5/27/2023 0 Comments Heroku postgres psequel![]() ![]() Semaphore Groups define the precise quantity to decrement. There is a final ambiguity in the previous example: we defined decr as decrementing zero-or-more from a Semaphore, but left implicit is the derivation of how much to decrement. ![]() Here we can see that the Server will correctly begin a second re-configuration pass. Thus, the Formation actor modifies the state of its associated Server actors directly: The implementation avoids any synchronization construct not already put in place by Postgres. To show how simpler implementations have issues, we simplify the previous example, introducing problems. configuring) after the last such triggering input is submitted? add_node) that CloudPlane will run at least once the required action (e.g. The problem is: how can CloudPlane ensure that given some arbitrary number concurrent inputs (e.g. In the following, this document details the motivations for the design. While use of Semaphores is not complex, it is unorthodox. Whereas, the “consumer” checks ( pos?) and decrements ( decr) that same semaphore. Rephrased, the “producer” of work performs an increment ( incr) on the semaphore called :configure. state = :configuring save_changes end end def work_configuring configure_stuff sem. To show pseudo-code of the above example, one would have request code that looks like this:Ĭlass Server < Actor def work_running if sem. Making the above description more complete, the use of Semaphores looks like this. ![]() For example, when adding a node to a cluster, one must send addressing and credential information to every node in the cluster. We name each Semaphore, so different threads can change independent semaphores concurrently. SemaphoresĬloudPlane uses a table of Semaphores to trigger actions in Actors, avoiding concurrency hazards. ![]() work endįinally, Actor state changes and/or decrementation of semaphores must occur while holding the lock. stub_responses ( :run_instances, " ) old_state = actor. to eq ( 'stopping' ) end it 'makes an instance in creating' do s. uuid, state: 'creating', region: 'us-west-2' ) end it 'begins stopping if offline' do s. describe Server do before do # Better in spec_helper.rb in a real program. One can then run this state machine by hand, which is useful in experimentation: new ( region: region ) end aasm column: :state do state :creating, initial: true state :running state :stopping state :starting_instance state :wait_running event :created do transitions from: :creating, to: :wait_running end event :detect_online do transitions from: :wait_running, to: :running end event :detect_unavailable do transitions from: :running, to: :stopping end event :stopped do transitions from: :stopping, to: :starting_instance end event :started do transitions from: :starting_instance, to: :wait_running end end end Examples of what to do: read fresh telemetry data # or use Net::SSH to connect are examples of what to do here. start_instances ( instance_ids: ) started end def online? # A Stub. stop_instances ( instance_ids: ) stopped end def work_starting_instance ec2. instance_id created end def work_wait_running detect_online if online? end def work_running detect_unavailable unless online? end def work_stopping ec2. include AASM def work send ( 'work_' + state ) save_changes end def work_creating # Note how the client_token feature is used for idempotency. a reader for "id" or setter "instance_id=". First, the SQL schema:Ĭlass Server < Sequel :: Model # Sequel defines accessors for all fields on a table, # e.g. has easy fault injection via mocks and rspec.has crucial mature libraries: Sequel, Net::SSH, AWS SDKv2.has a REPL, and a syntax amenable to its use.Ruby carries the following advantages that one should try to maximize: The web requests provide input to the state machine. It is comparable in this respect to the most common-variety web application. One of those processes serves web requests. It requires Redis, Postgres, and two distinct processes running. With the abstract in mind, I’ll translate this into implementation as seen in CloudPlane.įirst, it’s worth glossing over CloudPlane’s physical resources. A retry of the steps within that state will follow. The net result of these two properties is that the state machine can crash, or error, at any point in execution. In practice, this is easy to adhere to in most cloud APIs. They must also raise errors if they fail, to end execution of a state body. It’s worth noting that each state’s side effects, the cloud_ operations, are idempotent. ![]()
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