

Lifecycle: This method can only be called if the Subscription has been initialized using a "Field Schema"

Throws:

if the Subscription was initialized with a "Field List" or was not initialized at all.

Type
IllegalStateException
Returns:

the "Field Schema" to be subscribed to through the server.

Type
String

getItemGroup() → {String}

Inquiry method that can be used to read the item group specified for this Subscription.

Lifecycle: This method can only be called if the Subscription has been initialized using an "Item Group"

Throws:

if the Subscription was initialized with an "Item List" or was not initialized at all.

Type
IllegalStateException
Returns:

the "Item Group" to be subscribed to through the server.

Type
String

getItems() → {Array.<String>}

Inquiry method that can be used to read the "Item List" specified for this Subscription.
Note that if a single item was specified in the constructor, this method will return an array of length 1 containing such item.

Lifecycle: This method can only be called if the Subscription has been initialized with an "Item List".

Throws:

if the Subscription was initialized with an "Item Group" or was not initialized at all.

Type
IllegalStateException
Returns:

the "Item List" to be subscribed to through the server.

Type
Array.<String>

getKeyPosition() → {Number}

Returns the position of the "key" field in a COMMAND Subscription.
This method can only be used if the Subscription mode is COMMAND and the Subscription was initialized using a "Field Schema".

Lifecycle: This method can be called at any time.

Throws:

if the Subscription mode is not COMMAND or if the SubscriptionListener#onSubscription event for this Subscription was not yet fired.

Type
IllegalStateException
Returns:

the 1-based position of the "key" field within the "Field Schema".

Type
Number

getListeners() → {Array.<SubscriptionListener>}

Returns an array containing the SubscriptionListener instances that were added to this client.

Returns:

an Array containing the listeners that were added to this client. Listeners added multiple times are included multiple times in the array.

Type
Array.<SubscriptionListener>

getMode() → {String}

Inquiry method that can be used to read the mode specified for this Subscription.

Lifecycle: This method can be called at any time.

Returns:

the Subscription mode specified in the constructor.

Type
String

getRequestedBufferSize() → {String}

Inquiry method that can be used to read the buffer size, configured though Subscription#setRequestedBufferSize, to be requested to the Server for this Subscription.

Lifecycle: This method can be called at any time.

Returns:

the buffer size to be requested to the server.

Type
String

getRequestedMaxFrequency() → {String}

Inquiry method that can be used to read the max frequency, configured through Subscription#setRequestedMaxFrequency, to be requested to the Server for this Subscription.

Lifecycle: This method can be called at any time.

Returns:

A decimal number, representing the max frequency to be requested to the server (expressed in updates per second), or the strings "unlimited" or "unfiltered", or null.

Type
String

getRequestedSnapshot() → {String}

Inquiry method that can be used to read the snapshot preferences, configured through Subscription#setRequestedSnapshot, to be requested to the Server for this Subscription.

Lifecycle: This method can be called at any time.

Returns:

the snapshot preference to be requested to the server.

Type
String

getSelector() → {String}

Inquiry method that can be used to read the selctor name
specified for this Subscription through Subscription#setSelector.

Lifecycle: This method can be called at any time.

Returns:

the name of the selector.

Type
String

getValue(itemIdentifier, fieldIdentifier) → {String}

Returns the latest value received for the specified item/field pair.
It is suggested to consume real-time data by implementing and adding a proper SubscriptionListener rather than probing this method. In case of COMMAND Subscriptions, the value returned by this method may be misleading, as in COMMAND mode all the keys received, being part of the same item, will overwrite each other; for COMMAND Subscriptions, use Subscription#getCommandValue instead.
Note that internal data is cleared when the Subscription is unsubscribed from.

Lifecycle: This method can be called at any time; if called to retrieve a value that has not been received yet, then it will return null.

