doc/external-dependency-solver-protocol.txt

   1 # APT External Dependency Solver Protocol (EDSP) - version 0.3
   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
  30 `/usr/lib/apt/solvers`. The naming scheme is
  31 `/usr/lib/apt/solvers/NAME`, where `NAME` is the name of the external
  32 solver.
  33
  34 Each file under `/usr/lib/apt/solvers` corresponding to an external
  35 solver must be executable.
  36
  37 No non-solver files must be installed under `/usr/lib/apt/solvers`, so
  38 that an index of available external solvers can be obtained by listing
  39 the content of that directory.
  40
  41
  42 ## Configuration
  43
  44 Several APT options can be used to affect dependency solving in APT. An
  45 overview of them is given below. Please refer to proper APT
  46 configuration documentation for more, and more up to date, information.
  47
  48 - **APT::Solver::Name**: the name of the solver to be used for
  49   dependency solving. Defaults to `internal`
  50
  51 - **APT::Solver::Strict-Pinning**: whether pinning must be strictly
  52   respected (as the internal solver does) or can be slightly deviated
  53   from. Defaults to `yes`.
  54
  55 - **APT::Solver::NAME::Preferences** (where NAME is a solver name):
  56   solver-specific user preference string used during dependency solving,
  57   when the solver NAME is in use. Check solver-specific documentation
  58   for what is supported here. Defaults to the empty string.
  59
  60
  61 ## Protocol
  62
  63 When configured to use an external solver, APT will resort to it to
  64 decide which packages should be installed or removed.
  65
  66 The interaction happens **in batch**: APT will invoke the external
  67 solver passing the current status of installed and available packages,
  68 as well as the user request to alter the set of installed packages. The
  69 external solver will compute a new complete set of installed packages
  70 and gives APT a "diff" listing of which *additional* packages should be
  71 installed and of which currently installed packages should be
  72 *removed*. (Note: the order in which those actions have to be performed
  73 will be up to APT to decide.)
  74
  75 External solvers are invoked by executing them. Communications happens
  76 via the file descriptors: **stdin** (standard input) and **stdout**
  77 (standard output). stderr is not used by the EDSP protocol. Solvers can
  78 therefore use stderr to dump debugging information that could be
  79 inspected separately.
  80
  81 After invocation, the protocol passes through a sequence of phases:
  82
  83 1. APT invokes the external solver
  84 2. APT send to the solver a dependency solving **scenario**
  85 3. The solver solves dependencies. During this phase the solver may
  86    send, repeatedly, **progress** information to APT.
  87 4. The solver sends back to APT an **answer**, i.e. either a *solution*
  88    or an *error* report.
  89 5. The external solver exits
  90
  91
  92 ### Scenario
  93
  94 A scenario is a text file encoded in a format very similar to the "Deb
  95 822" format (AKA "the format used by Debian `Packages` files"). A
  96 scenario consists of two distinct parts: a **request** and a **package
  97 universe**, occurring in that order. The request consists of a single
  98 Deb 822 stanza, while the package universe consists of several such
  99 stanzas. All stanzas occurring in a scenario are separated by an empty
 100 line.
 101
 102
 103 #### Request
 104
 105 Within a dependency solving scenario, a request represents the action on
 106 installed packages requested by the user.
 107
 108 A request is a single Deb 822 stanza opened by a mandatory Request field
 109 and followed by a mixture of action and preference fields.
 110
 111 The value of the **Request:** field is a string describing the EDSP
 112 protocol which will be used to communicate. At present, the string must
 113 be `EDSP 0.3`.
 114
 115 a unique request identifier, such as an
 116 UUID. Request fields are mainly used to identify the beginning of a
 117 request stanza; their actual values are otherwise not used by the EDSP
 118 protocol.
 119
 120 The following **action fields** are supported in request stanzas:
 121
 122 - **Install:** (optional, defaults to the empty string) A space
 123   separated list of package names, with *no version attached*, to
 124   install. This field denotes a list of packages that the user wants to
 125   install, usually via an APT `install` request.
 126
 127 - **Remove:** (optional, defaults to the empty string) Same syntax of
 128   Install. This field denotes a list of packages that the user wants to
 129   remove, usually via APT `remove` or `purge` requests.
 130
 131 - **Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
 132   `no`. When set to `yes`, an upgrade of all installed packages has been
 133   requested, usually via an APT `upgrade` request.
 134
 135 - **Dist-Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
 136   `no`. Same as Upgrade, but for APT `dist-upgrade` requests.
 137
 138 - **Autoremove:** (optional, defaults to `no`). Allowed values: `yes`,
 139   `no`. When set to `yes`, a clean up of unused automatically installed
 140   packages has been requested, usually via an APT `autoremove` request.
 141
 142 The following **preference fields** are supported in request stanzas:
 143
 144 - **Strict-Pinning:** (optional, defaults to `yes`). Allowed values:
 145   `yes`, `no`. When set to `yes`, APT pinning is strict, in the sense
 146   that the solver must not propose to install packages which are not APT
 147   candidates (see the `APT-Pin` and `APT-Candidate` fields in the
 148   package universe). When set to `no`, the solver does only a best
 149   effort attempt to install APT candidates. Usually, the value of this
 150   field comes from the `APT::Solver::Strict-Pinning` configuration
 151   option.
 152
 153 - **Preferences:** a solver-specific optimization string, usually coming
 154   from the `APT::Solver::Preferences` configuration option.
 155
 156
 157 #### Package universe
 158
 159 A package universe is a list of Deb 822 stanzas, one per package, called
 160 **package stanzas**. Each package stanzas starts with a Package
 161 field. The following fields are supported in package stanzas:
 162
 163 - All fields contained in the dpkg database, with the exception of
 164   fields marked as "internal" (see the manpage `dpkg-query (1)`). Among
 165   those fields, the following are mandatory for all package stanzas:
 166   Package, Version, Architecture.
 167
 168   It is recommended not to pass the Description field to external
 169   solvers or, alternatively, to trim it to the short description only.
 170
 171 - **Installed:** (optional, defaults to `no`). Allowed values: `yes`,
 172   `no`. When set to `yes`, the corresponding package is currently
 173   installed.
 174
 175   Note: the Status field present in the dpkg database must not be passed
 176   to the external solver, as it's an internal dpkg field. Installed and
 177   other fields permit to encode the most relevant aspects of Status in
 178   communications with solvers.
 179
 180 - **Hold:** (optional, defaults to `no`). Allowed values: `yes`,
 181   `no`. When set to `yes`, the corresponding package is marked as "on
 182   hold" by dpkg.
 183
 184 - **APT-ID:** (mandatory). Unique package identifier, according to APT.
 185
 186 - **APT-Pin:** (mandatory). Must be an integer. Package pin value,
 187   according to APT policy.
 188
 189 - **APT-Candidate:** (optional, defaults to `no`). Allowed values:
 190   `yes`, `no`. When set to `yes`, the corresponding package is the APT
 191   candidate for installation among all available packages with the same
 192   name.
 193
 194 - **APT-Automatic:** (optional, defaults to `no`). Allowed values:
 195   `yes`, `no`. When set to `yes`, the corresponding package is marked by
 196   APT as automatic installed. Note that automatic installed packages
 197   should be removed by the solver only when the Autoremove action is
 198   requested (see Request section).
 199
 200 ### Answer
 201
 202 An answer from the external solver to APT is either a *solution* or an
 203 *error*.
 204
 205 The following invariant on **exit codes** must hold true. When the
 206 external solver is *able to find a solution*, it will write the solution
 207 to standard output and then exit with an exit code of 0. When the
 208 external solver is *unable to find a solution* (and s aware of that), it
 209 will write an error to standard output and then exit with an exit code
 210 of 0. An exit code other than 0 will be interpreted as a solver crash
 211 with no meaningful error about dependency resolution to convey to the
 212 user.
 213
 214
 215 #### Solution
 216
 217 A solution is a list of Deb 822 stanzas. Each of them could be an
 218 install stanza (telling APT to install a specific package), a remove
 219 stanza (telling APT to remove one), or an autoremove stanza (telling APT
 220 about the *future* possibility of removing a package using the
 221 Autoremove action).
 222
 223 An **install stanza** starts with an Install field and supports the
 224 following fields:
 225
 226 - **Install:** (mandatory). The value is a package identifier,
 227   referencing one of the package stanzas of the package universe via its
 228   APT-ID field.
 229
 230 - All fields supported by package stanzas.
 231
 232 **Remove stanzas** are similar to install stanzas, but have **Remove**
 233 fields instead of Install fields.
 234
 235 **Autoremove stanzas** are similar to install stanzas, but have
 236 **Autoremove** fields instead of Install fields. Autoremove stanzas
 237 should be output so that APT can inform the user of which packages they
 238 can now autoremove, as a consequence of the executed action. However,
 239 this protocol makes no assumption on the fact that a subsequent
 240 invocation of an Autoremove action will actually remove the very same
 241 packages indicated by Autoremove stanzas in the former solution.
 242
 243 In terms of expressivity, install and remove stanzas can carry one
 244 single field each, as APT-IDs are enough to pinpoint packages to be
 245 installed/removed. Nonetheless, for protocol readability, it is
 246 recommended that solvers either add unconditionally the fields Package,
 247 Version, and Architecture to all install/remove stanzas or,
 248 alternatively, that they support a `--verbose` command line flag that
 249 explicitly enables the output of those fields in solutions.
 250
 251
 252 #### Error
 253
 254 An error is a single Deb 822 stanza, starting the field Error. The
 255 following fields are supported in error stanzas:
 256
 257 - **Error:** (mandatory). The value of this field is ignored, although
 258   it should be a unique error identifier, such as a UUID.
 259
 260 - **Message:** (mandatory). The value of this field is a text string,
 261   meant to be read by humans, that explains the cause of the solver
 262   error. Message fields might be multi-line, like the Description field
 263   in the dpkg database. The first line conveys a short message, which
 264   can be explained in more details using subsequent lines.
 265
 266
 267 ### Progress
 268
 269 During dependency solving, an external solver may send progress
 270 information to APT using **progress stanzas**. A progress stanza starts
 271 with the Progress field and might contain the following fields:
 272
 273 - **Progress:** (mandatory). The value of this field is a date and time
 274   timestamp, in RFC 2822 format. The timestamp provides a time
 275   annotation for the progress report.
 276
 277 - **Percentage:** (optional). An integer from 0 to 100, representing the
 278   completion of the dependency solving process, as declared by the
 279   solver.
 280
 281 - **Message:** (optional). A textual message, meant to be read by the
 282   APT user, telling what is going on within the dependency solving
 283   (e.g. the current phase of dependency solving, as declared by the
 284   solver).
 285
 286
 287 # Future extensions
 288
 289 Potential future extensions to this protocol, listed in no specific
 290 order, include:
 291
 292 - fixed error types to identify common failures across solvers and
 293   enable APT to translate error messages
 294 - structured error data to explain failures in terms of packages and
 295   dependencies