Documentation

[TOC]

Overview

Aedis is a high-level Redis client library built on top of Asio. Some of its distinctive features are

• Support for the latest version of the Redis communication protocol RESP3.
• Support for STL containers, TLS and Redis sentinel.
• Serialization and deserialization of your own data types.
• Back pressure, cancellation and low latency.

In addition to that, Aedis hides most of the low-level Asio code away from the user. For example, the coroutine below retrieves Redis hashes in a std::map and quits the connection (see containers.cpp)

auto hgetall(std::shared_ptr<connection> conn) -> net::awaitable<void>
{
   // A request contains multiple Redis commands.
   request req;
   req.get_config().cancel_on_connection_lost = true;
   req.push("HELLO", 3);
   req.push("HGETALL", "hset-key");
   req.push("QUIT");

   // Tuple elements will hold the response to each command in the request.
   std::tuple<aedis::ignore, std::map<std::string, std::string>, aedis::ignore> resp;

   // Executes the request and reads the response.
   co_await conn->async_exec(req, adapt(resp));

   // Uses the map ...
}

The execution of calls to connection::async_exec like above are triggered by the connection::async_run member function, which is required to be running concurrently for as long as the connection stands. For example, the code below uses a short-lived connection to execute the coroutine above

net::awaitable<void> async_main()
{
   auto conn = std::make_shared<connection>(co_await net::this_coro::executor);

   // Resolves and connects (from examples/common.hpp to avoid vebosity)
   co_await connect(conn, "127.0.0.1", "6379");

   // Runs and executes the request.
   co_await (conn->async_run() || hgetall(conn));
}

Long-lived connections follow the same principle (see the examples below) and will be discussed in more detail later. The role of the async_run is to coordinate IO and ensure the connection is always reading from the socket. The reationale behind this design is

• Provide quick reaction to disconnections and hence faster failovers.
• Support server pushes and requests in the same connection object, concurrently.

In the following sections we will discuss with more details the main entities Aedis users are concerned with, namely

• aedis::resp3::request: A container of Redis commands.
• aedis::adapt(): A function that adapts data structures to receive Redis responses.
• aedis::connection: A connection to the Redis server.

before that however, users might find it helpful to skim over the examples, to gain a better feeling about the library capabilities

• intro.cpp: The Aedis hello-world program. It sends one command to Redis and quits the connection.
• intro_tls.cpp: Same as intro.cpp but over TLS.
• containers.cpp: Shows how to send and receive stl containers and 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.
• echo_server.cpp: A simple TCP echo server.
• chat_room.cpp: A command line chat room built on Redis pubsub.

Requests

Redis requests are composed of one or 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", "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_range("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 performed with aedis::connection::async_exec as already stated.

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 define a to_bulk function like this

// Example struct.
struct mystruct {...};

// Serialize your data structure here.
void to_bulk(std::pmr::string& to, mystruct const& obj)
{
   std::string dummy = "Dummy serializaiton string.";
   aedis::resp3::to_bulk(to, dummy);
}

Once to_bulk is defined and visible 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

Aedis uses the following strategy to support Redis responses

• Static: For aedis::resp3::request whose sizes are known at compile time std::tuple is supported.
• Dynamic: Otherwise use std::vector<aedis::resp3::node<std::string>>.

For example, below is a request with a compile time size

request req;
req.push("PING");
req.push("INCR", "key");
req.push("QUIT");

To read the response to this request users can use the following tuple

std::tuple<std::string, int, std::string>

The pattern may have become apparent to the user, the tuple must have the same size as the request (exceptions below) and each element must be able to store the response to the command it refers to. To ignore responses to individual commands in the request use the tag aedis::ignore

// Ignore the second and last responses.
std::tuple<std::string, aedis::ignore, std::string, aedis::ignore>

The following table provides the response types of some commands

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

To map these RESP3 types into a C++ data structure use the table below

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, the response to the request

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");

can be read in the tuple below

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

Where both are passed to async_exec as showed elsewhere

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

If the intention is to ignore the response to all commands altogether use adapt() without arguments instead

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

Responses that contain nested aggregates or heterogeneous data types will be given special treatment later in The general 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.

