AWS database services part 2

Part one https://osamaoracle.com/2023/01/03/aws-database-services/

Amazon RDS

Amazon RDS is a web service that makes it easy to set up, operate, and scale a relational database in the cloud. It provides cost-efficient and resizable capacity while managing time-consuming database administration tasks. This allows you to focus on your applications and business. Amazon RDS gives you access to the full capabilities of a MySQL, Oracle, SQL Server, or Aurora database engines. This means that the code, applications, and tools you already use today with your existing databases can be used with Amazon RDS. 

Amazon RDS automatically patches the database software and backs up your database. It stores the backups for a user-defined retention period and provides point-in-time recovery. You benefit from the flexibility of scaling the compute resources or storage capacity associated with your relational DB instance with a single API call.

Amazon RDS is available on six database engines, which optimize for memory, performance, or I/O. The database engines include:

• Amazon Aurora
• PostgreSQL
• MySQL
• MariaDB
• Oracle Database
• SQL Server

Amazon RDS Multi-AZ deployments

Amazon RDS Multi-AZ deployments provide enhanced availability and durability for DB instances, making them a natural fit for production database workloads. When you provision a Multi-AZ DB instance, Amazon RDS synchronously replicates the data to a standby instance in a different Availability Zone. 

You can modify your environment from Single-AZ to Multi-AZ at any time. Each Availability Zone runs on its own physically distinct, independent infrastructure and is engineered to be highly reliable. Upon failure, the secondary instance picks up the load. Note that this is not used for read-only scenarios.

Read replicas

With Amazon RDS, you can create read replicas of your database. Amazon automatically keeps them in sync with the primary DB instance. Read replicas are available in Amazon RDS for Aurora, MySQL, MariaDB, PostgreSQL, Oracle, and Microsoft SQL Server. Read replicas can help you:

• Relieve pressure on your primary node with additional read capacity.
• Bring data close to your applications in different AWS Regions.
• Promote a read replica to a standalone instance as a disaster recovery (DR) solution if the primary DB instance fails.

You can add read replicas to handle read workloads so your primary database doesn’t become overloaded with read requests. Depending on the database engine, you can also place your read replica in a different Region from your primary database. This gives you the ability to have a read replica closer to a particular locality.

You can configure a source database as Multi-AZ for high availability and create a read replica (in Single-AZ) for read scalability. With RDS for MySQL and MariaDB, you can also set the read replica as Multi-AZ, and as a DR target. When you promote the read replica to be a standalone database, it will be replicated to multiple Availability Zones.

Amazon DynamoDB tables

DynamoDB is a fully managed NoSQL database service. DynamoDB uses primary keys to uniquely identify each item in a table and secondary indexes to provide more querying flexibility. When creating a table, you must specify a table name and a primary key. These are the only two required entities.

There are two types of primary keys supported:

• Simple primary key: A simple primary key is composed of just one attribute designated as the partition key. If you use only the partition, no two items can have the same value.
• Composite primary key: A composite primary key is composed of both a partition key and a sort key. In this case the partition key value for multiple items can be the same, but their sort key values must be different.

You work with the core components: tables, items, and attributes. A table is a collection of items, and each item is a collection of attributes. In the example above, the table includes two items, with primary keys Nikki Wolf and John Stiles. The item with the primary key Nikki Wolf includes three attributes: Role, Year, and Genre. The primary key for John Stiles includes a Height attribute, and it does not include the Genre attribute.

Amazon DynamoDB consistency options

When your application writes data to a DynamoDB table and receives an HTTP 200 response (OK), the write has occurred and is durable. The data is eventually consistent across all storage locations, usually within one second or less. DynamoDB supports eventually consistent and strongly consistent reads.

DynamoDB uses eventually consistent reads, unless you specify otherwise. Read operations (such as GetItem, Query, and Scan) provide a ConsistentRead parameter. If you set this parameter to true, DynamoDB uses strongly consistent reads during the operation.

EVENTUALLY CONSISTENT READS

When you read data from a DynamoDB table, the response might not reflect the results of a recently completed write operation. The response might include some stale data. If you repeat your read request after a short time, the response should return the latest data.

STRONGLY CONSISTENT READS

When you request a strongly consistent read, DynamoDB returns a response with the most up-to-date data, reflecting the updates from all prior write operations that were successful. A strongly consistent read might not be available if there is a network delay or outage.

Amazon DynamoDB global tables

A global table is a collection of one or more DynamoDB tables, all owned by a single AWS account, identified as replica tables. A replica table (or replica, for short) is a single DynamoDB table that functions as part of a global table. Each replica stores the same set of data items. Any given global table can only have one replica table per Region, and every replica has the same table name and the same primary key schema.

DynamoDB global tables provide a fully managed solution for deploying a multi-Region, multi-active database, without having to build and maintain your own replication solution. When you create a global table, you specify the AWS Regions where you want the table to be available. DynamoDB performs all the necessary tasks to create identical tables in these Regions and propagate ongoing data changes to all of them.

Database Caching

Without caching, EC2 instances read and write directly to the database. With a caching, instances first attempt to read from a cache which uses high performance memory. They use a cache cluster that contains a set of cache nodes distributed between subnets. Resources within those subnets have high-speed access to those nodes.

