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`. Request fields are mainly used to identify the beginning
 118 of a request stanza; their actual values are otherwise not used by the
 119 EDSP protocol.
 120
 121 The following **action fields** are supported in request stanzas:
 122
 123 - **Install:** (optional, defaults to the empty string) A space
 124   separated list of package names, with *no version attached*, to
 125   install. This field denotes a list of packages that the user wants to
 126   install, usually via an APT `install` request.
 127
 128 - **Remove:** (optional, defaults to the empty string) Same syntax of
 129   Install. This field denotes a list of packages that the user wants to
 130   remove, usually via APT `remove` or `purge` requests.
 131
 132 - **Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
 133   `no`. When set to `yes`, an upgrade of all installed packages has been
 134   requested, usually via an APT `upgrade` request.
 135
 136 - **Dist-Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
 137   `no`. Same as Upgrade, but for APT `dist-upgrade` requests.
 138
 139 - **Autoremove:** (optional, defaults to `no`). Allowed values: `yes`,
 140   `no`. When set to `yes`, a clean up of unused automatically installed
 141   packages has been requested, usually via an APT `autoremove` request.
 142
 143 The following **preference fields** are supported in request stanzas:
 144
 145 - **Strict-Pinning:** (optional, defaults to `yes`). Allowed values:
 146   `yes`, `no`. When set to `yes`, APT pinning is strict, in the sense
 147   that the solver must not propose to install packages which are not APT
 148   candidates (see the `APT-Pin` and `APT-Candidate` fields in the
 149   package universe). When set to `no`, the solver does only a best
 150   effort attempt to install APT candidates. Usually, the value of this
 151   field comes from the `APT::Solver::Strict-Pinning` configuration
 152   option.
 153
 154 - **Preferences:** a solver-specific optimization string, usually coming
 155   from the `APT::Solver::Preferences` configuration option.
 156
 157
 158 #### Package universe
 159
 160 A package universe is a list of Deb 822 stanzas, one per package, called
 161 **package stanzas**. Each package stanzas starts with a Package
 162 field. The following fields are supported in package stanzas:
 163
 164 - All fields contained in the dpkg database, with the exception of
 165   fields marked as "internal" (see the manpage `dpkg-query (1)`). Among
 166   those fields, the following are mandatory for all package stanzas:
 167   Package, Version, Architecture.
 168
 169   It is recommended not to pass the Description field to external
 170   solvers or, alternatively, to trim it to the short description only.
 171
 172 - **Installed:** (optional, defaults to `no`). Allowed values: `yes`,
 173   `no`. When set to `yes`, the corresponding package is currently
 174   installed.
 175
 176   Note: the Status field present in the dpkg database must not be passed
 177   to the external solver, as it's an internal dpkg field. Installed and
 178   other fields permit to encode the most relevant aspects of Status in
 179   communications with solvers.
 180
 181 - **Hold:** (optional, defaults to `no`). Allowed values: `yes`,
 182   `no`. When set to `yes`, the corresponding package is marked as "on
 183   hold" by dpkg.
 184
 185 - **APT-ID:** (mandatory). Unique package identifier, according to APT.
 186
 187 - **APT-Pin:** (mandatory). Must be an integer. Package pin value,
 188   according to APT policy.
 189
 190 - **APT-Candidate:** (optional, defaults to `no`). Allowed values:
 191   `yes`, `no`. When set to `yes`, the corresponding package is the APT
 192   candidate for installation among all available packages with the same
 193   name.
 194
 195 - **APT-Automatic:** (optional, defaults to `no`). Allowed values:
 196   `yes`, `no`. When set to `yes`, the corresponding package is marked by
 197   APT as automatic installed. Note that automatic installed packages
 198   should be removed by the solver only when the Autoremove action is
 199   requested (see Request section).
 200
 201 ### Answer
 202
 203 An answer from the external solver to APT is either a *solution* or an
 204 *error*.
 205
 206 The following invariant on **exit codes** must hold true. When the
 207 external solver is *able to find a solution*, it will write the solution
 208 to standard output and then exit with an exit code of 0. When the
 209 external solver is *unable to find a solution* (and s aware of that), it
 210 will write an error to standard output and then exit with an exit code
 211 of 0. An exit code other than 0 will be interpreted as a solver crash
 212 with no meaningful error about dependency resolution to convey to the
 213 user.
 214
 215
 216 #### Solution
 217
 218 A solution is a list of Deb 822 stanzas. Each of them could be an
 219 install stanza (telling APT to install a specific package), a remove
 220 stanza (telling APT to remove one), or an autoremove stanza (telling APT
 221 about the *future* possibility of removing a package using the
 222 Autoremove action).
 223
 224 An **install stanza** starts with an Install field and supports the
 225 following fields:
 226
 227 - **Install:** (mandatory). The value is a package identifier,
 228   referencing one of the package stanzas of the package universe via its
 229   APT-ID field.
 230
 231 - All fields supported by package stanzas.
 232
 233 **Remove stanzas** are similar to install stanzas, but have **Remove**
 234 fields instead of Install fields.
 235
 236 **Autoremove stanzas** are similar to install stanzas, but have
 237 **Autoremove** fields instead of Install fields. Autoremove stanzas
 238 should be output so that APT can inform the user of which packages they
 239 can now autoremove, as a consequence of the executed action. However,
 240 this protocol makes no assumption on the fact that a subsequent
 241 invocation of an Autoremove action will actually remove the very same
 242 packages indicated by Autoremove stanzas in the former solution.
 243
 244 In terms of expressivity, install and remove stanzas can carry one
 245 single field each, as APT-IDs are enough to pinpoint packages to be
 246 installed/removed. Nonetheless, for protocol readability, it is
 247 recommended that solvers either add unconditionally the fields Package,
 248 Version, and Architecture to all install/remove stanzas or,
 249 alternatively, that they support a `--verbose` command line flag that
 250 explicitly enables the output of those fields in solutions.
 251
 252
 253 #### Error
 254
 255 An error is a single Deb 822 stanza, starting the field Error. The
 256 following fields are supported in error stanzas:
 257
 258 - **Error:** (mandatory). The value of this field is ignored, although
 259   it should be a unique error identifier, such as a UUID.
 260
 261 - **Message:** (mandatory). The value of this field is a text string,
 262   meant to be read by humans, that explains the cause of the solver
 263   error. Message fields might be multi-line, like the Description field
 264   in the dpkg database. The first line conveys a short message, which
 265   can be explained in more details using subsequent lines.
 266
 267
 268 ### Progress
 269
 270 During dependency solving, an external solver may send progress
 271 information to APT using **progress stanzas**. A progress stanza starts
 272 with the Progress field and might contain the following fields:
 273
 274 - **Progress:** (mandatory). The value of this field is a date and time
 275   timestamp, in RFC 2822 format. The timestamp provides a time
 276   annotation for the progress report.
 277
 278 - **Percentage:** (optional). An integer from 0 to 100, representing the
 279   completion of the dependency solving process, as declared by the
 280   solver.
 281
 282 - **Message:** (optional). A textual message, meant to be read by the
 283   APT user, telling what is going on within the dependency solving
 284   (e.g. the current phase of dependency solving, as declared by the
 285   solver).
 286
 287
 288 # Future extensions
 289
 290 Potential future extensions to this protocol, listed in no specific
 291 order, include:
 292
 293 - fixed error types to identify common failures across solvers and
 294   enable APT to translate error messages
 295 - structured error data to explain failures in terms of packages and
 296   dependencies