Author: Jonathan Moore Liles, Nils Hilbricht

# New Session Manager - API

Jonathan Moore Liles, Nils Hilbricht
version API 1.1.2
License CC-By-SA v2.5

Important | "New Session Manager" is a community version of the "Non Session Manager" by Jonathan Moore Liles, who also wrote the majority of this API document, especially the API itself. The API is the same. Any technical changes or differences in behaviour are described in API Versions and Behaviour Changes. All other changes to this document can be reviewed by accessing the git log. This document is licensed under CC-By-Sa v2.5. See LICENSE

Important | The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The "New Session Manager"-API is used by many music and audio programs in Linux distributions to allow any number of independent programs to be managed together as part of a logical session (i.e. a song). Thus, operations such as loading and saving are synchronized.

The API comprises a simple Open Sound Control (OSC) based protocol, along with some behavioral guidelines, which can easily be implemented by various applications.

This project contains a program called `nsmd` which is an implementation of the server side of the NSM API. `nsmd` can be controlled by direct OSC messages, or (more commonly) a GUI: Included in this package is the `nsm-legacy-gui`, which gets symlinked to "non-session-manager`. Another GUI is "Agordejo". Other applications exist that (partially) support the NSM API and are able to load clients, but they do not use the New-Session-Manager (or Non-Session-Manager) implementation and are therefore out of scope for this document.

However, the same server-side API can also be implemented by other programs (such as Carla), although consistency and robustness will likely suffer if non-NSM compliant clients are allowed to participate in a session.

There is no direct dependency for client implementations, as long as they can send and receive OSC. Some clients use `liblo` (the OSC library), which becomes a dependency if you choose to implement NSM-support with the provided header file `nsm.h` (`extras/nsm.h/nsm.h` in the git repository). Some clients use the provided single-file python library `pynsm` (`extras/pynsm/nsmclient.py` in the git repository) which has no dependencies outside the Python3 standard library.

The aim of this project is to thoroughly define the behavior required of clients. Often the difficulty with other session-management approaches has been not in implementing code-support for them, but in not defining rules and behaviour clearly enough.

As written above unambiguous rules are created by using RFC 2119 in this document. For the good of the user, these rules are meant to be followed and are non-negotiable. If an application does not conform to this specification it should be considered broken. Consistency across applications under session management is very important for a good user experience.

## 1. Client Behavior Under Session Management

Most graphical applications make available to the user a common set of file operations, typically presented under a File or Project menu.

These are: New, Open, Save, Save As, Close and Quit or Exit.

The following sub-sections describe how these options should behave when the application is part of an NSM session. These rules only apply when session management is active, that is, after the `announce` handshake described in the NSM OSC Protocol section. In order to provide a consistent and predictable user experience, it is critically important for applications to adhere to these guidelines.

### 1.1. File Menu

#### 1.1.1. New

This option MAY empty/reset the current file or project (possibly after user confirmation). It MUST NOT allow the user to create a new project/file in another location.

#### 1.1.2. Open

This option MUST be disabled.

The application MAY elect to implement an option called "Import into Session", which creates a copy of a file/project which is then saved at the session path provided by NSM.

#### 1.1.3. Save

This option should behave as normal, saving the current file/project as established by the NSM `open` message.

This option MUST NOT present the user with a choice of where to save the file.

#### 1.1.4. Save As

This option MUST be disabled.

The application MAY elect to implement an option called 'Export from Session', which creates a copy of the current file/project which is then saved in a user-specified location outside of the session path provided by NSM.

#### 1.1.5. Close (as distinguished from Quit or Exit)

This option MUST be disabled unless its meaning is to disconnect the application from session management.

#### 1.1.6. Quit or Exit

This option MAY behave as normal (possibly asking the user to confirm exiting), or MAY do nothing to only allow quit from the session-manager control. When the client supports :optional-gui: this option SHOULD be replaced with hiding the client’s GUI so a quit by window manager hides.

### 1.2. Data Storage

#### 1.2.1. Internal Files

All project specific data created by a client MUST be stored in the per-client storage area provided by NSM. This includes all recorded audio and MIDI files, snapshots, etc. Only global configuration items, exports, and renders of the project may be stored elsewhere (wherever the user specifies).