Push

Commands that have push response like

• "SUBSCRIBE"
• "PSUBSCRIBE"
• "UNSUBSCRIBE"

must be not be included in the tuple. For example, the request below

request req;
req.push("PING");
req.push("SUBSCRIBE", "channel");
req.push("QUIT");

must be read in this tuple std::tuple<std::string, std::string>, that has size two.

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::tuple<
   std::optional<A>,
   std::optional<B>,
   ...
   > response;

co_await conn->async_exec(req, adapt(response));

Everything else stays pretty much the same.

Transactions

To read responses to transactions we must first observe that Redis will queue its commands and send their responses to the user as elements of an array, after the EXEC command comes. For example, to read the response to this request

req.push("MULTI");
req.push("GET", "key1");
req.push("LRANGE", "key2", 0, -1);
req.push("HGETALL", "key3");
req.push("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<
   aedis::ignore,  // multi
   aedis::ignore,  // get
   aedis::ignore,  // lrange
   aedis::ignore,  // hgetall
   exec_resp_type, // exec
> resp;

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

For a complete example see containers.cpp.

Deserialization

As mentioned in \ref serialization, it is common practice to serialize data before sending it to Redis e.g. as json strings. For performance and convenience reasons, we may also want to deserialize it directly in its final data structure when reading them back from Redis. 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 aedis::resp3::node type abstraction, that is the most general form of an element in a response, be it a simple RESP3 type or the element of 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 not different than using other types

// Receives any RESP3 simple or aggregate data type.
std::vector<node<std::string>> resp;
co_await conn->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 and other data structures in general.

Connection

The aedis::connection is a class that provides async-only communication with a Redis server by means of three member functions

• connection::async_run: Starts read and write operations and remains suspended until the connection it is lost.
• connection::async_exec: Executes commands.
• connection::async_receive: Receives server-side pushes.

In general, these operations will be running concurrently in user application, where, for example

1. Run: One coroutine will call async_run, perhaps in a loop and with healthy checks.
2. Execute: Multiple coroutines will call async_exec independently and without coordination (e.g. queuing).
3. Receive: One coroutine will loop on async_receive to receive server-side pushes (required only if the app expects server pushes).

Each of these operations can be performed without regards to the others as they are independent from each other. Below we will cover the points above with more detail.

Run

The code snipet in the overview section has shown us an example that used connection::async_run in short-lived connection, in the general case however, applications will connect to a Redis server and hang around for as long as possible, until the connection is lost for some reason. When that happens, simple setups will want to wait for a short period of time and try to reconnect. To support this usage pattern Aedis connections can be reconnected while there are pending requests and receive operations. The general form of a reconnect loop looks like this (see subscriber.cpp)

auto async_main() -> net::awaitable<void>
{
   auto ex = co_await net::this_coro::executor;
   auto conn = std::make_shared<connection>(ex);
   signal_set_type sig{ex, SIGINT, SIGTERM};
   timer_type timer{ex};

   request req;
   req.get_config().cancel_on_connection_lost = true;
   req.push("HELLO", 3);
   req.push("SUBSCRIBE", "channel");

   // Loop to reconnect on connection lost. To exit type Ctrl-C twice.
   for (;;) {
      co_await connect(conn, "127.0.0.1", "6379");

      // Starts async_run and other operations.
      co_await ((conn->async_run() || healthy_checker(conn) || sig.async_wait() ||
               receiver(conn)) && conn->async_exec(req));

      // Prepare for a reconnect.
      conn->reset_stream();

      // Waits some time before reconnecting.
      timer.expires_after(std::chrono::seconds{1});
      co_await timer.async_wait();
   }
}

It is important to emphasize that Redis servers use the old communication protocol RESP2 by default, therefore it is necessary to send a HELLO 3 command everytime a connection is established. Another common scenarios for reconnection is, for example, a failover with sentinels, covered in resolve_with_sentinel.cpp example.

