Question

I would like to have a master-slave setup of Erlang nodes, where read and write operations happen on the master node only. Slave nodes are only kept as hot-standbys.

As I understand the default behavior of Mnesia is to acquire the lock synchronously on all nodes before executing the write operation. This would result in high latency especially for geographically distributed nodes.

My question is: does Mnesia support asynchronous transactions, where locks are only acquired on the master node, and write operations are propagated afterwards towards slave nodes?

Answer 1

I think you will be happier if you build this off-site replication using a message queue system (rabbitmq perhaps) updating the replicated db yourself from the message queue feed. WAN links are more likely to become congested or go down, and message-queue protocols have ways to handle that. Erlang distribution just give up and you have to spill the updates into a file until the replica comes up and can consume it.

For best symmetry, have posting-to-the-message queue be the primary method to update the db. So even the master is updated by consuming from the message queue. If a response is needed, the current master can send a message back to the issuer of the message.

Mnesia does have a few different kinds of mnesia transaction contexts but nothing that really fit exactly with what you want.

Answer 2

Maybe your application can benefit using sticky locks. I guess that it is quite close to your needs, but...not exactly what you wanted http://www.erlang.org/documentation/doc-5.8.3/lib/mnesia-4.4.17/doc/html/Mnesia_chap4.html#id70700

Answer 3

Interesting Q and equally interesting A!

Basically, what you are suggesing, Christian, is e.g. to have a gen_server - serializing the access to the DB. First time I did that and then I realized: hang on! Mnesia is transactional so it sounds a bit odd to first serialize access and then sort of do it again by updating the DB via a transaction.

I still am a bit puzzled, however, given that mnesia enforces transactional semantics I tend to take that as a hint that you should not have to serialize access yourselves, especially since the implementors of mnesia probably know the system better than I do ;)

I understand that this is not quite a direct answer to your question, however, I d say use mnesia + memorynodes + disknodes. The memorynodes for quick takeover and the disknodes for recovering after a crash/backup.

HTH, haavee

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