redis-ipc

redis-ipc is an example of how redis can be used as an advanced IPC mechanism on an embedded Linux system, for instance as a substitute for the more common choice of dbus.

redis-ipc is intended to make communication among different logical components of a system convenient. It is not intended to replace shared memory for high data-rate transfers between processes, where lowest possible overhead is key, but to provide a convenient and reliable way to implement the following IPC mechanisms:

• command queues
• settings
• status
• event channels

"But, but... redis for embedded applications??"

Quick Start Package Install

redis-ipc comes in 2 flavors, a python class module and a lightweight C library implementation (this repo). The python module has moved to a new home.

Packages are available for Ubuntu, and the latest can be installed on Gentoo using the ebuilds in this portage overlay. To build from source, see Building redis-ipc below.

Making Changes & Contributing

This repo is now pre-commit enabled for various linting and format checks. The checks run automatically on commit and will fail the commit (if not clean) with some checks performing simple file corrections.

If other checks fail on commit, the failure display should explain the error types and line numbers. Note you must fix any fatal errors for the commit to succeed; some errors should be fixed automatically (use git status and `` git diff`` to review any changes).

See the pre-commit docs under docs/dev/ for more information:

• pre-commit-config
• pre-commit-usage

You will need to install pre-commit before contributing any changes; installing it using your system's package manager is recommended, otherwise install with pip into your usual virtual environment using something like:

$ sudo emerge pre-commit  --or--
$ pip install pre-commit

then install it into the repo you just cloned:

$ git clone https://github.com/VCTLabs/redis-ipc
$ cd redis-ipc/
$ pre-commit install

It's usually a good idea to update the hooks to the latest version:

pre-commit autoupdate

Prerequisites

A supported linux distribution, mainly something that uses either .ebuilds (eg, Gentoo or funtoo) or .deb packages, starting with at least Ubuntu bionic or Debian stretch (see the above PPA package repo on Launchpad).

Make sure you have the add-apt-repository command installed and then add the PPA:

$ sudo apt-get install software-properties-common
$ sudo add-apt-repository -y -s ppa:nerdboy/embedded
$ sudo apt-get install libredis-ipc-dev redis-tools redis-server

Note

Since the package series currently published are for bionic/focal, building from source is recommended if installing on Debian.

If you get a key error you will also need to manually import the PPA signing key like so:

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PPA_KEY>

where <PPA_KEY> is the key shown in the launchpad PPA page under "Adding this PPA to your system", eg, 41113ed57774ed19 for Embedded device ppa.

Build and test with Tox

As long as you have git and at least Python 3.6, then the "easy" dev install is to clone this repository and install tox.

After cloning the repository, you can run the current tests using either cmake or autotools with the tox command. It will build a virtual python environment with most of the build dependencies (except the shared libraries above and the autotools bits) and then run the tests. For cmake plus test coverage, you would first install your toolchain, the required json-c and hiredis libraries, redis-server, and tox, then run the following commands:

$ git clone https://github.com/VCTLabs/redis-ipc
$ cd redis-ipc
$ tox -e ctest

The above will start the redis server, build and run the tests with coverage display, and stop the redis server. Alternatively, you can manipulate the server manually and use tox -e tests instead. To run the autotools build, you may need most or all of the following packages installed; for example on Ubuntu you might need:

• build-essential, make, libjson-c-dev, libhiredis-dev, redis-server (from above)
• autoconf, autoconf-archive, automake, pkg-config, libtool

Note

The default libjson-c version (0.13) on Ubuntu focal is broken, so you should add the PPA and install the 0.15 package instead.

There are several tox -e environment commands available:

• ctest - build/run tests using ctest (with redis-server handling)
• tests - build/run tests using cmake (without redis-server handling)
• bionic - build/run tests using cmake (with redis-server handling)
• grind - build/run using cmake and valgrind (with redis-server handling)
• clean - clean the cmake build/ directory/files
• auto - build/run tests using autotools (with redis-server handling)
• autoclean - clean all the autotools cruft
• lint - run the cpplint style checks

With the additional dependencies of LLVM/Clang >= 12 you can try the LLVM source-based coverage alternative to gcov/lcov-based coverage. First install the above toolchain, then run the following:

$ CC=clang CXX=clang++ tox -e clang

If you installed a newer version than 12, eg, 13, then prepend the version using ENV_LLVM_VER=13 to the above command.

