iHeavy Insights 78 – Degrade Gracefully

Your recent social media campaign has gone viral.  It’s what you’ve been dreaming about, pinning your hopes on, and all of your hard work is now coming to fruition.  Tens of thousands of internet users, hoards of them in fact, are now descending on your website.  Only one problem, it went down!!

That’s a situation you want to avoid.  Luckily there are some best practices for avoiding scenarios like the one I described.  In engineering it’s termed “degrade gracefully”.  That is continue functioning but with the heaviest features disabled.

Browsing Only, But Still Functioning

One way to do this is for your site to have a browsing only mode.  On the database side you can still be functioning with a read-only database.  With a switch like that, your site will continue to function while pointed to any of your read-only replication slaves.  What’s more you can load balance across those easily, and keep your site up and running.

Decoupling

In software development, decoupling involves breaking apart components or pieces of an application that should not depend on one another.  One way to do this is to use a queuing system such as Amazon’s SQS to allow pieces of the application to queue up work to be done.  This makes those pieces asynchronous, ie they’ll return right away.  Another way is to expose services internal to your site through web services.  These individual components can then be scaled out as needed.  This makes them more highly available, and reduces the need to scale your memcache, webservers or database servers – the hardest ones to scale.

Identify Features You Can Disable

Typically your application will have features that are more superfluous, or that are not part of the core functionality.  Perhaps you have star ratings, or some other components that are heavy.  Work with the development and operations teams to identify those areas of the application that are heaviest, and that would warrant disabling if the site hits heavy storms.

Once you’ve done all that, document how to disable and reenable those features, so other team members will be able to flip the switches if necessary.

Continue reading iHeavy Insights 78 – Degrade Gracefully

Cloud Computing – Disciplined Deployments

With traditional managed hosting solutions, we have best practices, we have business continuity plans, we have disaster recovery, we document our processes and all the moving parts in our infrastructure.  At least we pay lip service to these goals, though from time to time we admit to getting side tracked with bigger fish to fry, high priorities and the emergency of the day.  We add “firedrill” to our todo list, promising we’ll test restoring our backups.  But many times we find it is in the event of an emergency that we are forced to find out if we actually have all the pieces backed up and can reassemble them properly.

** Original article — Intro to EC2 Cloud Deployments **

Cloud Computing is different.  These goals are no longer be lofty ideals, but must be put into practice.  Here’s why.

  1. Virtual servers are not as reliable as physical servers
  2. Amazon EC2 has a lower SLA than many managed hosting providers
  3. Devops introduces new paradigm, infrastructure scripts can be version controlled
  4. EC2 environment really demands scripting and repeatability
  5. New flexibility and peace of mind

Unreliable Servers

EC2 virtual servers can and will die.  Your spinup scripts and infrastructure should consider this possibility not as some far off anomalous event, but a day-to-day concern.  With proper scripts and testing of various scenarios, this should become manageable.  Use snapshots to backup EBS root volumes, and build spinup scripts with AMIs that have all the components your application requires.  Then test, test and test again.

Amazon EC2’s SLA – Only 99.95%

The computing industry throws around the 99.999% or five-nines uptime SLA standard around a lot.  That amounts to less than six minutes of downtime.  Amazon’s 99.95% allows for 263 minutes of downtime.  Greater downtime merely gets you a credit on your account.  With that in mind, repeatable processes and scripts to bring your infrastructure back up in different availability zones or even different datacenters is a necessity.  Along with your infrastructure scripts, offsite backups also become a wise choice.  You should further take advantage of availability zones and regions to make your infrastructure more robust.  By using private IP addresses and network, you can host a MySQL database slave in a separate zone, for instance.  You can also do GDLB or Geographically Distributed Load Balancing to send customers on the west coast to that zone, and those on the east coast to one closer to them.  In the event that one region or availability zone goes out, your application is still responding, though perhaps with slightly degraded performance.

Devops – Infrastructure as Code

With traditional hosting, you either physically manage all of the components in your infrastructure, or have someone do it for you.  Either way a phone call is required to get things done.  With EC2, every piece of your infrastructure can be managed from code, so your infrastructure itself can be managed as software.  Whether you’re using waterfall method, or agile as your software development lifecycle, you have the new flexibility to place all of these scripts and configuration files in version control.  This raises manageability of your environment tremendously.  It also provides a type of ongoing documentation of all of the moving parts.  In a word, it forces you to deliver on all of those best practices you’ve been preaching over the years.

EC2 Environment Considerations

When servers get restarted they get new IP addresses – both private and public.  This may affect configuration files from webservers to mail servers, and database replication too, for example.  Your new server may mount an external EBS volume which contains your database.  If that’s the case your start scripts should check for that, and not start MySQL until it finds that volume.  To further complicate things, you may choose to use software raid over a handful of EBS volumes to get better performance.

The more special cases you have, the more you quickly realize how important it is to manage these things in software.  The more the process needs to be repeated, the more the scripts will save you time.

