doc/external-dependency-solver-protocol.txt

   1 # APT External Dependency Solver Protocol (EDSP) - version 0.4
   2
   3 This document describes the communication protocol between APT and
   4 external dependency solvers. The protocol is called APT EDSP, for "APT
   5 External Dependency Solver Protocol".
   6
   7
   8 ## Components
   9
  10 - **APT**: we know this one.
  11 - APT is equipped with its own **internal solver** for dependencies,
  12   which is identified by the string `internal`.
  13 - **External solver**: an *external* software component able to resolve
  14   dependencies on behalf of APT.
  15
  16 At each interaction with APT, a single solver is in use.  When there is
  17 a total of 2 or more solvers, internals or externals, the user can
  18 choose which one to use.
  19
  20 Each solver is identified by an unique string, the **solver
  21 name**. Solver names must be formed using only alphanumeric ASCII
  22 characters, dashes, and underscores; solver names must start with a
  23 lowercase ASCII letter. The special name `internal` denotes APT's
  24 internal solver, is reserved, and cannot be used by external solvers.
  25
  26
  27 ## Installation
  28
  29 Each external solver is installed as a file under Dir::Bin::Solvers (see
  30 below), which defaults to `/usr/lib/apt/solvers`. We will assume in the
  31 remainder of this section that such a default value is in effect.
  32
  33 The naming scheme is `/usr/lib/apt/solvers/NAME`, where `NAME` is the
  34 name of the external solver.
  35
  36 Each file under `/usr/lib/apt/solvers` corresponding to an external
  37 solver must be executable.
  38
  39 No non-solver files must be installed under `/usr/lib/apt/solvers`, so
  40 that an index of available external solvers can be obtained by listing
  41 the content of that directory.
  42
  43
  44 ## Configuration
  45
  46 Several APT options can be used to affect dependency solving in APT. An
  47 overview of them is given below. Please refer to proper APT
  48 configuration documentation for more, and more up to date, information.
  49
  50 - **APT::Solver**: the name of the solver to be used for
  51   dependency solving. Defaults to `internal`
  52
  53 - **APT::Solver::Strict-Pinning**: whether pinning must be strictly
  54   respected (as the internal solver does) or can be slightly deviated
  55   from. Defaults to `yes`.
  56
  57 - **APT::Solver::NAME::Preferences** (where NAME is a solver name):
  58   solver-specific user preference string used during dependency solving,
  59   when the solver NAME is in use. Check solver-specific documentation
  60   for what is supported here. Defaults to the empty string.
  61
  62 - **Dir::Bin::Solvers**: absolute path of the directory where to look for
  63   external solvers. Defaults to `/usr/lib/apt/solvers`.
  64
  65 ## Protocol
  66
  67 When configured to use an external solver, APT will resort to it to
  68 decide which packages should be installed or removed.
  69
  70 The interaction happens **in batch**: APT will invoke the external
  71 solver passing the current status of installed and available packages,
  72 as well as the user request to alter the set of installed packages. The
  73 external solver will compute a new complete set of installed packages
  74 and gives APT a "diff" listing of which *additional* packages should be
  75 installed and of which currently installed packages should be
  76 *removed*. (Note: the order in which those actions have to be performed
  77 will be up to APT to decide.)
  78
  79 External solvers are invoked by executing them. Communications happens
  80 via the file descriptors: **stdin** (standard input) and **stdout**
  81 (standard output). stderr is not used by the EDSP protocol. Solvers can
  82 therefore use stderr to dump debugging information that could be
  83 inspected separately.
  84
  85 After invocation, the protocol passes through a sequence of phases:
  86
  87 1. APT invokes the external solver
  88 2. APT send to the solver a dependency solving **scenario**
  89 3. The solver solves dependencies. During this phase the solver may
  90    send, repeatedly, **progress** information to APT.
  91 4. The solver sends back to APT an **answer**, i.e. either a *solution*
  92    or an *error* report.