Note

Without the PPA, Bionic has an older GTest package and needs an extra cmake arg.

See the Github workflow files for more details on the packages installed for each runner OS environment.

Quick Start Dev Environment

Packages should eventually be available in Conda Forge but you can always use Conda's devenv support to build/install locally inside a Conda environment. This is the recommended method if you can't use the PPA or Gentoo overlay. Set your default shell to bash if not already set.

Prerequisites

Install either Anaconda or Miniconda (we recommend miniconda) and add the conda-forge channel, then install the conda-devenv package.

• Download the miniconda linux-64 installer and run it

• Let the installer add the conda init bits to your .bashrc

• Source your shell environment: source ~/.bashrc

• Install conda-devenv:

  conda config --append channels conda-forge
  conda install -n base conda-devenv
  

• Clone this repository:

  git clone https://github.com/VCTLabs/redis-ipc.git
  

• Create a new conda devenv environment:

  cd redis-ipc/
  conda devenv
  

This command will create the conda environment called redis-ipc-test, which can take a few minutes to complete the first time. This will install the conda toolchain and all required dependencies to build from source (see the contents of the environment.devenv.yml file for details).

• Activate the environment:

  conda activate redis-ipc-test
  

Now you can use the usual cmake configure and build steps (see the Cmake build section below) or you can run the following one-liner for a quick build-and-test:

ctest --build-config RelWithDebInfo --build-generator "Ninja" \
  --build-and-test . build --build-options -DRIPC_DISABLE_SOCK_TESTS=1 \
  -DCMAKE_INSTALL_PREFIX=$CONDA_PREFIX -DCMAKE_INSTALL_LIBDIR=lib \
  --test-command ctest -V --build-config RelWithDebInfo

Note

The above command will omit running the socket tests, but if already have a running redis server available, you can set the RIPC_DISABLE_SOCK_TESTS argument to 0 instead.

Whenever the above dependencies change or you alter your local conda environment, rebuild your devenv:

conda devenv

When finished, deactivate the environment:

conda deactivate

Digression into the wonders of redis

redis is a powerful communications protocol that has been undeservedly typecast, in the entertainment industry sense, as a backend service (database replacement / database caching layer) for web applications. Many non-web developers have not even heard of redis, and if they have, it was probably in the context of peeking at some no-sql discussions to see what those crazy big-server/cloud web-service developers are up to nowadays -- without the realization that they have just stumbled across a general-purpose technology that could likely be put to good use in one of their own non-webcentric projects someday.

Many of the features that make redis appealing for web applications are still relevant for IPC among local applications:

• low overhead -- maxes out at thousands of operations per second on a 1GHz ARM and 10s of thousands of operations per second on a desktop-class CPU, so system load is low for practical usage (even hundreds of operations per second)
• wide variety of language bindings -- name your favorite non-toy language, and there is a good chance the redis binding already exists
• convenient monitoring for troubleshooting -- snoop all the incoming commands as a human-readable stream of text, rather than needing specialized debugging tools to decode binary messages in order to make any sense of them (and having to update said tools every time a new custom message is defined)
• atomic operations -- along with the ability to turn on monitoring, can make developing multi-threaded and multi-process applications so much easier

and finally one more which is specifically relevant for embedded software:

• portable -- cross-compiles nicely, available as openembedded recipe

Important caveat regarding redis security

After covering the many attractions of redis, it is only fair to point out an important limitation: the lack of security features (toy authentication and no ability to restrict capabilities of connected clients) makes it highly unsuitable for access by untrusted users.

Security-wise (and performance-wise, for that matter) it is better to use unix sockets than a locally-bound tcp socket, so that filesystem permissions can be used to restrict socket access to a certain user or group. However always keep in mind that a rogue process running as that authorized user or group gains full admin powers over the server, including snooping of all redis activity and making runtime changes to the config.

For that reason, never use redis in security-sensitive environments unless there are solid external mechanisms for restricting access (sandboxing, custom SELinux policy limiting redis connections to specific trusted applications), and for security-critical tasks the principle of layered defense calls for a more secure store as an additional line of defense, eg, credit card info cached in an unencrypted redis store would be such a juicy target for any attackers who made it onto the server!