New Flexibility in the Cloud

Ultimately if you take into consideration less reliable virtual servers, and mitigate that with zones and regions, and automated scripts, you can then enjoy all the new benefits of the cloud.

  • autoscaling
  • easy test & dev environment setup
  • robust load & scalability testing
  • vertically scaling servers in place – in minutes!
  • pause a server – incurring only storage costs for days or months as you like
  • cheaper costs for applications with seasonal traffic patterns
  • no huge up-front costs

MySQL Cluster In The Cloud – Managers Guide

The term clustering is often used loosely in the context of enterprise databases.  In relation to MySQL in the cloud you can configure:

  1. Master-master active/passive
  2. Sharded MySQL Database
  3. NDB Cluster

Master-Master active/passive replication

Also sometimes known as circular replication.  This is used for high availability. You can perform operations on the inactive node (backups, alter tables or slow operations) then switch roles so inactive becomes active.  You would then perform the same operations on the former master.  Applications sees “zero downtime” because they are always pointing at the active master database.  In addition the inactive master can be used as a read-only slave to run SELECT queries and large reporting queries.  This is quite powerful as typical web applications tend to have 80% or more of their work performed with read-only queries such as browsing, viewing, and verifying data and information.

Sharded MySQL Database

This is similar to what in the Oracle world is called “application partitioning”.   In fact before Oracle 10 most Parallel server and RAC installations required you to do this.  For example a user table might be sharded by putting names A-F on node A, G-L on node B and so forth.

You can also achieve this somewhat transparently with user_ids.  MySQL has an autoincrement column type to handle serving up unique ids.  It also has a cluster-friendly feature called auto_increment_increment.  So in an example where you had *TWO* nodes, all EVEN numbered IDs would be generated on node A and all ODD numbered IDs would be generated on node B.  They would also be replicating changes to eachother, yet avoid collisions.

Obviously all this has to be done with care, as the database is not otherwise preventing you from doing things that would break replication and your data integrity.

One further caution with sharding your database is that although it increases write throughput by horizontally scaling the master, it ultimately reduces availability.   An outage of any server in the cluster means at least a partial outage of the cluster itself.

NDB Cluster

This is actually a storage engine, and can be used in conjunction with InnoDB and MyISAM tables.  Normally you would use it sparingly for a few special tables, providing availability and read/write access to multiple masters.  This is decidedly *NOT* like Oracle RAC though many mistake it for that technology.

MySQL Clustering In The Cloud

The most common MySQL cluster configuration we see in the Amazon EC2 environment is by far the Master-Master configuration described above.  By itself it provides higher availability of the master node, and a single read-only node for which you can horizontally scale your application queries.  What’s more you can add additional read-only slaves to this setup allowing you to scale out tremendously.

Migrating MySQL to Oracle Guide

Also find Sean Hull’s ramblings on twitter @hullsean.

Migrating from MySQL to Oracle can be as complex as picking up your life and moving from the country to the city.  Things in the MySQL world are often just done differently than they are in the Oracle world.  Our guide will give you a birds eye view of the differences to help you determine what is the right path for you.

** See also: Oracle to MySQL Migration Considerations **

MySQL comes from a more open-source or DIY background.  One of Unix and Linux administrators and even developers carrying the responsibility of a DBA.

  1. Installation & Administration Considerations
  2. Query and Optimizer Differences
  3. Security Strengths and Weaknesses
  4. Replication & High Availability
  5. Table Types & Storage Engines
  6. Applications, Connection Pooling, Stored Procedures and More
  7. Backups & Disaster Recovery
  8. Community – MySQL & Oracle Differences
  9. TCO, Licensing, and Cloud Considerations
  10. Advanced Oracle Features – Missing in MySQL

Check back soon as we update each of these sections.

Oracle to MySQL Migration Considerations

There are a lot of forms of transportation, from walking to bike riding, motorcycles and cars to busses, trains and airplanes.  Each mode of transport will get you from point a to point b, but one may be faster, or more comfortable and another more cost effective.  It’s important to keep in mind when comparing databases like Oracle and MySQL that there are indeed a lot of feature differences, a lot of cultural differences, and a lot of cost differences.  There are also a lot of impassioned people on both sides arguing at the tomfoolery of the other.  Hopefully we can dispel some of the myths and discuss the topic fairly.

** See also: Migrating MySQL to Oracle Guide **

As a long time Oracle DBA turned MySQL expert, I’ve spent time with clients running both database engines and many migrating from one to the other.  I can speak to many of the differences between the two environments.  I’ll cover the following:

  1. Query & Optimizer Limitations
  2. Security Differences
  3. Replication & HA Are Done Differently
  4. Installation & Administration Simplicity
  5. Watch Out – Triggers, Stored Procedures, Materialized Views & Snapshots
  6. Huge Community Support – Open-source Add-ons
  7. Enter The Cloud With MySQL
  8. Backup and Recovery
  9. Miscellaneous Considerations

Check back again as we edit and publish the various sections above.