  93 5. The external solver exits
  94
  95
  96 ### Scenario
  97
  98 A scenario is a text file encoded in a format very similar to the "Deb
  99 822" format (AKA "the format used by Debian `Packages` files"). A
 100 scenario consists of two distinct parts: a **request** and a **package
 101 universe**, occurring in that order. The request consists of a single
 102 Deb 822 stanza, while the package universe consists of several such
 103 stanzas. All stanzas occurring in a scenario are separated by an empty
 104 line.
 105
 106
 107 #### Request
 108
 109 Within a dependency solving scenario, a request represents the action on
 110 installed packages requested by the user.
 111
 112 A request is a single Deb 822 stanza opened by a mandatory Request field
 113 and followed by a mixture of action and preference fields.
 114
 115 The value of the **Request:** field is a string describing the EDSP
 116 protocol which will be used to communicate. At present, the string must
 117 be `EDSP 0.4`.
 118
 119 a unique request identifier, such as an
 120 UUID. Request fields are mainly used to identify the beginning of a
 121 request stanza; their actual values are otherwise not used by the EDSP
 122 protocol.
 123
 124 The following **action fields** are supported in request stanzas:
 125
 126 - **Install:** (optional, defaults to the empty string) A space
 127   separated list of package names, with *no version attached*, to
 128   install. This field denotes a list of packages that the user wants to
 129   install, usually via an APT `install` request.
 130
 131 - **Remove:** (optional, defaults to the empty string) Same syntax of
 132   Install. This field denotes a list of packages that the user wants to
 133   remove, usually via APT `remove` or `purge` requests.
 134
 135 - **Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
 136   `no`. When set to `yes`, an upgrade of all installed packages has been
 137   requested, usually via an APT `upgrade` request.
 138
 139 - **Dist-Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
 140   `no`. Same as Upgrade, but for APT `dist-upgrade` requests.
 141
 142 - **Autoremove:** (optional, defaults to `no`). Allowed values: `yes`,
 143   `no`. When set to `yes`, a clean up of unused automatically installed
 144   packages has been requested, usually via an APT `autoremove` request.
 145
 146 The following **preference fields** are supported in request stanzas:
 147
 148 - **Strict-Pinning:** (optional, defaults to `yes`). Allowed values:
 149   `yes`, `no`. When set to `yes`, APT pinning is strict, in the sense
 150   that the solver must not propose to install packages which are not APT
 151   candidates (see the `APT-Pin` and `APT-Candidate` fields in the
 152   package universe). When set to `no`, the solver does only a best
 153   effort attempt to install APT candidates. Usually, the value of this
 154   field comes from the `APT::Solver::Strict-Pinning` configuration
 155   option.
 156
 157 - **Preferences:** a solver-specific optimization string, usually coming
 158   from the `APT::Solver::Preferences` configuration option.
 159
 160
 161 #### Package universe
 162
 163 A package universe is a list of Deb 822 stanzas, one per package, called
 164 **package stanzas**. Each package stanzas starts with a Package
 165 field. The following fields are supported in package stanzas:
 166
 167 - All fields contained in the dpkg database, with the exception of
 168   fields marked as "internal" (see the manpage `dpkg-query (1)`). Among
 169   those fields, the following are mandatory for all package stanzas:
 170   Package, Version, Architecture.
 171
 172   It is recommended not to pass the Description field to external
 173   solvers or, alternatively, to trim it to the short description only.
 174
 175 - **Installed:** (optional, defaults to `no`). Allowed values: `yes`,
 176   `no`. When set to `yes`, the corresponding package is currently
 177   installed.
 178
 179   Note: the Status field present in the dpkg database must not be passed
 180   to the external solver, as it's an internal dpkg field. Installed and
 181   other fields permit to encode the most relevant aspects of Status in
 182   communications with solvers.
 183
 184 - **Hold:** (optional, defaults to `no`). Allowed values: `yes`,
 185   `no`. When set to `yes`, the corresponding package is marked as "on
 186   hold" by dpkg.
 187
 188 - **APT-ID:** (mandatory). Unique package identifier, according to APT.
 189
 190 - **APT-Pin:** (mandatory). Must be an integer. Package pin value,
 191   according to APT policy.
 192
 193 - **APT-Candidate:** (optional, defaults to `no`). Allowed values:
 194   `yes`, `no`. When set to `yes`, the corresponding package is the APT
 195   candidate for installation among all available packages with the same
 196   name.
