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A Worker Model for Designate¶
Thesis: Gratuitous unnecessary complexity exists within the Designate code, and in the operation of Designate as a service. Making Designate a producer-worker type of project will vastly simplify the development and operation, and align it more to the true nature of the service it provides (DNS).
designate-pool-manager does a reasonably good job at pushing Create/Delete
changes out to nameservers, but the process gets a bit less shiny after that.
Polling to see if state is live is done via asynchronous and synchronous RPC with another component.
Cache usage is a mess (storing mutliple keys, keeping one key around forever, storing a different key for each type of operation).
Periodic Sync/Recovery are very unreliable as the number of changes grows.
The update_status and logic for calculating consensus is heavy-handed, too eager, and too complex.
The state machine is very foggy, and the logic for updating status that gets pushed into central is obfuscated.
Pool Managers are tied to one pool
designate-zone-manager does a good job at executing periodic timers for the
zones it manages. However:
One zone-manager process is responsible for a certain set of zones, if the operations for that set of zones gets heavy, a single zone manager process could become overwhelmed.
We rely on
toozto manage the extremely delicate task of ensuring balance and coverage of all zones by zone manager processes.
Certain work (export) that’s in the critical path of operations has already crept into the component that wasn’t really meant for that. As a substitute for proper workers, the zone-manager is looking like the current answer.
designate-mdns is a DNS server written in Python. It works well for small
amounts of traffic, but as traffic grows, we may realize that we need it to be
more specialized, as a DNS server written in Python should be. The logic for
sending NOTIFYs and polling of changes seems less likely to belong in mdns in
the future. If those bits were removed,
designate-mdns could be rewritten
to make use of a better tool for the problem.
A change to the underlying architecture of executing actual work on DNS servers
and the running of other tasks. Essentially, removing
designate-zone-manager, replacing them with
designate-producer (names up for debate) and
removing certain logic from
designate-mdns. All of the actual “work” would
be put in the scalable
designate-worker process, which has work produced
by the API/Central, and
designate-mdns gets back
to it’s roots, and only answers AXFRs. Callbacks over queues that don’t involve
the API are eliminated, simplifying the code in all components that deal with
No changes to the API or Database are required, with minimal changes to
To the end user, the results of this change would be relatively simple:
Scalability limited only by DNS servers, datastores, and queues. If at any point Designate starts to slow down in some aspect, unless there’s an issue with those services listed above, the problem can be solved by throwing more Designate processes at the problem.
Fault tolerance. One or more Designate processes dying would be inivisible in almost every case. An operator wouldn’t have to fear a certain process dying, because no one process handles responsiblities others will not, as long as there is a minimal amount of redundancy.
Simplicity. The Designate architecture becomes much cleaner and easier to understand. The questions of “what is the difference between pool-manager and zone-manager” and “what’s synchronous and asynchronous” become less muddy, or disappear altogether.
Operationally, this opens the door for simpler scenarios for small deployments where a customer need only scale a couple components (even on the same machine) to get more performance. You could even go so far as to deploy app nodes that only shared datastore (db, cache) and had their own designate components and queues.
These are the services that would remain present:
designate-api- To receive JSON and parse it for Designate
designate-central- To do validation/storage of zone/record data and send CRUD tasks to
designate-mdns- To only perfrom AXFRs from Designate’s database
designate-worker- Any and all tasks that Designate needs to produce state on nameservers
designate-producer- To run periodic/timed jobs, and produce work for
designate-workerthat is out the normal path of API operations. For example: Periodic recovery.
Other necessary components:
Queue - Usually RabbitMQ
Database - Usually MySQL
Cache - (encouraged, although not necessary) Memcached, MySQL, Redis
toozbackend (Zookeeper, Memcahed, Redis)
Services/components that are no longer required:
The scope of
designate-worker’s duties are essentially any and all tasks
that Designate needs to take action to perform. For example:
Create, Update, and Delete zones on pool targets via backend plugins
Poll that the change is live
Update a cache with the serial number for a zone/target
Emit zone-exists events for billing
Flatten Alias Records
Clean up deleted zones
The service essentially exposes a vast RPCAPI that contains
An important difference to Designate’s current model is that all of these tasks do not call back. They are all fire-and-forget tasks that will be shoved on a queue and await worker action.
tasks are essentially functions, that given relatively simple input, make
the desired income happen on either nameservers, or the Designate database.
The cache performs a similar function to the current pool manager cache now.
It will store state for each different type of task that a worker can use
to decide if it needs to continue with a
task received from the queue, or
simply drop it and move on to the next task.
This varies by task, some are relatively simple, knowing whether to perform a zone update to a certain serial number is knowable by seeing the serial number of a zone on each target in a pool. For DNSSEC zone signing, a key would probably be placed to indicate that a certain worker was working on resigning a zone, as it’s a more long-running process.
In the absence of such a cache, each worker will act naive and try to complete each task it receives.
Each task will be idempotent, to the degree that it is possible.
As mentioned in the
Cache section, to a certain degree, tasks could be
able to know if they need to complete work based on information in the cache.
But they should also make an effort to not duplicate work, for instance, if it’s trying to delete a zone that’s already gone, it should interpret the zone being gone as a sign that the delete is successful and move on.
On the whole these tasks would simply be lifted from where they currently exist in the code, and wouldn’t change all that much.
A slight change might be that during the course of the task, we may recheck that the work that is being undertaken still needs to be done.
As an example:
An API customer creates many recordsets very quickly. The work being dispatched
designate-worker processes would go to a lot of different places, and one of
the first updates to actually reach a nameserver might contain all the changes
necessary to bring the zone up-to-date. The other tasks being worked should
check before they send their NOTIFY that the state is still behind, and check
again after they’ve sent their NOTIFY, but before they’ve began polling, so
that they can cut down on unnecessary work for themselves, and the nameservers.
