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Discover how to optimize data retrieval from multiple Redis hashes in one request using Lua scripting in StackExchange.Redis.
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This video is based on the question https://stackoverflow.com/q/71951308/ asked by the user 'Romulus Urakagi Ts'ai' ( https://stackoverflow.com/u/290163/ ) and on the answer https://stackoverflow.com/a/71951658/ provided by the user 'Marc Gravell' ( https://stackoverflow.com/u/23354/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Efficiently HGET From Multiple Hashes in StackExchange.Redis

If you're working with Redis and using the StackExchange.Redis library, you may encounter a situation where you need to retrieve values from multiple hashes that share the same key. For example, let's say you have several hashes, and you're particularly interested in all values associated with a specific key across them. Doing this efficiently is crucial, especially if you're handling a large number of hashes. This guide will guide you through a solution that allows you to fetch all relevant values in one network request instead of multiple round trips to the Redis server.

The Problem: Redundant Network Requests

Imagine you have a scenario where you maintain around 1000 hashes, and multiple hashes may contain the same key. For instance:

Hash1: {1: 1, 2: 1, 3: 5, 4: 7 ...}

Hash2: {1: 4, 3: 5, 9: 7 ...}

Hash3: {5: 4, 8: 5, 9: 7 ...}

You want to fetch the values corresponding to key 1 from all these hashes. A straightforward approach might involve iterating over each hash key and making individual requests, which leads to redundancy. Here's what that might look like in C-:

[[See Video to Reveal this Text or Code Snippet]]

This method results in hashKeys.Count() network calls, which can be inefficient depending on the number of hashes.

The Solution: Utilizing Lua Scripting

While Redis does not provide a built-in API to fetch values from multiple hashes in one go, you can leverage Lua scripting to achieve this goal effectively. By using Lua, you can combine all your retrievals into a single script that executes on the Redis server, reducing network overhead.

Step-by-Step Guide:

Create a Lua Script: You will write a Lua script that accepts multiple hash keys and iteratively retrieves the value for the specified key.

Use the ScriptEvaluate Method: You will pass your Lua script to the ScriptEvaluate method provided by the StackExchange.Redis client.

Handling the Results: The Lua script will collect results into an array that you can return after execution.

Here's an illustrative example of how your Lua script might look:

[[See Video to Reveal this Text or Code Snippet]]

Implementing in C-

Now, let’s put this into practice with C- code to execute your Lua script:

[[See Video to Reveal this Text or Code Snippet]]

Benefits of This Approach

Reduced Latency: By executing the retrieval as a single operation, you significantly cut down on the number of network requests.

Server-Side Efficiency: Processing on the server reduces the load on your application and network resources.

Scalability: This method scales better with larger datasets, allowing for quicker data retrieval as your application grows.

Conclusion

Using Lua scripting within StackExchange.Redis allows you to efficiently retrieve values from multiple hashes with shared keys, overcoming the limitations of making numerous network requests. Whether you're managing a handful of hashes or scaling to thousands, this approach will enhance your application’s performance and responsiveness. Happy coding!