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+# APT External Dependency Solver Protocol (EDSP) - version 0.4
+
+This document describes the communication protocol between APT and
+external dependency solvers. The protocol is called APT EDSP, for "APT
+External Dependency Solver Protocol".
+
+
+## Components
+
+- **APT**: we know this one.
+- APT is equipped with its own **internal solver** for dependencies,
+  which is identified by the string `internal`.
+- **External solver**: an *external* software component able to resolve
+  dependencies on behalf of APT.
+  
+At each interaction with APT, a single solver is in use.  When there is
+a total of 2 or more solvers, internals or externals, the user can
+choose which one to use.
+
+Each solver is identified by an unique string, the **solver
+name**. Solver names must be formed using only alphanumeric ASCII
+characters, dashes, and underscores; solver names must start with a
+lowercase ASCII letter. The special name `internal` denotes APT's
+internal solver, is reserved, and cannot be used by external solvers.
+
+
+## Installation
+
+Each external solver is installed as a file under Dir::Bin::Solvers (see
+below), which defaults to `/usr/lib/apt/solvers`. We will assume in the
+remainder of this section that such a default value is in effect.
+
+The naming scheme is `/usr/lib/apt/solvers/NAME`, where `NAME` is the
+name of the external solver.
+
+Each file under `/usr/lib/apt/solvers` corresponding to an external
+solver must be executable.
+
+No non-solver files must be installed under `/usr/lib/apt/solvers`, so
+that an index of available external solvers can be obtained by listing
+the content of that directory.
+
+
+## Configuration
+
+Several APT options can be used to affect dependency solving in APT. An
+overview of them is given below. Please refer to proper APT
+configuration documentation for more, and more up to date, information.
+
+- **APT::Solver**: the name of the solver to be used for
+  dependency solving. Defaults to `internal`
+
+- **APT::Solver::Strict-Pinning**: whether pinning must be strictly
+  respected (as the internal solver does) or can be slightly deviated
+  from. Defaults to `yes`.
+
+- **APT::Solver::NAME::Preferences** (where NAME is a solver name):
+  solver-specific user preference string used during dependency solving,
+  when the solver NAME is in use. Check solver-specific documentation
+  for what is supported here. Defaults to the empty string.
+
+- **Dir::Bin::Solvers**: absolute path of the directory where to look for
+  external solvers. Defaults to `/usr/lib/apt/solvers`.
+
+## Protocol
+
+When configured to use an external solver, APT will resort to it to
+decide which packages should be installed or removed.
+
+The interaction happens **in batch**: APT will invoke the external
+solver passing the current status of installed and available packages,
+as well as the user request to alter the set of installed packages. The
+external solver will compute a new complete set of installed packages
+and gives APT a "diff" listing of which *additional* packages should be
+installed and of which currently installed packages should be
+*removed*. (Note: the order in which those actions have to be performed
+will be up to APT to decide.)
+
+External solvers are invoked by executing them. Communications happens
+via the file descriptors: **stdin** (standard input) and **stdout**
+(standard output). stderr is not used by the EDSP protocol. Solvers can
+therefore use stderr to dump debugging information that could be
+inspected separately.
+
+After invocation, the protocol passes through a sequence of phases:
+
+1. APT invokes the external solver
+2. APT send to the solver a dependency solving **scenario**
+3. The solver solves dependencies. During this phase the solver may
+   send, repeatedly, **progress** information to APT.
+4. The solver sends back to APT an **answer**, i.e. either a *solution*
+   or an *error* report.
+5. The external solver exits
+
+
+### Scenario
+
+A scenario is a text file encoded in a format very similar to the "Deb
+822" format (AKA "the format used by Debian `Packages` files"). A
+scenario consists of two distinct parts: a **request** and a **package
+universe**, occurring in that order. The request consists of a single
+Deb 822 stanza, while the package universe consists of several such
+stanzas. All stanzas occurring in a scenario are separated by an empty
+line.
+
+
+#### Request
+
+Within a dependency solving scenario, a request represents the action on
+installed packages requested by the user.
+
+A request is a single Deb 822 stanza opened by a mandatory Request field
+and followed by a mixture of action and preference fields.
+
+The value of the **Request:** field is a string describing the EDSP
+protocol which will be used to communicate. At present, the string must
+be `EDSP 0.4`.
+
+a unique request identifier, such as an
+UUID. Request fields are mainly used to identify the beginning of a
+request stanza; their actual values are otherwise not used by the EDSP
+protocol.
+
+The following **action fields** are supported in request stanzas:
+
+- **Install:** (optional, defaults to the empty string) A space
+  separated list of package names, with *no version attached*, to
+  install. This field denotes a list of packages that the user wants to
+  install, usually via an APT `install` request.
+
+- **Remove:** (optional, defaults to the empty string) Same syntax of
+  Install. This field denotes a list of packages that the user wants to
+  remove, usually via APT `remove` or `purge` requests.
+
+- **Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
+  `no`. When set to `yes`, an upgrade of all installed packages has been
+  requested, usually via an APT `upgrade` request.
+
+- **Dist-Upgrade:** (optional, defaults to `no`). Allowed values: `yes`,
+  `no`. Same as Upgrade, but for APT `dist-upgrade` requests.
+
+- **Autoremove:** (optional, defaults to `no`). Allowed values: `yes`,
+  `no`. When set to `yes`, a clean up of unused automatically installed
+  packages has been requested, usually via an APT `autoremove` request.
