Apache Ignite internal locking for cache and PME Updates

Question

I want to understand the internal locking mechanism used by Apache Ignite for Cache and PME updates:

• Scenario 1 : Persistent cache

  For a persistent cache, it requires lock during checkpointing and put operation. I am trying to understand at what level ignite takes this lock and if it is read or write lock. Is it for whole cache on all objects or across all caches or just on the object/objects being updated by checkpointing and cache updates.

• Scenario 2 : In memory Cache

  For In memory cache, there will be no checkpointing. So only locking will happen during cache updates. Is this lock taken at cache level or across all caches or just on the object/objects being updated by cache updates. If it is read or write lock?

As PME requires write lock across all caches, I am trying to get clarity on locking due to cache operations.

Any pointers on above points will be helpful.

Answer 1

Checkpoint lock

I guess checkpoint lock is, in fact, very important to Ignite's operation with enabled persistence, and you ask about it specifically, so let's talk about it in detail.

Ignite will take a checkpoint read lock on a put operation, and on a very large number of other operations. Any operation that needs to access data or metadata will first acquire a checkpoint read lock.

The checkpoint write lock is acquired only by one thread that starts the checkpointing process, every 3 minutes by default (checkpointTimeout property). It will only hold the write lock while it creates a list of pages to be checkpointed which normally takes milliseconds at most.

The purpose of all this is to make sure that the checkpoint process can safely find the dirty pages that need to be written to disk.

Other locks

There are actually many different locks you need to get when doing reads and writes - segment locks, page locks, entry locks... Ignite is a highly concurrent system written in a threaded model, and because of that, it requires lots of locking. Basically, every component introduces its own concurrency and locking to the whole system.

If you really want to learn about the details, I suggest you start with the design documents on the Ignite wiki. Here are a few of particular interest:

• Persistence
• Page Memory
• Partition Map Exchange (PME)
• Transactions