repro 1.x : Updated downstream example and added info for Sequential execution.

content/docs/command-reference/repro.md

@@ -54,8 +54,7 @@ other options.
 
 It saves all the data files, intermediate or final results into the <abbr>DVC
 cache</abbr> (unless the `--no-commit` option is used), and updates the hash
-values of changed dependencies and outputs in the DVC files (`dvc.lock` and
-`.dvc`).
+values of changed dependencies and outputs in the `dvc.lock` and `.dvc` files.
 
 ### Parallel stage execution
 
@@ -83,11 +82,12 @@ $ dvc dag
 ```
 
 This pipeline consists of two parallel branches (`A` and `B`), and the final
-"result" stage, where the branches merge. To reproduce both branches at the same
-time, you could run `dvc repro A2` and `dvc repro B2` at the same time (e.g. in
-separate terminals). After both finish successfully, you can then run
-`dvc repro train`: DVC will know that both branches are already up-to-date and
-only execute the final stage.
+`train` stage, where the branches merge. If you run `dvc repro` at this point,
+it would reproduce each branch sequentially before `train`. To reproduce both
+branches simultaneously, you could run `dvc repro A2` and `dvc repro B2` at the
+same time (e.g. in separate terminals). After both finish successfully, you can
+then run `dvc repro train`: DVC will know that both branches are already
+up-to-date and only execute the final stage.
 
 ## Options
 
@@ -151,7 +151,8 @@ only execute the final stage.
   each execution, meaning the cache cannot be trusted for such stages.
 
 - `--downstream` - only execute the stages after the given `targets` in their
-  corresponding pipelines, including the target stages themselves.
+  corresponding pipelines, including the target stages themselves. This option
+  has no effect if `targets` are not provided.
 
 - `-h`, `--help` - prints the usage/help message, and exit.
 
@@ -239,7 +240,7 @@ If we now run `dvc repro`, we should see this:
 $ dvc repro
 Stage 'filter' didn't change, skipping
 Running stage 'count' with command:
-        python3 process.py numbers.txt > count.txt
+        python process.py numbers.txt > count.txt
 Updating lock file 'dvc.lock'
 ```
 
@@ -262,31 +263,42 @@ The answer to universe is 42
 - The Hitchhiker's Guide to the  Galaxy
 ```
 
-Now, using the `--downstream` option results in the following output:
+Let's say we want to print the filename also in the description and so we update
+the `process.py` as:
+
+```python
+print(f'Number of lines in {sys.argv[1]}:')
+print(num_lines)
+```
+
+Now, using the `--downstream` option with `count` as a target stage, results in
+the following output:
 
 ```dvc
-$ dvc repro --downstream
-Data and pipelines are up to date.
+$ dvc repro --downstream count
+Running stage 'count' with command:
+        python process.py numbers.txt > count.txt
+Updating lock file 'dvc.lock'
 ```
 
-The reason being that the `text.txt` file is not a dependency in the last stage
-of the pipeline, used as the default target by `dvc repro`. `text.txt` is a
-dependency of the `filter` stage, which happens earlier (shown in the figure
-below), so it's skipped given the `--downstream` option.
+The change in the `text.txt` file is ignored because that file is a dependency
+in the `filter` stage, which did not get updated in the above command. This is
+because `filter` happens before `count` in the pipeline (shown below) and the
+`--downstream` option only execute the stages after a given target stage
+(`count` in this case), including the target stage itself.
 
 ```dvc
 $ dvc dag
-    .------------.
-    |   filter   |
-    `------------'
-           *
-           *
-           *
-      .---------.
-      |  count  |
-      `---------'
+
+  +--------+
+  | filter |
+  +--------+
+      *
+      *
+      *
+  +-------+
+  | count |
+  +-------+
 ```
 
-> Note that using `dvc repro --downstream` without a target will always have a
-> similar effect, where all previous stages are ignored — only if the last stage
-> is changed will it have any effect.
+> Refer to `dvc dag` for more details on that command.