 197
 198 - **APT-Automatic:** (optional, defaults to `no`). Allowed values:
 199   `yes`, `no`. When set to `yes`, the corresponding package is marked by
 200   APT as automatic installed. Note that automatic installed packages
 201   should be removed by the solver only when the Autoremove action is
 202   requested (see Request section).
 203
 204 ### Answer
 205
 206 An answer from the external solver to APT is either a *solution* or an
 207 *error*.
 208
 209 The following invariant on **exit codes** must hold true. When the
 210 external solver is *able to find a solution*, it will write the solution
 211 to standard output and then exit with an exit code of 0. When the
 212 external solver is *unable to find a solution* (and s aware of that), it
 213 will write an error to standard output and then exit with an exit code
 214 of 0. An exit code other than 0 will be interpreted as a solver crash
 215 with no meaningful error about dependency resolution to convey to the
 216 user.
 217
 218
 219 #### Solution
 220
 221 A solution is a list of Deb 822 stanzas. Each of them could be an
 222 install stanza (telling APT to install a specific package), a remove
 223 stanza (telling APT to remove one), or an autoremove stanza (telling APT
 224 about the *future* possibility of removing a package using the
 225 Autoremove action).
 226
 227 An **install stanza** starts with an Install field and supports the
 228 following fields:
 229
 230 - **Install:** (mandatory). The value is a package identifier,
 231   referencing one of the package stanzas of the package universe via its
 232   APT-ID field.
 233
 234 - All fields supported by package stanzas.
 235
 236 **Remove stanzas** are similar to install stanzas, but have **Remove**
 237 fields instead of Install fields.
 238
 239 **Autoremove stanzas** are similar to install stanzas, but have
 240 **Autoremove** fields instead of Install fields. Autoremove stanzas
 241 should be output so that APT can inform the user of which packages they
 242 can now autoremove, as a consequence of the executed action. However,
 243 this protocol makes no assumption on the fact that a subsequent
 244 invocation of an Autoremove action will actually remove the very same
 245 packages indicated by Autoremove stanzas in the former solution.
 246
 247 In terms of expressivity, install and remove stanzas can carry one
 248 single field each, as APT-IDs are enough to pinpoint packages to be
 249 installed/removed. Nonetheless, for protocol readability, it is
 250 recommended that solvers either add unconditionally the fields Package,
 251 Version, and Architecture to all install/remove stanzas or,
 252 alternatively, that they support a `--verbose` command line flag that
 253 explicitly enables the output of those fields in solutions.
 254
 255
 256 #### Error
 257
 258 An error is a single Deb 822 stanza, starting the field Error. The
 259 following fields are supported in error stanzas:
 260
 261 - **Error:** (mandatory). The value of this field is ignored, although
 262   it should be a unique error identifier, such as a UUID.
 263
 264 - **Message:** (mandatory). The value of this field is a text string,
 265   meant to be read by humans, that explains the cause of the solver
 266   error. Message fields might be multi-line, like the Description field
 267   in the dpkg database. The first line conveys a short message, which
 268   can be explained in more details using subsequent lines.
 269
 270
 271 ### Progress
 272
 273 During dependency solving, an external solver may send progress
 274 information to APT using **progress stanzas**. A progress stanza starts
 275 with the Progress field and might contain the following fields:
 276
 277 - **Progress:** (mandatory). The value of this field is a date and time
 278   timestamp, in RFC 2822 format. The timestamp provides a time
 279   annotation for the progress report.
 280
 281 - **Percentage:** (optional). An integer from 0 to 100, representing the
 282   completion of the dependency solving process, as declared by the
 283   solver.
 284
 285 - **Message:** (optional). A textual message, meant to be read by the
 286   APT user, telling what is going on within the dependency solving
 287   (e.g. the current phase of dependency solving, as declared by the
 288   solver).
 289
 290
 291 # Future extensions
 292
 293 Potential future extensions to this protocol, listed in no specific
 294 order, include:
 295
 296 - fixed error types to identify common failures across solvers and
 297   enable APT to translate error messages
 298 - structured error data to explain failures in terms of packages and
 299   dependencies