#### 1.2.2. External Files

Files required by the project but external to it (typically read-only data such as audio samples) SHOULD be referenced by creating a symbolic link within the assigned session area, and then referring to the symlink. This allows sessions to be archived and transported simply (e.g. with "tar -h") by tools that have no knowledge of the project formats of the various clients in the session. The symlinks thus created should, at the very least, be named after the files they refer to. Some unique component may be required to prevent collisions.

#### 1.2.3. Session Root and Session Directories

Client programs MUST NOT handle the following themselves. This section is background-information.

NSM follows the XDG Base Directory Specifications

All existing and new sessions are directories below the session-root, which defaults to `$XDG_DATA_HOME/nsm/`, which usually results in `$HOME/.local/share/nsm/`.

Each session directory contains a file `session.nsm` with one client per line `name:executable:UID\n` For example:

    JACKPatch:jackpatch:nBEIQ
    jack_mixer:jack_mixer:nTXHV
    Carla-Rack:carla-rack:nFAOD

`nsmd` loads and saves this file, client names are their self-reported names. The file format is final and frozen. Additions or changes SHALL NOT be made.

##### 1.2.3.1. Subdirectories / Hierarchical Structure

Subdirectories MAY be made to organize sessions into meaningful structures, such as album/track or composer/genre/piece. For example: `Johann Sebastian Bach/Kantaten/Wie schön leuchtet der Morgenstern`. Which results in the same directory structure on disk. Session names can contain any characters that are supported by the underlying file system, usually UTF-8.

Subdirectories are created by either `nsmd` itself or by the users themselves, through their file manager or a GUI (while the session is not open).

The project_name from `/nsm/server/new s:project_name` accepts the format `a/b/c/d`.

Any session itself MUST be a "leaf" in this directory tree. A session MUST NOT contain further session subdirectories: any directory that contains a file `session.nsm` is the final element in the hierarchy.

##### 1.2.3.2. Write-Protection for Session Templates

Write protection for a whole session directory can either happen by "accident" (files from another user, a network mount etc.) or on purpose, to protect a session template against accidental changes. The latter is possible with a recursive `chown`, `chmod` or `chattr -R +i session-dir`.

nsmd itself just checks if `session.nsm` is read-only. In this case it will not send the save command to it’s session clients. This does not prevent hypothetical problems when the user triggers a clients internal save command in a write protected directory. Clients SHOULD handle their write protected save files themselves.

Advanced contraptions, like overlay filesystems or copy-on-write hardlinks to create read-only sessions without the clients noticing, are out of scope for nsm.

##### 1.2.3.3. Lockfiles

Because multiple `nsmd` can run at the same time we need to prevent accidental write-access to the same session by different nsm-daemons, and subsequently GUIs.

Therefore each currently open session creates a lockfile under `$XDG_RUNTIME_DIR/nsm/` (usually `/run/user/XXXX/nsm/`) that tells `nsmd` to not open such a locked session. This directory gets cleaned by the operating system, preventing sessions to stay locked after e.g. a power failure.

If the system is not XDG-compliant nsmd will try to fallback to `/run/user/XXXX`, which must exist according to the Linux Filesystem Hierarchy Standard. I this also fails the user has to set $XDG_RUNTIME_DIR manually to start nsmd.

The lockfile is named after the simple session name combined with a numeric ID for the session root. It is possible that two `nsmd` opened two different session roots, both with the same simple session name, e.g. "my song". Lockfiles are able to distinguish between those and will not prevent access in this scenario. The numeric ID is a djb2 hash modulo (%) 65521 of the session root directory (see `src/file.cpp` function `simple_hash()`).

The lockfile contains, on separate lines:

- The absolute path to the session, including the root-dir, which could be overriden by `nsmd --session-root`, allowing two sessions of the same basic name in different roots.

- the OSC URL of the server that runs this session, the same as `$NSM_URL`.

- the PID of `nsmd`

Example:

    /home/johann/.local/share/nsm/cantatas/easter1751
    osc.udp://myuser.localdomain:11287/
    3022

##### 1.2.3.4. Daemon Discovery

Each running `nsmd`, per user, creates a state file under `$XDG_RUNTIME_DIR/nsm/d/` (usually `/run/user/XXXX/nsm/d/`) that can be used to look up running daemons, even if no session is loaded. The name of the file is `nsmd` PID and the files contain their daemons osc.udp URL that is compatible with the --nsm-url parameter of the GUI.

