Add "topic" channel

docs/channel.md

@@ -424,6 +424,47 @@ Y
 
 See also: [fromList](#fromlist) factory method.
 
+(channel-topic)=
+
+### topic
+
+:::{versionadded} 23.11.0-edge
+:::
+
+:::{note}
+This feature requires the `nextflow.preview.topic` feature flag to be enabled.
+:::
+
+The `topic` method is used to create a queue channel that receives the items from all process outputs in a particular "topic".
+
+A process output can be assigned to a topic using the `topic` option on an output:
+
+```groovy
+process foo {
+  output:
+  val('foo'), topic: 'my-topic'
+}
+
+process bar {
+  output:
+  val('bar'), topic: 'my-topic'
+}
+```
+
+Then, the `topic` method can be used to consume all items in the topic:
+
+```groovy
+Channel.topic('my-topic').view()
+```
+
+This approach is a convenient way to collect related items from many different sources without explicitly defining the channel logic (the `topic` method is essentially an implicit {ref}`operator-mix` operation). You can name topics however you want, and you can use different names to collect items for different "topics", as long as your process outputs and channel logic are consistent with each other.
+
+:::{warning}
+Any process that consumes a channel topic should not send any outputs to that topic, or else the pipeline will hang forever.
+:::
+
+See also: {ref}`process-additional-options` for process outputs.
+
 (channel-value)=
 
 ### value

docs/config.md

@@ -1809,3 +1809,12 @@ Some features can be enabled using the `nextflow.enable` and `nextflow.preview`
 : *Experimental: may change in a future release.*
 
 : When `true`, enables process and workflow recursion. See [this GitHub discussion](https://github.com/nextflow-io/nextflow/discussions/2521) for more information.
+
+`nextflow.preview.topic`
+
+: :::{versionadded} 23.11.0-edge
+  :::
+
+: *Experimental: may change in a future release.*
+
+: When `true`, enables {ref}`channel topics <channel-topic>`.

docs/process.md

@@ -1166,62 +1166,59 @@ process foo {
 ```
 :::
 
-### Optional outputs
+(process-additional-options)=
 
-In most cases, a process is expected to produce an output for each output definition. However, there are situations where it is valid for a process to not generate output. In these cases, `optional: true` may be added to the output definition, which tells Nextflow not to fail the process if the declared output is not produced:
+### Additional options
 
-```groovy
-output:
-    path("output.txt"), optional: true
-```
+The following options are available for all process outputs:
 
-In this example, the process is normally expected to produce an `output.txt` file, but in the cases where the file is legitimately missing, the process does not fail. The output channel will only contain values for those processes that produce `output.txt`.
+`emit: <name>`
 
-(process-multiple-outputs)=
+: Defines the name of the output channel, which can be used to access the channel by name from the process output:
 
-### Multiple outputs
+  ```groovy
+  process FOO {
+      output:
+      path 'hello.txt', emit: hello
+      path 'bye.txt', emit: bye
+
+      """
+      echo "hello" > hello.txt
+      echo "bye" > bye.txt
+      """
+  }
 
-When a process declares multiple outputs, each output can be accessed by index. The following example prints the second process output (indexes start at zero):
+  workflow {
+      FOO()
+      FOO.out.hello.view()
+  }
+  ```
 
-```groovy
-process FOO {
-    output:
-    path 'bye_file.txt'
-    path 'hi_file.txt'
+  See {ref}`workflow-process-invocation` for more details.
 
-    """
-    echo "bye" > bye_file.txt
-    echo "hi" > hi_file.txt
-    """
-}
+`optional: true | false`
 
-workflow {
-    FOO()
-    FOO.out[1].view()
-}
-```
+: Normally, if a specified output is not produced by the task, the task will fail. Setting `optional: true` will cause the task to not fail, and instead emit nothing to the given output channel.
 
-You can also use the `emit` option to assign a name to each output and access them by name:
+  ```groovy
+  output:
+  path("output.txt"), optional: true
+  ```
 
-```groovy
-process FOO {
-    output:
-    path 'bye_file.txt', emit: bye_file
-    path 'hi_file.txt',  emit: hi_file
+  In this example, the process is normally expected to produce an `output.txt` file, but in the cases where the file is missing, the task will not fail. The output channel will only contain values for those tasks that produced `output.txt`.
 
-    """
-    echo "bye" > bye_file.txt
-    echo "hi" > hi_file.txt
-    """
-}
+: :::{note}
+  While this option can be used with any process output, it cannot be applied to individual elements of a [tuple](#output-type-tuple) output. The entire tuple must be optional or not optional.
+  :::
 
-workflow {
-    FOO()
-    FOO.out.hi_file.view()
-}
-```
+`topic: <name>`
+
+: :::{versionadded} 23.11.0-edge
+  :::
+
+: *Experimental: may change in a future release.*
 
-See {ref}`workflow-process-invocation` for more details.
+: Defines the {ref}`channel topic <channel-topic>` to which the output will be sent.
 
 ## When
 

docs/workflow.md

@@ -104,7 +104,7 @@ workflow {
 }
 ```
 