Example of sensible scenarios for redis deployment:

• Use redis to coordinate processes in regression test framework for assessing current development status of an embedded device. Connections are limited to localhost, and the only other users with accounts on the workstation are trusted fellow teammates on the project.
• Use redis to maintain settings and status for an embedded device. Connections are again limited to localhost, and in normal operations (i.e. not development mode) there are NO network logins enabled to the device.

Building redis-ipc

Now back to the star of this show, namely redis-ipc, starting with how to build and install it on your Linux development box (redis-ipc now supports both autotools and CMake build systems, so in the following steps choose one or the other).

• Install build dependencies

  • C/C++ toolchain
  • pkg-config
  • make
  • cmake --or-- automake/autoconf/libtool
  • hiredis
  • json-c

  On a development system with Debian/Ubuntu/Mint, this generally amounts to:

  apt-get install libhiredis-dev libjson0-dev
  

  with Gentoo:

  emerge dev-libs/hiredis dev-libs/json-c
  

  with Fedora (or CentOS/RHEL using EPEL):

  yum install hiredis-devel json-c-devel
  

• Check out redis-ipc source code (no tarball releases yet):

  git clone https://github.com/VCTLabs/redis-ipc.git
  cd redis-ipc/
  

CMake build

The cmake tools can be run in several ways, and follow the standard set of (cmake) out-of-tree build steps.

• Create the build directory:

  mkdir build && cd build/
  

• Configure the build:

  cmake -DCMAKE_BUILD_TYPE=Debug ..
  

• Build it:

  make
  

• Run the tests:

  make check
  

Autotools build

The autotools build will create the standard set of Makefiles and the configure script.

• Generate and run configure:

  ./autogen.sh && ./configure
  

• Run the compile

  • native build:

    # also builds the library, in addition to some simple example apps
    make
    

  • cross-compile build:

    # also builds the library, in addition to some simple example apps
    make CROSS_COMPILE=<toolchain prefix> SYSROOT=<cross-compile staging area>
    

    • CROSS_COMPILE is everything up to (and including) the last '-' in the tool names, e.g. if the C compiler is arm-none-linux-gnueabi-gcc then

      CROSS_COMPILE=arm-none-linux-gnueabi-

    • SYSROOT is the base path of your staging area that has cross-compiled versions of the dependency libraries, e.g. if the cross-compiled hiredis library is under

      /home/sjl/yocto/build/tmp/sysroots/armv5te-poky-linux-gnueabi/usr/lib

      then

      SYSROOT=/home/sjl/yocto/build/tmp/sysroots/armv5te-poky-linux-gnueabi/

Running redis-ipc

After building redis-ipc for the desired platform, try running it against a redis server. The redis server needs to be configured to use a unix socket, the path of which defaults to $RPC_RUNTIME_DIR/socket, where RPC_RUNTIME_DIR defaults to /tmp/redis-ipc but may be overridden at compile time. The socket path may also be overridden at runtime with the environment variable RIPC_SERVER_PATH.

redis.conf excerpt:

# Accept connections on the specified port, default is 6379.
# If port 0 is specified Redis will not listen on a TCP socket.
port 0

# Specify the path for the unix socket that will be used to listen for
# incoming connections. There is no default, so Redis will not listen
# on a unix socket when not specified.

unixsocket /tmp/redis-ipc/socket
# this allows connections by the user who starts the server (or by root, of course)
unixsocketperm 500
# alternatively, allow connections from the group under which server was started
#unixsocketperm 550

Next verify the combination of redis-server and redis-ipc library are working together by running the simple example programs, which each end in _test. Use LD_LIBRARY_PATH to run the programs from the build directory (or, if cross-compiling, a copy of build directory loaded on the embedded target) prior to a system-wide install

LD_LIBRARY_PATH=. ./command_result_test

Expected output for each example app has been provided in files ending in _test.out.

Developing with redis-ipc

As mentioned in the intro, redis-ipc implements the following mechanisms:

• command queues
• settings
• status
• event channels

Command queues are a method for any component to request an action from another component, and receive a result after the command has been processed. Each component that exports actions to other components would own one or more command queues. When sending a command, the queue is specified by component and "subqueue" to allow components to manage multiple queues that are processed with different priorities.

Settings are hashes representing the current configuration of each component. The settings for a single component can all be read atomically and written atomically, to avoid bugs where one component gets into an inconsistent state by reading settings when partially updates by another component. Note that settings changes across multiple components are not atomic, so complicated designs where settings consistency depends on updating multiple components at the same time would need to implement that separately, e.g. with some form of locking.

