Think Cloud portable! Let Applications drive the Model

Richard Harding

Richard Harding

on 16 October 2017

In our last intro to Modeling with Juju post we didn’t pay any attention to the hardware needed to run our workloads. We ran with Juju default values for what the hardware characteristics of the cloud instances should be. Rather than define a bunch of YAML about each machine needed to run the infrastructure, Juju picks sane defaults that allow you to prototype and test things out with a lot less work.

The reason this is important is that Juju is a modeling tool. When it comes time to making decisions about your hardware infrastructure, it’s key to focus on the needs of the Applications in your Model. If we’re going to operate PostgreSQL in production then we need to look at the needs of PostgreSQL. How much disk space do we need to provide to adequately store our data and provide space for backups and the like? How much CPU power do we need in our PostgreSQL instance? How much memory is going to be required to make sure PostgreSQL has the best chance of achieving hits to data already in memory?

All of these questions fall under characteristics required to run the specific Application. It’ll be different for each Application we operate as part of our infrastructure. Juju shines at this because of the way that it’s build around the Cloud idea of infrastructure as an API. There’s no plugins required. Juju natively is put together so that whenever you deploy an Application that you want to run the API calls are made to make sure you get the right type of hardware using the underlying Cloud API calls. Infrastructure is loaded as-needed and there’s no requirement to pre-load software on the machines before you use them.

Juju provides a tool called “Constraints” to manage these details in the Model. In their simplest form, a Constraint is just an option you can pass during the deploy command. Let’s compare the outcome of these two commands:

$ juju deploy postgresql 
juju deploy postgresql pgsql-constrained --constraints mem=32G

$ juju status
App                Version  Status  Scale  Charm       Store       Rev  OS      Notes
pgsql-constrained  9.5.9    active      1  postgresql  jujucharms  163  ubuntu
postgresql         9.5.9    active      1  postgresql  jujucharms  163  ubuntu

Unit                  Workload  Agent  Machine  Public address   Ports     Message
pgsql-constrained/0*  active    idle   1   5432/tcp  Live master (9.5.9)
postgresql/0*         active    idle   0  5432/tcp  Live master (9.5.9)

Machine  State    DNS              Inst id        Series  AZ          Message
0        started  juju-dd8186-0  xenial  us-east1-b  RUNNING
1        started   juju-dd8186-1  xenial  us-east1-c  RUNNING

From juju status these commands don’t look like they did a whole lot different. They’ve both introduced a PostgreSQL server into our Model and it’s available to be part of Relations to other Applications where it will provide them with a database to use. Juju encourages focusing on what’s running on the Application level over the minutia of hardware. However, if we compare the machine details we can see these commands definitely changed PostgreSQL’s ability to perform for us.

$ juju show-machine 0
    hardware: arch=amd64 cores=1 cpu-power=138 mem=1700M root-disk=10240M availability-zone=us-east1-b

$ juju show-machine 1
    constraints: mem=32768M
    hardware: arch=amd64 cores=8 cpu-power=2200 mem=52000M root-disk=10240M availability-zone=us-east1-c

Notice the different output? The constrained machine knows that the Model states that there’s an active constraint. We want 32G of memory for this PostgreSQL Application and if I were to run a juju add-unit command the next machine would know there’s an active Constraint and respect it. In this way Constraints are part of the active Model. If we were to remove the Unit pgsql-constrained the Constraint is still in there as part of the Model.

The next thing to note is that we’ve got more memory than I asked for. We wanted to make sure my PostgreSQL had 32G of RAM vs the default instance type which has 1.7G. In order to do that Juju had to request a larger instance which also brought in 8 cores and we ended up with 52G of ram. Looking at the GCE instances available, their instance n1-standard-8 only has 30G of memory. So Juju went looking for a better match and pulled up the n1-highmem-8 instance type. In this way we receive the best matching unit that meets or exceeds my Constraints we’ve requested. Juju walks through all the various instance types to help do this.

As a user, we focus on what our Application needs to run at its best and Juju takes care of finding the most cost-effective instance type that will meet these goals. This is key to allowing my Model to remain cloud agnostic and maintain performance expectations, regardless of what Cloud we use this Model on.  Let’s try this same deploy on AWS and see what happens.

