Comparison with other Redis clients :: Boost.Redis

# Comparison with other Redis clients

## Comparison

The main reason for why I started writing Boost.Redis 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

### Boost.Redis vs Redis-plus-plus

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

- The latest version of the communication protocol RESP3. Without that 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 Boost.Redis there is no difference between sending one command, a pipeline or a transaction because requests are decoupled from the I/O 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).

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