Summary
Amazon RDS has two main different types of replicas, Multi-AZ Replica and Read Replica, and it's easily to find their difference.
However, Read Replica had supported Multi-AZ deployment at JAN, 2018.
What is the main difference between "Multi-AZ Deployment" and "Read Replica Version Multi-AZ Deployment"?
The two ways to add the Multi-AZ Deployment at the current database are as follow:
Situation 1: (Original, Multi-AZ Deployment)
Instance Action
→ Modify
→ specified the "Multi-AZ deployment" option
Situation 2: (Read Replica Version Multi-AZ Deployment)
Instance Action
→ Create read replica
→ specified the "Multi-AZ deployment" option
An RDS read replica instance is an asynchronous read-only replica of an upstream primary ("master") database instance. It can be used by your application for any query that does not require changing data, thus relieving load from the master. If the replica crashes or fails, it has no impact on the master but the replica itself can no longer handle any traffic.
Multi-AZ means the database instance has a standby spare server machine and spare hard drive in a different availability zone of the same region. This is a synchronous replica, but cannot be accessed by you. If the active server fails, the spare server takes over and starts handling traffic more quickly than would be possible without the spare.
Multi-AZ is a deployment strategy for higher reliability.
It reduces the downtime required for version upgrades, and reduces the impact of backup snapshots and creation of replicas, since snapshots can be done from the spare (by the service). It doubles the cost of the instance because of the hot standby capacity it provides.
Multi-AZ typically used only on the master instance, for fast recovery.
Historically, this was the only variant of Multi-AZ, but a Multi-AZ read replica is now possible, and is what it sounds like: a replica with Multi-AZ. It will recover more quickly from faults and failures because it has spare hardware. The active and spare are synchronous replicas of each other but are still asynchronous replicas of the master, as all non-Aurora replicas are in RDS/MySQL.
Combining Read Replicas with Multi-AZ enables you to build a resilient disaster recovery strategy and simplify your database engine upgrade process.
Amazon RDS Read Replicas enable you to create one or more read-only copies of your database instance within the same AWS Region or in a different AWS Region. Updates made to the source database are then asynchronously copied to your Read Replicas. In addition to providing scalability for read-heavy workloads, Read Replicas can be promoted to become a standalone database instance when needed.
https://aws.amazon.com/about-aws/whats-new/2018/01/amazon-rds-read-replicas-now-support-multi-az-deployments/
In summary, Multi-AZ on the master gets you one server with an invisible hot spare that is used for failure recovery but is not a usable database replica. It is a good strategy for resiliency.
Multi-AZ on a replica is an expensive way of speeding recovery time on a crashed instance. It is a separate server, so can be accessed by you, but so can a non-Multi-AZ read replica.
A multi-AZ deployment has a Master database in one AZ and a Standby (or Secondary) database in another AZ. Only the Master database serves traffic. If the Master fails, then the Secondary takes over.
A Read Replica is a read-only copy of the database. It is actively running and apps can use it for read-only queries. A Read Replica can be in a different AZ or even in a different region.
In terms of Highly Available, Multi-AZ has higher availability over Read-replica. As Multi-AZ provide a backup writer in other AZ, so both read and write is not affected on Single AZ fails.
Related
I would like to setup a PostgreSQL database with read-only standby with automatic failover ability.
Does Amazon RDS with Multi-AZ have automatic failover ability?
If yes, will the endpoint/DNS automatically point to the new standby database?
There are 2 scenarios to this question as it was little ambiguous in the question.
scenario 1 :- rds with multi AZ.
Here standby instance will have automatic failover.
Here the endpoint/DNS automatically point to the new standby database, no manual intervention is needed.
But read and write is being taken by only the master DB and sync replication takes with standby database.
scenario 2 :- rds with multi AZ where read replicas is also there.
In this scenario read replicas are not treated like standby database database by default.howver they can be used like standby.
failover is not automatic.
You will need to manually update your dns.
article from aws says
A Read Replica in a different region than the source database can be used as a standby database and promoted to become the new production database in case of a regional disruption.
also a similar question on stack should also help you Difference between "Multi-AZ Deployment" and "Read Replica Verison Multi-AZ Deployment"
Scenario 2 seems to have slightly evolved since 2021.
Now this has become Multi-AZ DB Cluster
Reader DB instances act as automatic failover targets and also serve
read traffic to increase application read throughput. If an outage
occurs on your writer DB instance, RDS manages failover to one of the
reader DB instances. RDS does this based on which reader DB instance
has the most recent change record.
