Bridge Four is a simple, functional, effectful, single-leader, multi worker, distributed compute system optimized for embarrassingly parallel workloads.

The name is inspired by Brandon Sanderson's "The Stormlight Archive".

It is explained in detail on my blog, where I'm building out this system as a learning exercise instead of "grinding Leetcode".

This is a work in progress, missing key features. It is, however, surprisingly [resilient](#Chaos Monkey).

• Single leader, multi-worker
• Eventually consistent
• Redundant workers, SPOF in the leader
• "Self"-healing (workers can be killed and re-started during execution, the leader will recover)

• A job reads files, performs computations, and writes output files
• Each job executes N tasks (N > 0)
• Each task executes on a worker (aka a spren)
• Each worker has a set number of slots with each can execute exactly 1 task in parallel

General purpose diagram for "starting a job":

1. POST to /start to submit a job
2. Leader accepts or rejects the job, adds it to the cache
3. If a free slot on the cluster is available, it will assign the job; if the cluster capacity is < the required processing capacity for the job, it will do so piecemeal
4. The user can GET /status/$jobId or GET /data/$jobId, which will either return an ExecutionStatus or the result of the job

The user facing interface therefore is eventually consistent.

(From the blog)

Instead of the leader assigning lines of input to mappers that emit individual key-value pairs that get moved deterministically (shuffled) and then reduced to an output, our system is more naive:

It reads pre-split files (assume e.g. a Spark output with properly sized files) which can be read in parallel and write independent outputs, which the leader at the end doing the final aggregation, akin to e.g. a Spark collect() call that sends data to the Driver, with the idea being that each worker's output is a fraction of the original input file. Please see below for a caveat on that in the "Breaking it" section.

That, of course, is not realistic - outside of embarrassingly parallel problems, most systems would run into amassive bottleneck at the leader's end. [1]

However, consider a more computationally intense problem, such as distributed.net - brute forcing ciphers. With minimal adjustments - namely, a mechanism for a worker to report definitive success, which would cause the leader to ignore other worker's output - our system here could feasibly model this, assuming we can pre-generate a set of seed files:

For a naive brute force, task A tries all combinations starting with the letter "a", task B tries all starting with "b" and so on - the input file can be a single letter in atxt file. "Naive" is definitely the word of the hourhere.

Lastly, please check the last section for a retrofit of Partitioning into Bridge Four, making this look more similar to classic Map/Reduce.

• Partitioning: File assignment is greedy and not optimal
• Worker Stages: Jobs start and complete, the leader only checks their state, not intermediate stages (i.e., we can't build a shuffle stage like Map/Reduce right now)
• A sane job interface and a way to provide jars - the BridgeFourJob trait an ADT is pretty dumb and nothing but a stop gap. See the article for details
• Global leader locks: The BackgroundWorker is concurrency-safe, but you can start two jobs that work on the same data, causing races - the leader job controller uses a simple Mutex[F] to compensate
• Atomic operations / 2 Phase Commits
• Consensus: Leader is a Single Point of Failure
• I've been very heavy handed with Sync[F].blocking, which often isn't the correct effect
• File System Abstraction: This assumes a UNIX-like something like NFS to be available, which isn't ideal and has it's own locking problems

sbt docker:publishLocal
docker-compose up

And run sbin/wordcount_docker.sh to run a sample job.

In separate terminals (or computers, provided you have an NFS share mounted - see above), run:

BRIDGEFOUR_PORT=5553 WORKER_ID=0 sbt worker/run 
BRIDGEFOUR_PORT=5554 WORKER_ID=1 sbt worker/run 
WORKER1_PORT=5554 WORKER2_PORT=5553 sbt leader/run 

sbin/wordcount_docker.sh will showcase a execution with more tasks than workers:

1. Download "War and Peace" by Leo Tolstoy
2. Run a word count job on it, with an artificial delay of 30s per task, with 5 tasks on 4 workers
3. Print the output

Sample output:

# ...
{"id":1,"slots":[{"id":{"id":0,"workerId":1},"available":true,"status":{"type":"Done"},"taskId":{"id":1200393588,"jobId":-1368283400}},{"id":{"id":1,"workerId":1},"available":true,"status":{"type":"Done"},"taskId":{"id":1049728891,"jobId":-1368283400}}],"allSlots":[0,1],"availableSlots":[0,1],"runningTasks":[]}
Sleeping for 10 seconds
{"type":"Done"}
Job done
Checking results
{
  "the": 31714,
  "and": 20560,
  "": 16663,
  "to": 16324,

sbin/wordcount_chaos_monkey.sh will gradually murder your cluster and then recover by restarting a single worker, finishing the job in a degraded cluster state.

Run curl --location 'http://localhost:6550/cluster' to see the cluster state.