urbit/pub/docs/user/clay.md

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true	6	Filesystem handbook

Filesystem handbook

Urbit has its own revision-controlled filesystem, the %clay vane. %clay is like a simplified git, but more reactive, and also typed. Okay, this makes no sense.

The most common way to use %clay is to mount a %clay node in a Unix directory. The Urbit process will watch this directory and automatically record edits as changes, Dropbox style. The mounted directory is always at the root of your pier directory.

Commands

Note that in both commands and generators, a currently unbound case (such as a version in the future) will make the calculation block, not complete. A remote case will cause a network request. A remote, unbound case will cause a waiting subscription.

Mounting to Unix

|mount [pax=path pot=$|(~ [knot ~])]

Mount the path pax at the Unix mount point pot, the name of a subdirectory in your pier.

|mount %/pub/doc %documents

with a $PIER of /home/nixon/urbit/fintud-macrep, will mount %/pub/doc in /home/nixon/urbit/fintud-macrep/documents.

The mount point is optional; if it's not supplied, the last knot in the path (%doc) will be used.

|unmount [mon=$|(term [knot path]) ~]

Undo a mount, either by specifying the path or the mount point:

|unmount %/pub/doc
|unmount %documents

It's a good habit to also delete the Unix subtree, but Urbit doesn't do it for you.

Revision-control operations

|merge [syd=desk src=beak how=$|(~ [germ ~])]

Merge the beak src into the desk syd, with optional merge strategy how.

The src beak can be a desk (%home); a plot-desk cell ([~doznec %home]); or a plot-desk-case path (/=home=).

|merge %home-work /=home= %fine
|merge %home-work /=home=

|sync [syd=desk her=plot org=$|(~ [desk ~])]

Activate autosync from the plot her and source desk org, into the desk syd. If org is omitted, it's the same as syd:

|sync %home-local ~doznec %home
|sync %home ~doznec

Note that |merge takes a path because it needs a source case (revision), which would make no sense for |sync.

|label [syd=desk lab=term]

Label the current version of desk syd:

|unsync [syd=desk her=plot org=desk ~]

Turn off autosync. The argument needs to match the original |sync perfectly, or Urbit will become angry and confused.

Filesystem manipulation

|rm [pax=path]

Remove the leaf at pax.

|rm /===/pub/fab/nixon/hoon

Remember that folders in %clay are a consequence of the tree of leaves; there is no rmdir or mkdir.

|cp [fro=path too=path]

Copy the leaf at fro to too.

|mv [fro=path too=path]

In %clay, |mv is just a shorthand for |cp then |rm. The |rm doesn't happen unless the |cp succeeds, obviously -- it's good to be transactional.

Filesystem generators

+cat [paz=(list path)]

Produce the noun, if any, at each of these (global) paths.

+ls [pax=path ~]

Produce the list of names in the folder at pax.

Because generators aren't passed the dojo's default path, unlike the current directory in Unix, it's not possible to build an trivial +ls that's the equivalent of Unix ls. You always have to write +ls %.

A quick overview of %clay

%clay is a typed, global revision-control system. Or in other words, a typed, global referentially transparent namespace. It's difficult to understate how awesome this is.

(Actually, in Layer 4 and 5 code, you can use the Hoon .^ rune to literally dereference this namespace. And in Layer 5, a generator will even block until the resource is available.)

(Another awesome global immutable namespace is IPFS. But IPFS is distributed, whereas %clay is just decentralized. IPFS stores resources around the network in a DHT, like Freenet or Bittorrent; %clay stores resources on the publisher's server, like HTTP or git.)

Path format

As a noun, a path in %clay is a (list knot), where each segment is an @ta atom -- URL-safe text, restricted to [a z], [0 9], ., -, _ and ~. The list is a tuple terminated with a Hoon null, ~.

As an ordinary Hoon noun, [%foo %bar %baz] has this structure. But Hoon also supports the Unix path syntax: /foo/bar/baz is the same noun.

Relative paths

The Hoon path syntax is always defined relative to a default path, which is configuration state in the Hoon parser. In :dojo, this works a little like the Unix current directory.

(But note that in Unix, relative paths are expanded by the application, which can read the current directory from the environment. In Urbit, the current directory and variables are hidden by the dojo from any code it runs. The parser generates the absolute path -- more like the way a Unix shell parser unglobs *.)

