A Lightweight Alternative to ApacheMQ or RabbitMQ: OCamlMQ
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Join For Freeocamlmq is a messaging system written in OCaml (as you might have
guessed) but it can be used with virtually any language (a STOMP
protocol) implementation
is required). It has a small footprint and it was written using around
1,200 lines of code. The message broker's author, Mauricio Fernández,
says that ocamlmq is
especially suitable for creating task queues and communication between
subsystems.
Because ActiveMQ and RabbitMQ (with STOMP adapters) didn't work the way Fernández needed, he created created ocamlmq. The problems he ran into included excessive memory footprint, performance issues with ApacheMQ's scalable storage backends (KahaDB, JDBC), and performance issues with RabbitMQ's topic message dispatch. Fernández says the memory footprint for ocamplmq is under 3MB at startup while ApacheMQ is over 120MB. He also mentions that RabbitMQ was doing a linear scan of the subscription table per dispatch during its topic message dispatch.
ocamlmq can scale over arbitrarily large numbers of queues with constant memory usage, and it also has strong message saving guarantees. It features message priorities and a per-subscription prefetch limit for queue messages. There is error handling and ACK timeout, so if a subscriber doesn't ACK a message after its timeout, the message will be sent to a different subscriber. If a subscriber dies and its connection expires, messages will also be re-sent automatically.
Millions of subscriptions cause no problems for ocamlmq. It also has
topic subscriptions with prefix matching, and simple extensions within
the STOMP protocol that can report the number of messages in a queue and
the subscribers to a queue or topic. ocamlmq does have a few
limitations outside of its intended domain, which is persistent queues
and transient topic destinations. It's not designed to perform well when
scaled beyond a thousand simultaneous connections and there is no flow
control for topic messages (in the proper use case, topics are small and
processed fast). ocamlmq currently implements PostgreSQL's backend
which can only persist a few thousand messages per second, but it allows
message bursts of more than 50k per second in async mode. It also
doesn't allow very high message rates (20k/s on 3GHz AMD64).
However, ocamlmq is highly scalable, and it can support millions of queues and topic subscriptions with a low memory footprint. It usually only needs around 150 bytes per subscription. Remember, ActiveMQ requires 120MB on startup. Queries have no additional memory cost, so you are free to use plenty without regard for memory.
The ocamlmq source code can be found on GitHub. You'll need a working OCaml environment and three other libraries.
Because ActiveMQ and RabbitMQ (with STOMP adapters) didn't work the way Fernández needed, he created created ocamlmq. The problems he ran into included excessive memory footprint, performance issues with ApacheMQ's scalable storage backends (KahaDB, JDBC), and performance issues with RabbitMQ's topic message dispatch. Fernández says the memory footprint for ocamplmq is under 3MB at startup while ApacheMQ is over 120MB. He also mentions that RabbitMQ was doing a linear scan of the subscription table per dispatch during its topic message dispatch.
ocamlmq can scale over arbitrarily large numbers of queues with constant memory usage, and it also has strong message saving guarantees. It features message priorities and a per-subscription prefetch limit for queue messages. There is error handling and ACK timeout, so if a subscriber doesn't ACK a message after its timeout, the message will be sent to a different subscriber. If a subscriber dies and its connection expires, messages will also be re-sent automatically.

However, ocamlmq is highly scalable, and it can support millions of queues and topic subscriptions with a low memory footprint. It usually only needs around 150 bytes per subscription. Remember, ActiveMQ requires 120MB on startup. Queries have no additional memory cost, so you are free to use plenty without regard for memory.
The ocamlmq source code can be found on GitHub. You'll need a working OCaml environment and three other libraries.
Memory (storage engine)
Flow control (data)
Message broker
Timeout (computing)
Protocol (object-oriented programming)
OCaml
Database
Connection (dance)
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