[PATCH 6/6] doc/technical_overview: Describe binary cache

[Alexander Smirnov <asmirnov@ilbers.de>]

Describe Isar binary cache feature.

Signed-off-by: Alexander Smirnov <asmirnov@ilbers.de>
---
 doc/technical_overview.md | 59 +++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 55 insertions(+), 4 deletions(-)

diff --git a/doc/technical_overview.md b/doc/technical_overview.md
index 20e08b4..b3265dc 100644
--- a/doc/technical_overview.md
+++ b/doc/technical_overview.md
@@ -100,6 +100,43 @@ Target filesystem lifecycle can be described as following:
  - According to the list of custom packages in bitbake recipes, the initial
    filesystem will be populated by successfully built packages.
 
+## 2.5 Binary Cache
+
+As mentioned above, apart from fetching binary packages from upstream apt
+repositories, Isar is able to build custom packages from sources. To simplify
+further custom packages management, Isar implements binary caching.
+
+Binary cache is a typical apt repository created by Isar to store newly built
+packages. This repository has mainly two goals:
+
+ - Simplify image filesystem populating and distributing binary packages to
+   other systems.
+
+ - Reuse these binaries in next builds to avoid re-building from the
+   scratch everytime.
+
+Current Isar tree provides default cache repository: `meta-isar-bin`.
+Isar binary cache is described by the following files:
+
+ - `meta-isar-bin/conf/layer.conf`
+
+ - `meta-isar-bin/files/distributions.in`
+
+After system build is complete, this repo contains binary packages for all the
+distros and architectures, requested in multiconfig. So this folder can be
+published and used as apt repo for already installed systems.
+
+An important note, that Isar creates local repositories using dedicated Debian
+tool reprepro. This tool takes care about:
+
+ - Maintenance of multiple architectures and distros.
+
+ - Internal packages database management.
+
+ - Repo signatures management.
+
+ - Requests about available packages in repo (name, version etc...).
+
 # 3 Isar Internal Processes
 
 ## 3.1 General Overview
@@ -174,10 +211,17 @@ contain debian folder. The build process is implemented in
 
    3. Run dpkg-buildpackage
 
-4. Task `do_install`: install successfully built packages
+4. Further package processing depends on binary cache option:
+
+   1. Task `do_install_to_deploy`: install successfully built packages if
+   binary cache is disabled:
    `${BUILDCHROOT_DIR}/home/build/${PN}/*.deb` to deploy directory
    `${DEPLOY_DIR_DEB}`
 
+   2. Task `do_install_to_cache`: install successfully built packages to
+   binary cache repository:
+   `${BUILDCHROOT_DIR}/home/build/${PN}/*.deb` to `${DIR_CACHE}`
+
 ## 3.5 Populate Target Filesystem
 
 Each target image can be extended by custom packages listed in IMAGE_INSTALL
@@ -185,11 +229,18 @@ variable. Task `do_populate` performs the following:
 
 1. Parse IMAGE_INSTALL variable.
 
-2. Find respective packages in `${DEPLOY_DIR_DEB}`.
+2. Further steps depends in binary cache option.
+If binary cache is disabled:
+
+   1. Find respective packages in `${DEPLOY_DIR_DEB}`.
+
+   2. Copy packages to deb folder in dedicated target filesystem.
+
+   3. Execute dpkg command in chroot for all the copied packages.
 
-3. Copy them to deb folder in dedicated target filesystem.
+If binary cache is enabled:
 
-4. Execute dpkg command in chroot for all the copied packages.
+   1. Call apt-get install in chroot in target image filesystem.
 
 ## 3.6 Generate Bootable Image
 
-- 
2.1.4