This stream processing stage establishes a continuous source of server-sent events from the given URI.

A single source of server-sent events is obtained from the URI. Once completed, either normally or by failure, a next one is obtained thereby sending a Last-Evend-ID header if available. This continues in an endless cycle.

The shape of this processing stage is a source of server-sent events; to take effect it must be connected and run. Progress (including termination) is controlled by the connected flow or sink, e.g. a retry delay can be implemented by streaming the materialized values of the handler via a throttle.

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                                              +---------------------+
|                                             |       trigger       | |
                                              +----------o----------+
|                                                        |            |
                                           Option[String]|
|                                                        v            |
             Option[String]                   +----------o----------+
|            +------------------------------->o        merge        | |
             |                                +----------o----------+
|            |                                           |            |
             |                             Option[String]|
|            |                                           v            |
  +----------o----------+                     +----------o----------+
| | currentLastEventId  |                     |    eventSources     | |
  +----------o----------+                     +----------o----------+
|            ^                                           |            |
             |      (EventSource, Future[Option[String]])|
|            |                                           v            |
             |                                +----------o----------+
|            +--------------------------------o        unzip        | |
             Future[Option[String]]           +----------o----------+
|                                                        |            |
                                              EventSource|
|                                                        v            |
                                              +----------o----------+
|                                  +----------o       flatten       | |
                    ServerSentEvent|          +---------------------+
|                                  v                                  |
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