Relative path syntax: % is the default path (Unix .). %% is the parent path (Unix ..). Unix does not have ..., ...., etc. But Urbit has %%%, %%%%, etc. Urbit has no local relative paths; in Unix, foo/bar is a shorthand for ./foo/bar, but in Urbit you have to write %/foo/bar.

Unix has no top-level substitution syntax, but Urbit does. If the default path is /foo/bar/baz, /=/moo means /foo/moo, and /=/moo/=/goo means /foo/moo/baz/goo. Also, instead of /=/=/zoo or /=/=/=/voo, write /==zoo or /===voo. Your fingers have enough miles on them already.

Beak

The top three knots in a %clay path are /plot/desk/case, where plot is of course an urbit; desk is a branch name; and case is a revision identity, which is either (a) a label, (b) a date, or (c) a change number. For obscure reasons, this prefix is called the beak.

Spur

The rest of the path, or spur, navigates a tree of node nouns. A node is like an inode in a Unix filesystem, but different.

An inode is either a file or a directory. A node is both a folder (which may be empty) and an optional leaf (a noun).

There is no rmdir or mkdir; an empty node is automatically pruned, and creating a node creates its path. The absence of a file-or-directory mode bit eliminates all kinds of strange corner cases, especially in merging.

Leaf

%clay is a typed filesystem, or more precisely a marked one. When we sync Unix and Urbit paths, we convert a Unix file extension (an informal specifications) into a Urbit *mark* (an executable specification)

The mark name is actually the last knot in the path. Or to put it differently: if any %clay node has a leaf, its name within its parent is its mark.

This is ridiculously confusing without examples. Suppose we have the following Unix files, with directories to match:

doc.md
doc/intro.md
doc/start.md

These become the Urbit files

%/doc/md
%/doc/intro/md
%/doc/start/md

The folder map of the %/doc node contains three entries: %md, %intro, %start. The folder of %/doc/intro and that of %/doc/start each contain one entry: %md (the mark of an atom in Markdown syntax).

Perhaps this example helps explains why %clay uses this node design. One, it's a simple index-page model for any kind of published tree. Two, this tree can expand its leaves smoothly, just by adding content: if we decided %/doc/start was not a leaf but a tree, we could just add %/doc/start/child/md.

And three, the %clay node structure syncs invertibly with an equivalent, and not unduly weird, Unix inode layout.

Mounting to Unix

The most convenient way of interacting with %clay is mounting it to Unix, and modifying it with Unix tools. The mount directory is a flat subdirectory of your Urbit pier.

When you have a live mount point, Urbit monitors it with inotify() or equivalent. (It would be neat to have a FUSE driver, but we don't.) If you shut your urbit off, it will recheck the mount point when it reloads.

Unix files beginning with ., with no extension, with an extension that doesn't map to an Urbit mark, or containing data that doesn't validate to the mark, are ignored. Depending on the extension, there may be a more or less complex conversion from the Unix length/bytestream pair to the Urbit noun.

More about desks and marks

The Hoon source code for a mark like %md is in /===/mar/md/hoon. But relative to what beak? What's in the /===?

The mark source of a leaf in %clay is always relative to its own plot, desk and case. For example, a leaf at

`/~fintud-macrep/home/31/pub/doc/hello/md`

is controlled by the mark source

`/~fintud-macrep/home/31/mar/md/hoon`

If there is no such file or it doesn't compile, the mark is effectively treated as %noun, ie, an arbitrary value.

(Note that when updating a mark, any update which shrinks the set of nouns in that mark needs to at least adapt old nouns to new. Also, mark source updates should be very slow, but aren't. They should validate all nouns against the new mark, but don't.)

What can you do with a mark? Validate an arbitrary noun; perform diffs, patches, and and conflict merges; transform to or from another mark. The %ford vane, which builds and converts nouns, can even discover and apply multi-step conversion paths.

Marks are also used to describe network messages. In this case, the mark source beak is the beak of the receiving urbit.

Desks and merging

As in any git-shaped revision control system, the core operation of the system is merging.

One of the effects of same-beak marks is that it doesn't make sense to create an empty desk. You can't populate an empty desk properly with typed files. Instead, a new desk should be merged from an existing desk -- normally the default desk, %home.

It's also generally bad style to edit directly in the desk you want to modify. Your Unix filesystem changes will appear as a stream of small, unstructured changes. You should be editing a working desk. Conventionally, to change %home, merge %home into %home-work, edit there, and merge back as a "commit." Ideally, your "commits" include modifications to a text file that acts as a changelog.