Execute

The basic idea about async_exec was stated above already: execute Redis commands. One of the most important things about it however is that it can be called multiple times without coordination, for example, in a HTTP or Websocket server where each session calls it independently to communicate with Redis. The benefits of this feature are manifold

• Reduces code complexity as users won't have to implement queues every time e.g. different HTTP sessions want to share a connection to Redis.
• A small number of connections improves the performance associated with pipelines. A single connection will be indeed enough in most of cases.

There are some important things about connection::async_exec that are worth stating here

• connection::async_exec will write a request and read the response directly in the data structure passed by the user, avoiding temporaries altogether.
• Requests belonging to different async_exec will be coalesced in a single payload (pipelined) and written only once, improving performance massively.
• Users have full control whether async_exec should remain suspended if a connection is lost, (among other things). See aedis::resp3::request::config.

The code below illustrates this concepts in a TCP session of the echo_server.cpp example

auto echo_server_session(tcp_socket socket, std::shared_ptr<connection> db) -> net::awaitable<void>
{
   request req;
   std::tuple<std::string> response;

   for (std::string buffer;;) {
      // Reads a user message.
      auto n = co_await net::async_read_until(socket, net::dynamic_buffer(buffer, 1024), "\n");

      // Echos it through Redis.
      req.push("PING", buffer);
      co_await conn->async_exec(req, adapt(response));

      // Writes is back to the user.
      co_await net::async_write(socket, net::buffer(std::get<0>(response)));

      // Cleanup
      std::get<0>(response).clear();
      req.clear();
      buffer.erase(0, n);
   }
}

Notice also how the session above provides back-pressure as the coroutine won't read the next message from the socket until a cycle is complete.

Receive

Receiving Redis pushes works similar to the async_exec discussed above but without the request. The example below was taken from subscriber.cpp

net::awaitable<void> push_receiver(std::shared_ptr<connection> conn)
{
   for (std::vector<node<std::string>> resp;;) {
      co_await conn->async_receive(adapt(resp));
      print_push(resp);
      resp.clear();
   }
}

In general, it is advisable to all apps to keep a coroutine calling async_receive as an unread push will cause the connection to stall and eventually timeout. Notice that the same connection that is being used to send requests can be also used to receive server-side pushes.

Cancellation

Aedis supports both implicit and explicit cancellation of connection operations. Explicit cancellation is supported by means of the aedis::connection::cancel member function. Implicit cancellation, like those that may happen when using Asio awaitable operators && and || will be discussed with more detail below.

co_await (conn.async_run(...) && conn.async_exec(...))

• Useful when implementing reconnection.
• async_exec is responsible for sending the HELLO command and optionally for subscribing to channels.

co_await (conn.async_run(...) || conn.async_exec(...))

• Useful for short-lived connections that are meant to be closed after a command has been executed.

co_await (conn.async_exec(...) || time.async_wait(...))

• Provides a way to limit how long the execution of a single request should last.
• The cancellation will be ignored if the request has already been written to the socket.
• It is usually a better idea to have a healthy checker that adding per request timeout, see subscriber.cpp for an example.

co_await (conn.async_run(...) || time.async_wait(...))

• Sets a limit on how long the connection should live.

co_await (conn.async_exec(...) || conn.async_exec(...) || ... || conn.async_exec(...))

• This works but is considered an antipattern. Unless the user has set aedis::resp3::request::config::coalesce to false, and he shouldn't, the connection will automatically merge the individual requests into a single payload anyway.

