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Protocol Documentation

The protocol is a simple transport for other APIs. It's a simple wrapper over some other protocols, like DBus, REST, and so on. At the present time this is unstable, but will need to become stable in the near future.


Each message is part of a channel, which has a id. This channel id identifies the type of data in the message payload, the host where it is headed to/from and things like the service or user credentials.

An empty/missing channel id represents a special channel called the control channel. It contains command messages. These are described below. Initially only the control channel exists. Additional channels are opened and closed via command messages.

Channels operate across all participants, and they help a forwarding layer like cockpit-ws to know where to send messages.


The channel id is a UTF-8 valid string. The channel id string comes first in the message, then a new line, then payload.

For example if you had a payload of the string 'abc', and it was being sent through the channel 'a5', that might look like:


When a message is sent over a stream transport that does not have distinct messages (such as SSH or stdio), the message is also prefixed with a base 10 integer and new line, which represents the length of the following message.

An example. When going over a stream transport with a payload of the 3 byte string 'abc', and a channel of 'a5', would have a message length of 6: 3 bytes of payload, 2 bytes for the channel number, 1 for the new line. It would look like this in a stream:


Control Messages

Control messages let the various components such as cockpit-web, cockpit-ws and cockpit-bridge communicate about what's going on.

Control messages are always sent in the control channel. They always have an empty channel. The payload of a control message is always a json object. There is always a "command" field:

    "command": <command>,
    "channel": <channel>,

If a control message pertains to a specific channel it has a "channel" field containing the id of the channel. It is invalid to have a present but empty "channel" field.

Unknown control messages are ignored. Control messages that have a channel are forwarded to the correct endpoint. Control messages without a channel are not forwarded, but only travel a single hop.

Command: init

The "init" command is the first message sent over a new transport. It is an error if any other message is received first. The transport is not considered open until the "init" message has been received.

The following fields are defined:

  • "version": The version of the protocol. Currently 1, and stable.
  • "capabilities": Optional array of strings advertizing capabilities.
  • "channel-seed": A seed to be used when generating new channel ids.
  • "host": The host being communicated with.
  • "problem": A problem occurred during init.
  • "csrf-token": The web service will send a csrf-token for external channels.
  • "os-release": The bridge sends fields from /etc/os-release which identify the system.
  • "packages": The bridge sends a list of package names on the system.

If a problem occurs that requires shutdown of a transport, then the "problem" field can be set to indicate why the shutdown will be shortly occurring.

The "init" command message may be sent multiple times across an already open transport, if certain parameters need to be renegotiated.

Command: open

The "open" command opens a new channel for payload.

The following fields are defined:

  • "binary": If present set to "raw"
  • "channel": A uniquely chosen channel id
  • "payload": A payload type, see below
  • "host": The destination host for the channel, defaults to "localhost"
  • "user": Optional alternate user for authenticating with host
  • "superuser": Optional. Use "require" to run as root, or "try" to attempt to run as root.
  • "group": An optional channel group
  • "capabilities": Optional, array of capability strings required from the bridge
  • "session": Optional, set to "private" or "shared". Defaults to "shared"
  • "flow-control": Optional boolean whether the channel should throttle itself via flow control.

If "binary" is set to "raw" then this channel transfers binary messages.

After the command is sent, then the channel is assumed to be open. No response is sent. If for some reason the channel shouldn't or cannot be opened, then the recipient will respond with a "close" message.

The channel id must not already be in use by another channel.

An example of an open:

    "command": "open",
    "channel": "a4",
    "payload": "stream",
    "host": "localhost"

This message is sent to the cockpit-bridge.

The caller can control whether to open a new bridge session or not by setting the "session" field to "private" or "shared". A "private" session will always start another bridge just for this channel. If this option is not specified then typically the channel will share an already existing session. There are certain heuristics for compatibility, where it defaults to private.

