372 lines
16 KiB
Markdown
372 lines
16 KiB
Markdown
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# Table of contents
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* [Hands-on experiments with Machi and Humming Consensus](#hands-on)
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* [Using the network partition simulator and convergence demo test code](#partition-simulator)
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<a name="hands-on">
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# Hands-on experiments with Machi and Humming Consensus
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## Prerequisites
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Please refer to the
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[Machi development environment prerequisites doc](./dev-prerequisites.md)
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for Machi developer environment prerequisites.
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If you do not have an Erlang/OTP runtime system available, but you do
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have [the Vagrant virtual machine](https://www.vagrantup.com/) manager
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available, then please refer to the instructions in the prerequisites
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doc for using Vagrant.
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<a name="clone-compile">
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## Clone and compile the code
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Please see the
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[Machi 'clone and compile' doc](./dev-clone-compile.md)
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for the short list of steps required to fetch the Machi source code
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from GitHub and to compile & test Machi.
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## Running three Machi instances on a single machine
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All of the commands that should be run at your login shell (e.g. Bash,
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c-shell) can be cut-and-pasted from this document directly to your
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login shell prompt.
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Run the following command:
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make stagedevrel
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This will create a directory structure like this:
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|-dev1-|... stand-alone Machi app + subdirectories
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|-dev-|-dev2-|... stand-alone Machi app + directories
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|-dev3-|... stand-alone Machi app + directories
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Each of the `dev/dev1`, `dev/dev2`, and `dev/dev3` are stand-alone
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application instances of Machi and can be run independently of each
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other on the same machine. This demo will use all three.
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The lifecycle management utilities for Machi are a bit immature,
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currently. They assume that each Machi server runs on a host with a
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unique hostname -- there is no flexibility built-in yet to easily run
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multiple Machi instances on the same machine. To continue with the
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demo, we need to use `sudo` or `su` to obtain superuser privileges to
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edit the `/etc/hosts` file.
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Please add the following line to `/etc/hosts`, using this command:
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sudo sh -c 'echo "127.0.0.1 machi1 machi2 machi3" >> /etc/hosts'
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Next, we will use a shell script to finish setting up our cluster. It
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will do the following for us:
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* Verify that the new line that was added to `/etc/hosts` is correct.
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* Modify the `etc/app.config` files to configure the Humming Consensus
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chain manager's actions logged to the `log/console.log` file.
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* Start the three application instances.
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* Verify that the three instances are running correctly.
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* Configure a single chain, with one FLU server per application
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instance.
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Please run this script using this command:
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./priv/humming-consensus-demo.setup.sh
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If the output looks like this (and exits with status zero), then the
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script was successful.
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Step: Verify that the required entries in /etc/hosts are present
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Step: add a verbose logging option to app.config
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Step: start three three Machi application instances
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pong
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pong
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pong
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Step: configure one chain to start a Humming Consensus group with three members
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Result: ok
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Result: ok
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Result: ok
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We have now created a single replica chain, called `c1`, that has
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three file servers participating in the chain. Thanks to the
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hostnames that we added to `/etc/hosts`, all are using the localhost
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network interface.
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| App instance | Pseudo | FLU name | TCP port |
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| directory | Hostname | | number |
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|--------------+----------+----------+----------|
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| dev1 | machi1 | flu1 | 20401 |
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| dev2 | machi2 | flu2 | 20402 |
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| dev3 | machi3 | flu3 | 20403 |
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The log files for each application instance can be found in the
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`./dev/devN/log/console.log` file, where the `N` is the instance
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number: 1, 2, or 3.
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## Understanding the chain manager's log file output
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After running the `./priv/humming-consensus-demo.setup.sh` script,
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let's look at the last few lines of the `./dev/dev1/log/console.log`
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log file for Erlang VM process #1.