Common caching strategies

There are multiple strategies for keeping information in the cache in sync with the database. Two common caching strategies include lazy loading and write-through.

Lazy loading

In lazy loading, updates are made to the database without updating the cache. In the case of a cache miss, the information retrieved from the database can be subsequently written to the cache. Lazy loading ensures that the data loaded in the cache is data needed by the application but can result in high cache-miss-to-cache-hit ratios in some use cases.

Write-through

An alternative strategy is to write through to the cache every time the database is accessed. This approach results in fewer cache misses. This improves performance but requires additional storage for data, which may not be needed by the applications.

Managing your cache

As your application writes to the cache, you need to consider cache validity and make sure that the data written to the cache is accurate. You also need to develop a strategy for managing cache memory. When your cache is full, you determine which items should be deleted by setting an eviction policy.

CACHE VALIDITY

Lazy loading allows for stale data but doesn’t fail with empty nodes. Write-through ensures that data is always fresh but can fail with empty nodes and can populate the cache with superfluous data. By adding a time to live (TTL) value to each write to the cache, you can ensure fresh data without cluttering up the cache with extra data. 

TTL is an integer value that specifies the number of seconds or milliseconds, until the key expires. When an application attempts to read an expired key, it is treated as though the data is not found in cache, meaning that the database is queried and the cache is updated. This keeps data from getting too stale and requires that values in the cache are occasionally refreshed from the database.

MANAGING MEMORY

When cache memory is full, the cache engine removes data from memory to make space for new data. It chooses this data based on the eviction policy you set. An eviction policy evaluates the following characteristics of your data:

• Which were accessed least recently?
• Which have been accessed least frequently?
• Which have a TTL set and the TTL value?

Amazon Elasticache

Amazon ElastiCache is a web service that makes it easy to set up, manage, and scale a distributed in-memory data store or cache environment in the cloud. When you’re using a cache for a backend data store, a side-cache is perhaps the most commonly known approach. Redis and Memcached are general-purpose caches that are decoupled from the underlying data store.

Use ElastiCache for Memcached for data-intensive apps. The service works as an in-memory data store and cache to support the most demanding applications requiring sub-millisecond response times. It is fully managed, scalable, and secure—making it an ideal candidate for cases where frequently accessed data must be in memory. The service is a popular choice for web, mobile apps, gaming, ad tech, and e-commerce. 

ElastiCache for Redis is an in-memory data store that provides sub-millisecond latency at internet scale. It can power the most demanding real-time applications in gaming, ad tech, e-commerce, healthcare, financial services, and IoT. 

ElastiCache engines

	ElastiCache for Memcached	ElastiCache for Redis
Simple cache to offload database burden	Yes	Yes
Ability to scale horizontally for writes and storage	Yes	Yes (if cluster mode is enabled)
Multi-threaded performance	Yes	–
Advanced data types	–	Yes
Sorting and ranking data sets	–	Yes
Pub and sub capability	–	Yes
Multi-AZ with Auto Failover	–	Yes
Backup and restore	–	Yes

Amazon DynamoDB Accelerator

DynamoDB is designed for scale and performance. In most cases, the DynamoDB response times can be measured in single-digit milliseconds. However, there are certain use cases that require response times in microseconds. For those use cases, DynamoDB Accelerator (DAX) delivers fast response times for accessing eventually consistent data.

DAX is an Amazon DynamoDB compatible caching service that provides fast in-memory performance for demanding applications.

AWS Database Migration Service

AWS Database Migration Service (AWS DMS) supports migration between the most widely used databases like Oracle, PostgreSQL, SQL Server, Amazon Redshift, Aurora, MariaDB, and MySQL. AWS DMS supports both homogeneous (same engine) and heterogeneous (different engines) migrations.

• The service can be used to migrate between databases on Amazon EC2, Amazon RDS, and on-premises. Either the target or the source database must be located in Amazon EC2. It cannot be used to migrate between two on-premises databases.
• AWS DMS automatically handles formatting of the source data for consumption by the target database. It does not perform schema or code conversion.
• For homogenous migrations, you can use native tools to perform these conversions. For heterogeneous migrations, you can use the AWS Schema Conversion Tool (AWS SCT).

AWS Schema Conversion Tool

The AWS Schema Conversion Tool (AWS SCT) automatically converts the source database schema and a majority of the database code objects. The conversion includes views, stored procedures, and functions. They are converted to a format that is compatible with the target database. Any objects that cannot be automatically converted are marked so that they can be manually converted to complete the migration.

Source databases		Target databases on AWS
Oracle database Oracle data warehouse Azure SQL SQL server Teradata IBM Netezza Greenplum HPE Vertica MySQL and MariaDB PostgreSQL Aurora IBM DB2 LUW Apache Cassandra SAP ASE	AWS SCT	MySQL PostgreSQL Oracle AmazonDB RDS for MySQL Aurora for MySQL RDS for PostgreSQL Aurora PostgreSQL

The AWS SCT can also scan your application source code for embedded SQL statements and convert them as part of a database schema conversion project. During this process, the AWS SCT performs cloud native code optimization by converting legacy Oracle and SQL Server functions to their equivalent AWS service, modernizing the applications at the same time of migration. 

Regards

Osama