Status are also hashes, but represent a component's current runtime state instead of representing how a component has been configured. While settings are likely written by a single component, each component maintains its own status with any state info that is of interest to one or more other components.

Event channels are an efficient way to broadcast events from one component to any others that might be interested (i.e. "subscribers). At the toplevel, event channels are grouped into normal channels and debug channels that are accessed by a separate set of calls. This segregation of normal events from debug messages makes it obvious in the code which messages are only meant for debugging the component, and makes it easy to log/observe detailed debug info while normal subscribers can listen to normal events without having to discard a flood of debug events when debugging is enabled (by runtime configuration or special debug compile).

Most data handled by redis-ipc (commands, command replies, settings, status, and events) is formatted into JSON objects, meaning associative arrays containing key/value pairs. The only exception is that an individual field within a setting or status object can be accessed as a cstring. json-c library is used as the JSON implementation. Actually, debug events are another exception, being specified with a numeric priority level and a message with printf-style format + arguments.

As typical for a C library dealing with dynamically created objects, reference counting is used to ensure memory is released at the proper time. redis-ipc returns new JSON objects with one reference that the caller is responsible for freeing with json_object_put(). C++ applications can make use of the json.hh wrapper supplied in redis-ipc that takes and drops references on the underlying json-c json_object when appropriate

#include "json.hh"

...

void monitor_printer()
{
  redis_ipc_subscribe_events("printer", NULL);
  // does not take a new reference on json_object being wrapped
  // because redis_ipc_get_message_blocking() already took one
  json next_printer_event(redis_ipc_get_message_blocking());
  cout << "Event priority:" << next_printer_event.get_field("priority");
}
// reference to  json_object dropped when next_printer_event goes out of scope

Common API

Every thread and process using redis-ipc must individually call the init function prior to any of the other calls

int redis_ipc_init(const char *this_component, const char *this_thread);

Example:

// monitor process (or thread) of printer software component
redis_ipc_init("printer", "monitor");

When redis-ipc is no longer needed, there is a corresponding function to free resources

int redis_ipc_cleanup(pid_t tid);

Examples:

// single process closing down
redis_ipc_cleanup(getpid());

// one thread of multi-thread process closing down
// see gettid() definition in redis_ipc.c if your libc lacks it
redis_ipc_cleanup(gettid());

Command queue API

Command queues currently have a blocking implementation.

The JSON object for a command automatically gets 2 attributes added as a part of submission

• command_id : unique ID for command, including component name and thread id of the submitter
• results_queue : name of queue on which the result object should be pushed when command has been processes, also based on component name and thread id (each thread submitting commands has its own queue to wait on)

The JSON object for reporting back a command result to the submitter automatically gets the command_id added, to ensure commands and their results can be reliably associated.

Important note: To avoid memory leaks, callers of command queue API must drop references to command objects and result objects when finished with them.

Processes/threads that execute commands block until a command arrives

json_object * redis_ipc_receive_command_blocking(const char *subqueue,
                                            unsigned int timeout);

then when another process/thread submits a command, it will block until the command has been completed (or timeout for waiting has expired)

json_object * redis_ipc_send_command_blocking(const char *dest_component,
                                            const char *subqueue,
                                            json_object *command,
                                            unsigned int timeout);

which happens when the executing process/thread reports back the command results with

int redis_ipc_send_result(const json_object *completed_command, json_object *result);

Example:

// printer component has 2 queues, "print" and "cancel"
// because cancel commands need a separate queue that is checked even
// while printing or else an in-progress job can't be canceled

// non-printer component requests printing of file
json_object *command = json_object_new_object();
json_object_object_add(command, "pagesize",
                       json_object_new_string("A4"));
json_object_object_add(command, "file",
                       json_object_new_string("/tmp/job1231.pdf"));
json_object *result = redis_ipc_send_command_blocking("printer", "print", command, timeout);
json_object *job_id_obj = json_object_object_get(result, "job-id");
char *job_id_str = json_object_get_string(job_id_obj);
json_object_put(command);
json_object_put(result);
json_object_put(job_id_obj);