The open fields are also used with external channels. External channels are channels that have their payload sent via a separate HTTP request or another WebSocket. In this case the "channel" field must not be present, and an "external" field may be present with the following arguments:

  • "content-disposition": a Content-Disposition header for GET responses
  • "content-type": a Content-Type header for GET responses
  • "protocols": an array of possible protocols for a WebSocket

Channel "group" fields can be used to group channels into groups. You should prefix your groups with a reverse domain name so they don't conflict with other or Cockpit's group names. Group names without any punctuation are reserved.

One such special group name is "default", which contains all channels which were opened without specifying a group.

Another one is the "fence" group. While any channels are open in the "fence" group, any channels opened after that point will be blocked and wait until all channels in the "fence" group are closed before resuming.

The "flow-control" option controls whether a channel should attempt to throttle itself via flow control when sending or receiving large amounts of data. The current default (when this option is not provided) is to not do flow control. However, this default will likely change in the future.

Host values

Because the host parameter is how cockpit maps url requests to the correct bridge, cockpit may need to additional information to route the message correctly. For example you want to connect to a container my-container running on "".

To allow this the host parameter can encode a key/value pair that will be expanded in the open command json. The format is host+key+value. For example

    "command": "open",
    "channel": "a4",
    "payload": "stream",
    "host": ""

will be expanded to

    "command": "open",
    "channel": "a4",
    "payload": "stream",
    "host": "",
    "host-container": "my-container",
    "host": ""

Command: close

The "close" command closes a channel.

The following fields are defined:

  • "channel": The id of the channel to close
  • "problem": A short problem code for closure, or not present for a normal close

The channel id must be set. An example of a close:

    "command": "close",
    "channel" : "5x",
    "problem": "access-denied"

Any protocol participant can send this message. The cockpit-bridge and cockpit-ws backends will send this message when a channel closes whether because of an error or a normal closure. The frontend cockpit-web will send this when it wants to close a channel normally. Once either end has sent this message the channel is considered closed.

See below for a list of problem codes.

When a channel closing is the result of a ssh transport closing cockpit-ws reports on the authentication methods used while attempting to authenticate by adding a "auth-method-results" object to the close object. This is mainly useful to provide helpful error messages and suggestions to users.

The "auth-method-results" object contains a key for each method that cockpit-ws is able to atempt authentication with as well as the result of the atempt. For example:

    "password": "denied"

This possible "result" values are:

  • "no-server-support": The target server does not support this method.
  • "not-provided": cockpit-ws doesn't have a credential to try so this method was skipped.
  • "succeeded": Authentication with this method was successful.
  • "denied": Authentication with this method was denied.
  • "partial": The server wants more authentication.
  • "error": Unexpected error occurred when using this method.
  • "not-tried": This methods was not tried, usually due to an earlier method succeeding.

Other fields may be present in a close message.

Command: ready

The "ready" command indicates that the channel implementation (usually the bridge) is in a ready and settled state. It is not normally necessary to listen to this control message, since it is possible to start sending payload over a channel immediately after the open message has been sent.

The following fields are defined:

  • "channel": The id of the channel

Other fields may be present in a ready message.

Command: done

The "done" command indicates that no more messages will be sent on the channel in the same direction as the "done" was sent.

The following fields are defined:

  • "channel": The id of the channel

Either or both endpoints of a channel can send this message. It may only be sent once.

After it is sent no more messages may be sent in that direction. It is an error to send further messages, or send another "done" message.

Command: options

The "options" command sends further channel options on the fly. The contents of the message depends on the channel payload.

The following fields are defined:

  • "channel": The id of the channel

Command: ping

The "ping" command is simply a keep alive.

No additional fields are defined, but any are allowed. If a "channel" is set this "ping" will be forwarded. Otherwise it be limited to a single hop.

An example of a ping:

    "command": "ping",

Any protocol participant can send a "ping". It is responded to by sending a "pong" with identical options as a reply. If a "ping" is sent with a "channel" field set to a channel that is not currently open, the "ping" will be ignored as expected, and no "pong" will be sent.

