If you are unfamiliar with Redis, the best place to start is https://redis.io. Inpacient users can skim over the examples before proceeding to the next sections

intro.cpp: This is the Aedis hello-world program. It sends one command to Redis and quits the connection.
intro_sync.cpp: Synchronous version of intro.cpp.
containers.cpp: Shows how to send and receive stl containers. Also shows how to use transactions.
serialization.cpp: Shows how to serialize types using Boost.Json.
subscriber.cpp: Shows how to implement pubsub that reconnects and resubscribes when the connection is lost.
subscriber_sentinel.cpp: Subscriber that resolves the master address with sentinels.
subscriber_sync.cpp: Synchronous version of subscriber.cpp.
echo_server.cpp: A simple TCP echo server that uses coroutines.
chat_room.cpp: A simple chat room that uses coroutines.

Tutorial

We will start with asynchronous interface as that is the basis for the synchronous API.

Async

The basic Aedis functionality resolves around calling these three functions from the aedis::connection class

connection::async_run: Stablishes a connection the Redis server and performs healthy checks.
connection::async_exec: Send commands and receives their responses.
connection::async_receive_push: Receives server side pushes.

In the simplest cases stablising a connection is as simple as

using connection = aedis::connection<net::ip::tcp::socket>;

int main()
{
   net::io_context ioc;
   connection conn{ioc};

   conn.async_run({"127.0.0.1", "6379"}, [](auto const& ec) {
      std::clog << "Connection lost: " << ec.message() << std::endl;
   });

   ...

   ioc.run();
}

For a complete example see intro.cpp. async_run completes only when the connection is lost. When that happens the operations async_exec or async_receive_push won't be affected and will be processed once a new connection is stablished or are canceled with connection::cancel. The feature makes it possible to implement reconnection, for example

subscriber.cpp: Reconnects to the same server instance when the connection is lost, for example, because of a server crash or restart.
subscriber_sentinel.cpp: Implements failover Redis sentinels.

Executing commands

Executing commands in Aedis is also straitforward, for example, the coroutine below will send a Ping command and print the echoed message to the screen

net::awaitable<void> ping(std::shared_ptr<connection> conn)
{
   // Request
   request req;
   req.push("PING", "some message");

   // Response
   std::tuple<std::string> resp;

   // Execution
   co_await conn->async_exec(req, adapt(resp));
   std::cout << "Response: " << std::get<0>(resp) << std::endl;
}

The general proceedure to execute commands is as follows

Use aedis::request to create requests where you can add as many commands as you like.
Use a std::tuple to receive the response. Each element of the tuple will receive the response to its respective command, in the order they have been sent.
Execute the command.

If, by change, the client is disconnected at the moment connection::async_exec is called, the operation will wait until a new connection is stablished.

For further details see the Requests and Responses.

Receiving pushes

The code snippet below show how to receive pushes

net::awaitable<void> receive_pushes(connection& db)
{
   for (std::vector<node<std::string>> resp;;) {
      co_await db->async_receive_push(adapt(resp));
      // Process the push in resp.
      resp.clear();
   }
}

Sync

Synchronous communication is supported in Aedis by the wrapper class aedis::sync.

Requests

Redis requests are composed of one of more Redis commands (in Redis documentation they are called pipelines). For example

request req;

// Command with variable length of arguments.
req.push("SET", "key", "some value", value, "EX", "2");

// Pushes a list.
std::list<std::string> list
   {"channel1", "channel2", "channel3"};
req.push_range("SUBSCRIBE", list);

// Same as above but as an iterator range.
req.push_range2("SUBSCRIBE", std::cbegin(list), std::cend(list));

// Pushes a map.
std::map<std::string, mystruct> map
   { {"key1", "value1"}
   , {"key2", "value2"}
   , {"key3", "value3"}};
req.push_range("HSET", "key", map);

Sending a request to Redis is then peformed with the following function

co_await db->async_exec(req, adapt(resp));

Serialization

The push and push_range functions above work with integers e.g. int and std::string out of the box. To send your own data type defined a to_bulk function like this

struct mystruct {
   // Example struct.
};

void to_bulk(std::string& to, mystruct const& obj)
{
   std::string dummy = "Dummy serializaiton string.";
   aedis::resp3::to_bulk(to, dummy);
}

Once to_bulk is defined and accessible over ADL mystruct can be passed to the request

request req;

std::map<std::string, mystruct> map {...};

req.push_range("HSET", "key", map);

Example serialization.cpp shows how store json string in Redis.

