Database Transactions
Scaling Redis writes with cluster mode
Summarized using AI
Scaling Redis writes with cluster mode
by KJ TsanaktsidisIn the talk titled 'Scaling Redis writes with cluster mode,' KJ Tsanaktsidis from Zendesk discusses the challenges and solutions encountered while migrating to Redis Cluster mode for their write-heavy workload. The presentation outlines the various methods of deploying Redis, from a single instance to Redis Sentinel and Redis Distributed, before arriving at Redis Cluster. Key points include:
Understanding Redis Architecture: KJ explains the simple deployment of a single Redis instance versus more complex architectures like Redis Sentinel for high availability, which doesn't work for write operations in a write-heavy context.
Redis Distributed: This method splits the keyspace across multiple Redis servers but can lead to high availability issues as losing a server means losing half the keys.
Redis Cluster: The talk highlights Redis Cluster’s capability to support horizontal scalability by dividing the keyspace into 16,000 slots and electing masters per slot. This allows for both read and write operations across multiple nodes, though it requires client library support for routing keys correctly.
Challenges Faced: KJ recounts the initial attempt to scale using Redis Cluster, which faced issues like 'cross-slot pipelining errors' due to improper configuration. The understanding of Redis operations and the architecture was essential to troubleshoot these errors.
Contributions to Redis: Zendesk engaged with the Redis open-source community to improve transaction support in the Redis gem, emphasizing the importance of collaboration and maintaining upstream dependencies. They aimed to refine their implementations rather than resorting to hacks.
Concluding, KJ provides valuable lessons about utilizing Redis for write-heavy workloads and engaging with open-source libraries:
- Use Redis Cluster for scaling writes and keep anecdotal historical data in mind.
- Report issues and stay updated with gem versions to benefit from improvements.
- Collaborate with upstream maintainers as everyone has something to offer to open-source initiatives.
00:00:03.439
Hello everyone! Yes, my name is KJ, and as she introduced me, I’m an engineer at Zendesk.
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I’m a committer on the Ruby core team, and today I'm talking about our fun adventures scaling our Redis writes.
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I didn’t have time to rename the talk to something like "Valkyrie" or the other title "Rict", so it’s just "R" here.
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Please feel free to interrupt me if something I say doesn’t make sense, because I'm good at not making sense.
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Also, I should talk a bit about what we do at Zendesk for those who don’t know. We are a customer service software platform.
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We deal with everything from email-based ticketing workflows to advanced AI chat features, and all that good stuff.
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So what is our story about today? Here’s a photo of our office in San Francisco, which is relevant to this talk.
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I work on a team responsible for the overall health, reliability, and well-being of our big million-line monolith application.
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We’re a distributed team, with some members in Australia and many others spread far and wide throughout the United States.
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We usually do all the remote work things, but we had the opportunity last year to come together at our office in San Francisco.
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This team offsite was great; we went bowling, had some food, and accomplished a lot of productive work, like setting our vision for the future and engaging in pair programming.
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However, there was one thing we got to do together in San Francisco that I was not expecting: in-person incident response.
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It felt like the good old days where everyone is in a war room trying to fix an ongoing incident.
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The incident around that table was related to one of our many large instances affectionately known as "MK".
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MK probably gives you an idea of what goes on there. I need to thank my colleagues for suggesting I needed more memes in my presentation.
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The problem with MK was that something had gone wrong — it was using too much CPU.
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The alarm went off, we got paged, and upon investigation, it looked like the issue was related to usage.
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This Redis instance was actually an ElastiCache cluster mode-enabled Redis, but it only had one shard.
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So it wasn't really functioning as a proper cluster with just one node. We considered that, figured it was time to write a check to Mr. Bezos.
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We clicked the button there and said we wanted more shards, but this didn’t fix everything; in fact, it made everything much worse.
00:03:03.920
The original problem wasn’t that severe, but when we scaled out the Redis cluster, we started to see these weird and wonderful errors.
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These were errors we had never seen before, such as the "cross slot pipelining error" and "move 3016" in this picture. They were very serious, causing real problems, and we had no idea why they were happening.
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So, we undid the scaling and reverted back to a one-node cluster.
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After this, we were left questioning why things had gone so wrong.
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We had to spend some time learning how Redis works and understanding what cluster mode entails and why it didn’t work well for us.
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I’m going to give you a speed run of the different ways to deploy Redis that we learned about during this fun learning exercise.
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The simplest version is a single Redis instance.
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If you want something more complex, you could have one server with Redis on it. Choose your server provider of choice, install Redis, and there you have it.
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This is simple and cheap; if it's sufficient, you can save yourself thirty minutes.
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However, sometimes we need more robustness than this setup can provide; we might need high availability.
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If our single Redis server goes down, we need to ensure we still have access to Redis.
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Sometimes, there could be a risk of data loss here, and we might need replication as well.
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This would mean creating a setup where if the Redis server goes down, we don't lose the data it was holding.