Command: authorize

The "authorize" command is for communication of authentication and authorization challenges and responses between cockpit-bridge and cockpit-ws. Despite the name this is also used for authentication.

For challenge/response authentication, the following fields are defined:

  • "cookie": a unique string sent with a challenge that must be present in the corresponding response.
  • "challenge": a challenge string present in messages from a bridge to cockpit-ws
  • "response": a response string present in messages from cockpit-ws to a bridge

Example authorize challenge and response messages:

    "command": "authorize",
    "cookie": "555",
    "challenge": "crypt1:74657374:$6$2rcph,noe92ot..."

    "command": "authorize",
    "cookie": "555",
    "response": "crypt1:$6$r0oetn2039ntoen..."

Authorize messages are used during authentication by authentication commands (ei: cockpit-session, cockpit-ssh) to obtain the users credentials from cockpit-ws. An authentication command can send a authorize message with a response but no cookie. For example

    "command": "authorize",
    "response": "Basic ..."

In that case cockpit-ws will store the response and use it in a reply to a subsequent challenge.

For credential cache authorization, the following fields are defined:

  • "credential": Empty or "password"

When set to "password" then a "password" field should also be present which represents a new password to cache, which replaces any current passwords. If no password field is present then the passwords are cleared.

Command: kill

The "kill" command terminates a whole set of channels. It is sent by the frontend and processed by cockpit-ws.

The following fields are defined:

  • "host": optional string kills channels with the given host
  • "group": optional string to select only channels opened with the given "group"

If no fields are specified then all channels are terminated.

Command: logout

The "logout" command is sent by the shell to cockpit-ws. It discards credentials for the logged in user. Optionally it disconnects the user.

The following fields are defined:

  • "disconnect": if set to true then disconnect the user

Example logout message:

    "command": "logout",
    "disconnect": true

The "logout" command is broadcast to all bridge instances.

Command: hint

The "hint" command provides hints to other components about the state of things or what's going to happen next. The remainder of the fields are extensible.

Payload: null

A channel opened with this payload type will never send data, and will ignore all data it receives.

Payload: echo

A channel opened with this payload type will send back all data that it receives. It sends an "done" when it receives one.

Payload: dbus-json3

DBus messages are encoded in JSON payloads by cockpit-web, and decoded in cockpit-bridge. The 'dbus-json1' and 'dbus-json2' protocols are no longer supported.

Additional "open" command options are needed to open a channel of this type:

  • "bus": The DBus bus to connect to either "session", "system" or "none". Defaults to "system" if not present. If set to "none" you must also provide the address parameter.
  • "name": A service name of the DBus service to communicate with by default. Always omit this field if "bus" is "none". If omitted and "bus" is not "none" then a "name" field must be specified explicitly on other messages.
  • "address": A dbus supported address to connect to. This option is only used when bus is set to "none". Accepts any valid DBus address or "internal" to communicate with the internal bridge DBus connection.

The DBus bus name is started on the bus if it is not already running. If it could not be started the channel is closed with a "not-found". If the DBus connection closes, the channel closes with "disconnected".

DBus messages are encoded as JSON objects. If an unrecognized JSON object is received, then the channel is closed with a "protocol-error".

Method calls are a JSON object with a "call" field, whose value is an array, with parameters in this order: path, interface, method, in arguments.

    "call": [ "/path", "org.Interface", "Method", [ "arg0", 1, "arg2" ] ],
    "id": "cookie"

All the various parameters must be valid for their use. arguments may be null if no DBus method call body is expected. If a "type" field is specified then it is expected to be the DBus method type signature (no tuple). If a "flags" field is a string, then this includes message flags. The default flags option has the "i" flag set:

  • "i": Allow interactive authentication.

If no "name" field was specified in the "open" call, indicating which DBus service to talk to, then one must be specified here along with the "call".

If a DBus method call fails an "error" message will be sent back. An error will also be sent back in parameters or arguments in the "call" message are invalid.

