Settings View Source Link Resolution

Some functions in Spear have :resolve_links? options. Projected streams such as streams beginning with "$" or custom-made projected streams do not copy event bodies literally from linked events. For example, the "$all" projected stream does not contain a copy of every event in the eventstore. Rather projected streams are comprised of link events which are very slim references to the source events. This is conceptually similar to pointers in a language like C.

When we read a projected stream (in this example an event-type stream) with :resolve_links? set to false, we see

iex> import Spear.Records.Streams
iex> Spear.stream!(conn, "$et-grpc-client", chunk_size: 1, resolve_links?: false, raw?: true) |> Enum.take(1)
[
  read_resp(
    content: {:event,
     read_resp_read_event(
       event: read_resp_read_event_recorded_event(
         data: "0@es_supported_clients"
       ),
       link: :undefined
     )
    )
  )
]

(Note that the read-response bodies we see in this section are simplified to only show the parts interesting to link resolution.)

The link field on the read event is nil and the event field has a strange data body of 0@es_supported_clients. With link resolution turned off, we are telling the EventStoreDB that we'd like to read the stream literally: to receive just the links themselves.

When we turn link resolution on, we see a different picture

iex> import Spear.Records.Streams
iex> Spear.stream!(conn, "$et-grpc-client", chunk_size: 1, resolve_links?: true, raw?: true) |> Enum.take(1)
[
  read_resp(
    content: {:event,
     read_resp_read_event(
       event: read_resp_read_event_recorded_event(
         data: "{\"languages\":[\"typescript\",\"javascript\"],\"runtime\":\"NodeJS\"}",
       ),
       link: read_resp_read_event_recorded_event(
         data: "0@es_supported_clients",
       )
     ))
  )
]

Now the :link field contains the reference to the original event and the :event contains the full data for the original event.

What happens if you try to resolve links for an EventStoreDB stream which is not a projected stream?

iex> import Spear.Records.Streams
iex> Spear.stream!(conn, "es_supported_clients", chunk_size: 1, resolve_links?: true, raw?: true) |> Enum.take(1)
[
  read_resp(
    content: {:event,
     read_resp_read_event(
       event: read_resp_read_event_recorded_event(
         data: "{\"languages\":[\"typescript\",\"javascript\"],\"runtime\":\"NodeJS\"}",
       ),
       link: :undefined
     ))
  )
]

Nothing! The events from non-projected streams are unaffected by link resolution choice. Hence the :resolve_links? option is consistently defaulted to true.