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2016-03-09 10:16:35.676 [info] <0.105.0>@machi_lifecycle_mgr:process_pending_flu:422 Started FLU f1 with supervisor pid <0.128.0>
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2016-03-09 10:16:35.676 [info] <0.105.0>@machi_lifecycle_mgr:move_to_flu_config:540 Creating FLU config file f1
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2016-03-09 10:16:35.790 [info] <0.105.0>@machi_lifecycle_mgr:bootstrap_chain2:312 Configured chain c1 via FLU f1 to mode=ap_mode all=[f1,f2,f3] witnesses=[]
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2016-03-09 10:16:35.790 [info] <0.105.0>@machi_lifecycle_mgr:move_to_chain_config:546 Creating chain config file c1
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2016-03-09 10:16:44.139 [info] <0.132.0> CONFIRM epoch 1141 <<155,42,7,221>> upi [] rep [] auth f1 by f1
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2016-03-09 10:16:44.271 [info] <0.132.0> CONFIRM epoch 1148 <<57,213,154,16>> upi [f1] rep [] auth f1 by f1
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2016-03-09 10:16:44.864 [info] <0.132.0> CONFIRM epoch 1151 <<239,29,39,70>> upi [f1] rep [f3] auth f1 by f1
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2016-03-09 10:16:45.235 [info] <0.132.0> CONFIRM epoch 1152 <<173,17,66,225>> upi [f2] rep [f1,f3] auth f2 by f1
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2016-03-09 10:16:47.343 [info] <0.132.0> CONFIRM epoch 1154 <<154,231,224,149>> upi [f2,f1,f3] rep [] auth f2 by f1
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Let's pick apart some of these lines. We have started all three
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servers at about the same time. We see some race conditions happen,
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and some jostling and readjustment happens pretty quickly in the first
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few seconds.
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* `Started FLU f1 with supervisor pid <0.128.0>`
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* This VM, #1,
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started a FLU (Machi data server) with the name `f1`. In the Erlang
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process supervisor hierarchy, the process ID of the top supervisor
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is `<0.128.0>`.
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* `Configured chain c1 via FLU f1 to mode=ap_mode all=[f1,f2,f3] witnesses=[]`
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* A bootstrap configuration for a chain named `c1` has been created.
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* The FLUs/data servers that are eligible for participation in the
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chain have names `f1`, `f2`, and `f3`.
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* The chain will operate in eventual consistency mode (`ap_mode`)
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* The witness server list is empty. Witness servers are never used
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in eventual consistency mode.
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* `CONFIRM epoch 1141 <<155,42,7,221>> upi [] rep [] auth f1 by f1`
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* All participants in epoch 1141 are unanimous in adopting epoch
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1141's projection. All active membership lists are empty, so
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there is no functional chain replication yet, at least as far as
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server `f1` knows
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* The epoch's abbreviated checksum is `<<155,42,7,221>>`.
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* The UPI list, i.e. the replicas whose data is 100% in sync is
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`[]`, the empty list. (UPI = Update Propagation Invariant)
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* The list of servers that are under data repair (`rep`) is also
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empty, `[]`.
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* This projection was authored by server `f1`.
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* The log message was generated by server `f1`.
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* `CONFIRM epoch 1148 <<57,213,154,16>> upi [f1] rep [] auth f1 by f1`
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* Now the server `f1` has created a chain of length 1, `[f1]`.
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* Chain repair/file re-sync is not required when the UPI server list
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changes from length 0 -> 1.
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* `CONFIRM epoch 1151 <<239,29,39,70>> upi [f1] rep [f3] auth f1 by f1`
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* Server `f1` has noticed that server `f3` is alive. Apparently it
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has not yet noticed that server `f2` is also running.
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* Server `f3` is in the repair list.
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* `CONFIRM epoch 1152 <<173,17,66,225>> upi [f2] rep [f1,f3] auth f2 by f1`
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* Server `f2` is apparently now aware that all three servers are running.
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* The previous configuration used by `f2` was `upi [f2]`, i.e., `f2`
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was running in a chain of one. `f2` noticed that `f1` and `f3`
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were now available and has started adding them to the chain.
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* All new servers are always added to the tail of the chain in the
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repair list.
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* In eventual consistency mode, a UPI change like this is OK.