An optional "id" field indicates that a response is desired, which will be sent back in a "reply" or "error" message with the same "id" field.

If an interface has been exported, then the bridge will send "call" messages in its direction for calls received for on that interface.

Method reply messages are JSON objects with a "reply" field whose value is an array, the array contains another array of out arguments, or null if the DBus reply had no body.

    "reply": [ [ "arg0", 1, 2 ] ],
    "id": "cookie"

If the call had a "type" field, then the reply will have one too containing the DBus type signature of the arguments. If a "flags" field was present on the call, then "flags" will also be present on the reply. Valid out flags are:

  • ">": Big endian message
  • "<": Little endian message

An error message is JSON object with an "error" field whose value is an array. The array contains: error name, error arguments

    "error": [ "org.Error", [ "Usually a message" ] ]
"id": "cookie"

To receive signals you must subscribe to them. This is done by sending a "add-match" message. It contains various fields to match on. If a field is missing then it is treated as a wildcard.

"path" limits the signals to those sent by that DBus object path, it must be a valid object path according to the specification. "path_namespace" limits signals to the subtree of DBus object paths. It must be a valid object path according to the specification, and may not be specified together with "path".

"interface" limits the signals to those sent on the given DBus interface, and must be a valid interface name. "member" limits the signal to those of that signal name. "arg0" limits the signals to those that have the given string as their first argument. If any of the values are not valid according to the dbus specification, the channel will close with a "protocol-error".

    "add-match": {
        "name": "org.the.Name",
        "path": "/the/path",
        "interface": "org.Interface",
        "member": "SignalName",
        "arg0": "first argument",

If the "name" field is omitted, it will be populated from the "open" message. If no "name" was specified in the "open" message, then DBus messages from any bus name will be matched.

To unsubscribe from DBus signals, use the "remove-match" message. If a match was added more than once, it must be removed the same number of times before the signals are actually unsubscribed.

The form of "remove-match" is identical to "add-match".

    "remove-match": {
        "name": "org.the.Name",
        "path": "/the/path",
        "interface": "org.Interface",
        "member": "SignalName",
        "arg0": "first argument",

Signals are sent in JSON objects that have a "signal" field, which is an array of parameters: path, interface, signal name, and arguments. arguments may be null if the DBus signal had no body.

    "signal": [ "/the/path", "org.Interface", "SignalName", [ "arg0", 1, 2 ] ]

If a signal message is sent to the bridge, the signal will be emitted. In addition a "destination" field may be present to indicate whether the signal should be broadcast or not.

    "signal": [ "/the/path", "org.Interface", "SignalName", [ "arg0", 1, 2 ] ]

If the bus name of the sender of the signal does not match the "name" field of the "open" message, then a "name" field will be included with the "signal" message.

Properties can be watched with the "watch" request. Either a "path" or "path_namespace" can be watched. Property changes are listened for with DBus PropertiesChanged signals. If a "path_namespace" is watched and path is a DBus ObjectManager, then it is used to watch for new DBus interfaces, otherwise DBus introspection is used. The "id" field is optional, if present a "reply" will be sent with this same "id" when the watch has sent "notify" messages about the things being watched.

    "watch": {
        "name": "org.the.Name",
    "path": "/the/path/to/watch",
        "interface": org.Interface
"id": 5

To remove a watch, pass the identical parameters with an "unwatch" request.

    "unwatch": {
        "path": "/the/path/to/watch"

If the "name" field is omitted, it will be populated from the "open" message. Either a "name" field must be specified here or in the "open" message.

Property changes will be sent using a "notify" message. This includes addition of interfaces without properties, which will be an empty interface object, or interfaces removed, which will be null. Only the changes since the last "notify" message will be sent.

"notify": {
        "/a/path": {
            "org.Interface1": {
                "Prop1": x,
                "Prop2": y
	"org.Interface2": { }
        "/another/path": {
            "org.Removed": null

If the bus name of the sender of the signal does not match the "name" field of the "open" message, then a "name" field will be included with the "notify" message.