Parameters:
Name Type Description
itemIdentifier String

a String representing an item in the configured item list or a Number representing the 1-based position of the item in the specified item group. (In case an item list was specified, passing the item position is also possible).
fieldIdentifier String

a String representing a field in the configured field list or a Number representing the 1-based position of the field in the specified field schema. (In case a field list was specified, passing the field position is also possible).
Throws:

if an invalid item name or field name is specified or if the specified item position or field position is out of bounds.

Type
IllegalArgumentException
Returns:

the current value for the specified field of the specified item (possibly null), or null if no value has been received yet.

Type
String

isActive() → {boolean}

Inquiry method that checks if the Subscription is currently "active" or not. Most of the Subscription properties cannot be modified if a Subscription is "active".
The status of a Subscription is changed to "active" through the
LightstreamerClient#subscribe method and back to "inactive" through the LightstreamerClient#unsubscribe one.

Lifecycle: This method can be called at any time.

See:
Returns:

true/false if the Subscription is "active" or not.

Type
boolean

isSubscribed() → {boolean}

Inquiry method that checks if the Subscription is currently subscribed to through the server or not.
This flag is switched to true by server sent Subscription events, and back to false in case of client disconnection, LightstreamerClient#unsubscribe calls and server sent unsubscription events.

Lifecycle: This method can be called at any time.

Returns:

true/false if the Subscription is subscribed to through the server or not.

Type
boolean

removeListener(listener)

Removes a listener from the Subscription instance so that it will not receive events anymore.

Lifecycle: a listener can be removed at any time.

Parameters:
Name Type Description
listener SubscriptionListener

The listener to be removed.

setCommandSecondLevelDataAdapter(dataAdapter)

Setter method that sets the name of the second-level Data Adapter (within the Adapter Set used by the current session) that supplies all the second-level items. All the possible second-level items should be supplied in "MERGE" mode with snapshot available. The Data Adapter name is configured on the server side through the "name" attribute of the <data_provider> element, in the "adapters.xml" file that defines the Adapter Set (a missing attribute configures the "DEFAULT" name).

Default value: The default Data Adapter for the Adapter Set, configured as "DEFAULT" on the Server.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
dataAdapter String

the name of the Data Adapter. A null value is equivalent to the "DEFAULT" name.
See:
Throws:

setCommandSecondLevelFields(fields)

Setter method that sets the "Field List" to be subscribed to through Lightstreamer Server for the second-level items. It can only be used on COMMAND Subscriptions.
Any call to this method will override any "Field List" or "Field Schema" previously specified for the second-level.
Calling this method enables the two-level behavior:
in synthesis, each time a new key is received on the COMMAND Subscription, the key value is treated as an Item name and an underlying Subscription for this Item is created and subscribed to automatically, to feed fields specified by this method. This mono-item Subscription is specified through an "Item List" containing only the Item name received. As a consequence, all the conditions provided for subscriptions through Item Lists have to be satisfied. The item is subscribed to in "MERGE" mode, with snapshot request and with the same maximum frequency setting as for the first-level items (including the "unfiltered" case). All other Subscription properties are left as the default. When the key is deleted by a DELETE command on the first-level Subscription, the associated second-level Subscription is also unsubscribed from.
Specifying null as parameter will disable the two-level behavior.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
fields Array.<String>

An array of Strings containing a list of fields to be subscribed to through the server.
Ensure that no name conflict is generated between first-level and second-level fields. In case of conflict, the second-level field will not be accessible by name, but only by position.
See:
Throws:

setCommandSecondLevelFieldSchema(schemaName)