This enables you to e.g. start nsmd at boot with a random free port. Server-control programs such as GUIs can then use this to look for running servers without requiring the user to look up and input an osc URL manually as command line parameter.

## 2. NSM OSC Protocol

All message parameters are REQUIRED. All messages MUST be sent from the same socket as the `announce` message, using the `lo_send_from` method of liblo or its equivalent, as the server uses the return addresses to distinguish between clients.

Clients MUST create thier OSC servers using the same protocol (UDP,TCP) as found in `NSM_URL`. `nsmd` itself is using UDP only.

### 2.1. Establishing a Connection

#### 2.1.1. Announce

At launch, the client MUST check the environment for the value of `NSM_URL`. If present, the client MUST send the following message to the provided address as soon as it is ready to respond to the `/nsm/client/open` event:

    /nsm/server/announce s:application_name s:capabilities s:executable_name i:api_version_major i:api_version_minor i:pid

If `NSM_URL` is undefined, invalid, or unreachable, then the client should proceed assuming that session management is unavailable.

`api_version_major` and `api_version_minor` must be the two parts of the version number of the NSM API as defined by this document.

Note that if the application intends to register JACK clients, `application_name` MUST be the same as the name that would normally be passed to `jack_client_open`. For example, Non-Mixer sends "Non-Mixer" as its `application_name`. Applications MUST NOT register their JACK clients until receiving an `open` message; the `open` message will provide a unique client name prefix suitable for passing to JACK. This is probably the most complex requirement of the NSM API, but it isn’t difficult to implement, especially if the application simply wishes to delay its initialization process briefly while awaiting the `announce` reply and subsequent `open` message.

`capabilities` MUST be a string containing a colon separated list of the special capabilities the client possesses. e.g. `:dirty:switch:progress:`

`executable_name` MUST be the executable name that the program was launched with. For C programs, this is simply the value of `argv[0]`. Note that hardcoding the name of the program here is not the same as using, as the user may have launched the program from a script with a different name using exec, or have created a symlink to the program. Getting the correct value in scripting languages like Python can be more challenging.

Table 1. Available Client Capabilities
Name | Description
--- | ---
switch | client is capable of responding to multiple `open` messages without restarting
dirty | client knows when it has unsaved changes
progress | client can send progress updates during time-consuming operations
message | client can send textual status updates
optional-gui | client has an optional GUI

#### 2.1.2. Response

The server will respond to the client’s announce message with the following message:

    /reply "/nsm/server/announce" s:message s:name_of_session_manager s:capabilities

`message` is a welcome message.

The value of `name_of_session_manager` will depend on the implementation of the NSM server. It might say "New Session Manager", or it might say "Non Session Manager" etc. This is for display to the user.

`capabilities` will be a string containing a colon separated list of special server capabilities.

Presently, the server `capabilities` are:

Table 2. Available Server Capabilities
Name | Description
--- | ---
server-control | client-to-server control
broadcast | server responds to /nsm/server/broadcast message
optional-gui | server responds to optional-gui messages. This capability is always present and MUST be supported by any server implementation.

A client should not consider itself to be under session management until it receives this response. For example, the Non applications activate their "SM" blinkers at this time.

If there is an error, a reply of the following form will be sent to the client:

    /error "/nsm/server/announce" i:error_code s:error_message

The following table defines possible values of `error_code`:

Table 3. Response codes
Code | Meaning
--- | ---
ERR_GENERAL | General Error
ERR_INCOMPATIBLE_API | Incompatible API version
ERR_BLACKLISTED | Client has been blacklisted.

### 2.2. Server to Client Control Messages

Compliant clients MUST accept the client control messages described in this section. All client control messages REQUIRE a response. Responses MUST be delivered back to the sender (`nsmd`) from the same socket used by the client in its `announce` message (by using `lo_send_from`) AFTER the action has been completed or if an error is encountered. The required response is described in the subsection for each message.

