diff --git a/CHANGES.md b/CHANGES.md
index f47b5730a1..3fe09d0f5c 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -62,6 +62,9 @@ New:
 
 Changed:
 
+- Reimplemented `request.once`, `single` and more using `source.dynamic`. Removed experiment
+  flag on `source.dynamic`. The operator is considered stable enough to define advanced sources
+  but the user should be careful when using it.
 - Mute SDL startup messages (#2913).
 - `int` can optionally raises an error when passing `nan` or `infinity`, `int(infinity)`
   now returns `max_int` and `int(-infinity)` returns `min_int`. (#3407)
diff --git a/src/core/builtins/builtins_source.ml b/src/core/builtins/builtins_source.ml
index 1497693389..4bd5ad1f18 100644
--- a/src/core/builtins/builtins_source.ml
+++ b/src/core/builtins/builtins_source.ml
@@ -41,6 +41,15 @@ let _ =
       s#set_name n;
       Lang.unit)
 
+let _ =
+  Lang.add_builtin ~base:source "last_metadata" ~category:(`Source `Liquidsoap)
+    ~descr:"Return the last metadata from the source."
+    [("", Lang.source_t (Lang.univ_t ()), None, None)]
+    (Lang.nullable_t Lang.metadata_t)
+    (fun p ->
+      let s = Lang.to_source (List.assoc "" p) in
+      match s#last_metadata with None -> Lang.null | Some m -> Lang.metadata m)
+
 let _ =
   Lang.add_builtin ~base:source "skip" ~category:(`Source `Liquidsoap)
     ~descr:"Skip to the next track."
diff --git a/src/core/operators/dyn_op.ml b/src/core/operators/dyn_op.ml
index 426f65f874..01c60d48ac 100644
--- a/src/core/operators/dyn_op.ml
+++ b/src/core/operators/dyn_op.ml
@@ -20,84 +20,85 @@
 
  *****************************************************************************)
 
-class dyn ~init ~track_sensitive ~infallible ~resurection_time ~self_sync f =
+class dyn ~init ~track_sensitive ~infallible ~self_sync ~merge next_fn =
   object (self)
     inherit Source.source ~name:"source.dynamic" ()
-
-    inherit
-      Source.generate_from_multiple_sources
-        ~merge:(fun () -> false)
-        ~track_sensitive ()
-
+    inherit Source.generate_from_multiple_sources ~merge ~track_sensitive ()
     method fallible = not infallible
     val mutable activation = []
-    val source : Source.source option Atomic.t = Atomic.make init
+    val current_source : Source.source option Atomic.t = Atomic.make init
+    method current_source = Atomic.get current_source
     val mutable last_select = Unix.gettimeofday ()
-    val proposed = Atomic.make None
-    method propose s = Atomic.set proposed (Some s)
 
-    method private prepare s =
+    method private no_source =
+      if infallible then
+        Lang.raise_error ~pos:[]
+          ~message:
+            (Printf.sprintf
+               "Infallible source.dynamic %s was not able to prepare a source \
+                in time! Make sure to either define infallible sources in the \
+                source's dynamic function or mark the source as fallible.."
+               self#id)
+          "failure";
+      None
+
+    method prepare s =
       Typing.(s#frame_type <: self#frame_type);
       Clock.unify ~pos:self#pos s#clock self#clock;
-      s#wake_up;
-      (match Atomic.exchange source (Some s) with
-        | Some s -> s#sleep
-        | None -> ());
-      if s#is_ready then Some s else None
+      s#wake_up
+
+    method private exchange s =
+      self#log#info "Switching to source %s" s#id;
+      self#prepare s;
+      Atomic.set current_source (Some s);
+      if s#is_ready then Some s else self#no_source
 
     method private get_next reselect =
       self#mutexify
         (fun () ->
-          match Atomic.exchange proposed None with
-            | Some s -> self#prepare s
+          last_select <- Unix.gettimeofday ();
+          let s =
+            Lang.apply next_fn [] |> Lang.to_option |> Option.map Lang.to_source
+          in
+          match s with
             | None -> (
-                last_select <- Unix.gettimeofday ();
-                let s =
-                  Lang.apply f [] |> Lang.to_option |> Option.map Lang.to_source
-                in
-                match s with
-                  | None -> (
-                      match Atomic.get source with
-                        | Some s
-                          when self#can_reselect
-                                 ~reselect:
-                                   (match reselect with
-                                     | `Force -> `Ok
-                                     | v -> v)
-                                 s ->
-                            Some s
-                        | _ -> None)
-                  | Some s -> self#prepare s))
+                match self#current_source with
+                  | Some s
+                    when self#can_reselect
+                           ~reselect:
+                             (match reselect with `Force -> `Ok | v -> v)
+                           s ->
+                      Some s
+                  | _ -> self#no_source)
+            | Some s -> self#exchange s)
         ()
 
