The Read/Write Path

What I’m going to be showing you right now is an example of what happens when we read or write data to the database. And again, this is what’s happening under the hood. So this is for you to understand how ScyllaDB works. As you saw in the lab that. I showed you at the beginning, when I interact with the database, when I perform a query, I don’t have to worry about all this, how ScyllaDB replicates data, how it knows which node is responsible for the data and so on. But it is important to understand how it works. Mostly, so you can create the data model for your application and understand different trade-offs of how you can use the database. Great. So we have an example here.. We have a cluster with five different nodes V, W, X, Y, and Z. The replication factor here is defined as three. RF equals three. And if you remember, that means that each piece of data is going to be replicated to three different nodes. So we’re going to hold three copies of each piece of data and each copy is going to be stored in a different node. The consistency level is defined as quorum. And that means that in order for an operation or a query to be successful, it would require that quorum, which in our case is two out of three nodes, acknowledge that query. If that happens, the query is going to be successful. So let’s see the example. What happens here?. We have a client. That client sends a request to one of the nodes in this example, Node V, and it wants to write a piece of data. Let’s call that piece of data “one” just to simplify things and not look at the entire row. For this specific example node V is designated the coordinator node. It’s the coordinator node only for this specific request and that means that it’s responsible for executing this query. Now, if you remember what I showed you before, node V would execute the consistent hash function or the practitioner on the partition key of the piece of data “one.” According to the hash, it would know which of the three nodes in the cluster are responsible for this piece of data. In this example, say these are the nodes W, X and Z so the coordinator node would pass that information along, the request write the piece of data “one” to those three replica nodes. And in this case, we can see that the data is indeed added to those three nodes. They write the data and they send an acknowledgment back to our coordinator node. They send the ack back. Now, in this specific example, the acknowledgment from nodes. W and Z indeed passed back to the coordinator node, but for some reason the acknowledgment from node X did not pass to the coordinator node. Say there is a network failure and node X cannot pass the information along. So what do you think would happen in this case? If you can quickly write that in the chat window, would knode. V return success to the client? Or would the operation fail? If you can quickly write in the chat window what you think would happen. So I see a few people think it would be successful. I see one person thinks it’s a failure. Okay, so I see most of you think that it’s going to be successful. Some think it’s going to fail. So the answer is that this specific operation is going to be considered successful and the coordinator node V passes an acknowledgment back to the client. The operation was successful. The reason for this being that we define the consistency level to be quorum. So two out of three nodes, three replica nodes need to acknowledge the request for it to be successful, and that’s why the operation is successful. And we return success to the client. If in this same exact example we would define the operation to have a consistency level of All instead of quorum, in that case, then the operation would be considered a failure. Node V would not return an acknowledgment and success to the client and the operation would fail. Okay, so it really depends on the replication factor and the consistency level and that’s something to keep in mind when you’re writing your application, when you create your data model. And I’m going to touch more on this in the next talk “Core concepts in ScyllaDB” the one in the essentials track right after this one.