If there is an error and the action cannot be completed, then `error_code` MUST be set to a valid error code (see Error Code Definitions) and `message` to a string describing the problem (suitable for display to the user).

The reply can take one of the following two forms, where path MUST be the `path` of the message being replied to (e.g. "nsm/client/save":

    /reply s:path s:message

    /error s:path i:error_code s:message

#### 2.2.1. Quit

There is no message for this. Clients will receive the Unix SIGTERM signal and MUST close cleanly IMMEDIATELY, without displaying any kind of dialog to the user and regardless of whether or not unsaved changes would be lost. When a session is closed the application will receive this signal soon after having responded to a `save` message.

#### 2.2.2. Open

    /nsm/client/open s:path_to_instance_specific_project s:display_name s:client_id

`path_to_instance_specific_project` is a path name in the form client_name.ID, assigned to the client for storing its project data. The client MUST choose one of the four strategies below to save, so that every file in the session can be traced back to a client and, vice versa, a client name.ID can be used to look up all its files. (For example to clean up the session dir)

- The client has no state and does not save at all

  - and it MUST NOT misuse e.g. ~/.config to save session specific information e.g. synth-instrument settings

- The client may use the path client_name.ID directly, resulting in a file client_name.ID in the session directory

- The client may append its native file extension (e.g. `.json`) to the path client_name.ID

- The client may use the path as directory, creating arbitrary files below, for example recorded .wav.

  - and it MUST NOT use the client ID below this point. This way the data stays transferable by hand to another client instance (in another session).

  - best case practice is to always use the same file names, for example `client_name.ID/savefile.json`

If a project exists at the path, the client MUST immediately open it.

If a project does not exist at the path, then the client MUST immediately create and open a new one at the specified path or, for clients which hold all their state in memory, store the path for later use when responding to the `save` message.

No file or directory will be created at the specified path by the server. It is up to the client to create what it needs.

For clients which HAVE NOT specified the `:switch:` capability, the `open` message will only be delivered once, immediately following the `announce` response.

For clients which HAVE specified the `:switch:` capability, the client MUST immediately switch to the specified project or create a new one if it doesn’t exist.

Clients which are incapable of switching projects or are prone to crashing upon switching MUST NOT include `:switch:` in their capability string.

If the user the is allowed to run two or more instances of the application simultaneously then such an application MUST PRE-PEND the provided `client_id` string, followed by "/", to any names it registers with common subsystems (e.g. JACK client names). This ensures that multiple instances of the same application can be restored in any order without scrambling the JACK connections or causing other conflicts.

The provided `client_id` will be a concatenation of the value of `application_name` sent by the client in its `announce` message and a unique identifier.

Therefore, applications which create single JACK clients can use the value of `client_id` directly as their JACK client name.

Applications which register multiple JACK clients (e.g. Carla or Non-Mixer) MUST PRE-PEND `client_id` value, followed by "/", to the client names they register with JACK and the application determined part MUST be unique for that (JACK) client.

For example, Carla is a plugin-host that loads each plugin as JACK client. Suitable JACK client names are: `carla-jack-multi.nBAF/ZynAddSubFx` or `carla-jack-multi.nBAF/Helm` Please note that ZynAddSubFx and Helm are not ports but clients. Each of them can have any number of audio and midi ports below them.

Note that this means that the application MUST NOT register with JACK (or any other subsystem requiring unique names) until it receives an `open` message from NSM. Likewise, applications with the `:switch:` capability should close their JACK clients and re-create them with using the new `client_id` (renaming JACK-clients is not possible, only ports).

A response is REQUIRED as soon as the open operation has been completed. Ongoing progress MAY be indicated by sending messages to `/nsm/client/progress`.

##### 2.2.2.1. Response

The client MUST respond to the 'open' message with:

    /reply "/nsm/client/open" s:message

Or

    /error "/nsm/client/open" i:error_code s:message

Table 4. Response codes
Code | Meaning
--- | ---
ERR | General Error
ERR_BAD_PROJECT | An existing project file was found to be corrupt
ERR_CREATE_FAILED | A new project could not be created
ERR_UNSAVED_CHANGES | Unsaved changes would be lost
ERR_NOT_NOW | Operation cannot be completed at this time

#### 2.2.3. Save

    /nsm/client/save

This message will only be delivered after a previous `open` message, and may be sent any number of times within the course of a session (including zero, if the user aborts the session).