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* When performing a read, a client must read from both tail of the
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UPI list and also from all repairing servers.
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* When performing a write, the client writes to both the UPI
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server list and also the repairing list, in that order.
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* I.e., the client concatenates both lists,
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`UPI ++ Repairing`, for its chain configuration for the write.
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* Server `f2` will trigger file repair/re-sync shortly.
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* The waiting time for starting repair has been configured to be
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extremely short, 1 second. The default waiting time is 10
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seconds, in case Humming Consensus remains unstable.
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* `CONFIRM epoch 1154 <<154,231,224,149>> upi [f2,f1,f3] rep [] auth f2 by f1`
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* File repair/re-sync has finished. All file data on all servers
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are now in sync.
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* The UPI/in-sync part of the chain is now `[f2,f1,f3]`, and there
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are no servers under repair.
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## Let's create some failures
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Here are some suggestions for creating failures.
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* Use the `./dev/devN/bin/machi stop` and `./dev/devN/bin/machi start`
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commands to stop & start VM #`N`.
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* Stop a VM abnormally by using `kill`. The OS process name to look
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for is `beam.smp`.
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* Suspend and resume a VM, using the `SIGSTOP` and `SIGCONT` signals.
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* E.g. `kill -STOP 9823` and `kill -CONT 9823`
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The network partition simulator is not (yet) available when running
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Machi in this mode. Please see the next section for instructions on
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how to use partition simulator.
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<a name="partition-simulator">
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# Using the network partition simulator and convergence demo test code
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This is the demo code mentioned in the presentation that Scott Lystig
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Fritchie gave at the
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[RICON 2015 conference](http://ricon.io).
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* [slides (PDF format)](http://ricon.io/speakers/slides/Scott_Fritchie_Ricon_2015.pdf)
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* [video](https://www.youtube.com/watch?v=yR5kHL1bu1Q)
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## A complete example of all input and output
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If you don't have an Erlang/OTP 17 runtime environment available,
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please see this file for full input and output of a strong consistency
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length=3 chain test:
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https://gist.github.com/slfritchie/8352efc88cc18e62c72c
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This file contains all commands input and all simulator output from a
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sample run of the simulator.
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To help interpret the output of the test, please skip ahead to the
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"The test output is very verbose" section.
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## Prerequisites
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If you don't have `git` and/or the Erlang 17 runtime system available
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on your OS X, FreeBSD, Linux, or Solaris machine, please take a look
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at the [Prerequisites section](#prerequisites) first. When you have
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installed the prerequisite software, please return back here.
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## Clone and compile the code
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Please briefly visit the [Clone and compile the code](#clone-compile)
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section. When finished, please return back here.
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## Run an interactive Erlang CLI shell
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Run the following command at your login shell:
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erl -pz .eunit ebin deps/*/ebin
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If you are using Erlang/OTP version 17, you should see some CLI output
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that looks like this:
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Erlang/OTP 17 [erts-6.4] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]
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Eshell V6.4 (abort with ^G)
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1>
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## The test output is very verbose ... what are the important parts?
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The output of the Erlang command
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`machi_chain_manager1_converge_demo:help()` will display the following
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guide to the output of the tests.
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A visualization of the convergence behavior of the chain self-management
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algorithm for Machi.
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1. Set up some server and chain manager pairs.
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2. Create a number of different network partition scenarios, where
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(simulated) partitions may be symmetric or asymmetric. Then stop changing
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the partitions and keep the simulated network stable (and perhaps broken).
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3. Run a number of iterations of the algorithm in parallel by poking each
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of the manager processes on a random'ish basis.
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4. Afterward, fetch the chain transition changes made by each FLU and
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verify that no transition was unsafe.
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During the iteration periods, the following is a cheatsheet for the output.
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See the internal source for interpreting the rest of the output.
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'SET partitions = '
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A pair-wise list of actors which cannot send messages. The
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list is uni-directional. If there are three servers (a,b,c),
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and if the partitions list is '[{a,b},{b,c}]' then all
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messages from a->b and b->c will be dropped, but any other
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sender->recipient messages will be delivered successfully.