// non-printer component later decides to cancel print job
command = json_object_new_object();
json_object_object_add(command, "job-id",
                       json_object_new_string(job_id_str));
json_object *result = redis_ipc_send_command_blocking("printer", "cancel", command, timeout);
json_object_put(command);
json_object_put(result);

Settings API

Multiple settings for a single component can be updated atomically as multiple key/value pairs in a JSON object

int redis_ipc_write_setting(const char *owner_component, const json_object *fields);

or a single setting can be updated by name, with both name and value supplied as strings

int redis_ipc_write_setting_field(const char *owner_component, const char *field_name,
                                  const char *field_value);

Similarly, all settings belonging to a single component can be read as JSON object containing key/value pairs

json_object * redis_ipc_read_setting(const char *owner_component);

or a single setting can be requested by name, with both name and returned value as strings

char * redis_ipc_read_setting_field(const char *owner_component, const char *field_name);

Status API

Multiple status for a single component can be updated atomically as multiple key/value pairs in a JSON object

int redis_ipc_write_status(const json_object *fields);

or a single status can be updated by name, with both name and value supplied as strings

int redis_ipc_write_status_field(const char *field_name, const char *field_value);

Similarly, all settings belonging to a single component can be read as JSON object containing key/value pairs

json_object * redis_ipc_read_status(const char *owner_component);

or a single setting can be requested by name, with both name and returned value as strings

char * redis_ipc_read_status_field(const char *owner_component, const char *field_name);

Event API

Event channels currently have a blocking implementation for event listeners.

Channels for normal events are grouped according to component that generates the event. When a component sends a normal message it must also supply a "subchannel" as the most specific part of this addressing scheme, with each subchannel hopefully given a meaningful name to indicate what sort of messages subscribers should expect.

When a component sends a debug message, it supplies a debug level, so that the debug channels can skip sending debug messages that are higher than the currently configured debug verbosity (although, at the moment verbosity happens to be hard-coded to the value 5, meaning everything 5 and under gets broadcast...)

Listeners must sign up ahead of time to get the events of interest; there is no backlog for catching up on events posted to a channel before a listener subscribed. Event channels of interest are specified by the component generating the events and a subchannel name, where subchannel name may represent a topic that applies to multiple components.

Important note: To avoid memory leaks, callers of event API must drop references to event objects when finished with them.

Listeners can subscribe to channels with normal events

int redis_ipc_subscribe_events(const char *component, const char *subchannel)

and/or channels with debug events

int redis_ipc_subscribe_debug(const char *component);

Examples:

// subscribe to all printer-related events
redis_ipc_subscribe_events("printer", NULL);

// subscribe to all warnings that should be displayed to user
redis_ipc_subscribe_events(NULL, "warnings");

// subscribe specifically to warnings from printer component
redis_ipc_subscribe_events("printer", "warnings");

// subscribe to debug messages from printer component
redis_ipc_subscribe_debug("printer");

A component generates a normal event with one or more named attributes contained in a JSON object, and broadcasts it on one of its subchannels

int redis_ipc_send_event(const char *subchannel, json_object *message)

Example:

// printer component sends a low-on-paper event to its warning subchannel
json_object *event = json_object_new_object();
json_object_object_add(event, "severity",
                       json_object_new_string("2"));
json_object_object_add(event, "type",
                       json_object_new_string("LOW-ON-PAPER"));
redis_ipc_send_event("warnings", event);

or broadcasts a debug event with a debug level and printf-formatted message that then get turned into a JSON object

int redis_ipc_send_debug(unsigned int debug_level, const char *format, ...)

Example:

// completely hypothetical example, ahem...
redis_ipc_send_debug(RIPC_DBG_ERROR, "redis_ipc_send_command_blocking(): invalid result");

Listening components can retrieve the next normal/debug event

json_object * redis_ipc_get_message_blocking(void)

Example:

json object *message = redis_ipc_get_message_blocking();
// do stuff with message
json_object_put(message);

Testing/troubleshooting with redis-ipc

One of the great features of using redis for system-wide IPC is the ability to watch the interactions between components using the monitor command from redis-cli utility. Another great use is in unit testing of a single component, where a test script can push commands, update settings, check status and so forth. For both reasons it is useful to understand how each feature is implemented as redis data structures.

@@@TODO

Since redis-ipc requires the redis server to use a unix socket rather than tcp, remember to specify the socket path when running redis-cli

redis-cli -s /tmp/redis-ipc/socket