     method private get_source ~reselect () =
-      match (Atomic.get source, reselect) with
-        | None, _ | _, `Force -> self#get_next reselect
+      match (self#current_source, reselect) with
+        | None, _ | _, `Force | Some _, `After_position _ ->
+            self#get_next reselect
         | Some s, _ when self#can_reselect ~reselect s -> Some s
-        | Some _, _ when Unix.gettimeofday () -. last_select < resurection_time
-          ->
-            None
         | _ -> self#get_next reselect
 
     initializer
       self#on_wake_up (fun () ->
           Lang.iter_sources
             (fun s -> Typing.(s#frame_type <: self#frame_type))
-            f;
+            next_fn;
           ignore (self#get_source ~reselect:`Force ()));
       self#on_sleep (fun () ->
-          match Atomic.exchange source None with
+          match Atomic.exchange current_source None with
             | Some s -> s#sleep
             | None -> ())
 
     method remaining =
-      match Atomic.get source with Some s -> s#remaining | None -> -1
+      match self#current_source with Some s -> s#remaining | None -> -1
 
     method abort_track =
-      match Atomic.get source with Some s -> s#abort_track | None -> ()
+      match self#current_source with Some s -> s#abort_track | None -> ()
 
     method seek_source =
-      match Atomic.get source with
+      match self#current_source with
         | Some s -> s#seek_source
         | None -> (self :> Source.source)
 
@@ -106,7 +107,7 @@ class dyn ~init ~track_sensitive ~infallible ~resurection_time ~self_sync f =
         | Some v -> (`Static, self#source_sync v)
         | None -> (
             ( `Dynamic,
-              match Atomic.get source with
+              match self#current_source with
                 | Some s -> snd s#self_sync
                 | None -> None ))
   end
@@ -133,16 +134,13 @@ let _ =
         Lang.nullable_t Lang.bool_t,
         Some Lang.null,
         Some "For the source's `self_sync` property." );
-      ( "resurection_time",
-        Lang.nullable_t Lang.float_t,
-        Some (Lang.float 1.),
-        Some
-          "When track sensitive and the source is unavailable, how long we \
-           should wait before trying to update source again (`null` means \
-           never)." );
+      ( "merge",
+        Lang.getter_t Lang.bool_t,
+        Some (Lang.bool false),
+        Some "Set or return `true` to merge subsequent tracks." );
       ( "",
         Lang.fun_t [] (Lang.nullable_t (Lang.source_t frame_t)),
-        Some (Lang.val_fun [] (fun _ -> Lang.null)),
+        None,
         Some
           "Function returning the source to be used, `null` means keep current \
            source." );
@@ -152,17 +150,25 @@ let _ =
       "Dynamically change the underlying source: it can either be changed by \
        the function given as argument, which returns the source to be played, \
        or by calling the `set` method."
-    ~category:`Track ~flags:[`Experimental]
+    ~category:`Track
     ~meth:
       [
-        ( "set",
+        ( "current_source",
+          ([], Lang.fun_t [] (Lang.nullable_t (Lang.source_t frame_t))),
+          "Return the source currently selected.",
+          fun s ->
+            Lang.val_fun [] (fun _ ->
+                match s#current_source with
+                  | None -> Lang.null
+                  | Some s -> Lang.source s) );
+        ( "prepare",
           ([], Lang.fun_t [(false, "", Lang.source_t frame_t)] Lang.unit_t),
-          "Set the source.",
+          "Prepare a source that will be returned later.",
           fun s ->
             Lang.val_fun
               [("", "x", None)]
               (fun p ->
-                s#propose (List.assoc "x" p |> Lang.to_source);
+                s#prepare (List.assoc "x" p |> Lang.to_source);
                 Lang.unit) );
       ]
     (fun p ->
@@ -172,13 +178,10 @@ let _ =
       let track_sensitive = List.assoc "track_sensitive" p |> Lang.to_getter in
       let track_sensitive () = Lang.to_bool (track_sensitive ()) in
       let infallible = List.assoc "infallible" p |> Lang.to_bool in
-      let resurection_time =
-        List.assoc "resurection_time" p |> Lang.to_valued_option Lang.to_float
-      in
-      let resurection_time = Option.value ~default:(-1.) resurection_time in
+      let merge = Lang.to_getter (List.assoc "merge" p) in
+      let merge () = Lang.to_bool (merge ()) in
       let self_sync =
         Lang.to_valued_option Lang.to_bool (List.assoc "self_sync" p)
       in
       let next = List.assoc "" p in
-      new dyn
-        ~init ~track_sensitive ~infallible ~resurection_time ~self_sync next)
+      new dyn ~init ~track_sensitive ~infallible ~merge ~self_sync next)
diff --git a/src/libs/extra/source.liq b/src/libs/extra/source.liq
index 9fe862053f..727dc5cf60 100644
--- a/src/libs/extra/source.liq
+++ b/src/libs/extra/source.liq
@@ -195,7 +195,7 @@ end
 # @param ~every Duration of a track (in seconds).
 # @param ~metadata Metadata for tracks.
 # @param s The stream.
-def chop(~every=getter(3.), ~metadata=getter([]), s) =
+def chop(~id=null(), ~every=getter(3.), ~metadata=getter([]), s) =
   s = insert_metadata(s)
 