Setter method that sets the "Field Schema" to be subscribed to through Lightstreamer Server for the second-level items. It can only be used on COMMAND Subscriptions.
Any call to this method will override any "Field List" or "Field Schema" previously specified for the second-level.
Calling this method enables the two-level behavior:
in synthesis, each time a new key is received on the COMMAND Subscription, the key value is treated as an Item name and an underlying Subscription for this Item is created and subscribed to automatically, to feed fields specified by this method. This mono-item Subscription is specified through an "Item List" containing only the Item name received. As a consequence, all the conditions provided for subscriptions through Item Lists have to be satisfied. The item is subscribed to in "MERGE" mode, with snapshot request and with the same maximum frequency setting as for the first-level items (including the "unfiltered" case). All other Subscription properties are left as the default. When the key is deleted by a DELETE command on the first-level Subscription, the associated second-level Subscription is also unsubscribed from.
Specify null as parameter will disable the two-level behavior.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
schemaName String

A String to be expanded into a field list by the Metadata Adapter.
See:
Throws:

setDataAdapter(dataAdapter)

Setter method that sets the name of the Data Adapter (within the Adapter Set used by the current session) that supplies all the items for this Subscription.
The Data Adapter name is configured on the server side through the "name" attribute of the "data_provider" element, in the "adapters.xml" file that defines the Adapter Set (a missing attribute configures the "DEFAULT" name).
Note that if more than one Data Adapter is needed to supply all the items in a set of items, then it is not possible to group all the items of the set in a single Subscription. Multiple Subscriptions have to be defined.

Default value: The default Data Adapter for the Adapter Set, configured as "DEFAULT" on the Server.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
dataAdapter String

the name of the Data Adapter. A null value is equivalent to the "DEFAULT" name.
See:
Throws:

if the Subscription is currently "active".

Type
IllegalStateException

setFields(fields)

Setter method that sets the "Field List" to be subscribed to through Lightstreamer Server.
Any call to this method will override any "Field List" or "Field Schema" previously specified.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
fields Array.<String>

An array of Strings containing a list of fields to be subscribed to through the server.
Throws:

setFieldSchema(schemaName)

Setter method that sets the "Field Schema" to be subscribed to through Lightstreamer Server.
Any call to this method will override any "Field List" or "Field Schema" previously specified.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
schemaName String

A String to be expanded into a field list by the Metadata Adapter.
Throws:

if the Subscription is currently "active".

Type
IllegalStateException

setItemGroup(groupName)

Setter method that sets the "Item Group" to be subscribed to through Lightstreamer Server.
Any call to this method will override any "Item List" or "Item Group" previously specified.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
groupName String

A String to be expanded into an item list by the Metadata Adapter.
Throws:

if the Subscription is currently "active".

Type
IllegalStateException

setItems(items)

Setter method that sets the "Item List" to be subscribed to through Lightstreamer Server.
Any call to this method will override any "Item List" or "Item Group" previously specified.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
items Array.<String>

An array of Strings containing an "Item List" to be subscribed to through the server.
Throws:

setRequestedBufferSize(size)

Setter method that sets the length to be requested to Lightstreamer Server for the internal queueing buffers for the items in the Subscription. A Queueing buffer is used by the Server to accumulate a burst of updates for an item, so that they can all be sent to the client, despite of bandwidth or frequency limits. It can be used only when the subscription mode is MERGE or DISTINCT and unfiltered dispatching has not been requested. Note that the Server may pose an upper limit on the size of its internal buffers.

Default value: null, meaning to lean on the Server default based on the subscription mode. This means that the buffer size will be 1 for MERGE subscriptions and "unlimited" for DISTINCT subscriptions. See the "General Concepts" document for further details.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
size Number

The length of the internal queueing buffers to be used in the Server. If the string "unlimited" is supplied, then no buffer size limit is requested (the check is case insensitive). It is also possible to supply a null value to stick to the Server default (which currently depends on the subscription mode).
See:
Throws:

setRequestedMaxFrequency(freq)

Setter method that sets the maximum update frequency to be requested to Lightstreamer Server for all the items in the Subscription. It can be used only if the Subscription mode is MERGE, DISTINCT or COMMAND (in the latter case, the frequency limitation applies to the UPDATE events for each single key). For Subscriptions with two-level behavior (see Subscription#setCommandSecondLevelFields and Subscription#setCommandSecondLevelFieldSchema) , the specified frequency limit applies to both first-level and second-level items.
Note that frequency limits on the items can also be set on the server side and this request can only be issued in order to furtherly reduce the frequency, not to rise it beyond these limits.
This method can also be used to request unfiltered dispatching for the items in the Subscription. However, unfiltered dispatching requests may be refused if any frequency limit is posed on the server side for some item.