Why Aedis

The main reason for why I started writing Aedis was to have a client compatible with the Asio asynchronous model. As I made progresses I could also address what I considered weaknesses in other libraries. Due to time constraints I won't be able to give a detailed comparison with each client listed in the official list, instead I will focus on the most popular C++ client on github in number of stars, namely

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

Before we start it is important to mentioning some of the things redis-plus-plus does not support

• The latest version of the communication protocol RESP3. Without it it is impossible to support some important Redis features like client side caching, among other things.
• Coroutines.
• Reading responses directly in user data structures to avoid creating temporaries.
• Error handling with support for error-code.
• Cancellation.

The remaining points will be addressed individually. 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 in 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 this design (if at all).

Echo server benchmark

This document benchmarks the performance of TCP echo servers I implemented in different languages using different Redis clients. The main motivations for choosing an echo server are

• Simple to implement and does not require expertise level in most languages.
• I/O bound: Echo servers have very low CPU consumption in general and therefore are excelent to measure how a program handles concurrent requests.
• It simulates very well a typical backend in regard to concurrency.

I also imposed some constraints on the implementations

• It should be simple enough and not require writing too much code.
• Favor the use standard idioms and avoid optimizations that require expert level.
• Avoid the use of complex things like connection and thread pool.

To reproduce these results run one of the echo-server programs in one terminal and the echo-server-client in another.

Without Redis

First I tested a pure TCP echo server, i.e. one that sends the messages directly to the client without interacting with Redis. The result can be seen below

The tests were performed with a 1000 concurrent TCP connections on the localhost where latency is 0.07ms on average on my machine. On higher latency networks the difference among libraries is expected to decrease.

• I expected Libuv to have similar performance to Asio and Tokio.
• I did expect nodejs to come a little behind given it is is javascript code. Otherwise I did expect it to have similar performance to libuv since it is the framework behind it.
• Go did surprise me: faster than nodejs and libuv!

The code used in the benchmarks can be found at

• Asio: A variation of this Asio example.
• Libuv: Taken from here Libuv example .
• Tokio: Taken from here.
• Nodejs
• Go

With Redis

This is similar to the echo server described above but messages are echoed by Redis and not by the echo-server itself, which acts as a proxy between the client and the Redis server. The results can be seen below

The tests were performed on a network where latency is 35ms on average, otherwise it uses the same number of TCP connections as the previous example.

As the reader can see, the Libuv and the Rust test are not depicted in the graph, the reasons are

• redis-rs: This client comes so far behind that it can't even be represented together with the other benchmarks without making them look insignificant. I don't know for sure why it is so slow, I suppose it has something to do with its lack of automatic pipelining support. In fact, the more TCP connections I lauch the worse its performance gets.

• Libuv: I left it out because it would require me writing to much c code. More specifically, I would have to use hiredis and implement support for pipelines manually.

The code used in the benchmarks can be found at

• Aedis: code
• node-redis: code
• go-redis: code

Conclusion

Redis clients have to support automatic pipelining to have competitive performance. For updates to this document follow https://github.com/mzimbres/aedis.

Reference

• High-Level: Covers the topics discussed in this document.
• Low-Level: Covers low-level building blocks. Provided mostly for developers, most users won't need any information provided here.

Installation

Download the latest release on https://github.com/mzimbres/aedis/releases. Aedis is a header only library, so you can starting using it right away by adding the include subdirectory to your project and including

#include <aedis/src.hpp>

in no more than one source file in your applications. For example, to compile one of the examples manually

g++ -std=c++20 -pthread -I/opt/boost_1_79_0/include/ -Iinclude -Iexamples examples/intro.cpp examples/common.cpp

The requirements for using Aedis are

• Boost 1.79 or greater.
• C++17 minimum.
• Redis 6 or higher (must support RESP3).
• Optionally also redis-cli and Redis Sentinel.

The following compilers are supported

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

Acknowledgement

Acknowledgement to people that helped shape Aedis in one way or another.