So merges are important. Again as in git, merge strategies are important. That said, if you are not doing exciting things with %clay, you can skip the strategy subsection. By default, %clay will always use the %auto meta-strategy, which will always work if you're not doing exciting things.

Merge strategies

There are seven different merge strategies. Throughout our discussion, we'll say that the merge is from Alice's desk to Bob's.

Direct strategies

A %init merge should be used iff it's the first commit to a desk. The head of Alice's desk is used as the number 1 commit to Bob's desk. Obviously, the ancestry remains intact when traversing the parentage of the commit, even though previous commits are not numbered for Bob's desk.

A %this merge means to keep what's in Bob's desk, but join the ancestry. Thus, the new commit has the head of each desk as parents, but the data is exactly what's in Bob's desk. For those following along in git, this is the 'ours' merge strategy, not the '--ours' option to the 'recursive' merge strategy. In other words, even if Alice makes a change that does not conflict with Bob, we throw it away.

A %that merge means to take what's in Alice's desk, but join the ancestry. This is the reverse of %this.

A %fine merge is a "fast-forward" merge. This succeeds iff one head is in the ancestry of the other. In this case, we use the descendant as our new head.

For %meet, %mate, and %meld merges, we first find the most recent common ancestor to use as our merge base. If we have no common ancestors, then we fail. If we have multiple most recent common ancestors, then we have a criss-cross situation, which should be handled delicately. At present, we don't handle this kind of situation, but something akin to git's 'recursive' strategy should be implemented in the future.

There's a functional inclusion ordering on %fine, %meet, %mate, and %meld such that if an earlier strategy would have succeeded, then every later strategy will produce the same result. Put another way, every earlier strategy is the same as every later strategy except with a restricted domain.

A %meet merge only succeeds if the changes from the merge base to Alice's head (hereafter, "Alice's changes") are in different files than Bob's changes. In this case, the parents are both Alice's and Bob's heads, and the data is the merge base plus Alice's changed files plus Bob's changed files.

A %mate merge attempts to merge changes to the same file when both Alice and Bob change it. If the merge is clean, we use it; otherwise, we fail. A merge between different types of changes -- for example, deleting a file vs changing it -- is always a conflict. If we succeed, the parents are both Alice's and Bob's heads, and the data is the merge base plus Alice's changed files plus Bob's changed files plus the merged files.

A %meld merge will succeed even if there are conflicts. If there are conflicts in a file, then we use the merge base's version of that file, and we produce a set of files with conflicts. The parents are both Alice's and Bob's heads, and the data is the merge base plus Alice's changed files plus Bob's changed files plus the successfully merged files plus the merge base's version of the conflicting files.

Meta-strategies

There's also a meta-strategy %auto, which is the most common. If no strategy is supplied, then %auto is assumed. %auto checks to see if Bob's desk exists, and if it doesn't we use a %init merge. Otherwise, we progressively try %fine, %meet, and %mate until one succeeds.

If none succeed, we merge Bob's desk into a scratch desk. Then, we merge Alice's desk into the scratch desk with the %meld option to force the merge. For each file in the produced set of conflicting files, we call the ++mash function for the appropriate mark, which annotates the conflicts if we know how.

Finally, we display a message to the user informing them of the scratch desk's existence, which files have annotated conflicts, and which files have unannotated conflicts. When the user has resolved the conflicts, they can merge the scratch desk back into Bob's desk. This will be a %fine merge since Bob's head is in the ancestry of the scratch desk.

Autosync

Since %clay is reactive, it has a subscription interface. Changes to the filesystem create events which code at Layers 3 or 4 (vanes or apps) can listen to.

The :hood appliance uses subscriptions to implement "autosync". When one desk is synced to another, any changes to the first desk are automatically applied to the second -- for any two desks, on any two urbits.

Autosync isn't just mirroring. The target desk might have changes of its own. We use the full merge capabilities of %clay to try to make the merge clean. If there are conflicts, it'll notify you through :talk, and ask you to resolve.

There can be complex sync flows, many of which are useful. Often, many urbits will be synced to some upstream desk that is trusted to provide updates. Sometimes, it's useful to sync two desks to each other, so that changes to one or the other are mirrored. Cyclical sync structures are normal and healthy. Also, one desk can be the target of multiple autosyncs.

Cancelling a merge

Sometimes you start a merge that isn't going to finish quickly enough to suit your taste. If you find yourself in this awkward position,

|cancel %local-desk

will get you out of it.