##### 2.2.3.1. Response

    /reply "/nsm/client/save" s:message

Or

    /error "/nsm/client/save" i:error_code s:message

Table 5. Response codes
Code | Meaning
--- | ---
ERR | General Error
ERR_SAVE_FAILED | Project could not be saved
ERR_NOT_NOW | Operation cannot be completed at this time

### 2.3. Server to Client Informational Messages

#### 2.3.1. Session is Loaded

Accepting this message is optional. The intent is to signal to clients which may have some interdependence (say, peer to peer OSC connections) that the session is fully loaded and all their peers are available. Most clients will not need to act on this message. This message has no meaning when a session is being built or run; only when it is initially loaded. Clients who intend to act on this message MUST NOT do so by delaying initialization waiting for it.

    /nsm/client/session_is_loaded

This message does not require a response.

#### 2.3.2. Show Optional Gui

If the client has specified the `optional-gui` capability, then it may receive this message from the server when the user wishes to change the visibility state of the GUI. It doesn’t matter if the optional GUI is integrated with the program or if it is a separate program \(as is the case with SooperLooper\). When the GUI is hidden, there should be no window mapped and if the GUI is a separate program, it should be killed.

    /nsm/client/show_optional_gui

    /nsm/client/hide_optional_gui

This message does not require a response.

### 2.4. Client to Server Informational Messages

#### 2.4.1. Optional GUI

If the client has specified the `optional-gui` capability, then it MUST send this message whenever the state of visibility of the optional GUI has changed. It also MUST send this message after its announce message to indicate the initial visibility state of the optional GUI.

The client SHOULD always start hidden, if not saved as visible. That implies the first load, after adding to the session, SHOULD always be hidden.

It is the responsibility of the client to remember the visibility state of its GUI across session loads.

    /nsm/client/gui_is_hidden

    /nsm/client/gui_is_shown

No response will be delivered.

#### 2.4.2. Progress

    /nsm/client/progress f:progress

For potentially time-consuming operations, such as `save` and `open`, progress updates may be indicated throughout the duration by sending a floating point value between 0.0 and 1.0, 1.0 indicating completion, to the NSM server.

The server will not send a response to these messages, but will relay the information to the user.

Note that even when using the `progress` feature, the final response to the `save` or `open` message is still REQUIRED.

Clients which intend to send progress messages MUST include `:progress:` in their `announce` capability string.

#### 2.4.3. Dirtiness

    /nsm/client/is_dirty

    /nsm/client/is_clean

Some clients may be able to inform the server when they have unsaved changes pending. Such clients may optionally send `is_dirty` and `is_clean` messages.

Clients which have and use this capability MUST include `:dirty:` in their `announce` capability string.

#### 2.4.4. Status Messsages

    /nsm/client/message i:priority s:message

Clients may send miscellaneous status updates to the server for possible display to the user. This may simply be chatter that is normally written to the console. `priority` MUST be a number from 0 to 3, 3 being the most important.

Clients which have and use this capability MUST include `:message:` in their `announce` capability string.

### 2.5. Error Code Definitions

Table 6. Error Code Definitions
Symbolic Name | Integer Value
--- | ---
ERR_GENERAL | -1
ERR_INCOMPATIBLE_API | -2
ERR_BLACKLISTED | -3
ERR_LAUNCH_FAILED | -4
ERR_NO_SUCH_FILE | -5
ERR_NO_SESSION_OPEN | -6
ERR_UNSAVED_CHANGES | -7
ERR_NOT_NOW | -8
ERR_BAD_PROJECT | -9
ERR_CREATE_FAILED | -10

### 2.6. Client to Server Control

If the server publishes the `:server-control:` capability, then clients can also initiate action by the server. For example, a client might implement a 'Save All' option which sends a `/nsm/server/save` message to the server, rather than requiring the user to switch to the session management interface to effect the save.

### 2.7. Server Control API

The session manager not only manages clients via OSC, but it is itself controlled via OSC messages. The server responds to the following messages.

