While teaching the course of Using Oracle Machine Learning with Autonomous Database, I got the question from a participant that what are these services High, Medium and Low that we are using while connecting to the autonomous database. So here is a quick answer of that.
When you are working with Autonomous Database and you are intending to connect to it using a tool like SQL Developer, you will be required to download the wallet credential zip file. In that zip file, you will be able to see several files including TNSNAMES.ora file as well. If you are going to open that TNSNAMES.ora file, you will see that it contains entries of three pre-defined services with their description- High, Medium and Low. Here is an abbreviated entry from one of such wallet files.
xx_high = (description= (retry_count=20)(retry_delay=3)(address=(protocol=tcps)(port=1522)(host=somehost))(connect_data=(service_name=some_service_name))…
Similar to the above service, two more entries will also be there for Low and Medium. These three pre-defined services not only allows one to connect to the Autonomous database but also they give the ability to setup performance and concurrency levels for the users. Each of these three services provide different levels of numbers of concurrent SQL statements allowed to run along with the amount of CPU and IO resources to be available for them.
Hope that helps in understanding these 3 services in the Autonomous database. For more details, I shall highly recommend to read this section from the documentation of autonomous-data-warehouse-cloud.