I'm trying to evaluate AWS RDS Aurora as future replacement for our local MySQL databases, but I'm noticing some strange behaviors.
I have a basic cluster with a DB master (writer) and a replica (reader). My idea was to use the reader as an always available datasource, even when the writer in unavailable. But when I'm rebooting the master, it takes down the reader as well, making the setup quite worthless.
Looking at the reader replica log, this is what happens when the it notices that the writer is down:
Does anyone know how to have a Aurora read entry point that never goes down even if the writer is offline or busy for a brief time?
Or does the write/read "out of sync" always take down the reader entry points no matter the size of the cluster?
The only way to have a replica that remains available during a reboot of the master would be to have an asynchronous replica using conventional MySQL replication -- which Aurora does support.
Aurora replication is very different than MySQL (or Galera) replication. A loss of the master necessarily triggers a reorganization of the cluster, because the individual instances don't have their own copies of the data, they share a 6-way replicated storage volume -- that's how replication can remain in the 10-20 ms time range. What's actually replicated from the master is the transaction log LSN. Replacement of a master requires one replica to be promoted, verify that the on-disk data structures are clean after taking over, and then all of the other replicas start follow it.
If the DB cluster has one or more Aurora Replicas, then an Aurora Replica is promoted to the primary instance during a failure event. A failure event results in a brief interruption, during which read and write operations fail with an exception.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Aurora.Managing.html#Aurora.Managing.FaultTolerance
When an Aurora replica stops seeing updates from the master, it doesn't matter where the actual fault lies -- whether with the actual master or elsewhere in the infrastructure -- the replica stops serving queries because, best case, it no longer has access to authoritative data.
Where possible, zero-downtime patching appears to avoid a master restart during upgrades. Other than upgrades, there should not be a need to restart the master.
If a primary Aurora DB instance dies for some reason, and an existing replica gets promoted to the new primary, does a new replica instance get created so that I end up with the same number of read replicas?
If so, how long does it take for a new replica to be spun up on average?
There are two types of read replicas:
Backup replica (also known as slave) made by AWS when you deploy Multi-AZ RDS instance. That is synchronous read replica, but you can not use it.
Read replica created by you. Those are asynchronous replicas that you can use to offload some work.
A backup replica will be promoted to master automatically, usually it takes less than a minute. And yes, AWS will create new slave for the RDS instance that's now the master. It could take from several minutes to several hours depending on your workload and DB size.
Read replicas created by you will be just switched to the new master.
AWS Aurora is AWS's database with an architecture designed for cloud computing technologies. One of it's differences is that data is stored in a storage architecture similar to S3, in a cluster volume, which is a single, utilizes solid state disk (SSD) drives and consists of copies of the data across multiple Availability Zones in a single region. That has a few advantages, such as durability and also the fact that is distributed through in entire region, not just an AZ, helping with consistency between replicas and performance.
In case you have read replicas and your Master fails, one of them will become Master without downtime.
If you don't have a read replica, a new Master instance will be created and the process is really fast. Since data is on clusterized across the region, not on the server's disk, the process is fast, but there is downtime.
As AWS says:
To increase availability, you can use Aurora Replicas as failover
targets. That is, if the primary instance fails, an Aurora Replica is
promoted to the primary instance with only a brief interruption during
which read and write requests made to the primary instance fail with
an exception. If your Aurora DB cluster does not include any Aurora
Replicas, then the primary instance is recreated during a failure
event. However, promoting an Aurora Replica is much faster than
recreating the primary instance. For high-availability scenarios, we
recommend that you create one or more Aurora Replicas, of the same DB
instance class as the primary instance, in different Availability
Zones for your Aurora DB cluster. For more information on Aurora
Replicas as failover targets, see Fault Tolerance for an Aurora DB
Cluster.
You can read more on: http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Aurora.Replication.html"
I have a big database (~250GB) in Aurora getting lots of inserts. There's only one instance, so I'd like to create a replica for redundancy. While we are doing nightly snapshots, we would prefer a more fault tolerant system, and it appears that using aurora replicas would provide automatic failover.
My question: What exactly happens when I use the console and create a replica? Will a new instance come up and begin pulling data from the master instance? Could that affect database performance? I'm sure that it will take some time before the replica "catches up" and loads the 250GB; how will I know when it's "finished"?
