I’m evaluating Terracotta to help me scale up an application which is currently RAM-bounded. It is a collaborative filter and stores about 2 kilobytes of data per-user. I want to use Amazon’s EC2, which means I’m limited to 14GB of RAM, which gives me an effective per-server upper-bound of around 7 million users. I need to be able to scale beyond this.
Based on my reading so-far I gather that Terracotta can have a clustered heap larger than the available RAM on each server. Would it be viable to have an effective clustered heap of 30GB or more, where each of the servers only supports 14GB?
The per-user data (the bulk of which are arrays of floats) changes very frequently, potentially hundreds of thousands of times per minute. It isn’t necessary for every single one of these changes to be synchronized to other nodes in the cluster the moment they occur. Is it possible to only synchronize some object fields periodically?
I’d say the answer is a qualified yes for this. Terracotta does allow you to work with clustered heaps larger than the size of a single JVM although that’s not the most common use case.
You still need to keep in mind a) the working set size and b) the amount of data traffic. For a), there is some set of data that must be in memory to perform the work at any given time and if that working set size > heap size, performance will obviously suffer. For b), each piece of data added/updated in the clustered heap must be sent to the server. Terracotta is best when you are changing fine-grained fields in pojo graphs. Working with big arrays does not take the best advantage of the Terracotta capabilities (which is not to say that people don’t use it that way sometimes).
If you are creating a lot of garbage, then the Terracotta memory managers and distributed garbage collector has to be able to keep up with that. It’s hard to say without trying it whether your data volumes exceed the available bandwidth there.
Your application will benefit enormously if you run multiple servers and data is partitioned by server or has some amount of locality of reference. In that case, you only need the data for one server’s partition in heap and the rest does not need to be faulted into memory. It will of course be faulted if necessary for failover/availability if other servers go down. What this means is that in the case of partitioned data, you are not broadcasting to all nodes, only sending transactions to the server.
From a numbers point of view, it is possible to index 30GB of data, so that’s not close to any hard limit.