   # Track time in the source's context:
@@ -211,7 +211,7 @@ def chop(~every=getter(3.), ~metadata=getter([]), s) =
     end
   end
 
-  source.on_frame(s, f)
+  source.on_frame(id=id, s, f)
 end
 
 # Regularly skip tracks from a source (useful for testing skipping).
diff --git a/src/libs/request.liq b/src/libs/request.liq
index 511d312b5b..675c779481 100644
--- a/src/libs/request.liq
+++ b/src/libs/request.liq
@@ -43,9 +43,10 @@ def request.queue(
 ) =
   ignore(native)
   id = string.id.default(default="request.queue", id)
-  initial_queue = queue
+  initial_queue = ref(queue)
   queue = ref([])
   fetch = ref(fun () -> true)
+  started = ref(false)
 
   def next() =
     if
@@ -60,13 +61,23 @@ def request.queue(
   end
 
   def push(r) =
-    log.info(
-      label=id,
-      "Pushing #{r} on the queue."
-    )
-    queue := [...queue(), r]
-    fn = fetch()
-    ignore(fn())
+    if
+      started()
+    then
+      log.info(
+        label=id,
+        "Pushing #{r} on the queue."
+      )
+      queue := [...queue(), r]
+      fn = fetch()
+      ignore(fn())
+    else
+      log.info(
+        label=id,
+        "Pushing #{r} on the initial queue."
+      )
+      initial_queue := [...initial_queue(), r]
+    end
   end
 
   def push_uri(uri) =
@@ -110,12 +121,12 @@ def request.queue(
   end
 
   def set_queue(q) =
-    queue := q
+    if started() then queue := q else initial_queue := q end
     s.set_queue([])
   end
 
   def get_queue() =
-    [...s.queue(), ...(queue())]
+    [...s.queue(), ...initial_queue(), ...queue()]
   end
 
   s =
@@ -127,7 +138,14 @@ def request.queue(
       queue=get_queue
     }
 
-  s.on_wake_up({s.set_queue(initial_queue)})
+  s.on_wake_up(
+    fun () ->
+      begin
+        started := true
+        s.set_queue(initial_queue())
+        initial_queue := []
+      end
+  )
 
   source.set_name(s, "request.queue")
   fetch := s.fetch
@@ -432,10 +450,20 @@ def request.player(~simultaneous=true) =
         }
     }
   else
-    s = source.dynamic()
+    next_source = ref(null())
+
+    def next() =
+      s = next_source()
+      next_source := null()
+      s
+    end
+
+    s = source.dynamic(next)
 
     def play(r) =
-      s.set(request.once(r))
+      r = request.once(r)
+      s.prepare(r)
+      next_source := r
     end
 
     s.{play=play, length={1}}
diff --git a/src/libs/source.liq b/src/libs/source.liq
index 24526c93ea..d5c0e87576 100644
--- a/src/libs/source.liq
+++ b/src/libs/source.liq
@@ -173,39 +173,42 @@ def append(~id=null("append"), ~insert_missing=true, ~merge=false, s, f) =
   last_meta = ref(null())
   pending = ref(null())
 
-  def f(m) =
-    if
-      m["liq_append"] == "false"
-    then
-      last_meta := null()
-      pending := null()
-    elsif
-      last_meta() != m
-    then
-      last_meta := m
-      pending := (f(m) : source?)
-    end
-  end
-
-  s = if insert_missing then source.on_track(s, f) else s end
-  s = source.on_metadata(s, f)
-
   def next() =
     p = pending()
     pending := null()
     last_meta := null()
 