Don't want to have any downtime, so I'm a bit afraid to push the "create replica" button without knowing what it does...
What exactly happens when I use the console and create a replica?
A new instance is started as part of the cluster, and it has access to the master's data -- or, perhaps more precisely, the cluster's data. All Aurora instances are members of a "cluster," even if it's only a cluster of one master server. Aurora replication, within the same region, is starkly different than MySQL native replication.
Will a new instance come up and begin pulling data from the master instance?
Not really. As described above, the new instance will come up and be able to read from the master's backing store -- it doesn't have its own separate storage.
Aurora runs on 3 sets of 2 copies of the working data, mirrored and replicated across the availability zones in the region. This logical entity is called the Cluster Volume.
The cluster volume spans multiple Availability Zones in a single region, and each Availability Zone contains a copy of the cluster volume data.
http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Aurora.Managing.html
(The docs say each AZ contains "a copy," which is true, but it's mirrored.)
Aurora replicas read from this data -- for all practical purposes, synchronously.
Q: How far behind the primary will my replicas to be?
Since Amazon Aurora Replicas share the same data volume as the primary, there is virtually no replication lag. We typically observe lag times in the 10s of milliseconds.
— https://aws.amazon.com/rds/aurora/faqs/
Could that affect database performance?
It shouldn't.
I'm sure that it will take some time before the replica "catches up" and loads the 250GB; how will I know when it's "finished"?
No, it really shouldn't. Once the replica instance becomes accessible, it should be up-to-date, because it's reading the same data from the same place that the master is writing. Metrics related to Aurora replica lag are accessible in the console.
When launching an Aurora instance I have the option of "Multi-AZ Deployment", which it describes as "Specifies if the DB Instance should have a standby deployed in another Availability Zone."
However the Aurora documentation states that Aurora already automatically spreads the database across different availability zones?
Additionally, what is the difference between an Aurora Multi-AZ standby and an ordinary Aurora replica. Is that that an ordinary replica can be read from increasing performance whereas a standby cannot be read from?
Aurora replicates your data across three availability zones, at the storage layer... but the database server instance, itself, is still a virtual machine running on a single physical machine that is located in a single availability zone.
The Aurora storage layer is outside that instance, and is able to let access continue uninterrupted without data loss, even in the event of the loss of up to two AZs, but the loss of the zone containing the db instance will still cause an outage for you, if you only have a single Aurora instance in your cluster (1 master, 0 replicas). Loss of an entire availability zone is one of those things that is highly improbable but not impossible. Your db instance is still a single point of failure when you only have one.
Multi-AZ makes allowance for a complete redundant database instance, in a different AZ, which will automatically take over for the primary within one minute, if it works as designed, in case of the loss of the AZ hosting the primary instance or a catastrophic failure of the primary instance. It's a second virtual machine, on a second physical machine, in a second availability zone. It's always running, but you can't access it. It's in the background, managed and monitored by the RDS infrastructure, but it is only accessible to you in the case of primary instance failure. The secondary machine can also be used to reduce downtime in the event of a software upgrade or maintenance event on the primary. When failover occurs, if you are using DNS to connect to your database (as you should), you'll find that the DNS entry is automatically pointed to the secondary.
Contrast this to a read replica, which is accessible all the time and can thus provide a significant performance benefit, by allowing the offloading of reads. Failing over to a replica involves promoting it to become a standalone master (which permanently detaches it from its own former master) and reconfiguring your application to use the alternate endpoint. This, of course, is still faster than recovering from a failure in the master by using a point-in-time snapshot to create a replacement master instance.
https://aws.amazon.com/rds/details/multi-az/
Storage in Aurora is replicated across three availability zones. The database head node is a single instance. So, while your data is spread across multiple targets, the head node is not.
When you enable a multi-AZ deployment, we create an Aurora read replica that is available as a failover target. Any Aurora read replicas you create (up to a max of 15 at this time) are also available as failover targets.
There isn't any meaningful difference between Multi-AZ and other Aurora replicas. This is primarily a simplification in the user interface for customers accustomed to using Multi-AZ for other RDS engines.
AWS Management console.
The answer to this is straightforward.
You can create Multi-AZ in the management console or ignore it. Irrespective, the shared storage for Amazon Aurora is across three AZ (Multi-AZs) as it's the feature of Amazon Aurora however if we choose the Mult-AZ option then we will also have your instances of Amazon Aurora in multiple AZs.
Thus you should choose the Amazon console image option