Interface introspection data is sent using "meta" message. Before the first time an interface is sent using a "notify" message, a "meta" will be sent with that interface introspection info. Additional fields will be defined here, but this is it for now.

    "meta": {
        "org.Interface": {
            "methods": {
                "Method1": { },
                "Method2": { }
            "properties": {
                "Prop1": { "flags": "rw" },
                "Prop2": { "flags": "r" }

If the bus name of the sender of the signal does not match the "name" field of the "open" message, then a "name" field will be included with the "meta" message. Such meta information can also be sent to the bridge, in order to populate the cache of introspection data for the channel.

When the owner of the DBus "name" (specified in the open message) changes an "owner" message is sent. The owner value will be the id of the owner or null if the name is unowned.

    "owner": "1:"

A "publish" message can be used to export DBus interfaces on the bus. The bridge will then send "call" messages back to the frontend for each method invocation on the published interface and object path. The interface must first be described with DBus meta information. If a cookie is specified then a reply will be sent when the interface is published. If the interface is already published at the given path, it will be replaced.

{ "publish": [ "/a/path", "org.Interface" ], "id": "cookie" }

An "unpublish" message will unexport a DBus interface on the bus. It is not an error if no such interface has been published.

{ "unpublish": [ "/a/path", "org.Interface" ], }

DBus types are encoded in various places in these messages, such as the arguments. These types are encoded as follows:

  • byte, int16, uint16, int32, uint32, int64, uint64, double: encoded as a JSON number

  • boolean: encoded as a JSON boolean

  • string, object-path, signature: encoded as JSON strings

  • array of bytes: encoded as a base64 string

  • other arrays, struct/tuple: encoded as JSON arrays

  • dict: encoded as a JSON object, if the dict has string keys then those are used as property names directly, otherwise the keys are JSON encoded and the resulting string is used as a property name.

  • variant: encoded as a JSON object with a "v" field containing a value and a "t" field containing a DBus type signature.

    { "v": "value", "t": "s" }

Payload: http-stream2

This channel replesents a single HTTP request and response. The request HTTP headers are sent in the "open" message, and response headers are returned in a "response" control message.

The remaining messages make up the HTTP request and response bodies. When the channel is in text mode, the response body is automatically coerced to UTF-8.

The following HTTP methods are not permitted:


The following headers are not permitted on requests, some of these are added automatically as appropriate, and others are stripped from the response, in particular 'Content-Length':

  • 'Accept-Charset'
  • 'Accept-Ranges'
  • 'Connection'
  • 'Content-Length'
  • 'Content-MD5'
  • 'Content-Range'
  • 'Range'
  • 'TE'
  • 'Trailer'
  • 'Transfer-Encoding'
  • 'Upgrade'

The following are not accepted on non-binary channels:

  • 'Accept-Encoding'
  • 'Content-Encoding'

Additional "open" command options are needed to open a channel of this payload type:

  • "unix": Open a channel with the given unix socket.
  • "port": Open a channel with the given TCP port on localhost.
  • "address": Open a channel that communicates with the given address instead of localhost. This can be an IP address or a valid host name. To use this option you must also specify a port. This option should be used to communicate with external apis and not as a synonym for the host parameter.

You may also specify these options:

  • "connection": A stable connection identifier.
  • "tls": Set to a object to use an https connection.

The TLS object can have the following options:

  • "certificate": The client certificate to use, represented as an object described below.
  • "key": The client key to use, described below.
  • "authority": Certificate authority(s) to expect as signers of peer.
  • "validate": Validate the peer's certificate.

The "certificate", "key" and "authority" are objects with either of the following fields:

  • "file": String with a file name
  • "data": String with PEM encoded data

Any data to be sent should be sent via the channel, and then the channel should be closed without a problem.

Payload: websocket-stream1

This channel payload implements a WebSocket client. The data in the channel is the message frames.

Most of same options for http-stream2 apply here.