All of the following messages will be responded to, at the sender’s address, with one of the two following messages:

    /reply s:path s:message

    /error s:path i:error_code s:message

The first parameter of the reply is the path to the message being replied to. The `/error` reply includes an integer error code (non-zero indicates error). `message` will be a description of the error.

The possible errors are:

Table 7. Responses
Code | Meaning
--- | ---
ERR_GENERAL | General Error
ERR_LAUNCH_FAILED | Launch failed
ERR_NO_SUCH_FILE | No such file
ERR_NO_SESSION | No session is open
ERR_UNSAVED_CHANGES | Unsaved changes would be lost

- `/nsm/server/add s:executable_name`

  - Adds a client to the current session.

- `/nsm/server/save`

  - Saves the current session.

- `/nsm/server/open s:project_name`

  - Saves the current session and loads a new session.

- `/nsm/server/new s:project_name`

  - Saves the current session and creates a new session.

- `/nsm/server/duplicate s:new_project`

  - Saves and closes the current session, makes a copy, and opens it.

- `/nsm/server/close`

  - Saves and closes the current session.

- `/nsm/server/abort`

  - Closes the current session WITHOUT SAVING

- `/nsm/server/quit`

  - Saves and closes the current session and terminates the server.

- `/nsm/server/list`

  - Lists available projects. One `/reply` message will be sent for each existing project.

  - Afer listing the last session one final `/reply` with `/nsm/server/list, ""` will be send. That is an empty string.

#### 2.7.1. Client to Client Communication

If the server includes `:broadcast:` in its capability string, then clients may send broadcast messages to each other through the NSM server. Clients may send messages to the server at the path `/nsm/server/broadcast`.

The format of this message is as follows:

    /nsm/server/broadcast s:path [arguments...]

The message will then be relayed to all clients in the session at the path `path` (with the arguments shifted by one).

For example the message:

    /nsm/server/broadcast /tempomap/update "0,120,4/4:12351234,240,4/4"

Would broadcast the following message to all clients in the session (except for the sender), some of which might respond to the message by updating their own tempo maps.

    /tempomap/update "0,120,4/4:12351234,240,4/4"

The Non programs use this feature to establish peer to peer OSC communication by symbolic names (client IDs) without having to remember the OSC URLs of peers across sessions.

## 3. API Versions and Behaviour Changes

Here we will document all technical changes or differences in behaviour together with their API and project version numbers. The term "original" refers to Non Session Manager and "new" refers to New Session Manager.

Version numbers follow Semantic Versioning 2.0.0

Semantic Versioning Scheme

    Given a version number MAJOR.MINOR.PATCH, increment the:

    MAJOR version when you make incompatible API changes,
    MINOR version when you add functionality in a backwards compatible manner, and
    PATCH version when you make backwards compatible bug fixes.

Table 8. NSM Version Numbers
Subject | Version
--- | ---
Non Session Manager at moment of fork | 1.2 (June 2020)
Non Session Manager API | 1.0 NON nsmd.C
Original API Document | 1.0 non.tuxfamily.org/nsm/API.html
New Session Manager | 1.6.1
New Session Manager API | 1.1.2 NEW nsmd.cpp

### 3.1. Guidelines

The most important factor in decision making is to keep client compatibility at 100%. No client will ever receive an unrequested OSC message except those in API 1.0.0.

Messages that drastically change existing `/nsm/client/` or `/nsm/server` behaviour require an inrecement to `API_VERSION_MAJOR`, which we want to avoid.

`nsmd` checks if the clients `API_VERSION_MAJOR` is greater than its own and refuses the client with `ERR_INCOMPATIBLE_API`.

All changes (that concern client/server behaviour) that increment `API_VERSION_MINOR` will be request-only or gated by new capabilities (e.g. `:optional-gui:`). `nsmd` will not send any messages if a capability was not sent by the client in `announce`. This includes mostly optional features about requesting extra information.

New actions for server-control, for example a hypothetical `/nsm/server/save_as`, which would be triggered by the client and would only be answered by the server ("no unrequested message") will increment `API_VERSION_MINOR`.

All changes that increment `API_VERSION_PATCH` will not have any effect on behaviour, except to fix clear problems, where "problem" is defined by having a different effect than described in this document, which includes technical problems such as crashes.

