True Cache
Description
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Lois: Hello and welcome to the Oracle University Podcast. I’m Lois Houston, Director of Innovation Programs with Oracle University, and with me is Nikita Abraham, Principal Technical Editor.
Nikita: Hi everyone! Last week, we had quite a power-packed episode. We discussed the 23ai new feature for Automatic SQL Plan Management. We also looked at the 23ai automatic feature that enhances SecureFiles LOB Write Performance as well as the update to Wide Columns.
Lois: Yeah, and in today’s episode, we will look at True Cache, another 23ai new feature. To tell us all about it, we have Bill Millar back with us. Bill is a Senior Principal Database & MySQL Instructor with Oracle University. We'll ask Bill to give us an overview of True Cache, talk about its configuration and deployment, and discuss how to apply True Cache to our applications.
Nikita: To kick things off, Bill, can you give us a high-level overview of what True Cache is? How does it differ from other caching solutions like Redis or Memcached?
Bill: True Cache is an in-memory cache. It is read-only. True Cache is deployed in front of a primary database, and it is automatically managed. It keeps the most frequently accessed data in the cache, and it keeps the cache consistent with the primary database. They call it diskless, but it's not. It does require some space for SP file, redo logs, control files, and such. But it's very similar to Active Data Guard.
The queries can be offloaded to the True Cache for faster query response. And the data in the query cache is consistent. Unlike other mid-tier caches like Redis or Memcached, a query to the True Cache returns only committed data, and the data is always consistent. It's secure. Why? Because we implement our Oracle database security policies and you can control access to the cache.
Lois: So, why should we use True Cache?
Bill: Improve application performance without having to rewrite any applications. That can save considerable amount of time, effort, and expense. Reduces the application response time. So the closer the True Cache is to the application, the faster the response. Now, you do need a large amount of memory. We're talking memory here. It's an in-memory storage area, and depending on how you configure it, you can have it shared, you can have it divided. We mentioned it's automatically maintained. So there's no application changes required, and it is transparent to the application. Again, simplifies that development and maintenance.
Nikita: How does it impact application performance, and what kind of scenarios would benefit the most from implementing True Cache?
Bill: So at a high-level view, True Cache or primary database, the application configuration serves as other things that are going to decide where is it going to query the data from, from the True Cache or from the primary database.
The True Cache satisfies that query. And that's where the data will be fetched from. If not, then from the primary database. On start up, True Cache is empty. So it starts reading large chunks of data to populate the True Cache. So after a block is cached, then again, it can be automatically updated, apply the redo to it-- very similar to the Oracle Active Data Guard. In the data returned, it is always going to be consistent.
Lois: Is it going to be current data?
Bill: Maybe, maybe not. If it's been updated in the primary, if they redo apply hasn't occurred yet, then it's not the most consistent. But as far as the query cache is concerned, it is the most current because we only display consistent. You can have multiple True Caches. You can save the same database application service to the True Cache as you can partition it.
Nikita: I'm curious about the memory requirements, Bill. How crucial is memory for True Cache's performance?
Bill: You need to have significant amount of memory. Memory, memory, memory. So True Cache is completely memory, memory. So I want to have all my data possible in there. The more memory you have, the less likely something is going to age out. And of course, just like with the standard caching, you can also pin objects to stay in the True Cache.
Yeah, like I said, there are some requirements for storage, even though it's called diskless because of, again, redo log files, configuration files like the control files, SP file. And again it is read only.
Lois: Can you explain the differences between using physical and logical connections with True Cache? How does this impact the way applications interact with the database?
Bill: So with using the True Cache, we have two physical connections, and we can have one to the primary database and one to the True Cache. Each connection has a database application service associated with it, and it's going to choose which connection to use based whether it's going to go to the True Cache or to the primary database.
The second way is the application maintains one logical connection that uses the application service for the primary database. It's the JDBC thin driver, starting with Oracle Database 23 is available. It's going to maintain the physical connections to the primary database and the True Cache itself. Now, the logical connection, the one logical and one physical, is for Java applications only.
Applications that work with JSON, we extend the HTTP entity tag support for that. So a database GET request to the True Cache is going to compute the ETag, insert it into the return document.
Nikita: But what happens if there’s a mismatch when the modified document is put back into the primary database?
Bill: Well, then the database is going to verify. OK, what happens with that?
It's going to verify the document row still matches that ETag for that. If with that put command, let's say, I have new data here, the row is going to match that ETag that was automatically updated. If there's no match, another user has changed the data and the PUT request is rejected. So the PUT request can be retired using the new data.
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Nikita: Welcome back! Now, how do you configure True Cache, Bill?
Bill: You can configure True Cache one of two ways. You can either use the Database Configuration Assistant, which actually makes it a little simpler to configure it, and you can also manually create it.
You have some environment options. One is a uniform configuration where you can deploy identical True Cache that use the same database application service. Another way is partition configuration. The data is going to be divided across multiple True Caches, which, each cache is a different subset of the data. You can also deploy True Cache in a RAC environment. As one might expect, there are some additional configuration steps for a RAC environment.
You want to make sure you verify your configuration, that the database application services are working as expected after you configure it. And then, optionally, you can enable DML redirection. What that will do, it writes data to the primary database, and that data is automatically updated in the cache. It's very similar how to the Oracle Active Data Guard works. Because the DML redirection uses more resources, it's not recommended for update-intensi
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