Inheritance

If wildcard destinations are present in the configuration, an application’s destination might match multiple configured destinations. These are known as overlapping destinations.

Persistence Store and Cluster

Overlapping destinations must always use the same persistence store and cluster. It is an error to configure overlapping destinations with different persistence stores.

Subscription Properties

Overlapping destinations might specify different destination properties. When a topic subscription, queue, or map is created, FTL determines its properties as follows:

• If an exact match is found (for example, subscribe on foo.bar and foo.bar is in the config), FTL uses its properties. For example:

  • If >, foo.*, and foo.bar are configured, a subscriber on foo.bar uses the config from foo.bar.

• If a most-specific matching config is found, FTL uses its properties. A “most-specific” config is defined as one which is the strictest subset of all matching configs. For example:

  • If > and foo.* are configured, a subscriber on foo.bar uses the config from foo.*.

  • If *.* and foo.* are configured, a subscriber on foo.bar uses the config from foo.*.

  • If foo.* and *.bar are configured, there is no most-specific config, so FTL proceeds to tiebreaks.

• If multiple matching configs are found and a most specific cannot be determined, FTL determines the value for each property as follows, from the properties of all matching configs:

Property	Rule
message_ttl	Select lowest value (0 is treated as infinity)
durable_ttl	Select lowest value (0 is treated as infinity)
retention_time	Select lowest value
bytelimit	Select lowest value (0 is treated as infinity)
message_limit	Select lowest value (0 is treated as infinity)
swap_bytelimit	Select lowest value (-1 is treated as infinity; 0 as 0)
discard	Prefer ‘old’ over ‘new’
ack_settings	Prefer ‘sync’ over ‘async’
(ack) batch_size	Select lowest value
(ack) batch_time	Select lowest value
max_delivery	Select lowest value (0 is treated as infinity)

These rules enable you to designate a set of destination properties to use in generic cases, and other sets of destination properties to use for certain special cases.

 

Access Control

Overlapping destinations may specify different access control lists.

When an application program performs an operation on a destination (e.g. subscribe or publish), and you have enabled permissions in the FTL realm, FTL server will check that the application has permission to perform the requested operation on the specified destination.

When there are overlapping destinations in the configuration, FTL server will check the access control lists of all matching destinations. If any permission to perform the operation is granted for any matching destination, FTL server will allow the operation.

Put another way, the effective access control list for a specific destination is the union of the access control lists for all matching destinations. This allows you to specify broad permissions for more privileged users, and narrow permissions for less privileged users.

For example, if the configuration for foo.* indicates that user Bob is allowed to publish, while the configuration for foo.bar indicates that user Alice is allowed to publish, then:

• Bob is allowed to publish on foo.bar and foo.boo.

• Alice is allowed to publish on foo.bar but not on foo.boo.