Notes

HIGH AVAILABILITY

ELASTICITY & SCALABILITY 101

Elasticity
- used during short time period, such as hours or days
Scalability
-building out infrastructure to meet demands long term
-used over weeks days,moths and years

EC2
scalability - increase instance size as required using reserved instances
elasticity - increase the number of EC2 instances, based on auto scaling

DynamDB
- Scalability - unlimited amount of storage
- elasticity - Increase additional IOPS for for additional spikes in traffic. Decrease IOPS after spike

RDS
- scalability - increase instance size
-elasticity - not very elastic, cant scale rds based on demand

SCALE UP OR SCALE OUT?
Scaling UP
EC2 - increase the instance type for example from t2micro to t2Small

Scaling Out
EC2-adding additional EC2 instances and using autoscaling

RDS Multi AZ Failover
Multi AZ Deployments for MySQL, Oracle, and PostgreSQL engines use synchronous physical replication to keep data on the standby up-to-date with the primary.(done by AMAZON)

Multi AZ deployments for SQL server engine use synchronous logical replication, using SQL server-native Mirroring technology.
Both approaches safeguard in the event of DB instance failure of loss of AZ
exam tip- in the event of failover the rds endpoint(DNS Name not IP Address) Amazon will switch from primary to secondary.

RDS Multi-AZ Failover Advantages
- High Availability
-Backups are taken from secondary which avoids I/O suspension
- Restores are taken from secondary which avoid I/O suspension

Exam Tip: You can force a failover from one AZ to another by rebooting instance. Done by AWS console or RebootDBInstance API call.

RDS Mulit -AZ Failover is not a scaling solution!!!!
Read replicas are used to scale!!

RDS Read Replicas
- read only copies of your database
- used for read heavy database workloads
-can be created via AWS console or CreateDBInstanceReadReplica API
- Can create multiple read replicas and distribute read traffic accordingly
- Can have up to 5 read replicas per primary DB

When would you use Read Replicas?

-scaling beyond compute or I/O capacity of a single DB instance for read heavy database workloads. Excess read traffic can be directed to one or more Read replicas.
- serving read traffic while source DB instance is unavailable.(due to I/O suspension for backups or scheduled maintinence)
-Business reporting or data warehousing. You may want reporting queries to run against your read replica instead of primary prod db instance.

RDS Databases that Support Read replicas
-Mysql
-Postgresql
-MariaDB
-Amazon uses these engines native asynchronous replication to update the read replica.

Creating Read Replicas
AWS will take a snapshot of your database
- If Multi AZ is not enabled the snapshot will be of your primary database and can cause brief I/O suspension for around 1 minute.
- if Muliti AZ is enabled snapshot will be of secondary DB and will not result in performance degradation.

Connecting to Read Replica
- When a new read replica is created you will be able to connect to it using a new end point DNS address.

Read replicas Can be promoted
- You can promote a read replica to its own standalone database. This breaks replication link between primary and secondary.

Exam Tips!!
-You can have up-to 5 read replicas for MySQL, PostgreSQL, and MariaDB
-You can have read replicas in different REGIONS for all db's
-Replications is Asynchronous only, not synchronous
-Can be built off Multi AZ databases
- Read replicas cannot be Multi AZ currently
-You can have read replicas of read replicas. Beware of latency
- DB snapshots and Automated backups cannot be taken of Read Replicas
- Key metric is Replica Lag
- Know the difference between read replicas and Multi-AZ.
- For DR fail-over think Multi AZ

Automatic Backups need to be turned on to create a Read Replica

Exam Tips
-If you can't create a read replica you need to enable database backups first
- You can create Read replicas in Multiple regions
-Endpoints do not change if you modify a DB; they will change if you create a new DB from snapshot or if you create a read replica.
- You can manually fail-over a Multi AZ DB from one AZ to anther by rebooting it.

Bastion Host
-

Troubleshooting Auto Scaling
If instances are not launching into Security Group
-Key pair doesnt exist
-security group doesn't exist
-Autoscaling config not working corectly
- Autoscaling group not found
- Instance type specified is not supported in the AZ
- AZ is no longer supported
- Invalid EBS device mapping
- Autoscaling service is not enabled on your account
- Attempting to attach an EBS block store device to an instance store AMI