+    if null.defined(p) then null.get(p) else (s : source) end
+  end
+
+  d =
+    source.dynamic(
+      track_sensitive=true, merge={merge and null.defined(pending)}, next
+    )
+
+  def f(m) =
     if
-      null.defined(p)
+      d.current_source() == (s : source)
     then
-      p = null.get(p)
-      sequence(merge=merge, [p, s])
-    else
-      null()
+      if
+        m["liq_append"] == "false"
+      then
+        last_meta := null()
+        pending := null()
+      elsif
+        last_meta() != m
+      then
+        last_meta := m
+        s = f(m)
+        d.prepare(s)
+        pending := s
+      end
     end
   end
 
-  d = source.dynamic(track_sensitive=false, next)
+  d = if insert_missing then source.on_track(d, f) else d end
+  d = source.on_metadata(d, f)
+
   s = fallback(id=id, track_sensitive=false, [d, s])
   s.{
     pending=
diff --git a/src/libs/video.liq b/src/libs/video.liq
index 9797af4d58..b81e8cd1ff 100644
--- a/src/libs/video.liq
+++ b/src/libs/video.liq
@@ -34,27 +34,24 @@ def request.image(
   ~y=getter(0),
   req
 ) =
-  s = source.dynamic()
-
   last_req = ref(null())
 
-  def set_req() =
+  def next() =
     req = (getter.get(req) : request)
 
     if
       req != last_req()
     then
+      last_req := req
       image = request.single(id=id, fallible=fallible, req)
       image = video.crop(image)
-      image = video.resize(id=id, x=x, y=y, width=width, height=height, image)
-      s.set(image)
-      last_req := req
+      video.resize(id=id, x=x, y=y, width=width, height=height, image)
+    else
+      null()
     end
   end
 
-  s.on_wake_up(set_req)
-
-  source.on_frame(s, set_req)
+  source.dynamic(id=id, track_sensitive=false, next)
 end
 
 # Generate a source from an image file.
@@ -197,34 +194,45 @@ end
 # @category Source / Video processing
 # @param s Audio source whose metadata contain cover-art.
 def video.cover(s) =
-  video = source.dynamic()
+  last_filename = ref(null())
+  fail = (source.fail() : source)
+
+  def next() =
+    m = source.last_metadata(s ?? fail) ?? []
 
-  def read_cover(m) =
     filename = m["filename"]
-    cover =
-      if file.exists(filename) then file.cover(filename) else "".{mime=""} end
 
     if
-      null.defined(cover)
+      filename != last_filename()
     then
-      cover = null.get(cover)
-      ext = if cover.mime == "image/png" then ".png" else ".jpg" end
-      f = file.temp("cover", ext)
-      log.debug(
-        "Found cover for #{filename}."
-      )
-      file.write(data=cover, f)
-      video.set(request.once(request.create(temporary=true, f)))
+      last_filename := filename
+
+      cover =
+        if file.exists(filename) then file.cover(filename) else "".{mime=""} end
+
+      if
+        null.defined(cover)
+      then
+        cover = null.get(cover)
+        ext = if cover.mime == "image/png" then ".png" else ".jpg" end
+        f = file.temp("cover", ext)
+        log.debug(
+          "Found cover for #{filename}."
+        )
+        file.write(data=cover, f)
+        request.once(request.create(temporary=true, f))
+      else
+        log.debug(
+          "No cover for #{filename}."
+        )
+        fail
+      end
     else
-      log.debug(
-        "No cover for #{filename}."
-      )
-      video.set(source.fail())
+      null()
     end
   end
 
-  s.on_track(read_cover)
-  (video : source(video=canvas))
+  source.dynamic(track_sensitive=false, next)
 end
 
 let output.youtube = ()
@@ -652,7 +660,16 @@ def video.slideshow(
   id = string.id.default(default="video.slideshow", id)
   l = ref(l)
   n = ref(-1)
-  s = source.dynamic()
+
+  next_source = ref(null())
+
+  def next() =
+    s = next_source()
+    next_source := null()
+    s
+  end
+
+  s = source.dynamic(next)
 
   def current() =
     list.nth(l(), n())
@@ -664,7 +681,9 @@ def video.slideshow(
       0 <= n' and n' < list.length(l()) and n' != n()
     then
       n := n'
-      s.set(request.once(request.create(current())))
+      new_source = request.once(request.create(current()))
+      s.prepare(new_source)
+      next_source := new_source
     end
   end