All messages regarding GUI-communication that start with `/nsm/gui/…​` were undocumented in API 1.0.0 and only used by `non-session-manager` / `nsm-legacy-gui`. Until properly documented in this document this part of the API is considered unstable and may change at any time without notice. However, when changing already existing messages and behaviour it MAY increment `API_VERSION_MINOR` or `API_VERSION_PATCH`. In that case it will appear in the list below.

Last factor of compatibility is that any unknown message sent to `nsmd` will just print a warning message to stdout, but will otherwise be ignored. This secures a stable server, even when a client misbehaves and sends too-new messages outside of announced :capabilites:

### 3.2. Changes in API Version 1.1.0

Rewritten API document without code changes to adapt to existing code or existing client behaviour:

- Changed versioning scheme to Semantic Versioning with three positions Major.Minor.Patch

- Quit or Exit SHOULD hide instead of exiting when :optional-gui: is supported and MAY not act on the quit through menu otherwise.

- Open: Make clear that there are only certain possibilities for save paths. We added MUST because the rule was just implied before.

- Open: Make clear that the delimiter for multi-jack clients is "/".

- Optional GUI SHOULD start hidden, always after a fresh add to the session. After that saving the visibility state may override it for next time.

- Progress MUST be announced in :capabilities: . Before there was a lower case "should", which means nothing. Parallel-examples in the specs cleary say that supporting optional features must be announced first.

  - Same for Dirtiness and Status Messsages.

- Status Messsages have priority numbers between 0 and 3, so they MUST send that. It was never an arbitrary value.

Code changes:

- Server Control API: `/nsm/server/list` chain of single OSC messages, one for each session, is now finalized with sending and empty string "" as session name. Previously this was just a symbolically irrelevant console message `"Done."`

- Replies to `/nsm/server/save` etc. will now be sent back to the sender and not falsely to the last client who replied to `/nsm/client/save`. This alone would only require API_VERSION_PATCH increment, but we are already incrementing minor.

- Server Control API: `/nsm/server/add` was replying with an undocumented error code on success. Instead, as this document always specificed, it now sends `"/reply", path, "Launched."`. Again, this would have been just API_VERSION_PATCH on its own.

Undocumented (Unstable) `/nsm/gui` protocol

- Send client status after a GUI attaches to running server. This was not happening before, but it was the intention. It was just broken in nsmd.cpp. This alone would only require API_VERSION_PATCH increment, but we are already incrementing minor.

- Send label "launch error!" when a program is added (or loaded) that does not exist in $PATH. This requires no adaptation of any client, server or GUI because labels are arbitrary already and this is not meant for automatic parsing, but as user information.

- `/nsm/gui/session/name` will now always send the same parameter format, regardless of how the session was opened: simple-session-name, relative session path with subdirs below session-root.

- When a GUI announces itself to nsmd it will receive the absolute path to the session directory through the message `/nsm/gui/session/root`. This is not a new addition but was already in non-session-manager git.

### 3.3. Changes in API Version 1.1.1

- Server-capability :optional-gui: is now mandatory for SERVER implementations. Reasoning: This is an important core feature of NSM and thus will be treated as such by guaranteeing it to exist. After looking at all currently known clients and server-implementations it turns out that all servers support :optional-gui: and the vast majority of clients not only support it, but actually assume it and do not test for the server capability, as it was written in this document. There are now two choices: Adjust this document to the (good) reality or consider all clients broken. Summary: We consider this API document wrong and therefore fix it, thus increasing API version patch-level from 1.1.0 to 1.1.1

- Add API-section "Subdirectories / Hierarchical Structure" that explains the session directory. This behaviour was already the case for nsm-legacy-gui and nsmd 1.5.0 was patched to adhere to this behaviour more strictly as well, removing false session entries in 3rd party clients such as Agordejo.

### 3.4. Changes in API Version 1.1.2

- nsmd now follows the XDG Base Directory Specifications for it’s session root and lock files. This if of no consequence to clients but required documentation nevertheless, which was described as "background information" in the chapters for lock files and daemon disovery.

- nsmd now gracefully handles read-only `session.nsm` files. This theoretically enables read-only sessions and session-templates. It is included in the patch-level because this was marked as a long-standing `FIXME` in the code by the original author. Or in other words: just a bug fix.