$ juju add-model pgsql-aws aws
$ juju deploy postgresql pgsql-on-aws --constraints mem=32G
$ juju show-machine 0
    constraints: mem=32768M
    hardware: arch=amd64 cores=8 cpu-power=2800 mem=62464M root-disk=8192M availability-zone=us-east-1a

Notice here we ended up with an instance that’s got 8 cores and 62G of ram. That’s the best-fit instance-type on AWS for our Constraints we need to make sure our system performs at its best.

Other Constraints

As you’d expect we can set other constraints to customize the characteristics of the hardware our Applications leverage. We can start with the number of CPU cores. This is vital in today’s world of applications that are able to put those extra cores to use. It might also be key when we’re going to run LXD machine containers or other VM technology on a machine. Keeping a core per VM is a great way to leverage the hardware available.

$ juju deploy postgresql pgsql-4core --constraints cores=4
$ juju show-machine 2
    constraints: cores=4
    hardware: arch=amd64 cores=4 cpu-power=1100 mem=3600M root-disk=10240M availability-zone=us-east1-d

Voila, Juju helps get the best instance for those requirements. The most cost-effective instance available with 4 cores provides 3.6G of memory.

root-disk sizing

$ juju deploy postgresql pgsql-bigdisk --constraints root-disk=1T
$ show-machine 3 
    constraints: root-disk=1048576M
    hardware: arch=amd64 cores=1 cpu-power=138 mem=1700M root-disk=1048576M availability-zone=us-east1-b

One thing that’s not setup as part of the instance-type is the disk space allocated to the instance. Some Applications focus on CPU usage but others will want to track state and store that on disk. We can specify the size of the disk that the instances are allocated through the root-disk Constraint.

The root-disk Constraints only mean that Juju will make sure that the instance has a Volume of that size when it comes up. This is especially useful for Big Data Applications and Databases.

Remembering Constrains in the model

From here let’s combine our Constraints and produce a true production grade PostgreSQL server. Then we’ll create the bundle that will deploy this in a repeatable fashion that we can reuse on any Cloud that we want to take it to.

$ juju deploy postgresql pgsql-prod --constraints "cores=4 mem=24G root-disk=1T"

series: xenial
    charm: "cs:postgresql-163"
    num_units: 1
    constraints: "cores=4 mem=24576 root-disk=1048576"

Constraints on Controllers

Once we understand how Constraints work one area we’ll find it really valuable is in the sizing of your own Juju Controllers. If we’re operating a true multi-user Controller with many models to the folks in your business we’ll want to move up from the default instances sizes. Juju is conservative so that folks testing out and experimenting with Juju don’t end up running much larger instances than expected. There are a lot more test or trial controllers out there than Controllers that run true production. It is much like any product really.

Controller Constraints are supplied during bootstrap using the flag bootstrap-constraints. For example:

$ juju bootstrap --bootstrap-constraints="mem=16G cores=4 root-disk=400G" google

Some considerations when you’re operating a controller include:

  • root-disk – Juju leverages persistent database and tracks logs and such in that database. You want to make sure to provide space for running Juju backups, storage of Charms and Resources used in Models (they’re cached on the controller), and logs that are rotated over time, etc.
  • mem – as with any database, the more it can fit into memory the better. It’ll respond quicker to events through the system. During events such as upgrades and migrations there can be a lot of memory consumption as entire models are processed as part of those actions.
  • cores – speed, as you have more clients and units talking over the API the more cores and faster it can respond the better. Here cores is more important than raw CPU as most things are more about fetching/processing documents vs doing pure computation.

Cloud specific Constraints

There are cloud specific constraints that are supported. MAAS, for instance, allows deploying via tags because MAAS supports tags as a way of classifying bare metal machines where you might optimize a hardware purchase for network applications, storage applications, etc. Make sure to check out the documentation to see all available Constraints and what Clouds they are supported on.

The great thing about the Constraint system is that it keeps the focus on requirements on the Applications and what they actually need to get their jobs done. We’re not trying to force Applications onto existing hardware that might not fit it’s operating profile. It’s an idea that really excels in the IaaS Cloud world.

Give it a try and if you have any questions or hit any problems let us know. You can find us in the #juju IRC channel on Freenode or on the Juju mailing list. You can reach me on Twitter. Don’t hesitate to reach out.

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