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Relevant to this discussion, the goal is to scale out to get as much Redis throughput as possible from one server, but if we need more, a different approach will be required.
00:05:06.960
This brings us to Redis Sentinel, an early approach for high availability.
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Redis Sentinel is distributed as part of Redis but operates as a separate process. This means you have some servers with Redis running, and you can install Sentinel alongside them.
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The Sentinels will collaborate, have an election, and decide who will be the master and who will be the replicas.
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Sentinel then configures Redis to establish who is the master and has to be replicated.
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This setup requires application support as the application must know which of the Redis nodes is the master node.
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The application must ask Sentinel, which will provide the current master node's address, allowing the application to connect appropriately.
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This solution eliminates a single point of failure; if one of the Redis nodes fails, Sentinel promotes one of the replicas to master.
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The application learns about this promotion because it understands Sentinel.
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Additionally, this approach can be used to scale read operations; if you have a read-heavy workload and can tolerate slightly stale data, your application can opt to read from these replica nodes.
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Unfortunately, this multi-master setup doesn’t work for write operations under Sentinel; every write must still go through one Redis instance.
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This leaves us with a problem given our write-heavy workloads.
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Fortunately, there’s a solution in the Redis gem called Redis Distributed.
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Redis Distributed operates on the notion that we can take multiple Redis servers and split the keyspace between them.
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This is accomplished by using a hashing algorithm to determine which keys go where.
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The downside, however, is that this approach can lead to a loss of high availability.
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If one of those Redis servers goes down, we would lose half of our keys, which is not ideal.
00:07:04.479
To address the need for both distribution and high availability, we can combine the idea of Redis Sentinel with Redis Distributed.
00:07:13.920
This gives us independent Redis Sentinel clusters, allowing us to maintain multiple shards.
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With these powers combined, we can have horizontal write scalability; we increase throughput with additional nodes.
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However, we have to keep in mind how to add new nodes effectively.
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Redis Distributed has challenges when it comes to scaling; adding a new node requires you to determine how to rebalance the keyspace.
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The system doesn’t automatically accommodate the redistribution of keys.
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So while Redis Distributed allows you to start at a certain scale, it doesn’t allow easy changes to that scale.
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This limitation highlights the need for Redis Cluster, which is designed to solve the scalability issue.
00:08:01.599
In Redis Cluster mode, we divide the keyspace into 16,000 slots determined by a hashing function, specifically the CRC16 hash.
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The Redis nodes come together and elect a master for each slot, creating a proper cluster.
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In this arrangement, every slot is served by only one primary node, complemented by one or more replicas.
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Your application must know which keys are sent to which servers to ensure effective distribution.
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To support this architecture, we need client library support. The Redis gem provides the capability to understand how to route keys correctly.
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The gem can request cluster node details, returning a list of nodes and their responsible slots.
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This allows the application to send keys to the correct nodes according to the slot they hash to.
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Moreover, Redis Cluster allows changes to the topology; if we add a node, the system can redistribute the slots.
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This process involves copying the current values of the keys and using a consensus mechanism to declare new ownership of slots.
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The application will also be informed about these changes through redirection commands.
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If a request for a key goes to the wrong node, the node will indicate which node is responsible, enabling correct routing.
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This setup leads to horizontal scalability, as we can scale out writes and reads easily as we add more nodes.
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However, Redis Cluster requires complex client libraries to handle tricky situations.
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These situations may include what happens if keys are in the process of being moved or if there's a network partition.
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These issues do not arise when using a single Redis server, but higher complexity is required for clusters.
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At this point, we can understand the taxonomy of Redis and where Redis Cluster fits in.
00:10:00.279
Now, let's return to our incident and analyze what went wrong.
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Our Redis Cluster was configured with just one shard, which means all slots were served by a single master.
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In practical terms, this setup didn’t really function as a true cluster.
00:10:18.400
When we tried to scale out by adding more nodes, we consequently faced numerous issues.
00:10:24.400
Now let's examine the cross-slot pipelining error that we encountered.
00:10:30.480
Redis commands can operate on multiple keys at once. For instance, using Sidekiq Pro involves extensive multi-key operations.
00:10:42.920
Nevertheless, in Redis Cluster, a multi-key operation only works if all keys belong to the same slot.
00:10:55.080
For example, if you attempt to set values for two keys, but they fall into different slots, you’ll receive an error.
00:11:01.919
To work around this, you can use hashtags within your key names.
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This means that if part of your key name contains a brace, Redis only hashes the part inside the brace.
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This allows keys with matching hashtags to belong to the same slot, making them eligible for multi-key operations.
00:11:23.680
Pipelining is another efficiency approach in Redis.
00:11:31.239
Typically, you would request multiple keys' values one by one, but you can also send all the requests at once and collect responses later.
00:11:42.240
This minimizes the request-response roundtrip time and can result in a performance boost.
00:11:48.760
However, the cross-slot pipelining error can arise in situations where multiple keys are involved across different slots.