• Richard Hodges (madmongo1): For very helpful support with Asio, the design of asynchronous programs, etc.
• Vinícius dos Santos Oliveira (vinipsmaker): For useful discussion about how Aedis consumes buffers in the read operation.
• Petr Dannhofer (Eddie-cz): For helping me understand how the AUTH and HELLO command can influence each other.
• Mohammad Nejati (ashtum): For pointing out scenarios where calls to async_exec should fail when the connection is lost.
• Klemens Morgenstern (klemens-morgenstern): For useful discussion about timeouts, cancellation, synchronous interfaces and general help with Asio.

Changelog

v1.3.0

• Removes automatic sending of the HELLO command. This can't be implemented properly without bloating the connection class. It is now a user responsability to send HELLO. Requests that contain it have priority over other requests and will be moved to the front of the queue, see aedis::resp3::request::config

• Automatic name resolving and connecting have been removed from aedis::connection::async_run. Users have to do this step manually now. The reason for this change is that having them built-in doesn't offer enough flexibility that is need for boost users.

• Removes healthy checks and idle timeout. This functionality must now be implemented by users, see the examples. This is part of making Aedis useful to a larger audience and suitable for the Boost review process.

• The aedis::connection is now using a typeddef to a net::ip::tcp::socket and aedis::ssl::connection to net::ssl::stream<net::ip::tcp::socket>. Users that need to use other stream type must now specialize aedis::basic_connection.

• Adds a low level example of async code.

v1.2.0

• aedis::adapt supports now tuples created with std::tie. aedis::ignore is now an alias to the type of std::ignore.

• Provides allocator support for the internal queue used in the aedis::connection class.

• Changes the behaviour of async_run to complete with success if asio::error::eof is received. This makes it easier to write composed operations with awaitable operators.

• Adds allocator support in the aedis::resp3::request (a contribution from Klemens Morgenstern).

• Renames aedis::resp3::request::push_range2 to push_range. The suffix 2 was used for disambiguation. Klemens fixed it with SFINAE.

• Renames fail_on_connection_lost to aedis::resp3::request::config::cancel_on_connection_lost. Now, it will only cause connections to be canceled when async_run completes.

• Introduces aedis::resp3::request::config::cancel_if_not_connected which will cause a request to be canceled if async_exec is called before a connection has been established.

• Introduces new request flag aedis::resp3::request::config::retry that if set to true will cause the request to not be canceled when it was sent to Redis but remained unresponded after async_run completed. It provides a way to avoid executing commands twice.

• Removes the aedis::connection::async_run overload that takes request and adapter as parameters.

• Changes the way aedis::adapt() behaves with std::vector<aedis::resp3::node<T>>. Receiving RESP3 simple errors, blob errors or null won't causes an error but will be treated as normal response. It is the user responsibility to check the content in the vector.

• Fixes a bug in connection::cancel(operation::exec). Now this call will only cancel non-written requests.

• Implements per-operation implicit cancellation support for aedis::connection::async_exec. The following call will co_await (conn.async_exec(...) || timer.async_wait(...)) will cancel the request as long as it has not been written.

• Changes aedis::connection::async_run completion signature to f(error_code). This is how is was in the past, the second parameter was not helpful.

• Renames operation::receive_push to aedis::operation::receive.

v1.1.0-1

• Removes coalesce_requests from the aedis::connection::config, it became a request property now, see aedis::resp3::request::config::coalesce.

• Removes max_read_size from the aedis::connection::config. The maximum read size can be specified now as a parameter of the aedis::adapt() function.

• Removes aedis::sync class, see intro_sync.cpp for how to perform synchronous and thread safe calls. This is possible in Boost. 1.80 only as it requires boost::asio::deferred.

• Moves from boost::optional to std::optional. This is part of moving to C++17.

• Changes the behaviour of the second aedis::connection::async_run overload so that it always returns an error when the connection is lost.

• Adds TLS support, see intro_tls.cpp.

• Adds an example that shows how to resolve addresses over sentinels, see subscriber_sentinel.cpp.

• Adds a aedis::connection::timeouts::resp3_handshake_timeout. This is timeout used to send the HELLO command.