-When a process defines multiple output channels, each output can be accessed using the array element operator (`out[0]`, `out[1]`, etc.) or using *named outputs* (see below).
+When a process defines multiple output channels, each output can be accessed by index (`out[0]`, `out[1]`, etc.) or by name (see below).
 
 The process output(s) can also be accessed like the return value of a function:
 
@@ -144,7 +144,7 @@ workflow {
 }
 ```
 
-See {ref}`process-multiple-outputs` for more details.
+See {ref}`process outputs <process-additional-options>` for more details.
 
 ### Process named stdout
 

modules/nextflow/src/main/groovy/nextflow/Channel.groovy

@@ -121,6 +121,11 @@ class Channel  {
         return CH.queue()
     }
 
+    static DataflowWriteChannel topic(String name) {
+        if( !NF.topicChannelEnabled ) throw new MissingMethodException('topic', Channel.class, InvokerHelper.EMPTY_ARGS)
+        return CH.topic(name)
+    }
+
     /**
      * Create a empty channel i.e. only emits a STOP signal
      *

modules/nextflow/src/main/groovy/nextflow/NF.groovy

@@ -67,4 +67,8 @@ class NF {
     static boolean isRecurseEnabled() {
         NextflowMeta.instance.preview.recursion
     }
+
+    static boolean isTopicChannelEnabled() {
+        NextflowMeta.instance.preview.topic
+    }
 }

modules/nextflow/src/main/groovy/nextflow/NextflowMeta.groovy

@@ -43,6 +43,7 @@ class NextflowMeta {
         volatile float dsl
         boolean strict
         boolean recursion
+        boolean topic
 
         void setDsl( float num ) {
             if( num == 1 )
@@ -59,6 +60,12 @@ class NextflowMeta {
                 log.warn "NEXTFLOW RECURSION IS A PREVIEW FEATURE - SYNTAX AND FUNCTIONALITY CAN CHANGE IN FUTURE RELEASES"
             this.recursion = recurse
         }
+
+        void setTopic(Boolean value) {
+            if( topic )
+                log.warn "CHANNEL TOPICS ARE A PREVIEW FEATURE - SYNTAX AND FUNCTIONALITY CAN CHANGE IN FUTURE RELEASES"
+            this.topic = value
+        }
     }
 
     static class Features implements Flags {

modules/nextflow/src/main/groovy/nextflow/ast/NextflowDSLImpl.groovy

@@ -915,15 +915,16 @@ class NextflowDSLImpl implements ASTTransformation {
 
         /**
          * Transform a map entry `emit: something` into `emit: 'something'
+         * and `topic: something` into `topic: 'something'
          * (ie. as a constant) in a map expression passed as argument to
          * a method call. This allow the syntax
          *
          *   output:
-         *   path 'foo', emit: bar
+         *   path 'foo', emit: bar, topic: baz
          *
          * @param call
          */
-        protected void fixOutEmitOption(MethodCallExpression call) {
+        protected void fixOutEmitAndTopicOptions(MethodCallExpression call) {
             List<Expression> args = isTupleX(call.arguments)?.expressions
             if( !args ) return
             if( args.size()<2 && (args.size()!=1 || call.methodAsString!='_out_stdout')) return
@@ -936,6 +937,9 @@ class NextflowDSLImpl implements ASTTransformation {
                 if( key?.text == 'emit' && val ) {
                     map.mapEntryExpressions[i] = new MapEntryExpression(key, constX(val.text))
                 }
+                else if( key?.text == 'topic' && val ) {
+                    map.mapEntryExpressions[i] = new MapEntryExpression(key, constX(val.text))
+                }
             }
         }
 
@@ -955,7 +959,7 @@ class NextflowDSLImpl implements ASTTransformation {
                 // prefix the method name with the string '_out_'
                 methodCall.setMethod( new ConstantExpression('_out_' + methodName) )
                 fixMethodCall(methodCall)
-                fixOutEmitOption(methodCall)