Responses

To read responses effectively, users must know their RESP3 type, this can be found in the Redis documentation for each command (https://redis.io/commands). For example

Command	RESP3 type	Documentation
lpush	Number	https://redis.io/commands/lpush
lrange	Array	https://redis.io/commands/lrange
set	Simple-string, null or blob-string	https://redis.io/commands/set
get	Blob-string	https://redis.io/commands/get
smembers	Set	https://redis.io/commands/smembers
hgetall	Map	https://redis.io/commands/hgetall

Once the RESP3 type of a given response is known we can choose a proper C++ data structure to receive it in. Fortunately, this is a simple task for most types. The table below summarises the options

RESP3 type	Possible C++ type	Type
Simple-string	`std::string`	Simple
Simple-error	`std::string`	Simple
Blob-string	`std::string`, `std::vector`	Simple
Blob-error	`std::string`, `std::vector`	Simple
Number	`long long`, `int`, `std::size_t`, `std::string`	Simple
Double	`double`, `std::string`	Simple
Null	`std::optional<T>`	Simple
Array	`std::vector`, `std::list`, `std::array`, `std::deque`	Aggregate
Map	`std::vector`, `std::map`, `std::unordered_map`	Aggregate
Set	`std::vector`, `std::set`, `std::unordered_set`	Aggregate
Push	`std::vector`, `std::map`, `std::unordered_map`	Aggregate

For example

request req;
req.push("HELLO", 3);
req.push_range("RPUSH", "key1", vec);
req.push_range("HSET", "key2", map);
req.push("LRANGE", "key3", 0, -1);
req.push("HGETALL", "key4");
req.push("QUIT");

std::tuple<
   aedis::ignore,  // hello
   int,            // rpush
   int,            // hset
   std::vector<T>, // lrange
   std::map<U, V>, // hgetall
   std::string     // quit
> resp;

co_await db->async_exec(req, adapt(resp));

The tag @c aedis::ignore can be used to ignore individual elements in the responses. If the intention is to ignore the response to all commands in the request use @c adapt()

co_await db->async_exec(req, adapt());

Responses that contain nested aggregates or heterogeneous data types will be given special treatment later in \ref gen-case. As of this writing, not all RESP3 types are used by the Redis server, which means in practice users will be concerned with a reduced subset of the RESP3 specification.

Null

It is not uncommon for apps to access keys that do not exist or that have already expired in the Redis server, to deal with these cases Aedis provides support for std::optional. To use it, wrap your type around std::optional like this

std::optional<std::unordered_map<T, U>> resp;
co_await db->async_exec(req, adapt(resp));

Everything else stays the same.

Transactions

To read the response to transactions we have to observe that Redis queues the commands as they arrive and sends the responses back to the user as an array, in the response to the @c exec command. For example, to read the response to the this request

db.send("MULTI");
db.send("GET", "key1");
db.send("LRANGE", "key2", 0, -1);
db.send("HGETALL", "key3");
db.send("EXEC");

use the following response type

using aedis::ignore;

using exec_resp_type = 
   std::tuple<
      std::optional<std::string>, // get
      std::optional<std::vector<std::string>>, // lrange
      std::optional<std::map<std::string, std::string>> // hgetall
   >;

std::tuple<
   ignore,     // multi
   ignore,     // get
   ignore,     // lrange
   ignore,     // hgetall
   exec_resp_type, // exec
> resp;

co_await db->async_exec(req, adapt(resp));

Note that above we are not ignoring the response to the commands themselves but whether they have been successfully queued. For a complete example see containers.cpp.

Deserialization

As mentioned in \ref requests-serialization, it is common to serialize data before sending it to Redis e.g. to json strings. For performance and convenience reasons, we may also want to deserialize it directly in its final data structure. Aedis supports this use case by calling a user provided from_bulk function while parsing the response. For example

void from_bulk(mystruct& obj, char const* p, std::size_t size, boost::system::error_code& ec)
{
   // Deserializes p into obj.
}

After that, you can start receiving data efficiently in the desired types e.g. mystruct, std::map<std::string, mystruct> etc.

The general case

There are cases where responses to Redis commands won't fit in the model presented above, some examples are

Commands (like set) whose responses don't have a fixed RESP3 type. Expecting an int and receiving a blob-string will result in error.
RESP3 aggregates that contain nested aggregates can't be read in STL containers.
Transactions with a dynamic number of commands can't be read in a std::tuple.

To deal with these cases Aedis provides the resp3::node type, that is the most general form of an element in a response, be it a simple RESP3 type or an aggregate. It is defined like this

template <class String>
struct node {
   // The RESP3 type of the data in this node.
   type data_type;

   // The number of elements of an aggregate (or 1 for simple data).
   std::size_t aggregate_size;

   // The depth of this node in the response tree.
   std::size_t depth;

   // The actual data. For aggregate types this is always empty.
   String value;
};

Any response to a Redis command can be received in a std::vector<node<std::string>>. The vector can be seen as a pre-order view of the response tree. Using it is no different than using other types

// Receives any RESP3 simple data type.
node<std::string> resp;
co_await db->async_exec(req, adapt(resp));

// Receives any RESP3 simple or aggregate data type.
std::vector<node<std::string>> resp;
co_await db->async_exec(req, adapt(resp));

For example, suppose we want to retrieve a hash data structure from Redis with HGETALL, some of the options are

std::vector<node<std::string>: Works always.
std::vector<std::string>: Efficient and flat, all elements as string.
std::map<std::string, std::string>: Efficient if you need the data as a std::map.
std::map<U, V>: Efficient if you are storing serialized data. Avoids temporaries and requires from_bulk for U and V.