00:11:56.800
It should work seamlessly in theory, but in practice, we discovered it was not functioning correctly.
00:12:02.760
The good news is that the Redis gem v5 has improved cluster support under a new maintainer, and we learned we needed to upgrade to that version.
00:12:19.440
Thus, after implementing the upgrade in our testing environment, we stopped encountering those cross-slot pipelining errors.
00:12:27.080
However, new errors emerged that were related to transactions.
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Let’s now discuss transactions in Redis and what issues we faced.
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Transactions in Redis can be conceptualized as conditional execution.
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When you execute a transaction, it will only apply changes if the specified keys haven't been modified since the start of the transaction.
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In scenarios with competing modifications, the transaction will fail, preventing problems like negative ticket sales.
00:13:05.680
However, one limitation is that in a Redis cluster, transactions work only when all keys belong to the same slot.
00:13:14.880
While testing our new branch, we encountered ambiguous node errors, which we had experienced previously.
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In fact, transaction support in the Redis Cluster gem was incomplete.
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It facilitated multi-exec but lacked complete watch functionality necessary for conditional execution.
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We were faced with a choice; we could hack around this problem, but we aimed to contribute a proper solution upstream.
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We saw it as an opportunity not only to pay back to open source but also to ensure our solutions aligned well with the community.
00:14:03.359
The discussions also revolved around creating a stable foundation for future development, avoiding unnecessary workarounds.
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Our intention was to ensure that we could contribute meaningful improvements to the Redis ecosystem.
00:14:24.639
In providing clarity on how transactions should execute in Redis, we sought to engender mutual understanding.
00:14:31.040
While we may not have gotten every solution correct initially, each attempt presented vital learning opportunities.
00:14:39.920
I eventually engaged the maintainer in a more productive discussion to clarify our design objectives.
00:14:47.760
We learned the importance of breaking down changes into smaller, manageable pull requests.
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This ensures that both we and the maintainer can effectively analyze changes.
00:14:58.479
At the end of the day, collaboration and understanding remain essential when working with open source libraries.
00:15:06.399
The lesson learned is that engaging with existing projects and their maintainers is rewarding for all parties.
00:15:12.959
Regardless of your familiarity with the code, testing, reporting issues, or suggesting solutions carries immense value.
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Through this process, I found that successful open-source engagement produces benefits for both the contributor and the project.
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At Zendesk, as a company, we understood our responsibility in this ecosystem.
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We aimed to ensure the solutions we utilized were maintainable, rather than relying on specific hacks.
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The investment put into enhancing our libraries promotes long-term stability.
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Contributing to open-source gems is also valuable for personal knowledge growth.
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With the constant evolution of technology, nurturing expertise empowers engineers to solve problems.
00:16:05.760
Eventually, our collective contributions led to significant improvements in the Redis community.
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This culminated in finishing our first major upstream contribution.
00:16:18.840
As part of our efforts toward upgrading to Redis v5, we officially improved transaction support.
00:16:26.159
In our testing environment, we saw efficiency gains, alongside smoother operations.
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Now, the collective efforts have rewarded us to understand where open-source contributions impact our operational success.
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While we aimed to achieve stability and scaling with Redis, we also experienced positive performance optimizations.
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In fact, we observed our performance metrics improved — about 3 to 4 milliseconds gained in response times.
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Although that was not our primary goal, it served as a welcome surprise.
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However, we couldn’t forget there were still processes in progress for getting our patches fully merged.
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Initially, I was disappointed when I realized that we had to ship a fork to production.
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Working upstream can indeed be slower than anticipated.
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This experience taught me that maintainers often have limited capacity.
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We may not always receive quick turnaround on our contributions.
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However, contributing to a project in this manner remains a worthy endeavor.
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We must consider that maintaining open-source projects can be demanding work.
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We often miss the behind-the-scenes efforts that contributors invest, balancing their own responsibilities.
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For this reason, we need to exhibit patience while supporting these projects.
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As contributors, we must therefore be strategic about the implementation of upstream dependencies.
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The lesson learned is to engage with upstream contributors, testing and reporting issues.
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Our experiences not only provide insights for us but also improve the projects we use collectively.
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Let's summarize some key takeaways about Redis and contributions to open source.
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On the Redis front, if you're dealing with a write-heavy workload, you might want to consider using Redis Cluster.
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Start with a single node cluster and scale it out only when absolutely necessary, saving you costs.
00:19:00.760
If you pursue this path, the Redis Cluster client gem provides support for operations like pipelining and transactions.
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And if you encounter any issues, please report them so we can work together to resolve them.
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More generally, always keep your gems up to date; you don’t know what advantages you might be losing.
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Additionally, engage with upstream maintainers, as this can lead to fruitful collaborations.
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Remember, everyone has something to contribute to open-source projects.
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Even simple feedback, like sharing your usage experiences, is invaluable.
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Thank you for your attention, and feel free to find me later to discuss further.
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You can also follow me on social media, and I hope to connect with you soon!
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