4.5. Networking

To get a hang on the basics of sharing a dataset, you shared your DataLad-101 dataset with your room mate on a common, local file system. Your lucky room mate now has your notes and can thus try to catch up to still pass the course. Moreover, though, he can also integrate all other notes or changes you make to your dataset, and stay up to date. This is because a DataLad dataset makes updating shared data a matter of a single datalad update --how merge (manual) command.

But why does this need to be a one-way street? “I want to provide helpful information for you as well!”, says your room mate. “How could you get any insightful notes that I make in my dataset, or maybe the results of our upcoming mid-term project? It’s a bit unfair that I can get your work, but you cannot get mine.”

Consider, for example, that your room mate might have googled about DataLad a bit. In the depths of the web, he might have found useful additional information, such a script on dataset nesting. Because he found this very helpful in understanding dataset nesting concepts, he decided to download it from GitHub, and saved it in the code/ directory.

He does it using the DataLad command datalad download-url (manual) that you experienced in section Create a dataset already: This command will download a file just as wget, but it can also take a commit message and will save the download right to the history of the dataset that you specify, while recording its origin as provenance information.

Navigate into your dataset copy in mock_user/DataLad-101, and run the following command

$ # navigate into the installed copy
$ cd ../mock_user/DataLad-101

$ # download the shell script and save it in your code/ directory
$ datalad download-url \
  -d . \
  -m "Include nesting demo from datalad website" \
  -O code/nested_repos.sh \
download_url(ok): /home/me/dl-101/mock_user/DataLad-101/code/nested_repos.sh (file)
add(ok): code/nested_repos.sh (file)
save(ok): . (dataset)

Run a quick datalad status:

$ datalad status
nothing to save, working tree clean

Nice, the datalad download-url command saved this download right into the history, and datalad status (manual) does not report unsaved modifications! We’ll show an excerpt of the last commit here[1]:

$ git log -n 1 -p
commit 5b6e19a5✂SHA1
Author: Elena Piscopia <elena@example.net>
Date:   Tue Jun 18 16:13:00 2019 +0000

    Include nesting demo from datalad website

diff --git a/code/nested_repos.sh b/code/nested_repos.sh
new file mode 100644
index 0000000..f84c817
--- /dev/null
+++ b/code/nested_repos.sh
@@ -0,0 +1,59 @@

Suddenly, your room mate has a file change that you do not have. His dataset evolved.

So how do we link back from the copy of the dataset to its origin, such that your room mate’s changes can be included in your dataset? How do we let the original dataset “know” about this copy your room mate has? Do we need to install the installed dataset of our room mate as a copy again?

No, luckily, it’s simpler and less convoluted. What we have to do is to register a DataLad sibling: A reference to our room mate’s dataset in our own, original dataset.

Remote siblings

Git repositories can configure clones of a dataset as remotes in order to fetch, pull, or push from and to them. A datalad sibling (manual) is the equivalent of a git clone that is configured as a remote.

Let’s see how this is done.

First of all, navigate back into the original dataset. In the original dataset, “add” a “sibling” by using the datalad siblings (manual) command. The command takes the base command, datalad siblings, an action, in this case add, a path to the root of the dataset -d ., a name for the sibling, -s/--name roommate, and a URL or path to the sibling, --url ../mock_user/DataLad-101. This registers your room mate’s DataLad-101 as a “sibling” (we will call it “roommate”) to your own DataLad-101 dataset.

$ cd ../../DataLad-101
$ # add a sibling
$ datalad siblings add -d . \
  --name roommate --url ../mock_user/DataLad-101
.: roommate(+) [../mock_user/DataLad-101 (git)]

There are a few confusing parts about this command: For one, do not be surprised about the --url argument – it’s called “URL” but it can be a path as well. Also, do not forget to give a name to your dataset’s sibling. Without the -s/ --name argument the command will fail. The reason behind this is that the default name of a sibling if no name is given will be the host name of the specified URL, but as you provide a path and not a URL, there is no host name to take as a default.

As you can see in the command output, the addition of a sibling succeeded: roommate(+)[../mock_user/DataLad-101] means that your room mate’s dataset is now known to your own dataset as “roommate”.

$ datalad siblings
.: here(+) [git]
.: roommate(+) [../mock_user/DataLad-101 (git)]

This command will list all known siblings of the dataset. You can see it in the resulting list with the name “roommate” you have given to it.

What if I mistyped the name or want to remove the sibling?

You can remove a sibling using datalad siblings remove -s roommate

The fact that the DataLad-101 dataset now has a sibling means that we can also datalad update this repository. Awesome!

Your room mate previously ran a datalad update --how merge in the section Stay up to date. This got him changes he knew you made into a dataset that he so far did not change. This meant that nothing unexpected would happen with the datalad update --how merge.

But consider the current case: Your room mate made changes to his dataset, but you do not necessarily know which. You also made changes to your dataset in the meantime, and added a note on datalad update. How would you know that his changes and your changes are not in conflict with each other?

This scenario is where a plain datalad update becomes useful. If you run a plain datalad update (which uses the default option --how fetch), DataLad will query the sibling for changes, and store those changes in a safe place in your own dataset, but it will not yet integrate them into your dataset. This gives you a chance to see whether you actually want to have the changes your room mate made.

Let’s see how it’s done. First, run a plain datalad update without the --how merge option.

$ datalad update -s roommate
update(ok): . (dataset)

Note that we supplied the sibling’s name with the -s/--name option. This is good practice, and allows you to be precise in where you want to get updates from. It would have worked without the specification (just as a bare datalad update --how merge worked for your room mate), because there is only one other known location, though.

