The DynamicQueue plugin extends Oban's basic queue management by persisting queue changes across restarts, globally, across all connected nodes. It also boasts a declarative syntax for specifying which nodes a queue will run on.

DynamicQueues are ideal for applications that dynamically start, stop, or modify queues at runtime.

Using and Configuring

To begin using DynamicQueues, add the module to your list of Oban plugins in config.exs:

config :my_app, Oban,
  plugins: [Oban.Pro.Plugins.DynamicQueues]
  ...

Without providing a list of queues the plugin doesn't have anything to run. The syntax for specifying dynamic queues is identical to Oban's built-in :queues option, which means you can copy them over:

plugins: [{
  Oban.Pro.Plugins.DynamicQueues,
  queues: [
    default: 20,
    mailers: [global_limit: 20],
    events: [local_limit: 30, rate_limit: [allowed: 100, period: 60]]
  ]
}]

Prevent Queue Conflicts

Be sure to either omit any top level queues configuration to prevent a conflict between basic and dynamic queues.

Now, when DynamicQueues initializes, it will persist all of the queues to the database and start supervising them. The queues syntax is nearly identical to Oban's standard queues, with an important enhancement we'll look at shortly.

Each of the persisted queues are referenced globally, by all other connected Oban instances that are running the DynamicQueues plugin. Changing the queue's name, pausing, scaling, or changing any other options will automatically update queues across all nodes—and persist across restarts.

Persisted queues are referenced by name, so you can tweak a queue's options by changing the definition within your config. For example, to bump the mailer's global limit up to 30:

queues: [
  mailers: [global_limit: 30],
  ...
]

That isn't especially interesting—after all, that's exactly how regular queues work! Dynamic queues start to shine when you insert, update, or delete them dynamically, either through Oban Web or your own application code. But first, let's look at how to limit where dynamic queues run.

Limiting Where Queues Run

Dynamic queues can be configured to run on a subset of available nodes. This is especially useful when wish to restrict resource-intensive queues to only dedicated nodes. Restriction is configured through the only option, which you use like this:

queues: [
  basic: [local_limit: 10, only: {:node, :=~, "web|worker"}],
  audio: [local_limit: 5, only: {:node, "worker.1"}],
  video: [local_limit: 5, only: {:node, "worker.2"}],
  learn: [local_limit: 5, only: {:sys_env, "EXLA", "CUDA"}],
  store: [local_limit: 1, only: {:sys_env, "WAREHOUSE", true}]
]

In this example we've defined five queues, with the following restrictions:

• basic — will run on a node named web or worker
• audio — will only run on a node named worker.1
• video — will only run on a node named worker.2
• learn — will run wherever EXLA=CUDA is an environment variable
• store — will run wherever WAREHOUSE=true is an environment variable

Here are the various match modes, operators, and allowed patterns:

Modes

• :node — matches the node name as set in your Oban config. By default, node is the node's id in a cluster, the hostname outside a cluster, or a DYNO variable on Heroku.
• :sys_env — matches a single system environment variable as retrieved by System.get_env/1

Operators

• :== — compares the pattern and runtime value for equality as strings. This is the default operator if nothing is specified.
• :!= — compares the pattern and runtime value for inequality as strings
• :=~ — treats the pattern as a regex and matches it against a runtime value

Patterns

• boolean — either true or false, which is stringified before comparison
• string — either a literal pattern or a regular expression, depending on the supplied operator

Deleting Persisted Queues

It is possible to delete a persisted queue during initialization by passing the :delete option:

queues: [
  some_old_queue: [delete: true],
  ...
]

Multiple queues can be deleted simultaneously, if necessary. Deleting queues is also idempotent; nothing will happen if a matching queue can't be found.

In the next section we'll look at how to list, insert, update and delete queues dynamically at runtime.

Runtime Updates

Dynamic queues are persisted to the database, making it easy to manipulate them directly through CRUD operations, or indirectly with Oban's queue operations, i.e. pause_queue/2, scale_queue/2.

Explicit queue options will be overwritten on restart, while omitted fields are retained. For example, consider the following dynamic queue entry:

queues: [
  default: [limit: 10],
  ...
]

Scaling that default queue up or down at runtime wouldn't persist across a restart, because the definition will overwrite the limit. However, pausing the queue or changing the global_limit would persist because they aren't included in the queue definition.

# pause, global_limit, etc. will persist, but limit won't
default: [limit: 10]

# pause will persist, but limit and global_limit won't
default: [limit: 10, global_limit: 20]

# neither limits nor pausing will persist
default: [limit: 10, global_limit: 20, paused: true]

See function documentation for all/0, insert/1, update/2, and delete/1 for more information about runtime updates.

Enabling Polling Mode

In environments with restricted connectivity (where PubSub doesn't work) you can still use DynamicQueues at runtime through polling mode. The polling interval is entirely up to you, as it's disabled by default.

config :my_app, Oban,
  plugins: [{Oban.Pro.Plugins.DynamicQueues, interval: :timer.minutes(1)}]

With the interval above each DynamicQueues instance will wake up every minute, check the database for changes, and start new queues.

Isolation and Namespacing

All DynamicQueues functions have an alternate clause that accepts an Oban instance name for the first argument. This matches base Oban functions such as Oban.pause_queue/2, which allow you to seamlessly work with multiple Oban instances and across multiple database prefixes. For example, you can use all/1 to list all queues for the instance named ObanPrivate:

queues = DynamicQueues.all(ObanPrivate)

Likewise, to insert a new queue using the configuration associated with the ObanPrivate instance:

DynamicQueues.insert(ObanPrivate, private: limit: 10)