+
+The following **preference fields** are supported in request stanzas:
+
+- **Strict-Pinning:** (optional, defaults to `yes`). Allowed values:
+  `yes`, `no`. When set to `yes`, APT pinning is strict, in the sense
+  that the solver must not propose to install packages which are not APT
+  candidates (see the `APT-Pin` and `APT-Candidate` fields in the
+  package universe). When set to `no`, the solver does only a best
+  effort attempt to install APT candidates. Usually, the value of this
+  field comes from the `APT::Solver::Strict-Pinning` configuration
+  option.
+
+- **Preferences:** a solver-specific optimization string, usually coming
+  from the `APT::Solver::Preferences` configuration option.
+
+
+#### Package universe
+
+A package universe is a list of Deb 822 stanzas, one per package, called
+**package stanzas**. Each package stanzas starts with a Package
+field. The following fields are supported in package stanzas:
+
+- All fields contained in the dpkg database, with the exception of
+  fields marked as "internal" (see the manpage `dpkg-query (1)`). Among
+  those fields, the following are mandatory for all package stanzas:
+  Package, Version, Architecture.
+  
+  It is recommended not to pass the Description field to external
+  solvers or, alternatively, to trim it to the short description only.
+
+- **Installed:** (optional, defaults to `no`). Allowed values: `yes`,
+  `no`. When set to `yes`, the corresponding package is currently
+  installed.
+  
+  Note: the Status field present in the dpkg database must not be passed
+  to the external solver, as it's an internal dpkg field. Installed and
+  other fields permit to encode the most relevant aspects of Status in
+  communications with solvers.
+
+- **Hold:** (optional, defaults to `no`). Allowed values: `yes`,
+  `no`. When set to `yes`, the corresponding package is marked as "on
+  hold" by dpkg.
+
+- **APT-ID:** (mandatory). Unique package identifier, according to APT.
+
+- **APT-Pin:** (mandatory). Must be an integer. Package pin value,
+  according to APT policy.
+
+- **APT-Candidate:** (optional, defaults to `no`). Allowed values:
+  `yes`, `no`. When set to `yes`, the corresponding package is the APT
+  candidate for installation among all available packages with the same
+  name.
+
+- **APT-Automatic:** (optional, defaults to `no`). Allowed values:
+  `yes`, `no`. When set to `yes`, the corresponding package is marked by
+  APT as automatic installed. Note that automatic installed packages
+  should be removed by the solver only when the Autoremove action is
+  requested (see Request section).
+
+### Answer
+
+An answer from the external solver to APT is either a *solution* or an
+*error*.
+
+The following invariant on **exit codes** must hold true. When the
+external solver is *able to find a solution*, it will write the solution
+to standard output and then exit with an exit code of 0. When the
+external solver is *unable to find a solution* (and s aware of that), it
+will write an error to standard output and then exit with an exit code
+of 0. An exit code other than 0 will be interpreted as a solver crash
+with no meaningful error about dependency resolution to convey to the
+user.
+
+
+#### Solution
+
+A solution is a list of Deb 822 stanzas. Each of them could be an
+install stanza (telling APT to install a specific package), a remove
+stanza (telling APT to remove one), or an autoremove stanza (telling APT
+about the *future* possibility of removing a package using the
+Autoremove action).
+
+An **install stanza** starts with an Install field and supports the
+following fields:
+
+- **Install:** (mandatory). The value is a package identifier,
+  referencing one of the package stanzas of the package universe via its
+  APT-ID field.
+
+- All fields supported by package stanzas.
+
+**Remove stanzas** are similar to install stanzas, but have **Remove**
+fields instead of Install fields.
+
+**Autoremove stanzas** are similar to install stanzas, but have
+**Autoremove** fields instead of Install fields. Autoremove stanzas
+should be output so that APT can inform the user of which packages they
+can now autoremove, as a consequence of the executed action. However,
+this protocol makes no assumption on the fact that a subsequent
+invocation of an Autoremove action will actually remove the very same
+packages indicated by Autoremove stanzas in the former solution.
+
+In terms of expressivity, install and remove stanzas can carry one
+single field each, as APT-IDs are enough to pinpoint packages to be
+installed/removed. Nonetheless, for protocol readability, it is
+recommended that solvers either add unconditionally the fields Package,
+Version, and Architecture to all install/remove stanzas or,
+alternatively, that they support a `--verbose` command line flag that
+explicitly enables the output of those fields in solutions.
+
+
+#### Error
+
+An error is a single Deb 822 stanza, starting the field Error. The
+following fields are supported in error stanzas:
+
+- **Error:** (mandatory). The value of this field is ignored, although
+  it should be a unique error identifier, such as a UUID.
+
+- **Message:** (mandatory). The value of this field is a text string,
+  meant to be read by humans, that explains the cause of the solver
+  error. Message fields might be multi-line, like the Description field
+  in the dpkg database. The first line conveys a short message, which
+  can be explained in more details using subsequent lines.
+
+
+### Progress
+
+During dependency solving, an external solver may send progress
+information to APT using **progress stanzas**. A progress stanza starts
+with the Progress field and might contain the following fields:
+
+- **Progress:** (mandatory). The value of this field is a date and time
+  timestamp, in RFC 2822 format. The timestamp provides a time
+  annotation for the progress report.
+
+- **Percentage:** (optional). An integer from 0 to 100, representing the
+  completion of the dependency solving process, as declared by the
+  solver.
+
+- **Message:** (optional). A textual message, meant to be read by the
+  APT user, telling what is going on within the dependency solving
+  (e.g. the current phase of dependency solving, as declared by the
+  solver).
+
+
+# Future extensions
+
+Potential future extensions to this protocol, listed in no specific
+order, include:
+
+- fixed error types to identify common failures across solvers and
+  enable APT to translate error messages
+- structured error data to explain failures in terms of packages and
+  dependencies