You could get even smarter about the markers that you drop in a cache for these
tasks. For example, on a zone update, you could drop a key in the cache of the
zoneupdate-foo.com., and other if other zoneupdate tasks for the same
zone see that key, they could know to throw away their job and move on.
The partioning of certain elements that Designate had previously disappears. The worker service will send DNS queries, it will do cpu-bound tasks, but it will be one place to scale. It should be possible to have an extremely robust Designate architecture by simply scaling these workers.
designate-mdns will have it’s entire RPCAPI transferred to
designate-worker. This will vastly simplify the amount of work it needs
to do while it sits in the critical path of providing zone transfers to
nameservers Designate manages.
As a side-note, this would make this service much easier to optimize, or even rewrite in a faster programming language.
designate-producer is the place where jobs that produce tasks that
are outside of the normal path of API operations and operate on
some kind of timer live.
The key difference to the
zone-manager service, is that this service
simply generates work to be done, rather than actually doing the work.
designate-producer simply decides what needs to be done, and sends RPC
messages on the queue to
designate-worker to actually perform the work.
As we’ve grown Designate, we’ve seen the need for this grow vastly, and even more so in the future.
Deleted zone purging
Refreshing Secondary Zones
Emitting zone exists tasks and other billing events
DNSSEC signing of zones
Alias record flattening
We could move the
periodic_recovery tasks from the
Pool Manager to this service.
periodic_recovery tasks in the Pool Manager have
been a constant struggle to maintain and get right. This is due to a lot of
Making the generation of
tasks by periodic processes the job of only one
Designate component simplifies the architecture, and allows to solve the
problems it presents one time, one way, and generally do one thing well.
This service would essentially be a group of timers that wake up on a cadence
and create work to be put on the queue for
designate-worker processes to
The overhead is relatively low here, as we’re not actually doing the work, but more just scheduling the work to be done. This way we can focus on the unexpectedly difficult problem of dividing up the production of work that these processes will put on the queue.
To explain more clearly, the biggest problem we have in this service is making
it fault-tolerant, but not duplicating work for
to do. This was solved before by
tooz using the zone shards in the Designate
designate-zone-manager and it seems to work well.
designate-worker processes, as described above, will do a certain amount of
optimization so that they don’t duplicate work. But if we generate too much
cruft, those processes will be bogged down just by the task of seeing if they
need to do work. So we should work to minimize the amount of duplicate work we
One potential complication of this implementation is that, as the number of
timers and tasks that are out of Designate’s critical path of implementation
grow, they may get in the way of
designate-worker processes doing the tasks
that are most important, namely CRUD of zones and records.
We propose having queues/exchanges for each type of task, this would be an
optimal way to monitor the health of different types of tasks, and isolate the
sometimes long-running tasks that periodic timers will produce from the
relatively quicker, and more important CRUD operations. The algorithm for
choosing tasks from the various options could be customized by a particular
process if desired. But a good general default would be to handle CRUD
designate-central first. Or use a weighted random
choice algorithm, with the critical-path CRUD operations having higher weights.
Stand up a
Migrate CRUD zone operations to
designate-worker, reworking the cache implementation.
Migrate Pool Manager periodic tasks to
designate-producer, with small modifications to ensure they simply generate work for
designate-mdns’ NOTIFYing and polling to
Fix up the
Migrate all tasks from
zone-managerto a split of
producercreates the work on the queue and
workerexecutes it. Ensuring scalable logic for distributed work production using cache or some other method in
Upgrading to the next release with this change would introduce some operational changes. Mostly around the services that need to be deployed. The deployment need not be a cutover, deploying Newton Designate will work with or without the worker. This is because of a variety of compatibility measures taken:
designate-centralwill have a configurable “zone api” that it can swap between
designate-worker. If the worker process is enabled, central can send c/u/d zones events to the worker instead of the pool manager.
designate-worker’s ability to send NOTIFYs and Poll DNS servers can replace a portion of
designate-mdns’ responsibilities. For certain DNS servers, it’s theorized that they won’t behave well if a NOTIFY comes from a different server than it’s master that it zone transfers from. For this reason,
designate-worker’s ability to send NOTIFYs is a configurable element. Since the worker calls into a backend plugin to update zones, the NOTIFY via mdns logic in those backends can remain, and if the operator so chooses, the NOTIFY task in the worker can no-op. This works both ways. An operator can also choose to have the MiniDNS notify calls noop, and allow them to be completed by the worker process.
For those who choose to firewall all DNS traffic between Designate and DNS servers, it will be safest to deploy
designate-workerprocesses in close proximity to
designate-mdnsprocesses. So that the DNS polling that
designate-workerdoes can be completed, where
designate-mdnsused to do it.
As periodic-type processes are migrated to
designate-worker, they can be marked as “worker tasks” in
designate-zone-manager, that can be turned off behind a configuration flag.
The process for upgrading to the worker model code, after deploying Newton could look something like this:
(To account for firewalling dns traffic) Start
designate-workerprocesses operating on the same IPs as
designate-mdnsprocesses via proximity or proxy. The default configuration will still allow NOTIFYs and DNS polling to occur via
designate-mdns, and all other operations to work in
designate-pool-manager. No traffic will reach the worker.
Toggle configuration values
designate-mdnsprocesses so that mdns NOTIFY calls no-op, and central starts to use the worker instead of
designate-zone-manager::worker-tasksconfig flag and restart
designate-zone-managerso that it hands off periodic tasks to the producer/worker.
designate-producerprocess so that the worker starts doing recovery and other periodic tasks.
designate-pool-mangerprocesses, and if all processes are migrated out of
designate-zone-manager, that as well.
- Target Milestone for completion: