Throttle

Semaphores for distributed systems.

Motivation

Throttle provides semaphores as a service via an http interface. As the name indicates the primary usecase in mind is to throttle a systems access to a resource, by having the elements of that system to ask for permission (i.e. acquiring a lease) first. If the system consists of several process running on different machines, or virtual machines in the same Network, throttle might fit the bill.

Throttle aims to be easy to operate, well-behaved in edge cases and works without a persistence backend.

Features

• Server builds and runs on Windows, Linux, and OS-X.
• Python Client is available.
• Fairness (longer waiting peers have priority)
  • Peers acquiring locks with a large count, won't be starved by lots of others with a small one.
• Locks expire to prevent leaking semaphore count due to Network errors or client crashes.
• Locks can be prolonged indefinetly by sending heartbeats of the server.
• No persistence backend is required.
  • Server recovers state from heartbeats in case of reboot.

Work in progress, interfaces are still unstable.

Usage

Operating a Throttle server

Starting and Shutdown

Assuming the throttle executable to be in your path environment variable, you start a throttle sever by executing it. You can display the availible command line options using the --help flag. Starting it without any arguments is going to boot the server with default configuration.

throttle

This starts the server in the current process. Navigate with a browser to localhost:8000 to see a welcoming message. You can shut Throttle down gracefully by pressing Ctrl + C.

Default logging to stderr

Set the THROTTLE_LOG environment variable to see more output on standard error. Valid values are ERROR, WARN, INFO, DEBUG and TRACE.

In bash:

THROTTLE_LOG=WARN

or PowerShell:

$env:THROTLLE_LOG="WARN"

Starting the server now with an empty configuration yields a warning.

[2020-03-03T19:02:42Z WARN  throttle] No semaphores configured.

Hint: Enabling Gelf logging currently disables logging to standard error.

Toml configuration file

To actually serve semaphores, we need to configure their names and full count. By default Throttle is looking for a configuration in the working directories throttle.toml file should it exist.

# Sample throttle.cfg Explaining the options

# The time interval in which the litter collection backgroud thread checks for expired leases.
# Default is set to 5 minutes.
litter_collection_interval = "5min"

[semaphores]
# Specify name and full count of semaphores. Below line creates a semaphore named A with a full
# count of 42. Setting the count to 1 would create a Mutex.
A = 42


# Optional logging config, to log into graylog
[logging.gelf]
name = "MyThrottleServer"
host = "my_graylog_instance.cloud"
port = 12201
## Set this to either ERROR, WARN, INFO, DEBUG or TRACE.
level = "INFO"

Metrics

Throttle supports Prometheus metrics, via the /metrics endpoint. Depending on your configuration and state they may e.g. look like this:

# HELP throttle_count Accumulated count of all active leases
# TYPE throttle_count gauge
throttle_count{semaphore="A"} 0
# HELP throttle_full_count New leases which would increase the count beyond this limit are pending.
# TYPE throttle_full_count gauge
throttle_full_count{semaphore="A"} 42
# HELP throttle_longest_pending_sec Time the longest pending peer is waiting until now, to acquire a lock to a semaphore.
# TYPE throttle_longest_pending_sec gauge
throttle_longest_pending_sec{semaphore="A"} 0
# HELP throttle_num_404 Number of Get requests to unknown resource.
# TYPE throttle_num_404 counter
throttle_num_404 0
# HELP throttle_pending Accumulated count of all pending leases
# TYPE throttle_pending gauge
throttle_pending{semaphore="A"} 0

Python client

Throttle ships with a Python client. Here is how to use it in a nutshell.

from throttle_client import Client, lock

# Configure endpoint to throttle server
c = Client("http://localhost:8000")

# Use client configuraton to acquire a lock (amount 1) to semaphore A
with lock(c, "A"):
    # Do stuff while holding lock to "A"
    # ...

# A is released at the end of with block

Installation

Server

The server binary is published to crates.io and thus installable via cargo.

cargo install throttle-server

Python Client

Python client is publish to PyPi and can be installed using pip.

pip install throttle-client

Local build and test setup

• Install Rust compiler and Cargo. Follow the instructions on this site

• Checkout the sources with

  git clone https://github.com/pacman82/throttle.git
  

• You can run the build and run unit tests by executing

  cd throttle
  cargo test
  

• Execute integration test with clients

  cd python_client
  pip install -r test-requirements.txt
  pip install -e .
  pytest