The response headers are send back in a "response" control message.

Payload: stream

Raw data is sent back and forth to a socket. See cockpitstream.c. The boundaries of the messages are arbitrary, and depend on how the kernel and socket buffer things.

If the channel is not binary, then non-UTF-8 data is forced into UTF-8 with a replacement character.

Additional "open" command options should be specified with a channel of this payload type:

  • "unix": Open a channel with the given unix socket.
  • "batch": Batches data coming from the stream in blocks of at least this size. This is not a guarantee. After a short timeout the data will be sent even if the data doesn't match the batch size. Defaults to zero.
  • "latency": The timeout for flushing any cached data in milliseconds.
  • "spawn": Spawn a process and connect standard input and standard output to the channel. Should be an array of strings which is the process file path and arguments.
  • "internal": Open an internally defined stream.
    • "packages": A http-stream for serving package resources to cockpit-ws
    • "ssh-agent": A connection to the session user's ssh-agent, if one is available.

You can't specify both "unix" and "spawn" together. When "spawn" is set the following options can be specified:

  • "directory": The directory to spawn the process in.
  • "err": If "spawn" is set, and "err" is set to "out", then stderr is included in the payload data. If "err" is set to "ignore" then, the stderr output will be discarded. If "err" is set to "message" then it will be included in the close message. If "pty" is set then stderr is always included.
  • "environ": This is a list of additional environment variables for the new spawned process. The variables are in the form of "NAME=VALUE". The default environment is inherited from cockpit-bridge.
  • "pty": Execute the command as a terminal pty.
  • "window": An object containing "rows" and "cols" properties, which set the size of the terminal window. Values must be integers between 0 and 0xffff. This option is only valid if "pty" is true.

If an "done" is sent to the bridge on this channel, then the socket and/or pipe input is shutdown. The channel will send an "done" when the output of the socket or pipe is done.

If "spawn" is set and the channel is closed with a "problem" code, then the process will be sent a SIGTERM signal.

Additionally, a "options" control message may be sent in this channel to change the "batch", "latency", and "window" options.

Payload: fswatch1

You will get a stream of change notifications for a file or a directory.

The following options can be specified in the "open" control message:

  • "path": The path name to watch. This should be an absolute path to a file or directory.

Each message on the stream will be a JSON object with the following fields:

  • "event": A string describing the kind of change. One of "changed", "deleted", "created", "attribute-changed", "moved", or "done-hint".

  • "path": The absolute path name of the file that has changed.

  • "other": The absolute path name of the other file in case of a "moved" event.

  • "type": If the event was created this contains the type of the new file. Will be one of: file, directory, link, special or unknown.

In case of an error, the channel will be closed. In addition to the usual "problem" field, the "close" control message sent by the server might have the following additional fields:

  • "message": A string in the current locale describing the error.

Payload: fslist1

A channel of this type lists the files in a directory and will watch for further changes.

The following options can be specified in the "open" control message:

  • "path": The path name of the directory to watch. This should be an absolute path.
  • "watch": Boolean, when true the directory will be watched and signal on changes.

The channel will send a number of JSON messages that list the current content of the directory. These messages have a "event" field with value "present", a "path" field that holds the (relative) name of the file and a type field. Type will be one of: file, directory, link, special or unknown. After all files have been listed the "ready" control message will be sent.

Other messages on the stream signal changes to the directory, in the same format as used by the "fswatch1" payload type.

In case of an error, the channel will be closed. In addition to the usual "problem" field, the "close" control message sent by the server might have the following additional fields:

  • "message": A string in the current locale describing the error.

Payload: fsread1

Returns the contents of a file and its current 'transaction tag'.

The following options can be specified in the "open" control message:

  • "path": The path name of the file to read.

The channel will return the content of the file in one or more messages. As with "stream", the boundaries of the messages are arbitrary.

If the file is modified while you are reading it, the channel is closed with a "change-conflict" problem code. If the file is atomically replaced as with 'rename' when you are reading it, this is not considered an error and you will get the old content with a correct tag.

When all content has been sent or an error has occurred, the channel will be closed. In addition to the usual "problem" field, the "close" control message sent by the server might have the following additional fields:

  • "message": A string in the current locale describing the error.

  • "tag": The transaction tag for the returned file content. The tag for a non-existing file is "-".

It is not permitted to send data in an fslist1 channel. This channel sends a "done" when all file data was sent.

Payload: fsreplace1

Replace the content of a file.

The following options can be specified in the "open" control message:

  • "path": The path name of the file to replace.

  • "tag": The expected transaction tag of the file. When the actual transaction tag of the file is different, the write will fail. If you don't set this field, the actual tag will not be checked. To express that you expect the file to not exist, use "-" as the tag.

You should write the new content to the channel as one or more messages. To indicate the end of the content, send a "done" message.

If you don't send any content messages before sending "done", the file will be removed. To create an empty file, send at least one content message of length zero.

When the file does not have the expected tag, the channel will be closed with a "change-conflict" problem code.

The new content will be written to a temporary file and the old content will be replaced with a "rename" syscall when the channel is closed without problem code. If the channel is closed with a problem code (by either client or server), the file will be left untouched.

In addition to the usual "problem" field, the "close" control message sent by the server might have the following additional fields:

  • "message": A string in the current locale describing the error.

  • "tag": The transaction tag of the new content.

No payload messages will be sent by this channel.

Payload: metrics1

With this payload type, you can monitor a large number of metrics, such as all of the metrics exposed by PCP, the Performance Copilot.

In addition to monitoring live values of the metrics, you can also access archives of previously recorded values.

You specify which metrics to monitor in the options of the "open" command for a new channel.

One of the options is the "source" of the metrics. Many details of how to specify metrics and how the channel will work in detail depend on the type of the source, and are explained in specific sections below.

The channel guarantees that each sample returned by it contains values for all requested metrics.

The general open options are:

  • "source" (string): The source of the metrics:

    • "direct": PCP metrics from plugins that are loaded into the Cockpit bridge directly. Use this when in doubt.

    • "pcmd": PCP metrics from the local PCP daemon.

    • A string starting with "/": PCP metrics from one or more archives.

      The string is either the name of an archive, or the name of a directory. When it refers to a directory, the bridge will find all archives in that directory and merge them. The archives must not overlap in time.

    • "pcp-archive": PCP metrics from the default pmlogger archive.

      This is the same as using the name of the default pmlogger archive directory directly, but you don't have to know where it is.

  • "metrics" (array): Descriptions of the metrics to use. See below.

  • "instances" (array of strings, optional): When specified, only the listed instances are included in the reported samples.

  • "omit-instances" (array of strings, optional): When specified, the listed instances are omitted from the reported samples. Only one of "instances" and "omit-instances" can be specified.

  • "interval" (number, optional): The sample interval in milliseconds. Defaults to 1000.

  • "timestamp" (number, optional): The desired time of the first sample. This is only used when accessing archives of samples.

    This is either the number of milliseconds since the epoch, or (when negative) the number of milliseconds in the past.

    The first sample will be from a time not earlier than this timestamp, but it might be from a much later time.

  • "limit" (number, optional): The number of samples to return. This is only used when accessing an archive.

    When no "limit" is specified, all samples until the end of the archive are delivered.

You specify the desired metrics as an array of objects, where each object describes one metric. For example:

[ { name: "kernel.all.cpu.user",
    units: "millisec",
    derive: "rate"

A metric description can contain the following fields:

  • "name" (string): The name of the metric. Available metrics depend on the source.

  • "units" (string, optional): The units that values for this metric should be delivered in. If the given metric can not be converted into these units, the channel is closed. The format of the string depends on the source.

  • "derive" (string, optional): Optional computation. Possible values are "delta" and "rate". For "delta", the channel delivers the difference between the current and the previous value for a metric. For "rate", the channel delivers the change per millisecond of the metric.

