Currently what does the ipcluster plugin do when AWS shuts down one or more of the spot instance nodes? Is there any mechanism to re-start and then re-add these nodes back to the IPython cluster?
You need to use the loadbalance command in order to scale your cluster. If you know you want x nodes at all times, simply launch it with "--max_nodes x --min_nodes x" and it will try to add back the nodes as soon as they go away.
If your nodes go away, it's probably because of the spot price market fluctuations so your might have to wait for it to go below your SPOT_BID value before seeing them appear back.
I use the load balancer a lot with the SGE plugin, but never tried it with ipcluster, so I do not know how well it will behave.
Related
Well,maybe it seems a ridicilous question but I need some help and think that maybe someone else among approximately 8 billion human on the planet has same issue. Whenever I stop or delete some images or containers on docker dashboard, notification is filling so much space on screen and I cannot click on 'containers' or 'images' section again.
I just want to delete/stop containers/images quickly by clicking like "tiki-taka-tiki-taka" but not possible, I need to click 'X' to close it every single time and this process is so slow.
Usually this kind of products are made for enabling users to make things easier and faster instead of using command prompt but, for docker, using command prompt/terminal is much faster than using dashboard.
Do you know how to disable this notifications?(If it exists)
or like:
You should contact docker's developers for this issue.
For the meantime you could try Portainer to manage your images, volumes ecc...
I have a standard situation, two distributed Erlang nodes, one master one standby.
When I stop the master the standby comes on - failover, when I start the master the standby stops - takeover. Everything works fine as long as heart is not turned on and there is no network split.
However, when I disconnect the master from the network after 60 seconds or so the standby gives me an error message ** removing (timedout) connection ** and starts up as if the master node stopped. This makes sense to me, it doesn't know if the master is alive or not, and epmd can't connect to the master node so it is removed from the nodes() list. Lets pretend for a moment that this is the desired outcome.
The problem is that, when the connection is restored, I have master and standby running at the same time and the standby is oblivious to the fact that the master is running. Pinging the standby during the masters init does not solve the issue. I checked nodes() on the standby after doing so, it sees the master node but still it continues to run.
My solution for now has been to create a process, that monitors all nodes that are above each node in hierarchy and if any of them are online, can be pinged, the process calls erlang:halt() to terminate the standby node. It works for simple situations, but maybe someone can tell me if there is a better way? I found a similar problem described on Elixir forum so it probably a known erlang problem without an easy solution. https://elixirforum.com/t/distributed-application-network-split/10007
If during a network split you don't want to have two nodes running in parallel I'm guessing an outside monitoring application needs to be used?
The second major issue is heart. If heart is turned on, as is, the failover never happens. If heart is running with a sleep before it calls start it stops the failover node when it calls the application start. So even when it can't start the master, do to it not having access to vital resources for example, it stops the failover node, and doesn't bring it back up after it fails to start the master. I don't know if heart is not supposed to be used with a distributed application or if there is an option to run some erlang code to check if the resources are available before attempting a start the node and before stopping the failover node?
The documentation on heart is not great. Very hard to find any examples of HEART_COMMAND. I found a way to set the HEART_COMMAND to a script from within my application, but there is a limit to how long the argument can be, and it's not as long as stated in the documentation from what I can tell. This for example sets a sleep timer for 60 seconds before calling application start again. It doesn't solve any issues, because in 60 seconds it stops the failover node and hangs if master node can't start.
heart:set_cmd("sleep 60; ./bin/myapp start")
The solution I've ended up with for now is letting heart of the main release start another release, a pre-loader, which does a preliminary check that all resources are available and if they are it starts the main release-application, and if they are not it continues checking forever. This way the main app is running on the failover node without interruption. So the main release has heart turned on, and the pre-loader does not. I ended up using a bash script file because I needed to do more work than I could fit in the heart:set_cmd/1, so I'm still calling heart:set_cmd(Dir ++ "/priv/myHeartScript.sh " ++ Arg1 ++ " " ++ Arg2), but don't get carried away with the Args as there is a limit on size! I also used Environment Variables which I set in vm.args using -env to pass data to the script, such as the pre-loader path/name. This allowed me to avoid having to edit the scrip too during deployment.
If anyone has a better solution PLEASE let me know.
UPDATE
The team at Erlang Solutions was kind enough to shed some light onto the subject. Basically, nobody they know uses the Erlangs built in distributed model. Everything revolves around the data, and as long as it is available on redundant databases you can spin up new applications anytime. They recommend using the cloud hosts that can spin up new servers when one goes down or use a redundant node design, so have 5 nodes up in parallel and if a few go down you can restart them manually or by other means.