• Adds aedis::endpoint where in addition to host and port, users can optionally provide username, password and the expected server role (see aedis::error::unexpected_server_role).

• aedis::connection::async_run checks whether the server role received in the hello command is equal to the expected server role specified in aedis::endpoint. To skip this check let the role variable empty.

• Removes reconnect functionality from aedis::connection. It is possible in simple reconnection strategies but bloats the class in more complex scenarios, for example, with sentinel, authentication and TLS. This is trivial to implement in a separate coroutine. As a result the enum event and async_receive_event have been removed from the class too.

• Fixes a bug in connection::async_receive_push that prevented passing any response adapter other that adapt(std::vector<node>).

• Changes the behaviour of aedis::adapt() that caused RESP3 errors to be ignored. One consequence of it is that connection::async_run would not exit with failure in servers that required authentication.

• Changes the behaviour of connection::async_run that would cause it to complete with success when an error in the connection::async_exec occurred.

• Ports the buildsystem from autotools to CMake.

v1.0.0

• Adds experimental cmake support for windows users.

• Adds new class aedis::sync that wraps an aedis::connection in a thread-safe and synchronous API. All free functions from the sync.hpp are now member functions of aedis::sync.

• Split aedis::connection::async_receive_event in two functions, one to receive events and another for server side pushes, see aedis::connection::async_receive_push.

• Removes collision between aedis::adapter::adapt and aedis::adapt.

• Adds connection::operation enum to replace cancel_* member functions with a single cancel function that gets the operations that should be cancelled as argument.

• Bugfix: a bug on reconnect from a state where the connection object had unsent commands. It could cause async_exec to never complete under certain conditions.

• Bugfix: Documentation of adapt() functions were missing from Doxygen.

v0.3.0

• Adds experimental::exec and receive_event functions to offer a thread safe and synchronous way of executing requests across threads. See intro_sync.cpp and subscriber_sync.cpp for examples.

• connection::async_read_push was renamed to async_receive_event.

• connection::async_receive_event is now being used to communicate internal events to the user, such as resolve, connect, push etc. For examples see subscriber.cpp and connection::event.

• The aedis directory has been moved to include to look more similar to Boost libraries. Users should now replace -I/aedis-path with -I/aedis-path/include in the compiler flags.

• The AUTH and HELLO commands are now sent automatically. This change was necessary to implement reconnection. The username and password used in AUTH should be provided by the user on connection::config.

• Adds support for reconnection. See connection::enable_reconnect.

• Fixes a bug in the connection::async_run(host, port) overload that was causing crashes on reconnection.

• Fixes the executor usage in the connection class. Before theses changes it was imposing any_io_executor on users.

• connection::async_receiver_event is not cancelled anymore when connection::async_run exits. This change makes user code simpler.

• connection::async_exec with host and port overload has been removed. Use the other connection::async_run overload.

• The host and port parameters from connection::async_run have been move to connection::config to better support authentication and failover.

• Many simplifications in the chat_room example.

• Fixes build in clang the compilers and makes some improvements in the documentation.

v0.2.0-1

• Fixes a bug that happens on very high load. (v0.2.1)
• Major rewrite of the high-level API. There is no more need to use the low-level API anymore.
• No more callbacks: Sending requests follows the ASIO asynchronous model.
• Support for reconnection: Pending requests are not canceled when a connection is lost and are re-sent when a new one is established.
• The library is not sending HELLO-3 on user behalf anymore. This is important to support AUTH properly.

v0.1.0-2

• Adds reconnect coroutine in the echo_server example. (v0.1.2)
• Corrects client::async_wait_for_data with make_parallel_group to launch operation. (v0.1.2)
• Improvements in the documentation. (v0.1.2)
• Avoids dynamic memory allocation in the client class after reconnection. (v0.1.2)
• Improves the documentation and adds some features to the high-level client. (v.0.1.1)
• Improvements in the design and documentation.

v0.0.1

• First release to collect design feedback.