This plain datalad update “fetched” updates from the dataset. The changes however, are not yet visible – the script that he added is not yet in your code/ directory:

$ ls code/

So where is the file? It is in a different branch of your dataset.

If you do not use Git, the concept of a branch can be a big source of confusion. There will be sections later in this book that will elaborate a bit more what branches are, and how to work with them, but for now envision a branch just like a bunch of drawers on your desk. The paperwork that you have in front of you right on your desk is your dataset as you currently see it. These drawers instead hold documents that you are in principle working on, just not now – maybe different versions of paperwork you currently have in front of you, or maybe other files than the ones currently in front of you on your desk.

Imagine that a datalad update created a small drawer, placed all of the changed or added files from the sibling inside, and put it on your desk. You can now take a look into that drawer to see whether you want to have the changes right in front of you.

The drawer is a branch, and it is usually called remotes/origin/main. To look inside of it you can git checkout BRANCHNAME (manual), or you can do a diff between the branch (your drawer) and the dataset as it is currently in front of you (your desk). We will do the latter, and leave the former for a different lecture:

Please use ‘datalad diff –from main –to remotes/roommate/main’

Please use the following command instead:

$ datalad diff --from main --to remotes/roommate/main

This syntax specifies the main branch as a starting point for the comparison instead of the current adjusted/main(unlocked) branch.

$ datalad diff --to remotes/roommate/main
    added: code/nested_repos.sh (file)
 modified: notes.txt (file)

This shows us that there is an additional file, and it also shows us that there is a difference in notes.txt! Let’s ask git diff (manual) to show us what the differences in detail (note that it is a shortened excerpt, cut in the middle to reduce its length):

Please use ‘git diff main..remotes/roommate/main’

Please use the following command instead:

$ git diff main..remotes/roommate/main

This is Gits syntax for specifying a comparison between two branches.

$ git diff remotes/roommate/main
diff --git a/code/nested_repos.sh b/code/nested_repos.sh
deleted file mode 100644
index f84c817..0000000
--- a/code/nested_repos.sh
+++ /dev/null
@@ -1,59 +0,0 @@
-# This script was converted using cast2script from:
-# docs/casts/seamless_nested_repos.sh
-set -e -u
-export GIT_PAGER=cat
-# DataLad provides seamless management of nested Git repositories...
-# Let's create a dataset
-datalad create demo
-cd demo
diff --git a/notes.txt b/notes.txt
index 655be7d..ff02f68 100644
--- a/notes.txt
+++ b/notes.txt
@@ -59,3 +59,7 @@ The command "git annex whereis PATH" lists the repositories that have
 the file content of an annexed file. When using "datalad get" to
 retrieve file content, those repositories will be queried.

+To update a shared dataset, run the command "datalad update --how merge".
+This command will query its origin for changes, and integrate the
+changes into the dataset.

Let’s digress into what is shown here. We are comparing the current state of your dataset against the current state of your room mate’s dataset. Everything marked with a - is a change that your room mate has, but not you: This is the script that he downloaded!

Everything that is marked with a + is a change that you have, but not your room mate: It is the additional note on datalad update you made in your own dataset in the previous section.

Cool! So now that you know what the changes are that your room mate made, you can safely datalad update --how merge them to integrate them into your dataset. In technical terms you will “merge the branch remotes/roommate/main into main”. But the details of this will be stated in a standalone section later.

Note that the fact that your room mate does not have the note on datalad update does not influence your note. It will not get deleted by the merge. You do not set your dataset to the state of your room mate’s dataset, but you incorporate all changes he made – which is only the addition of the script.

$ datalad update --how merge -s roommate
merge(ok): . (dataset) [Merged roommate/main]
update.annex_merge(ok): . (dataset) [Merged annex branch]
update(ok): . (dataset)

The exciting question is now whether your room mate’s change is now also part of your own dataset. Let’s list the contents of the code/ directory and also peek into the history:

$ ls code/
$ git log --oneline
6ae8e71 Merge remote-tracking branch 'roommate/main'
4bb5d39 add note about datalad update
5b6e19a Include nesting demo from datalad website
adb4b5d add note on git annex whereis
1e73592 add note about cloning from paths and recursive datalad get

Wohoo! Here it is: The script now also exists in your own dataset. You can see the commit that your room mate made when he saved the script, and you can also see a commit that records how you merged your room mate’s dataset changes into your own dataset. The commit message of this latter commit for now might contain many words yet unknown to you if you do not use Git, but a later section will get into the details of what the meaning of “merge”, “branch”, “refs” or “main” is.

For now, you are happy to have the changes your room mate made available. This is how it should be! You helped him, and he helps you. Awesome! There actually is a wonderful word for it: Collaboration. Thus, without noticing, you have successfully collaborated for the first time using DataLad datasets.

Create a note about this, and save it.

$ cat << EOT >> notes.txt
To update from a dataset with a shared history, you need to add this
dataset as a sibling to your dataset. "Adding a sibling" means
providing DataLad with info about the location of a dataset, and a
name for it.
Afterwards, a "datalad update --how merge -s name" will integrate the
changes made to the sibling into the dataset. A safe step in between
is to do a "datalad update -s name" and checkout the changes with
"git/datalad diff" to remotes/origin/main

$ datalad save -m "Add note on adding siblings"
add(ok): notes.txt (file)
save(ok): . (dataset)