As for me, I can say that getting heart to start a pre-loader release/app gets the job done but it gets complicated fast. To launch the app now requires provisioning several extra sys.config/vm.args/rebar.config files. I will be looking into their suggestions for the next iteration.
UPDATE
Moved away from using Erlang distributed model. Using RabbitMQ to send heartbeats to all nodes, including itself. If a node is receiving heartbeats from itself and no other node it's the master, if receiving more than one use any attribute like node name to chose the master. You don't have to use RabbitMQ, but you need to make sure all nodes can reach the same destination and consume from it.
Also, devOps oppose using heart. They prefer to use standard Linux tools to monitor application status and restart it after crash or a server reboot.
If you start up a node as a slave, all its log output goes to the master. However, in my setup, I don't want to necessarily have a master, and I have nodes that automatically discover and join the cluster at will. I'd still like to have all the cluster's log output go to a single node, though. Is there a way to dynamically make a node's logging behave as if it were started as a slave? Otherwise, do I need to alter each installed error_handler to redirect output to where I want it to go?
Here would be my ideal setup: I flip a switch and all nodes in the cluster send everything that's going to any of the nodes' tty--io:format calls or sasl reports or what have you-- instead to one node where it is both displayed on the tty and logged in round robin files. What would make this a reality?
Use group_leader for this purpose. Have you checked this link?
I think http://jkvor.com/log-roller may be the answer, although it wouldn't capture io:format calls, I don't think. However, if you confine your log function calls to the error_logger module, it should work great.
I am running two erlang nodes with a replicated mnesia database. Whenever I tried to start one of them while mnesia IS NOT Running on the other one, mnesia:wait_for_tables(?TABS,?TIMEOUT), would hang on the node that its called from. I need to have a structure where (if both nodes are not running), I can start working with one while the other is down and later decide to bring the other one up yet continue to work well. I need to be sure that the first node that was running has updated the later when it gets up. Does this necessarily require me to have one as the master?
%%% Edited...........................................................................
Oh, I've got it. The database I was using had a couple of fragmented tables. Some of the fragments had been distributed across the network for load balancing. So, Mnesia on one host would try to load them across the network and would fail since mnesia on the other one is down!
I guess this has got nothing to do with a mnesia master node. But I still would love to understand the significance of the same because I've not used it before, yet, I always play with distributed schemas.
Thanks again...
Mnesia master nodes are used to resolve split-brain situations in a fairly brutal fashion. If mnesia discovers a split-brain situation, it will issue an event, "running partitioned network". One way to respond to this would be to set master nodes to the "island" that you want to keep, and then restart the other nodes. When they come back up, they will unconditionally load tables from the master nodes.
There is another mechanism in mnesia, called force_load. One should be very careful with it, but in the case where you have two nodes, A and B, terminate B (A logs B as down), then terminate A, then restart B, B will have no info about when A went down, so will refuse to load tables that have a copy on A. If you know that A is not coming back soon, you could choose to call mnesia:force_load_tables(Ts) on B, which will cause it to run with its own copies. Once A comes back up, it will detect that B is up, and will load tables from it. As you can see, there are several other scenarios where you can end up with an inconsistent database. Mnesia will not fix that, but tries to provide tools to resolve the situation if it arises. In the scenario above, unfortunately, mnesia will give you no hints, but it is possible to create an application that detects the problem.
My application runs in an erlang cluster - with usually two or more nodes. There's active monitoring between the nodes (using erlang:monitor_node) which works fine - I can detect and react to the fact that a node that was up is now down.
But how do I then find out that the node has restarted and is back in business? I can of course periodically ping the node until it is back up, but is there a better way that I've simply missed? Is process groups a better way of achieving this?
(Edited to add)
I think the answer to perform a technique like election of a supervisor is the thought process I was missing. I'll look into that and mark this question as done....
But how do I then find out that the node has restarted and is back in business? I can of course periodically ping the node until it is back up, but is there a better way that I've simply missed? Is process groups a better way of achieving this?
Just an idea, but how about having the restarting node itself explicitly inform the supervisor/monitoring node that it has finished restarting and that it is available again?
You could use a recurring "heartbeat message" for this purpose, or come up with a custom message specifically meant to be sent once after successful initialization. Something along the lines of:
start(SupervisorPID) ->
SuperVisorPID ! {hello, MyPID};
mainloop().
You could create a global_group then use the global_group:monitor_nodes(true) to monitor the other nodes within the same global group. The process that is monitoring the nodes will get nodeup and nodedown messages.