In addition to the above users can also use unordered versions of the containers. The same reasoning also applies to sets e.g. SMEMBERS.

Installation

Download the latest Aedis release from github

$ wget https://github.com/mzimbres/aedis/releases/download/v1.0.0/aedis-1.0.0.tar.gz

and unpack in your prefered location. Aedis is a header only library, so you can starting using it. For that include the following header

#include <aedis/src.hpp>

in no more than one source file in your applications (see intro.cpp for example). To build the examples, run the tests etc. cmake is also supported

$ BOOST_ROOT=/opt/boost_1_79_0/ cmake
$ make
$ make test

Notice you have to specify the compiler flags manually.

These are the requirements for using Aedis

Boost 1.78 or greater.
C++17. Some examples require C++20 with coroutine support.
Redis 6 or higher. Optionally also redis-cli and Redis Sentinel.

The following compilers are supported

Tested with gcc: 10, 11, 12.
Tested with clang: 11, 13, 14.

Why Aedis

At the time of this writing there are seventeen Redis clients listed in the official list. With so many clients available it is not unlikely that users are asking themselves why yet another one. In this section I will try to compare Aedis with the most popular clients and why we need Aedis. Notice however that this is ongoing work as comparing client objectively is difficult and time consuming.

The most popular client at the moment of this writing ranked by github stars is

https://github.com/sewenew/redis-plus-plus

Before we start it is worth mentioning some of the things it does not support

RESP3. Without RESP3 is impossible to support some important Redis features like client side caching, among other things.
Coroutines.
Reading responses directly in user data structures avoiding temporaries.
Error handling with error-code and exception overloads.
Healthy checks.

The remaining points will be addressed individually.

redis-plus-plus

Let us first have a look at what sending a command a pipeline and a transaction look like

auto redis = Redis("tcp://127.0.0.1:6379");

// Send commands
redis.set("key", "val");
auto val = redis.get("key"); // val is of type OptionalString.
if (val)
    std::cout << *val << std::endl;

// Sending pipelines
auto pipe = redis.pipeline();
auto pipe_replies = pipe.set("key", "value")
                        .get("key")
                        .rename("key", "new-key")
                        .rpush("list", {"a", "b", "c"})
                        .lrange("list", 0, -1)
                        .exec();

// Parse reply with reply type and index.
auto set_cmd_result = pipe_replies.get<bool>(0);
// ...

// Sending a transaction
auto tx = redis.transaction();
auto tx_replies = tx.incr("num0")
                    .incr("num1")
                    .mget({"num0", "num1"})
                    .exec();

auto incr_result0 = tx_replies.get<long long>(0);
// ...

Some of the problems with this API are

Heterogeneous treatment of commands, pipelines and transaction. This makes auto-pipelining impossible.
Any Api that sends individual commands has a very restricted scope of usability and should be avoided for performance reasons.
The API imposes exceptions on users, no error-code overload is provided.
No way to reuse the buffer for new calls to e.g. redis.get in order to avoid further dynamic memory allocations.
Error handling of resolve and connection not clear.

According to the documentation, pipelines in redis-plus-plus have the following characteristics

NOTE: By default, creating a Pipeline object is NOT cheap, since it creates a new connection.

This is clearly a downside of the API as pipelines should be the default way of communicating and not an exception, paying such a high price for each pipeline imposes a severe cost in performance. Transactions also suffer from the very same problem.

NOTE: Creating a Transaction object is NOT cheap, since it creates a new connection.

In Aedis there is no difference between sending one command, a pipeline or a transaction because requests are decoupled from the IO objects.

redis-plus-plus also supports async interface, however, async support for Transaction and Subscriber is still on the way.

The async interface depends on third-party event library, and so far, only libuv is supported.

Async code in redis-plus-plus looks like the following

auto async_redis = AsyncRedis(opts, pool_opts);

Future<string> ping_res = async_redis.ping();

cout << ping_res.get() << endl;

As the reader can see, the async interface is based on futures which is also known to have a bad performance. The biggest problem however with this async design is that it makes it impossible to write asynchronous programs correctly since it starts an async operation on every command sent instead of enqueueing a message and triggering a write when it can be sent. It is also not clear how are pipelines realised with the design (if at all).

Build status

Branch	GH Actions	codecov.io
`master`

Reference

See Reference

Acknowledgement

Some people that were helpful in the development of Aedis

Richard Hodges (madmongo1): For helping me with Asio and the design of asynchronous programs in general.
Vinícius dos Santos Oliveira (vinipsmaker): For useful discussion about how Aedis consumes buffers in the read operation (among other things).
Petr Dannhofer (Eddie-cz): For helping me understand how the AUTH and HELLO command can influence each other.

Languages

C++ 95.9%

Python 1.5%

CMake 1.2%

Java 0.3%

Rust 0.3%

Other 0.7%