    For both "delta" and "rate", the value for a metric will be "false" if there is no previous value to do the computation with.

Once the channel is open, it will send messages encoded as JSON. It will send two types of message: 'meta' messages that describe the metrics, and 'data' messages with the actual samples.

A 'meta' message applies to all following 'data' messages until the next 'meta' message. The first message in a channel will always be a 'meta' message.

The 'meta' messages are JSON objects; the 'data' messages are JSON arrays.

The 'meta' messages have at least the following fields:

  • "metrics" (array): This field provides information about the requested metrics. The array has one object for each metric, in the same order as the "metrics" option used when opening the channel.

    For each metric, the corresponding object contains at least the following field:

    • "instances" (array of strings, optional): This field lists the instances for instanced metrics. This field is not present for non-instanced metrics.

    • "units" (string): The units of the values for this metric.

    • "derive" (string): The post-processing mode, as specified in the "open" message.

  • "timestamp" (number): The point in time of the next 'data' message, in milliseconds since the epoch.

  • "interval" (number): The time interval between subsequent points in time in the 'data' messages, in milliseconds.

Depending on the source, more fields might be present in a 'meta' message, and more fields might be present in the objects of the "metrics" field.

The 'data' messages are nested arrays in this shape:

[  // first point in time
      // first metric (instanced, with two instances)
         // first instance
         // second instance
      // second metric (not instanced)
      // third metric
   // next point in time
      // same shape again as for the first point in time

Thus, a 'data' message contains data for one or more points in time where samples have been taken. A point in time is always one "interval" later than the previous point in time, even when they are reported in the same 'data' message.

For real time monitoring, you will generally only receive one point in time per 'data' message, but when accessing archived data, the channel might report multiple points in time in one message, to improve efficiency.

For each point in time, there is an array with samples for each metric, in the same order as the "metrics" option used when opening the channel.

For non-instanced metrics, the array contains the value of the metric. For instanced metrics, the array contains another array with samples for each instance, in the same order as reported in the "instances" field of the most recent 'meta' message.

In order to gain efficiency, 'data' messages are usually compressed. This is done by only transmitting the differences from one point in time to the next.

If a value for a metric or a instance is the same as at the previous point in time, the channel transmits a "null" value instead. Additionally, "null" values at the end of an array are suppressed by transmitting a shorter array.

For example, say the samples for three points in time are

1: [ 21354, [ 5,  5, 5 ], 100 ]
2: [ 21354, [ 5, 15, 5 ], 100 ]
3: [ 21354, [ 5, 15, 5 ], 100 ]

then the channel will send these array instead:

1: [ 21354, [    5,  5, 5 ], 100 ]
2: [  null, [ null, 15 ] ]
3: [  null, [ ] ]

This compression only happens when the last and current value belong to the same instance of the same metric. Thus, the client does not need to track layout changes when decompressing data messages.

Instead of a number of "null", a data message can also contain "false". This indicates an error of some kind, or an unavailable value.

PCP metric source

Cou can use "pminfo -L" to get a list of available PCP metric names for a "direct" source, for example. If no metric of this name exists, the channel is closed without delivering any message.

The format of the "units" member is the same as the one used by "pminfo -d".

The metric information objects in the 'meta' messages for PCP sources also contain these fields:

  • "semantics" (string): The semantics of this metric, one of "counter", "instant", or "discrete".

Only numeric metrics are currently supported. Non-numeric metrics have all their samples set to "false".

Problem codes

These are problem codes for errors that cockpit-web responds to. They should be self explanatory. It's totally not interesting to arbitrarily invent new codes. Instead the web needs to be ready to react to these problems. When in doubt use "internal-error".

  • "internal-error"
  • "no-cockpit"
  • "no-session"
  • "access-denied"
  • "authentication-failed"
  • "not-found"
  • "terminated"
  • "timeout"
  • "unknown-hostkey"
  • "no-forwarding"