+                fixOutEmitAndTopicOptions(methodCall)
             }
 
             else if( methodName in ['into','mode'] ) {

modules/nextflow/src/main/groovy/nextflow/extension/CH.groovy

@@ -1,5 +1,7 @@
 package nextflow.extension
 
+import static nextflow.Channel.*
+
 import groovy.transform.CompileStatic
 import groovy.transform.PackageScope
 import groovy.util.logging.Slf4j
@@ -15,8 +17,6 @@ import nextflow.Channel
 import nextflow.Global
 import nextflow.NF
 import nextflow.Session
-import static nextflow.Channel.STOP
-
 /**
  * Helper class to handle channel internal api ops
  *
@@ -30,7 +30,14 @@ class CH {
         return (Session) Global.session
     }
 
-    static private Map<DataflowQueue, DataflowBroadcast> bridges = new HashMap<>(10)
+    static class Topic {
+        DataflowBroadcast broadcaster = new DataflowBroadcast()
+        List<DataflowWriteChannel> sources = new ArrayList<>(10)
+    }
+
+    static final private Map<String, Topic> allTopics = new HashMap<>(10)
+
+    static final private Map<DataflowQueue, DataflowBroadcast> bridges = new HashMap<>(10)
 
     static DataflowReadChannel getReadChannel(channel) {
         if (channel instanceof DataflowQueue)
@@ -66,14 +73,41 @@ class CH {
     }
 
     static void broadcast() {
-        // connect all dataflow queue variables to associated broadcast channel 
+        // connect all broadcast topics, note this must be before the following
+        // "bridging" step because it can modify the final network topology
+        connectTopics()
+        // bridge together all broadcast channels
+        bridgeChannels()
+    }
+
+    static private void bridgeChannels() {
+        // connect all dataflow queue variables to associated broadcast channel
         for( DataflowQueue queue : bridges.keySet() ) {
             log.trace "Bridging dataflow queue=$queue"
             def broadcast = bridges.get(queue)
             queue.into(broadcast)
         }
     }
 
+    static private void connectTopics() {
+        for( Topic topic : allTopics.values() ) {
+            if( topic.sources ) {
+                // get the list of source channels for this topic
+                final ch = new ArrayList(topic.sources)
+                // the mix operator requires at least two sources, add an empty channel if needed
+                if( ch.size()==1 )
+                    ch.add(empty())
+                // map write channels to read channels
+                final sources = ch.collect(it -> getReadChannel(it))
+                // mix all of them 
+                new MixOp(sources).withTarget(topic.broadcaster).apply()
+            }
+            else {
+                topic.broadcaster.bind(STOP)
+            }
+        }
+    }
+
     static void init() { bridges.clear() }
 
     @PackageScope
@@ -102,6 +136,31 @@ class CH {
         return new DataflowQueue()
     }
 
+    static DataflowBroadcast topic(String name) {
+        synchronized (allTopics) {
+            def topic = allTopics[name]
+            if( topic!=null )
+                return topic.broadcaster
+            // create a new topic
+            topic = new Topic()
+            allTopics[name] = topic
+            return topic.broadcaster
+        }
+    }
+
+    static DataflowWriteChannel topicWriter(String name) {
+        synchronized (allTopics) {
+            def topic = allTopics.get(name)
+            if( topic==null ) {
+                topic = new Topic()
+                allTopics.put(name, topic)
+            }
+            final result = CH.create()
+            topic.sources.add(result)
+            return result
+        }
+    }
+
     static boolean isChannel(obj) {
         obj instanceof DataflowReadChannel || obj instanceof DataflowWriteChannel
     }