Erlang does have shared memory

I occasionally hear people say things such as "Erlang makes concurrency easy because it doesn't have shared memory" or "processes in Erlang communicate just by message passing." Just earlier today I came across one such comment on Steve Yegge's post Dynamic Languages Strike Back (you'll have some scrolling down to do -- it's a long article :) ).

For many purposes, it's "good enough" to think of Erlang as lacking shared memory. Erlang's built-in message passing semantics makes inter-process communications trivial to implement, and for many concurrent applications, it's all you need. Also, when you first learn Erlang, it's easy to get excited by the promise of not having to worry about the difficult shared memory problems you encounter in "traditional" concurrent programming. And that's for a good reason: Erlang indeed makes concurrent programming much easier (and more scalable, reliable, etc) than other languages. However, it's not true that Erlang processes can't share memory.

Erlang has (a kind of) shared memory: it's called ets.

From the ets documentation:

This module provides very limited support for concurrent updates. No locking is available, but the safe_fixtable/2 function can be used to guarantee that a sequence of first/1 and next/2 calls will traverse the table without errors and that each object in the table is visited exactly once, even if another process (or the same process) simultaneously deletes or inserts objects into the table. Nothing more is guaranteed; in particular any object inserted during a traversal may be visited in the traversal.

Multiple Erlang processes can simultaneously access and manipulate an ets table, which makes ets act very much like shared memory. ets isn't identical to shared memory in other languages, however. These are the main differences:

- Objects are copied when inserted into and looked-up from ets tables.
- Basic consistency is guaranteed. Individual ets records never get garbled.
- ets tables are not garbage collected (this lets you store massive amounts of data in RAM without incurring garbage collection penalties -- an important trait for soft real time performance.)

Despite these differences, as far as the programmer is concerned, ets is effectively shared memory. If you want to guarantee that multiple processes get a consistent snapshot of a set of objects in a plain ets table, you're out of luck.

The good news is that Erlang doesn't leave you to your own devices to figure out some subtle solution involving locking around critical regions to access ets tables safely from multiple processes. Instead, it provides a very nice tool for working with ets: Mnesia. Mnesia is is a kind of STM for Erlang, with some extra properties such as support for distributed and persistent storage. Mnesia has a simple transaction API you can use to ensure atomicity, isolation and consistency when accessing objects in an ets table.

Here's an example, taken for the Mnesia documentation, of how to raise an employee's salary in a transaction:


raise(Eno, Raise) ->
F = fun() ->
[E] = mnesia:read(employee, Eno, write),
Salary = E#employee.salary + Raise,
New = E#employee{salary = Salary},
mnesia:write(New)
end,
mnesia:transaction(F).


So, Erlang has shared memory, and it also has Mnesia, which provides easy transactional access to ets. Does that mean that concurrent programming in Erlang is just like other languages, but with using Mnesia for storing shared data? Not exactly. When you program in Erlang, you still use message passing in many situations where in other languages you would rely on semaphors/locks/monitors/signals/etc to enable inter-thread communications. In the simplest possible example, a producer/consumer application, you would use messaging in Erlang, not ets. In most other languages, you would probably use monitors to protect against concurrent access to a shared buffer (see the wikipedia examples).

Erlang isn't the only language that has message queues. In fact, you could use the basic concurrency facilities in most languages to implement message queues as higher-level abstractions for inter-thread communications (although you probably wouldn't be able to replicate Erlang's selective receive using pattern matching, and it probably wouldn't scale or perform as well as Erlang). This is exactly what some of the actor libraries out there do. However, you would have to be very careful to not send pointers/references to objects that would end up being shared between threads, or you would potentially run into nasty bugs when multiple threads modify the same object. In Erlang, all data is immutable, which makes such bugs impossible. And if even you figure out how to ensure copy semantics for message passing, you would still have your work cut out for you to allow processes to communicate between VMs...

Implementing full Erlang style concurrency isn't trivial. I don't think it can be added as a library to a language that doesn't have it by design with support from the runtime.

Erlang takes you as close as possible to concurrent (and distributed!) programming bliss -- but it does have (a kind of) shared memory: ets.