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'x uses:'
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The FLU x has made an internal state transition and is using
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this epoch's projection as operating chain configuration. The
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rest of the line is a summary of the projection.
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'CONFIRM epoch {N}'
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This message confirms that all of the servers listed in the
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UPI and repairing lists of the projection at epoch {N} have
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agreed to use this projection because they all have written
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this projection to their respective private projection stores.
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The chain is now usable by/available to all clients.
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'Sweet, private projections are stable'
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This report announces that this iteration of the test cycle
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has passed successfully. The report that follows briefly
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summarizes the latest private projection used by each
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participating server. For example, when in strong consistency
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mode with 'a' as a witness and 'b' and 'c' as real servers:
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%% Legend:
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%% server name, epoch ID, UPI list, repairing list, down list, ...
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%% ... witness list, 'false' (a constant value)
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[{a,{{1116,<<23,143,246,55>>},[a,b],[],[c],[a],false}},
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{b,{{1116,<<23,143,246,55>>},[a,b],[],[c],[a],false}}]
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Both servers 'a' and 'b' agree on epoch 1116 with epoch ID
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{1116,<<23,143,246,55>>} where UPI=[a,b], repairing=[],
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down=[c], and witnesses=[a].
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Server 'c' is not shown because 'c' has wedged itself OOS (out
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of service) by configuring a chain length of zero.
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If no servers are listed in the report (i.e. only '[]' is
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displayed), then all servers have wedged themselves OOS, and
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the chain is unavailable.
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'DoIt,'
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This marks a group of tick events which trigger the manager
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processes to evaluate their environment and perhaps make a
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state transition.
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A long chain of 'DoIt,DoIt,DoIt,' means that the chain state has
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(probably) settled to a stable configuration, which is the goal of the
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algorithm.
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Press control-c to interrupt the test....".
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## Run a test in eventual consistency mode
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Run the following command at the Erlang CLI prompt:
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machi_chain_manager1_converge_demo:t(3, [{private_write_verbose,true}]).
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The first argument, `3`, is the number of servers to participate in
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the chain. Please note:
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* Chain lengths as short as 1 or 2 are valid, but the results are a
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bit boring.
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* Chain lengths as long as 7 or 9 can be used, but they may
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suffer from longer periods of churn/instability before all chain
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managers reach agreement via humming consensus. (It is future work
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to shorten the worst of the unstable churn latencies.)
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* In eventual consistency mode, chain lengths may be even numbers,
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e.g. 2, 4, or 6.
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* The simulator will choose partition events from the permutations of
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all 1, 2, and 3 node partition pairs. The total runtime will
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increase *dramatically* with chain length.
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* Chain length 2: about 3 partition cases
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* Chain length 3: about 35 partition cases
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* Chain length 4: about 230 partition cases
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* Chain length 5: about 1100 partition cases
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## Run a test in strong consistency mode (with witnesses):
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*NOTE:* Due to a bug in the test code, please do not try to run the
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convergence test in strong consistency mode and also without the
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correct minority number of witness servers! If in doubt, please run
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the commands shown below exactly.
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Run the following command at the Erlang CLI prompt:
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machi_chain_manager1_converge_demo:t(3, [{private_write_verbose,true}, {consistency_mode, cp_mode}, {witnesses, [a]}]).
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The first argument, `3`, is the number of servers to participate in
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the chain. Chain lengths as long as 7 or 9 can be used, but they may
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suffer from longer periods of churn/instability before all chain
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managers reach agreement via humming consensus.
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Due to the bug mentioned above, please use the following
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commands when running with chain lengths of 5 or 7, respectively.
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machi_chain_manager1_converge_demo:t(5, [{private_write_verbose,true}, {consistency_mode, cp_mode}, {witnesses, [a,b]}]).
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machi_chain_manager1_converge_demo:t(7, [{private_write_verbose,true}, {consistency_mode, cp_mode}, {witnesses, [a,b,c]}]).
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