Edition Note: A further global frequency limit could also be imposed by the Server, depending on Edition and License Type; this specific limit also applies to RAW mode and to unfiltered dispatching. To know what features are enabled by your license, please see the License tab of the Monitoring Dashboard (by default, available at /dashboard).

Default value: null, meaning to lean on the Server default based on the subscription mode. This consists, for all modes, in not applying any frequency limit to the subscription (the same as "unlimited"); see the "General Concepts" document for further details.

Lifecycle: This method can can be called at any time with some differences based on the Subscription status:

  • If the Subscription instance is in its "inactive" state then this method can be called at will.
  • If the Subscription instance is in its "active" state then the method can still be called unless the current value is "unfiltered" or the supplied value is "unfiltered" or null. If the Subscription instance is in its "active" state and the connection to the server is currently open, then a request to change the frequency of the Subscription on the fly is sent to the server.

Parameters:
Name Type Description
freq Number

A decimal number, representing the maximum update frequency (expressed in updates per second) for each item in the Subscription; for instance, with a setting of 0.5, for each single item, no more than one update every 2 seconds will be received. If the string "unlimited" is supplied, then no frequency limit is requested. It is also possible to supply the string "unfiltered", to ask for unfiltered dispatching, if it is allowed for the items, or a null value to stick to the Server default (which currently corresponds to "unlimited"). The check for the string constants is case insensitive.
Throws:

  • if the Subscription is currently "active" and the current value of this property is "unfiltered".

    Type
    IllegalStateException

  • if the Subscription is currently "active" and the given parameter is null or "unfiltered".

    Type
    IllegalStateException

  • if the specified value is not null nor one of the special "unlimited" and "unfiltered" values nor a valid positive number.

    Type
    IllegalArgumentException

setRequestedSnapshot(required)

Setter method that enables/disables snapshot delivery request for the items in the Subscription. The snapshot can be requested only if the Subscription mode is MERGE, DISTINCT or COMMAND.

Default value: "yes" if the Subscription mode is not "RAW", null otherwise.

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
required String

"yes"/"no" to request/not request snapshot delivery (the check is case insensitive). If the Subscription mode is DISTINCT, instead of "yes", it is also possible to supply a number, to specify the requested length of the snapshot (though the length of the received snapshot may be less than requested, because of insufficient data or server side limits); passing "yes" means that the snapshot length should be determined only by the Server. Null is also a valid value; if specified no snapshot preference will be sent to the server that will decide itself whether or not to send any snapshot.
See:
Throws:

  • if the Subscription is currently "active".

    Type
    IllegalStateException

  • if the specified value is not "yes" nor "no" nor null nor a valid integer positive number.

    Type
    IllegalArgumentException

  • if the specified value is not compatible with the mode of the Subscription:

    • In case of a RAW Subscription only null is a valid value;
    • In case of a non-DISTINCT Subscription only null "yes" and "no" are valid values.
    Type
    IllegalArgumentException

setSelector(selector)

Setter method that sets the selector name for all the items in the Subscription. The selector is a filter on the updates received. It is executed on the Server and implemented by the Metadata Adapter.

Default value: null (no selector).

Lifecycle: This method can only be called while the Subscription instance is in its "inactive" state.

Parameters:
Name Type Description
selector String

name of a selector, to be recognized by the Metadata Adapter, or null to unset the selector.
Throws:

if the Subscription is currently "active".

Type
IllegalStateException